%Switch% Archives %%page%%

Tag Archive for 'Switch'

Open Shortest Path First – OSPF Fundamentals – DR and BDR

Published

on February 18, 2009

in BDR, BSCI, BSCI Notes, Certification, Cisco Systems, Concepts and Constructs, DR, OSPF and VLAN

. 8 Comments

When routers are connected to the same broadcast segment (I.O.W. several routers are in the same VLAN, on the same switch you getting the idea). One router is assigned the duty to maintain adjacencies with all other routers on the segment. This is the designated router (DR) and the DR router is selected using information in the Hello messages. For redundancy purposes a backup designated router (BDR) is also elected (There is a reason for this, read on).

DRs are created on multi-access links because the number of adjacencies grows at a quadratic rate. For a network of n routers, the number of adjacencies required would be:

Two (2) routers require the following adjacencies:

Four (4) routers require the following adjacencies:

Ten (10) Routers require the following adjacencies:

Maintaining a OSPF segment consumes more bandwidth and requires more processing resources (CPU and memory) as more routers are added onto a OSPF network (Due to keeping the tables updated and probability of changes occuring more frequently etc).

The DR and maintaining relationships

The purpose of a DR is to be the “one router” (sounds like the matrix) to which all other routers are adjacent (the router that has all the routes on the network). Using a DR reduces the number of adjacencies that consume bandwidth and processing to n – 1 (Larger networks will however still require more processing even if you are using a DR). With a DR the adjacencies scale more effectively and efficiently with the network (as one can see in the below figure and table).

To show this in a graphic way one can see how this “adjacency” relationship works without a DR, with a DR, and with a DR and BDR with a small example network using 5 routers.

Taking this a step further and plotting out the exponential growth requirements of OSPF adjacencies the table below shows the number of adjacencies needed for 1 – 10 routers (imagine the CPU and Memory requirements, not to mention the bandwidth consumption). Plan accordingly when implementing OSPF (at this point you generally use OSPF because you have a non-homogenous network environment and need the open standard because of this fact, I dont really see a point otherwise cause its such a resource hog and mission to setup).

The job of the DR

The role of the DR is to receive updates and distribute these updates to each segment router, making sure that each router acknowledges receipt and has a synchronized copy of the Link-State Database (LSDB).

Routers advertise changes to the “AllDRs” multicast address of 224.0.0.6 where the DR then advertise the Link-State advertisements (LSAs) using the “AllSPF” multicast address 224.0.0.5 where each router then ack receipt.

The BDR listens passively to this exchange and maintains a relationship with all the routers.

If the DR stops producing hellos, the BDR promotes itself and assumes the role of DR.

NB. DRs and BDRs are only useful on multi-access links because they reduce adjacencies. The concept of a DR is not used nor usefull on point-to-point links because there can only be one adjacency.

DRs are still however elected on Point-to-Point Ethernet links (most common type of links in networking these days) which is a rather pointless and resource waste/hog (as a DR is not really needed) which is why you will find that many design guides recommend changing Ethernet links to Point-to-Point mode to stop this from happening.

If a DR fails, the BDR is pomoted. The BDR is elected on the basis of highest OSPF priority, ties in OSPF priority are broken in favour of the highest IP ADDRESS.

The default priority is 1 and a priority of 0 (zero) prevents a router from being elected to the DR or BDR role.

Priority can be set from 0-255 (manually) to change the priority from default from the interface,

Router(config-if)#ip ospf priority number

DRs are inherently seen as stable entities once elected into the position, even if a Router joins a network with a “greater” priority the DR will not change.

To give an example of this an OSPF Segment with 5 Routers ( A – E, with different priorities 0 – 3). Taking what has been discussed previously A would be the DR, B the BDR, and E would never be elected. However this neglects the following set of circumstances:

Imagine the following sequence of events in this small segment,

Router C starts first.
1. Router C sends out Hellos and waits the dead time for a response from other routers.
2. Receiving no Response, Router C conducts an Election and becomes the BDR.
3. As there is no DR on this network, Router C then promotes itself to DR.
Router E starts (priority= 0)
1. Router E will not become the BDR due to its priority setting
Router B starts and becomes the BDR.
Router A starts
Router D starts

In the above scenario the startup sequence of the routers caused the election of the DR and BDR (namely Router C is DR and Router B is BDR) which is not what would have been expected. This is because designated routers do not preempt, the elected DR/BDR serves in its role until reboot/failure (DR and BDR are stable entities on the network once elected).

In this network as it stands now If Router C restarts, Router B promotes itself to DR and Router A is elected BDR while C is down. If Router B goes down, Router A promotes itself and elects Router C or Router D (whichever has the highest IP Address). Finally when the BDR is rebooted, Router B wins the election for BDR.

NOTE: In addition to rebooting, clearing the OSPS process using the the command clear ip ospf process * on the DR will force the DR and BDR election.

Notes and Notices: This is a part of my personal BSCI notes and research to assist myself in learning and understanding the concepts and theory for the BSCI exam. I learn by making notes reading and writing things down and wish to file them where I canâ€™t lose them. These notes are not to be seen, judged or mistaken for replacements to Cisco recognized and authorized training which I personally support and attend and suggest you undertake if you are going for the BSCI Certification.

CCIE Command Memorizer

Published

Deon Botha

on September 12, 2008

in Asides, Off-Topic, Support and Vine

. 2 Comments

About a two weeks ago David Bombal from Configure Terminal contacted me about doing a write up for CCIE Command Memorizer. As a current CCNP student there is obviously some of the CCIE content that still goes over my head but most of the stuff I am fairly okay with. David didn’t seem to mind that I wasn’t a CCIE as yet and I sure didn’t mind giving CCIE Command Memorizer a spin so below are my thoughts and feelings.

During my initial research on the CCIE Command Memorizer I found that CCIE Command Memorizer was geared towards the CCIE Routing and Switching (R&S) Lab. When I received the application Monday two weeks ago my initial impression was that it covers without many frills and spills in an effective and efficient manner the commands in a Do-It-Yourself fill in the blanks task to objective orientated format. There is no round-about, search for things “what now” moments; everything is straight and to the point. Another bonus is that the download isn’t enormous either.

The CCIE Command Memorizer application itself is written in an e-book format, which works for the content and in that format. If like myself you like using the keyboard when you get going on command line (IOW dislike moving between the mouse+keyboard) some of the shortcuts might feel “off” and take a bit to get use to. Take heart that once you get going it all starts flowing like second nature.

My feelings on how the CCIE Command Memorizer would fit into my studies it that as a study tool / study aid as it helps you practice commands anywhere at any time as long as you have your notebook handy (face it as a CCNP I realize I am a Geeks and my notebook goes on holiday with me so this pretty much means anywhere). My current situation with a LAB is that I am busy getting racks, cabling, and other kit together to put a LAB up, the actual LAB is non-existent. In my situation the CCIE Command Memorizer is my stop-gap for actual “command” time practice as I don’t really have kit and at times no block time to zero in on a specific Technology Area and do some quick revision work with a specific focus only on that technology. This is different from the LAB where you need to setup and configure peripheral services and technologies to get to the meat close to the bone, CCIE Command Memorizer lets you focus ONLY on those parts to give you practice on those parts.

In my opinion students that would find the CCIE Command Memorizer useful would be individuals who have problems with instant recall of information when under stress or duress (people with classic test condition issues that forget as they walk through the door when in normal day-to-day they are Uber networkers) and students who need to drill things to remember (repetition, repetition, repetition) information to get it to pass from short term into long term memory (this one would be me). Most other students may also find it handy to bridge the book theory and lab practical (where you just went through the theory to quickly go over the practical again to cement the knowledge just a bit more).

From my perspective as a CCNP student the CCIE Command Memorizer shares most (if not all) of the content material with the CCNP that I cared to check up on. In the two weeks I have been working through EIGRP and OSPF (notes to follow shortly) for the BSCI. Everything I covered in theory the CCIE Command Memorizer covered + some more.

To close off for those who want a one-size-fits-all study solution don’t mistake the CCIE Command Memorizer for something it wasn’t built to be. The CCIE Command Memorizer is a STUDY TOOL / STUDY AID and not a “Complete Blended Solution” or All-In-One (aio) Product like the guys from IP Expert offer. This will mean that you will probably either attend classes from a Cisco Learning Partner and/or do labs at home or through a Rack Rental Company and/or also have Cisco Press books that will ADD TO THE VALUE the CCIE Command Memorizer.

In summation I suggest taking a serious look at the CCIE Command Memorizer and how to integrate it into your study plans. Taking all things into consideration I would seriously recommend the CCIE Command Memorizer even to current CCNP level students. The value add is easy to find and it helps to have something that you can take along as you travel that does command line study effectively without the need to spend hours on “setting up other things” before you get to play with advanced topics.

Enhanced Interior Gateway Routing Protocol – Optional Configuration Commands for EIGRP – Tuning EIGRP

Published

Deon Botha

on September 2, 2008

in BSCI, BSCI Notes, Bandwidth, Certification, Cisco Systems and Hold Timer

. 0 Comments

Some South African/Anglo-African humour that is making me smile:

“Tune” to talk, especially to talk nonsense (“Are you tuning me?”)

But back to the topic at hand;

One can fine tune the EIGRP process in many ways. The most important of tuning methods would be the summarization of routes and load balancing. Other techniques however do exist and these include the frequency of the hello and hold timers and setting bandwidth.

The trade off to playing with timers would be that by decreasing hello traffic the network will take longer to notice failures, which in turn will delays convergence.

To go over some stuff from previous posts; EIGRP only sends updates when a new route is advertised or an existing route is withdrawn (changes state to down). A Link failure causes an interface to change state without delay (duh). But when a failed neighbour is not directly connected (on the other side of a Ethernet switch for example), the only way to notice failure would be that no hellos are received. The idea and concept of Neighbourship is important in EIGRP because it alerts the router to topology changes and because the router is responsible to the rest of the network to publicize the lost routes.

When fiddling with timers think about the wider ramifications. In most cases defaults are there for a reason. Instead of improving performance the opposite will most probably happen. (I.E. timers are changed per interface and changing timers on one side of a link and not the other side creates problems with neighbourship that forms and dissolves periodically).

Timer Values are based on the speed of the interface. Because the timers are assumed to be based on this speed, they will usually be the same (Timers are not communicated between neighbours and are not a requirement for neighbourship).

If Router A has a hello interval of 5 seconds and a hold time of 15 seconds (3x hello) and Router B has a hello interval of 30 seconds and a hold time of 90 seconds (3x hello), then the two routers will be neighbours for 15 seconds and then down for 15 seconds.

The Hello Timer

Tuning the Hello Timer directly affect the ability of the EIGRP Process to notice a change in the state of a neighbour. Only after a router’s interface is recognized as being down, or a router has failed to hear from a neighbour after a certain amount of time, does the router declare the neighbour dead and take action to update the Routing Table and neighbours.

For the above stated reasons, use of the

Router(config-if)#ip hello-interval eigrp autonomous-system-number seconds

command is typically used to decrease (AND NOT INCREASE) the amount of time between Hellos to ensure that the network converges QUICKER and not SLOWER (which would be done by INCREASING THE TIME). This however means MORE traffic devoted to EIGRP and more space used by EIGRP.

The defaults are as follows:

High Bandwidth links (every 5 seconds)
- Broadcast Media (Ethernet, Token Ring, FDDI)
- Point-to-Point Serial Links (PPP or HDLC Leased Circuits, Frame Relay Point-to-Point subinterfaces, and ATM)
- Point-to-point subinterfaces
- High Bandwidth (T1/E1 and greater) multipoint circuits (ISDN PRI and Frame Relay)
Lower Bandwidth Links (every 60 seconds)
- Multipoint Circuits (T1/E1 and slower, Frame Relay Multipoint interfaces, ATM multipoint interfaces, and ATM)
- Switched Virtual Circuits and ISDN BRIs

The Command to set how often hellos are sent to neighbours is applied to an interface and does not affect the ENTIRE EIGRP process:

Router(config)#interface serial 0/0 Router(config-if)#ip hello-interval eigrp autonomous-system-number seconds

To use this in an example we can change the hello timer of a WAN link, that is running on EIGRP AS 1. Doing so will not affect other interfaces running EIGRP AS 1 only this particular WAN link.

Router(config)#interface serial 0/0 Router(config-if)#ip hello-interval eigrp 1 10

The Hold Timer

The Hold Time as talked about here and is how long a router will wait for a hello before pronouncing the neighbour unavailable/dead. By Default the hold time is 3 times the hello time. TAKE NOTE that by changing the hello interval does not automatically change the hold time.

The hold timer for an interface must be changed manually using the following command:

Router(config-if)#ip hold-time eigrp autonomous-system-number seconds

Using this in the same example as above for the Hello time:

Router(config)#interface serial 0/0 Router(config-if)#ip hold-time eigrp 1 30

Authentication

EIGRP support two kinds of Authentication, simple passwords and MD5 hashes.

Simple passwords are sent as plain-text and matched to the key on the receiver. Simple passwords are not secure, because any listener can see this traffic and read the key value.
Hash keys, sent as MD5 values, are secure because the listener cannot use the value in one transmission to compute the key.

Using MD5 authentication, the router generates a had value for every EIGRP transmission and checks the hash of every received EIGRP packet.

To specify MD5 Authentication:

Router(config)#interface serial 0/0 Router(config-if)#ip authentication mode eigrp autonomous system md5

Once the MD5 authentication is set now comes the key:

Router(config-if)#ip authentication key-chain eigrp autonomous system chain-name

Then the key-chain is configured and the key is specified:

Router(config-if)#key chain chain-name Router(config-if)#key my-chain Router(config-keychain-if)#key-string key

An example using the WAN interface from above:

Router(config)#interface serial 0/0 Hello Interval Set Router(config-if)#ip hello-interval eigrp 1 10 Hold Interval Set Router(config-if)#ip hold-time eigrp 1 30 MD5 Authentication Set Router(config-if)#ip authentication mode eigrp 1 md5 MD5 Key Set Router(config-if)#ip authentication key-chain eigrp 1 My-Chain MD5 key-chain Set Router(config-if)#key chain My-Chain Router(config-if)#key 1 Router(config-keychain-if)#key-string cisco

Authentication results are not shown under show commands. A successful neighbourship means it works. You can however check command process using debug eigrp packets

Optional EIGRP Commands Over a WAN

EIGRP has some design and configuration issues when it comes to the WAN environment. In the WAN one must deal with limited capacity to a greater degree than at other points of the network (For example the LAN). EIGRP is limited in that it restricts its use of bandwidth to NO MORE than 1/2 the link capacity. This is superior to the considerations made by other protocols. Although EIGRP by default is usually sufficient, one might need to make small adjustments at times.

EIGRP Defaults in Bandwidth Utilization
Routers understand link capacity most of the time (MOST being important here). Serial interfaces are however problematic (and the exception to the rule) because they usually attach to a DSU. The router therefore assumes a default speed of 1544 kbps (which is in most cases on the WAN not true).

If the link is actually 56 kbps, then EIGRP would calculate incorrectly and -even limiting itself to 722 kbps -could saturate the link. This could result in dropped EIGRP and data packets because of congestion and dropped data.

The show interface command will allow you to check that the interface bandwidth is accurate. The output shows the configured bandwidth of the link.

The set bandwidth does not actually affect the speed of the link, but this value is used for routing protocol calculations and load calculations. Using the following command you can set the bandwidth:

Router(config)#interface serial 0/0 Router(config-if)#bandwidth speed-of-line

Configuring Bandwidth over an Non-Broadcast Multi-access (NBMA) Cloud

EIGRP plays well over WANs, including point-to-point and NBMA environments like Frame Relay and ATM. The NBMA topology can include either point-to-point subinterfaces or multipoint interfaces.

Cisco IDs three rules when configuring EIGRP over an NBMA cloud:

EIGRP traffic should not exceed the committed information rate (CIR) capacity of the virtual circuit (VC).
EIGRP aggregated traffic over all the VCs should not exceed the access line speed of the interface.
The bandwidth allocated to EIGRP on each VC must be the in the same directions.

Configuring Bandwidth over a Multipoint Network

In addition to being used in the EIGRP metric, the bandwidth command influences how EIGRP uses NBMA VCs. If a serial line has many VCs in a multipoint configuration, EIGRP will assume that each VC has an even share of the bandwidth. EIGRP will confine itself to using half that share for itself. This won’t work if a 56 kbps link has bandwidth set to 128 kbps because EIGRP will assume 64 kbps is for it’s own use.

The bandwidth command should reflect the access-link speed into the Frame Relay cloud. Your company might have five PVCs from your routers serial interface, each carrying 56 kbps. The access link will need a capacity of 5 * 56 kbps (280 kbps).

Configuring Bandwidth over a Hybrid Multipoint Network

If the multipoint network has different speeds allocated to the VCs, a more complex solution is needed.

Take the lowest CIR and multiply it by the total number of circuits. Apply the product (total) as the bandwidth of the physical interface. The problem with this configuration is that EIGRP will underutilize higher bandwidth links.
If possible, it is muse easier to configure and manage an environment that has used subinterfaces, where a VC is logically treated as a separate interface. The bandwidth command can be configured on each subinterface, which will allow different speeds on each VC. In this solution, subinterfaces are configured for each VC and the CIR is configured as the bandwidth. This is the preferred solution.

Configuring a Pure Point-to-Point Network

If there are many VCs, there might not be enough bandwidth at the access speed of the interface to support the aggregate EIGRP traffic. The subinterfaces should be configured with a bandwidth that is much lower than the real speed of the circuit. In this case, it is necessary to use the bandwidth-percent command that indicates to EIGRP that it can still function.

The ip bandwidth-percent eigrp command adjusts the percentage of capacity that EIGRP may use FROM THE default 50%. You would use the command because the bandwidth command does not reflect the TRUE speed of the link (The bandwidth command might have been altered to manipulate the routing metric and path selection of a routing protocol).

Router(config)#interface serial 0/0 Router(config-if)#ip bandwidth-percent eigrp autonomous-system-number percent

Software Study Resources:

The Command Memorizer was originally developed by a CCIE Candidate (David Bombal) for his own use and is now available to anyone who wants to use it.Command Memorizer helped him pass the CCIE Lab on the first attempt, and although I am not a CCIE candidate “officially” I have fiddling with it and finding it useful to test my command line retention and overall progress towards CCIE readiness as I do my current CCNP.The proof will be in the pudding as the Command Memorizer boasts 1000s of commands and hundreds of scenarios to test command line knowledge and retention. It has a section for EIGRP and I also like knowing where I am on my long road to Cisco.

Like most study aids / study tools this tool / aid has a specific focus. The Command Memorizer only works when used in conjunction with theoretical backing because you need to know what a command does and how it relates to the technology area. IOW You need to make the connection before you can start drilling actual commands repetitively to get them to start flowing and become second nature.

For a disclosure statement on my relationship with Configure Terminal.

Cisco Press Resources:

Stewart, B,D., Gough, C (2008). CCNP BSCI Official Exam Certification Guide, Fourth Edition. Indianapolis: Cisco Press.

Internetworking Technology Handbook – Intro to the Wan

Notes and Notices:

This is a part of my personal BSCI notes and research to assist myself in learning and understanding the concepts and theory for the BSCI exam. I learn by making notes reading and writing things down and wish to file them where I can’t lose them. These notes are not to be seen, judged or mistaken for replacements to Cisco recognized and authorized training which I personally support and attend and suggest you undertake if you are going for the BSCI Certification.

Linksys Brand to Disapear

Published

Deon Botha

on August 28, 2008

in Asides, Cisco Systems and Vine

. 1 Comment

Cisco acquired Linksys back in 2003 and the Linksys brand has been around in some way or form since then, kind of, I haven’t had problems with the product myself but have had logistics problems with the brand and this comes from up-channel from various distributors where they can’t promise due dates and shipping from Linksys.

This is a problem for the Linksys brand because although the brand as a whole has a great price point for Home, Home Office (SOHO) and Small, Medium Business (SMB) Market segments the availability sucks and not being able to promise delivery or give an indication of delivery makes using the brand as a plausible solution pointless. While an Enterprise customer might be willing to understand and “deal” that no stock is kept in a Emerging market of their class of products and that the lead time to delivery is longer that understanding is lacking with SMB customers where deals are lost on cents and the ability to start installation tomorrow.

There was talk about a year back from the channel and some of my networking buddies that the Linksys brand would be integrated into the Cisco “stable” for good, meaning that the Linksys brand would phase out totally and only one would emerge. There were obviously two views to this; while one said “Great Cisco all the way” and the other said “Linksys is a strong brand on its own, why kill it?”.

Be that as it may the first steps of the brand integration process has started. How this whole change management process will work is that soon the “Linksys a division of Cisco” will become “Linksys by Cisco” with Linksys and Cisco sharing as much product space and font size and finally only “Cisco” will be on the packaging and product. This process happens over years to get customers use to the idea and “new” packaging and branding and is the eventual process after the companies have assimilated into each other and adopted each others cultures and views.

Wasn’t around back in the day but I suppose the Catalyst Switching platform followed the same routine as this. I know that the IBM and Lexmark Printing and Imaging System did this back in the day.

Enhanced Interior Gateway Routing Protocol – Introduction

Published

Deon Botha

on August 5, 2008

in BSCI, BSCI Notes, Certification, Cisco Systems, Concepts and Constructs and EIGRP

. 2 Comments

This is the Introduction to Enhanced Interior Gateway Routing Protocol (EIGRP) most of this paragraph you will find here; moving swiftly along EIGRP is a Cisco Proprietary distance vector routing protocol that uses the same sophisticated metric that Interior Gateway Routing Protocol (IGRP) uses plus the Diffusing Update Algorithm (DUAL) convergences algorithm for loop-free routing. EIGRP is able to converge quickly and uses little bandwidth (like OSPF) because it separates keepalives, routing information and uses reliable updates. EIGRP is sometimes referred to as a hybrid routing protocol.

EIGRP was created (maybe read modified/updated) to solve scaling limitations that IGRP faced while still keeping the advantages of distance vector routing protocols (simplicity, economy of memory usage, and economy of processor resources). EIGRP is scalable in terms of hardware resources and network capacity. EIGRP is also very quick.

I use British English there will be a few small differences in spelling versus American English (the English Cisco Uses). Example: Neighbour vs Neighbor

Neighbourship and Reliable Incremental Updates

EIGRP supports several routed protocols independently (IP, IPX, Appletalk and IPv6) This means that each routed protocol has a best path that is not shared between other routed protocols.

EIGRP produces reliable (receiver ACKs the transmission was received and understood) updates by identifying its updates using IP protocol 88.

EIGRP uses five (5) types of packets to communicate:

Hello - Identifies neighbours; Hellos sent via multicast periodically and ACK.
Update – Advertises routes. Updates sent as multicast only when there is a change.
ACK – ACK receipt of an update.
Query – Used to ask about routes for which previous best path has been lost.
- If an update indicates that a path down, multicast queries used to ask other neighbours if they still have path.
- If querying router does not receive reply from each of its neighbours, it repeats query as a unicast to each unresponsive neighbour until it either gets a reply or gives up after sixteen (16) attempts.
Reply – Used to answer query. Each neighbour responds to the query with a unicast reply indicating an alternative path or that it does not have a path.

Neighbour Discovery and Recovery

EIGRP uses a reliable update procedure; this creates two problems,

The router needs to know how many other routers exist so that it knows how many ACK to expect.
The router needs to know whether a missing advertisement should be interpreted as “no new information” or “neighbour disconnected”.

EIGRP uses neighbourship to address these problems (periodic hellos).

The first hellos build a list of neighbours (Neighbour Table).
following hellos indicate that the neighbours are still alive.

If hellos are missed (for the period of the hold time) then the neighbour is removed from the EIGRP table and routing reconverges.

The discovery process begins with multicast advertisements being sent out and individual routers replying with unicast ACK. The neighbour table tracks replies to make sure that each neighbour responds. If a neighbour does not respond with an ACK a follow-up unicast message is sent, after 16 times attempts the neighbour is removed from the neighbour table and EIGRP continues with its next task.

Sophisticated Metric

EIGRP uses a sophisticated metric that takes into account bandwidth, load, reliability, and delay. The metric equation is:

EIGRP selects paths based on the fastest path (lowest value). To do that it uses K-values (K1 to K5 in the equation). The K-values are constants(don’t change) that are used to adjust the relative contribution of the various parameters to the total metric. The EIGRP K variables are set as follows:

Bandwidth – 107 kbps divided by the slowest link along the path. Because routing protocols select the lowest metric, inverting bandwidth makes faster paths have lower costs.
Load and reliability – 8-bit calculated values based on the performance of the link. Both are multiplied by a zero K-value (neither used).
Delay – a constant value on every interface type, and is stored in terms of microseconds (serial has a delay of 20,000 microseconds and Ethernet has a delay of 1000 microseconds). EIGRP uses a sum of all delays along the path, in microseconds.

By default:

K1 = K3 = 1 and
K2 = K4 = K5 = 0 (if you followed the maths if K5=0 then the metric equals 0).

Because the metric basically = 0 which will not be useful EIGRP ignores everything outside the parentheses.

Using the default K-values the equation then becomes:

Substituting the earlier description of variables, the equation becomes 10,000,000 divided by the chokepoint (worst/slowest link along the path) bandwidth plus the sum of delays:

Exercise to crystallize

This entire section is so that I understand how EIGRP selects the route using the below diagram (from Brent D, Stewarts CCNP book) lets plug in some values and see it work.

If we want to send traffic from Router A to Router D, which path would be used?

The top path ABCD has a chokepoint bandwidth of 768 Kbps and would go along 3 serial lines and look like this in the equation:

The bottom path AED has a chokepoint bandwidth of 512 Kbps and would go across 2 serial lines and look like this in the equation:

The result is that EIGRP chooses ABCD (top path) based on bandwidth.

Diffusing Update Algorithm (DUAL)

EIGRP uses the Diffusing update Algorithm (DUAL) which is a modification to the way distance-vector routing typically works. DUAL allows routers to identify loop-free failover paths. Using the same graphic as above lets do an exercise and figure out how DUAL works.

How DUAL works is that neighbouring routers advertise costs (using the below diagram. Lets say router A wants to send a packets to Router D). The two costs advertised by neighbours are as follows:

To send a packet from A to D the Advertised Distance (AD) is either via BCD or ED and excludes the first hop.
The other advertised metric is the Feasible Distance (FD) which is to send a packet the total distance ABCD or AED.

The idea that a path through a neighbour is loop free if the neighbour is closer is called the feasibility requirement and can be restated as “using a path where the neighbour’s advertised distance is less than our feasible distance will not result in a loop”.

The neighbour with the best path will be referred to as the successor. Neighbours that meet the feasibility requirements are called feasible successors. In emergencies, EIGRP knows that using feasible successors will not cause routing loops and instantly switches to the backup path.

Using the above diagram again I am going to be trying to reach Router D. What I did was plug in values using the same equation from the above exercise, just using each individual router (A, B, C, E) to get to D.

Queries

Having a Feasible Successor provides the best convergence. A feasible successor is a backup path and can be substituted should the active path go down at any point (without the need to change state and ask neighbours for a path). Should an active path go down and no Feasible Successor exist, a router will send out queries to remaining neighbours. If a neighbour does not know of a an alternative path, it will recursively ask neighbours.

Recursive queries can loop, forcing the router to time-out the query. This is known as stuck in active (SIA). EIGRP uses split horizon (a router should not advertise a network down a link from which it learned about the network – CCNA).

Queries will continue until an answer is found or until no one is left to query. When queries are produced the router changes to an Active State (actively querying for an alternative path) and sets a timer (3 minutes default). If the timer expires before an answer is returned the router is considered SIA. SIA typically occurs because queries are not properly limited to an area.

The primary way to limit how far queries travel (called query scoping) is to summarize (also allows quick convergence).

Incremental Updates

EIGRP periodically sends hellos to maintain neighbourship, but only sends updates when a change occurs. When a route is changed or withdrawn, an incremental update is sent including only those changes.

Multicast Addressing for Updates

EIGRP sends some packets using a reliable transport protocol (RTP). An example would be EIGRP sending a single multicast hello packet with an indicator that says it need not be ACK. Other types of packets like updates indicate that packet ACK is required.

EIGRP uses both multicast and unicast addressing.

Some packets are sent using Real-Time protocol (RTP), a Cisco Proprietary (?? Can’t find a source for this ??) protocol that oversees the communication of EIGRP packets. These packets are sent with sequence numbers to make the transmission of data reliable. Hellos and ACKs do not require acknowledgement.

Incremental Updates cannot be anticipated; update, query, and reply packets must be ACK by the receiving neighbour.

Updates are sent using reliable multicast (Reserved Class D address, 224.0.0.10). When a neighbour receives a multicast, it ACKs the receipt with an unreliable unicast.

Unequal-Cost load sharing

All IP routing protocols on Cisco routers support equal-cost load sharing. EIGRP is unique in its support for unequal-cost load sharing.

Unequal-cost load balancing takes the best FD and multiplies it by variance. Any other path with an FD less than this product (the product of multiplication read answer) is used for load sharing. EIGRP also does proportional unequal-cost load sharing.

EIGRP will pass a relative portion of the traffic to each interface (60/40) allowing links to a destination to be used to carry data without saturating the slower links or limiting the faster links.

Resources:

Stewart, Brent, D. 2008, CCNP BSCI Official Exam Certification Guide, 4th Ed. Indianapolis: Cisco Press.

Have a look at EIGRP Aragoen Celtdra notes on the same section of work

Introduction to EIGRP

Internetworking Technology Handbook – EIGRP

EIGRP Technology Whitepaper

The Dual Algorithm

Notes and Notices:

BSCI Design Foundation – Network Models

Published

Deon Botha

on July 25, 2008

in BSCI, BSCI Notes, Certification, Cisco Systems, Concepts and Constructs, ECNM, Enterprise Architecture, IIN and SONA

. 0 Comments

Design – Hierarchical

Where networks once were non-hierarchical (layer-1 design, layer-2 design, layer-3 design) they are generally now three-layer hierarchical in design (above). Cisco has been using this model for years and it gave a high-level overview of how a reliable network could be conceived but was largely conceptual because it did not provide specific guidance on “how-to” implement certain things, like:

Implementing redundancy,
Adding Internet Access,
Accounting for remote users,
Locating workgroup and enterprise services

Design – Enterprise Composite Network Model (ECNM)

Revisions to the hierarchical design showed redundant distribution and core devices and connections to make the hierarchical model more fault tolerant. The switch block design (above) explained how redundancy fit into a network, but still did not really adequately specify other parts of the network design. This lead to the Enterprise Composite Network Model (ECNM) development to address the failures of both the hierarchical model and switch block model.

This ECNM is broken into three large pieces:

Enterprise Campus,
Enterprise Edge,
Service Provider Edge.

Enterprise Composite Network Model

ECNM – Campus

The enterprise campus looks very much like the above switch block design with some added details:

Campus Backbone (like the core layer of the hierarchical model),
Building Distribution,
Building Access,
Management,
Server Farm (Enterprise Services).

The ECNM Campus builds onto the Switch block design but gives specific guidance as to where to place servers and management equipment. Take note that the servers look like a switch block and are redundantly attached (dual-homed) to the switches (not really shown nicely in the diagram).

ECNM – Enterprise Edge

The Enterprise edge shows the connections that the enterprise has with the wide area (other networks) and include:

E-Commerce,
Remote Access,
Internet Connectivity,
WAN (Internal links to other branches).

ECNM – Service Provider Edge

The service provider edge includes the public networks that facilitate wide area (other networks) connectivity:

Internet Service Provider (ISP),
Public Switched Telephone Network (PSTN) for dialup,
Frame Relay, ATM, and PPP for private connections.

Multiplexing

Historically voice traffic used one set of circuits and data traffic another. Also if you wanted more than one “number” the telecommunications company installed another physical line to your premises. If you wanted access to a data network they installed a data line for that purpose.

With line technologies like the T-carrier system (USA, Japan, Korea) 24 pulse-code modulated (I don’t know need to ask one the engineers about this), time-division multiplexed speech signals are carried over 2 copper pairs. This type of technology saved the telecommunications companies a lot of money in building out subscriber lines. The problem with T1 as a technology is that it cannot adjust as the customer usage requirements changes (see E-carrier system for Europe and other countries).

As technology changes so does the requirements from that technology; Modern networks are designed to carry voice, video, enterprise applications, normal LAN traffic and management traffic all on the same single secure infrastructure (convergence). The traffic is forced (statistically multiplexed) to share access to the network.

Service-Orientated Network Architecture (SONA) and Intelligent Information Network (IIN)

As covered above “Multiplexing” described the idea of a converged network as a system that integrates what was previously disparate systems (voice, video, data). The traffic types usually found on a converged network would include, but may not be limited to:

voice signalling and bearer traffic,
Core application traffic (ERP and CRM),
Transactional traffic related to database interactions (SQL),
Network management traffic for monitoring and maintaining the network structure (including routing protocol traffic),
Multicast multimedia,
Other traffic (web, e-mail, file transfer).

Each of the above traffic types has its own requirements and expectations that govern its successful execution. These requirements include security, QoS, transmission capacity, and delay.

To support this kind of multiplexed traffic, Cisco routers are able to implement filtering, compression, prioritization, and policing (dedicating network capacity). Except for the filtering process these processes are collectively known as QoS.

As an alternative to QoS, Cisco has an ideal called the Intelligent Information Network (IIN). This vision describes a network that integrates network and application functionality cooperatively allowing the network to be “smart” about how it handles traffic to minimize the footprint of applications. The IIN evolution is described in three phases:

Phase 1: Integrated Transport, deals with a converged network, built along a similar fashion of the ECNM and based on open standards (cross-compatibility)
Phase 2: Integrated Services, posits virtualization of resources such as servers, storage and network access; to move to an “on-demand” model. Don’t think marketing/advertising “virtualization” think practical virtualization the ISR routers (routing, switching, voice, network management, security and wireless) designed as an aio (all-in-one) appliance and Vitalizing Servers (if you have proper designed for the job servers) you can’t be trying this on SMB servers or try recycling 10 year old technology and thinking “bargain let’s load 5 operating systems on this”.
Phase 3: Integrated Applications, using application orientated networking (AON) to make the network “aware” allowing the network to actively monitor and participate in service delivery.

Service-Orientated Network Architecture (SONA) is the practical application or “how-to” of IIN in enterprise networks. SONA breaks down IIN into three layers;

SONA Infrastructure Layer is basically the same as IIN Phase 1,
SONA interactive Services Layer maps to IIN Phase 2,
SONA Application Layer has the same concepts as IIN Phase 3.

Resources:

Aragoen Celtdra on BSCI: Network Architecture and Design

Notes and Notices:

BSCI Design Foundation – Scalability

Published

Deon Botha

on July 25, 2008

in BSCI, BSCI Notes, Certification and Cisco Systems

. 0 Comments

As I posted after I passed BCMSN I was going to be doing things a little differently. This is the different I was talking about; I am going to crawl before I am going to try and fling myself off a building (baby steps). In this post I am going to go over the Foundation topics for Network Design.

Defining Scalability

As the course is called Building Scalable Cisco Internetworks (BSCI) one of the underlying topics covered is scalability. This can be defined as the capacity of a network to keep pace with changes and growth. An example of this would exponential (full-mesh topology) versus linear network growth (hub and spoke topology or Star Topology).

When one compares the two topologies a full-mesh topology grows exponential and thus just isn’t realistically scalable, this would be because of the cost element associated to build a large network using this topology.

To illustrate the exponential growth pattern of a network using this topology use the following equation:

Connections = Sites ( Sites – 1 ) ÷ 2

Using this equation a table can be drawn up (left) where one can see just how quickly a relatively small number of nodes (sites) quickly require many connections to keep the one node has a connection to all other nodes true.

This is the reason why this topology isn’t practically scalable in real life (IRL). Calculating the number of point-to-point connections and related hardware requirements needed there are very few companies that would be able to bankroll this sustainably (maybe if they own the underlying network architecture like a telecom or a company where the network itself is the business).

A hub and spoke topology grows incrementally as the network increases in size. Look at the above diagram and you see 5 nodes and a central switching station in the centre. This type of topology is realistically scalable because adding the 10th node would cost the same as adding the 100th node; the only cost incurred would be a point-to-point connection and network kit.

To illustrate the linear growth pattern of this network using this topology use the following equation:

Connections = Sites – 1

Resources:

Aragoen Celtdra on BSCI: Network Architecture and Design

Notes and Notices:

New Cisco IOS Software Activation and Licensing Workflows

Published

Deon Botha

on July 22, 2008

in Cisco Systems and Vine

. 7 Comments

I was introduced to this new Cisco IOS Software Activation and Licensing idea at a Partner Enablement session on the 6th of June and my first impression was “Windows Activation on steroids”. A post by Joe Harris has a visual workflow link that explains this process if this whole thing confuses you.

One of my questions to the presenter afterwards was what impact would this have on the simulation software packages like dynamips. My position being that I wasn’t able to build a lab simply for learning/studies even with Cisco discounts at this point in my career. He understood my point of view and knew of many other partners in similar positions but couldn’t really answer me.

What I gathered from my session on the 6th of June was that not all Router and Switch platforms will migrate to this new activation and licensing platform but it will happen over a period of time. This activation and licensing system is being implemented because the “gentlemans agreement” system Cisco has used in the past isn’t working where there is misuse of the current system.

That being said I did ask how this will directly affect me and the answer was that on initial order/sale not a whole lot. When a customer purchases a switch/router they generally purchase what they want with the licensing they want. On a scenario like that the IOS and feature is installed and activated before being shipped to the customer or me and then I go install.

This new system will start becoming a bother when upgrading from IP Base to another feature license. This will require the following steps:

The order of a Product Authorization Key (PAK) from Cisco
The Unique Device Identifier (UDI) from the Router/Switch
Entered this information into the Cisco Licensing Portal
Taking the information from the Portal and installing the license onto the Switch/Router

The installation of the license file can be done using the *.lic file that you receive from the Portal using the Command line interface or the Cisco License Manager software. Using the command line:

Switch#license install tftp://x.x.x.x/license.lic

Alternatively one can use the call-home feature and the PAK Number, this however would mean that you have an internet connection to the Router/Switch and you feel comfortable that you won’t have the *.lic file when things go wrong as the Switch/Router installs this directly from the License Portal:

Switch#license call-home install PAK PAK-NUMBER CCO Username: abcdef CCO Password: !...................... Follow the prompts to install the license

There was talk about an emergency license availability for “emergency purposes” where say for example a customer has a current desperate need for an upgrade feature set this very instant and the order cycle would take a couple of days to fulfil. The emergency license would take a couple of hours at most and last for a finite period while the order process ran its couse.

BCMSN Passed

Published

Deon Botha

on July 21, 2008

in Certification and Cisco Systems

. 17 Comments

I passed the BCMSN exam this morning; I am happy to have passed the exam that being the desired result of writing an exam, yet I am extremely unhappy with myself in that I didn’t feel “on the ball” with alot of the questions.

The testing centre venue was fine by all accounts and standards and the people friendly. You know that you visit enough when the testing people recognize you and know that you are the guy coming in for the Cisco exam of the morning. The exam room air-conditioning unit was set not too high and not too cold for the exam this morning which was actually rather pleasant for a change.

My biggest complaint of most places is not really related to this test venue but a general complaint of events venues. Us Africans don’t know how to use Air-conditioning inside buildings because it’s set either too cold or too hot in relation to the outside temperature. This means that you are either wearing too much or too little clothing and sitting in a closed room for 110 minutes freezing/boiling and your hands/ears/nose are going numb while you are trying to recall information is a pain and not really conducive. Another thing with air-conditioning is that it dries out your eyes and sinuses and when there is no other ventilation in the room half way through the exam all you start caring for is to get out the room because of the headache you getting. Makes one feel that they should start advertising the “room” temperature for the day so that you can dress accordingly for the climate.

The Preparation

My thinking going in for the exam 4 months ago was that the BCMSN switching content “in theory” is fairly straight forward and that it would be easy enough to try and use as a “test” case for the future CCNP courses. That assumption paid off because I think I wouldn’t have made the BSCI exam with the hitch I came up with today.

My test case did pay dividends and I have found something that I will have to sort out post haste for the BSCI and other future exams. My approach with my notes has been up until now focused on getting to know and becoming comfortable with the core theory of the subject matter. This approach served me well enough and enabled me to “pass” the exam although I want to not just pass but pass well and without a doubt and in my opinion what I kept stumbling on (over and over and over) was the deeper technical and practical base knowledge upon which the theory builds.

Changes

Because this stumbling block was not the exception but instead the rule I am going to be adding more in-depth technical and practical “appreciation” to my notes so that these topics will hopefully become part of my knowledge base (KB) and long term memory. This hopefully will help me when I sit the next exam and similar questions come up.

Network Community Online

Published

Deon Botha

on July 20, 2008

in Asides and Off-Topic

. 1 Comment

This post is kind-of off-topic but I feel it’s needed at this point. At the end of this month Network Ninja will have been online for 4 months, it’s hopefully going to be a double anniversary as it will hopefully also mark my first active step towards becoming a full fledged Cisco CCNP Certified bloke.

As to why I have been very quiet as of late when it comes to BCMSN topics I am booked in on Monday morning (tomorrow) for the BCMSN exam at 8:30am GMT+2 and I have been reading and re-reading my own notes (fixing spelling and typos while doing this). Hopefully I bring back good news otherwise its going to be a close call otherwise I am just going to make another booking and get back to the drawing board, I am at the moment looking at my own study limits to see what kind of time I need to give myself to make notes, study and get the material from my short term to long term memory, I feel prepared and feel good about this but with me and my horror history with exams who knows (I’m not a glass half full, glass half empty kind of person… There is no stupid glass, it’s a figment of your imagination).

Combined with all of the above I think its also time to say Thank You/Dankie/Ke a leboga/Ngiyabonga to all the online Cisco Networkers and people I have made contact with along the way that I have received active and passive support from (blog posts that helped me understand something, exhanged emails, twitters, IMs, skype, etc) in the last 4 months.

Thanks to blindhog.net – Josh Horton is the man behind Blindhog and his site is dedicated to helping people learn Cisco, Linux and VOIP technologies with the help of video tutorials. He has a good series of video-torials on GNS3 over at his blog head on over at check them out.

Tip of the hat to www.bitbucketblog.com – Is a blog by a CCIE member busy with his CCIE Security. Bitbucketblog has some good write ups and prep notes. Alot of the CCIE stuff still goes over my head but it’s valuable stuff none the less. Head on over and check it out!

Shout out to Baby, You can Route My World! – A fellow lamb to the CCNP slaughter Aragoen Celtdra is busy with the routing track of the CCNP while I am doing the Switching track. Aragoen is excellent at taking the core of the material and condensing it into great bullet form study sheets. If you don’t like my long winded notes head on over to his bog and give his notes a squiz.

A Networkers Blog – A CCIE blog full of tidbits and interesting posts. Well worth visiting.

Richard Bannister’s CCIE Blog – The CCIE notes and study blog of Richard Bannister, the blog showcases the trials and tribulations of a studying CCIE and what it takes on a weekly basis to study. Richard posts on his study schedule on a weekly basis, what he has covered and his thoughts on the weeks content.

The Life of a CCIE Training Advisor – The blog of Mike a training advisor over at IPexpert and Proctor Labs, really nice guy whose job it is to help the CCIE community at large get Blended Learning Solutions. Get in touch with Mike for some training material, labs etc. I’m sure he can help you out.

CCIE Pilot – The blog of Mar Apuhin a studying CCIE Routing and Switching that is in the last days before LAB. Head on over there and send your words of encouragement.

CCIE Pursuit Blog – A great blog filled to the brim with posts relating to things concerning CCIE study and all things CCIE.

Colin McNamara – The blog of Colin McNamara covering “Technical reviews and articles from a CCIE with extensive experience in designing and implementing converged enterprise networks”.

Arden Packeer – The blog of Arden Packeer a CCIE based in OZ. His blog description is almost like my blog name (never noticed that until I was writing this up). Arden has a pet project going called ccieMagazine head on over there and show some support.

Etherealmind – The blog of Greg Ferro a CCIE his blog covers not only CCIE topics and is well worth following; Greg has a really cool Network Dictionary and also a great style of posting.

Last but not least thanks goes to JP for the things that you pass on and have organized, really appreciate it.

That all having been said after tomorrow I will hopefully be charting a course for the next 4 months to be able to keep on track with my initial plans for my studies.

Network Ninja

Tag Archive for 'Switch'

Open Shortest Path First – OSPF Fundamentals – DR and BDR

CCIE Command Memorizer

Enhanced Interior Gateway Routing Protocol – Optional Configuration Commands for EIGRP – Tuning EIGRP

Linksys Brand to Disapear

Enhanced Interior Gateway Routing Protocol – Introduction

BSCI Design Foundation – Network Models

BSCI Design Foundation – Scalability

New Cisco IOS Software Activation and Licensing Workflows

BCMSN Passed

Network Community Online

Search

About

Latest

Archives

Categories