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Enhanced Interior Gateway Routing Protocol – Scalable EIGRP – Single interface EIGRP

Published

on September 2, 2008

in BSCI, BSCI Questions, Certification, Concepts and Constructs, EIGRP and passive-interface

. 0 Comments

Working from the my last couple of EIGRP posts I am going to try and crystallize some of the material found by working through questions found in Stewart, B,D., Gough, C (2008). CCNP BSCI Official Exam Certification Guide, Fourth Edition. Indianapolis: Cisco Press.

Imagine a router that has two interfaces -fastethernet0/0 is 10.1.1.1 and serial0/0 10.2.2.2 Normally, EIGRP would run with the “network 10.0.0.0″ command, which would enable both interfaces. There are two ways discussed in this chapter that could be used to specify that one interface run EIGRP and not the other. What are those two techniques?

The two techniques that would allow one interface to run EIGRP are:

The passive-interface command. This command prevents EIGRP from speaking on an interface; it does not send hello packets or advertisements, neighbours are not found on passive-interfaces and routes are not exchanged. However the prefix of the passive interface is exchanged to EIGRP neighbours on other interfaces.
Router(config)#interface fastethernet 0/0 Router(config-if)#ip address 10.1.1.1 255.255.255.0 Router(config-if)#exit Router(config)#interface serial 0/0 Router(config-if)#ip address 10.2.2.2 255.255.255.0 Router(config-if)#exit Router(config)#router eigrp 1 Router(config-router)#network 10.0.0.0 passive-interface command for FE0/0 Router(config-router)#passive-interface fastethernet 0/0 passive-interface command for S0/0 Router(config-router)#passive-interface serial 0/0
The second method would be to use the extended format of the network command by including the EIGRP wild card bits one can restrict the EIGRP process to a single network.
Router(config)#router eigrp 1 network command for FE0/0 Router(config-router)#network 10.1.1.1 0.0.0.0 network command for S0/0 Router(config-router)#network 10.2.2.2 0.0.0.0

Resources:

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

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.

Enhanced Interior Gateway Routing Protocol – Configuring EIGRP

Published

Deon Botha

on August 14, 2008

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

. 0 Comments

The basic requirements to get EIGRP running are as follows

The EIGRP Process and the the EIGRP Autonomous System (AS) number.

The EIGRP Process is the routing protocol that needs to be started on the router and the EIGRP Autonomous System (AS) number is a 16-bit arbitrary number and works by identifying and grouping routers with common AS numbers in the same administrative domain. The significance of this number is that a router will not become a neighbour with another router with a foreign/different AS.

To the enable EIGRP Process as an active routing protocol:

Router(config)#router eigrp autonomous-system-number

Used in an example

Router(config)#router eigrp 1

The Participating router interfaces can either be on one or all interfaces (EIGRP on internal interfaces and BGP on external interfaces). After the router command EIGRP is enabled but not active on any interfaces. EIGRP will not produce Hello packets not advertise the network until it is activated on particular links.

To active Interfaces to the EIGRP process one uses the network command. The network command is a pattern-matching tool (interfaces with matching IP addresses are active in EIGRP and subnets on those interfaces are advertised through EIGRP).

Router(config-router)#network network-number

Used in an Example

Router(config-router)#network 10.0.0.0

The Passive-interface command can be used on interfaces with no neighbours, or on interfaces that run another routing protocol (BGP). This command prevents EIGRP from speaking on an interface; it does not send hello packets or advertisements, neighbours are not found on passive-interfaces and routes are not exchanged. However the prefix of the passive interface is exchanged to EIGRP neighbours on other interfaces.

Router(config)#interface fastethernet 0/0
Router(config-if)#ip address 192.168.0.1 255.255.255.0
Router(config-if)#exit
Router(config)#interface fastethernet 0/1
Router(config-if)#ip address 192.168.0.2 255.255.255.0
Router(config-if)#exit
Router(config)#router eigrp 1
Router(config-router)#network 192.168.0.0 255.255.255.0
Router(config-router)#passive-interface fastethernet 0/1

Resources:

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

Notes and Notices:

Enhanced Interior Gateway Routing Protocol – Tables

Published

Deon Botha

on August 8, 2008

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

. 2 Comments

EIGRP builds and maintains three tables,

A Neighbour table – used to make sure all ACKs are received.
A Topology Table – used to understand paths through the network.
An IP Routing Table – the best paths from the Topology table.

Creating the Neighbour Table

As previously stated, the neighbour table is maintained through Hello packets (These are multicast announcements that the router is alive).

Hello packets place the router into an adjacent routers’ neighbour tables.
- Reciprocal Hellos build the local Neighbour Table.
- Once the Neighbour Table is built, Hellos continue periodically to maintain neighbourship.

Each Layer-3 Protocol supported by EIGRP (IPv4, IPv6, IPX and AppleTalk) has its own separate Neighbour Table. Information about neighbours, routes, or costs are not shared between protocols.

Contents of the Neighbour Table (Resource 1, 2)

The Layer-3 Address of the neighbour (IP Address)
The interface through which the neighbours Hello was heard (fe0/1)
The holdtime (how long the neighbour table waits without hearing a Hello from a neighbour before declaring the neighbour unavailable and purging the database). Holdtime is three times (x3) the value of the Hello timer by default.
The uptime (period since the router first heard from the neighbour).
The sequence number. The neighbour table tracks all the packets sent between neighbours (both the last sequence number sent to the neighbour and the last sequence number received from the neighbour).
Retransmission timeout (RTO), the time a router will wait on a connection-orientated protocol without ACK before retransmitting the packet.
Smooth Round Trip Time (SRTT), calculates the RTO. The SRTT is the time (milliseconds) that it takes a packet to be sent to a neighbour and a reply to be received.
The number of packets in a queue, which is a means by which administrators can monitor congestion on the network.

Becoming a Neighbour

All EIGRP routers periodically announce themselves with the Hello packet using multicast (224.0.0.10). On hearing a Hello (receiving) routers add an entry in the Neighbour Table (the continued receipt of Hello packets maintain the neighbour table).

If a Hello packet is not received from a neighbour within the holdtime (3x the Hello timer) the neighbour is removed from the Neighbour Table.

LAN = Hello timer 5 seconds, Holdtimer 15 seconds.
DS1 (1.5Mbps) or slower WAN links = Hello timer 60 second, Holdtimer 180 seconds.

To become a neighbour, the following conditions must be met:

The router muse hear a Hello packet from a neighbour,
The EIGRP Autonomous System (AS) number in the Hello packet must be the same as the receiving router,
the K-values used to calculate the metric must be the same.

Creating the Topology Table

After a router knows who neighbours are, it can create a Topology Table, assign Successors and Feasible Successors for each route (The Topology Table has a record of all routes not only Successors and Feasible Successors). The other routes are referred to as possibilities.

The topology table includes the following information:

Whether the route is passive or active.
Whether an update has been sent to the neighbour.
Whether a query packet has been sent to a neighbour
- if positive at least 1 route will be market active.
Whether a query packet has been sent
- if positive another field will track whether any replies have been received from neighbours.
That a reply packet has been sent in response to a query packet from a neighbour.
Prefixes, masks, interface, next-hop, and Feasible and Advertised Distance from remote networks.

The Topology Table is built from Update Packets that are exchanged by neighbours and by Replies to Queries sent by the router.

Queries and Responses used by EIGRP are sent reliably as multicast using RTP. If a router does not hear an ACK within the allotted time, it retransmits the packet as a unicast (16 times) after which the router marks the neighbour as dead.

Each time the router sends a packet, RTP increments the sequence number by one. The router must hear an ACK from EVERY router before it can send the next packet.

When all this is done the router has an understanding of the topology, it then runs DUAL to determine the BEST PATHS to the remote network. The result is entered into the Network Table.

Maintaining the Topology Table

The Topology Table may be recalculated because

A new network is added,
Successors change,
A network is lost.

Adding a Network to the Topology Table

As soon as Router A becomes aware of the new network (right),
1. It starts sending Hello packets out the new interface.
  1. No one answers (there is no router out the interface).
    - There will be no entries in the Neighbour Table because no neighbours responded to the Hello.
    - There is however a new entry in the Topology Table because it is attached to a new network.
EIGRP, sensing a change, must send an update to all neighbours on it’s old interface, informing neighbours of the change. These updates are tracked in the Topology Table and the Neighbour Table because updates are connection-orientated and ACKs from neighbours must be received within a timeframe.
Router A has completed its work.
1. Neighbours on the old network will update their sequence numbers in their Neighbour Tables and add the new network to the Topology Table.
  1. They will calculate FD and the Successor to place in the Routing Table.

Deleting a Path or Router from the Topology Table

If a network connected to Router A is disconnected (right),
1. Router A updates its Topology Table and Routing Table and sends an update to its neighbours.
When a neighbour receives the update ,
1. it updates the neighbour table and the topology table.
The neighbour searches for an alternate route to the network. It examines the Topology table for alternatives (none will be found there is only one path).
The neighbour then sends out a query to its neighbours requesting that they look in their tables for paths to the remote network.
1. This marks the route active in the Topology Table.
The query is tracked and when all replies are in the Topology Table and Neighbour Table is updated.
DUAL (which starts as soon as network change registers) runs to determine the best path, which is placed in the routing table.
Before routers respond, routers query their own neighbours (the search for alternative paths extends or diffuses throughout the entire organization).
If no alternative is found, the neighbours reply to the query stating that they have no path.
When no router can supply a path to the network, all the routers remove the network from their Routing Table and Topology Table.

Finding an alternate path to Remote Network

The router marks the routes that were reached by sending the traffic to that neighbour.
The router looks in the topology table to determine if there is an alternate route (Feasible Successor).
If a successor is found, the router adds the feasible successor to it’s routing table. If the router did not have a feasible successor, it would have placed the route into an active state while sending queries to neighbours for an alternate path.
After interrogating the topology table, if a feasible route is found, the neighbour replies with the alternative path. This path is placed in the Topology Table.
If no answer is heard, the messages are propagated through the network.

Creating the Routing Table

The Routing Table in EIGRP is built from the Topology Table using DUAL. The Topology Table holds all routing information known to the router and from this information successors and feasible successors are selected. Successors are passed to the Routing Table and used for routing decisions.

EIGRP Path Selection

Go here for more information on the metric.

Updating the Routing Table in Passive Mode with DUAL

When a path is lost, DUAL first looks in the Topology Table for a FD; If none the router stays in passive mode (as opposed to active mode where the router actively queries for alternative paths).

The FD from Router A to Router G is 10 ( A – D – G)
The AD from Router A to Router G is 5 (advertised from Neighbour D)
- Because 10 > 5 (FD > AD). The FD meets the feasibility condition allowing it to become FD.
- If the link between Router D and Router G goes down. Router A looks in its Topology Table.
- The Alternative Routes through Routers A to D to E to G (A-D-E-G) have an AD of 19
  - Because 10 < 19 (original FD), it does not qualify as a feasible successor.
- The Path through Router D to H to F to G (D-H-F-G) has an AD of 20
  - Because 10 < 20 (original FD), it does not qualify as a feasible successor.
- The Path through Router A to E to G has an AD of 7
  - Because 10 > 7 (original FD), it does qualify as a feasible successor.
- After the link between Router D and G dies, the Routing Table would be updated from the Topology Table while the router remains in Passive Mode.

Updating the Routing Table in Active Mode with DUAL

When no alternative route is found in the Routing Table, the following actions occur. The Topology Table of Router A starts with a path (successor) of A to D to G to X. The FD is 20, and the AD from Router D is 15. When Router D dies, Router A must find an alternate path to X.

The router rejects neighbours Router B, Router C, Router E and Router F as Feasible Successors.
- Router B 20 < 27
- Router C 20 < 27
- Router E 20 = 20
- Router F 20 < 21
  - Because all neighbours have a AD greater than or equal to the successors FD. They do not meet Feasibility requirements.
Router A goes into Active Mode and sends out queries.
Both Router E and F reply
- Router E 20 > 5
- Router F 21 > 5
  - The network returns to Passive Mode. The FD is acceptable, the Topology Table and Routing Table will be updated.
  - Router E is selected as the best route based on a lower FD
The result is placed in the Routing Table as the valid neighbouring router.
Router F will be the feasible successor.

EIGRP Network Design

EIGRP is designed to work in very large networks.
EIGRP is very design Sensitive.
Scaling a network properly is a major concern.
New demands are constantly driving the networks to use applications that require more bandwidth with less delay; while networks are becoming larger and more complex.

Factors that can affect of EIGRP include:

Amount of information sent between neighbours.
Number of routers that receive updates.
distance between neighbouring routers.
number of alternative paths to remote networks

Poorly scaled EIGRP networks result in:

A stuck-in-Active route
Network Congestion
Lost routing information
Flapping routes
Retransmission
Low Router memory
Over utilized Router CPU

Other factors (poor design) cause some of these symptoms because resources are overwhelmed with assigned tasks.

EIGRP Design Issues

Major concern in scaling an organizations network is controlling advertisements and limiting query range (NB over slow WAN links). Sending less information about the network there is more bandwidth available to clients and servers. This relieves the network and speeds convergence, it provides less information for alternate paths though.

EIGRP automatically summarizes at classful network boundaries because summarization is generally helpful and EIGRP is built to recognize opportunities such as this to optimize the network (Most Admins disable auto summarization because it does not match their needs, instead manually configure it at interface level).

Certain topologies pose problems for EIGRP networks. Like the hub-and-spoke design often used between remote sites and regional offices. Popular dual-hub configuration provides redundancy and allows for potential for routers to reflect queries back to one another. Summarization and filters make network design work well while also allowing queries to be managed effectively.

Guideline to Scaling Issues

Assign addresses and organize links so that natural points for summarization exist. A hierarhical network design IOW.
Provide sufficient hardware resources (mem and CPU) on network devices.
Use sufficient bandwidth on the WAN links.
Use filters to limit advertisements.
Monitor the network.

I’m very strange. Every time I type Hello, I have a voice in my head going “Hello Kitty”. So share my pain “Hello Kitty”!

Hello Kitty
I’m going to kick myself later when I read over this post again cause this is going to get stuck in my head again.

Resources:

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

Notes and Notices:

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 – Routing Protocols

Published

Deon Botha

on July 25, 2008

in BGP, BSCI, BSCI Notes, CIDR, Certification, Cisco Systems, Concepts and Constructs, EIGRP, IGRP, IS-IS, OSPF, RIP, RIPv2 and VLSM

. 2 Comments

Routing protocols employ one of two basic strategies to communicate/propagate routing information:

Distance vector routing protocols work by passing copies of their routing tables to their neighbours (a.k.a routing by rumour).
Link State routing protocols work by advertising a list of neighbours and the network attachment state to their neighbours until all routers have a copy of all the lists, routers then run the Shortest Path First Algorithm to analyse all paths and determine the best paths available.

Distance vector routing are less processor and memory intensive than link state routing, but can have loops because routing decisions are made on incomplete information.

Link state routing is loop-proof because routers know all possible routes, but link state routing requires more CPU time and memory.

Classless and Classful Routing

An important characteristic of routing protocols is how they advertise their routes. Older routing protocols (RIP and IGRP) assumed the subnet mask the same as the one the receiving on the interface or that it is the default one (Class A is /8, Class B is /16 and Class C is /24). This is called classful because the assumption is based on the Class of the IP address.

Modern routing protocols (OSPF, IS-IS, and EIGRP) explicitly advertise the mask. There is no assumption made with regard to the mask, it is clearly indicated. This is called classless because no assumption is made and an address alone is not a good indicator subnet mask.

Variable Length Subnet Masks (VLSM) refers to the property of a network that allows different subnet masks to be mixed throughout the network.

Classless Interdomain Routing (CIDR) is a property of a network that allows classful networks to be aggregated.

Classless routing protocols support both VLSM and CIDR.

Interior and Exterior Gateway Protocols

Most protocols are “Interior Gateway”, meaning that they are designed to be run inside a network (inside the trusted boundaries of the company).

BGP on the other hand is an exterior gateway protocol (EGP) and is used for routing between autonomous systems (AS) on the Internet (outside the trusted boundaries of the company). As BGP is the only EGP you will have to consider using it if you connect your network to the Internet.

Convergence Times

A distinguishing characteristic of routing protocols is the speed of convergence times. To explain convergence, when a routing protocol is forwarding data, it is converged. In this state the routing protocol has shared routing table information and each router in the topology knows the best paths available. If there was a change (a router going down, another router being added, etc) this would require all routers to share information again because there are routes they do not have information on. The time between network change and forwarding would be “convergence”. This is generally classed as either slow or fast.

Fast convergence would mean that the routing protocol is able to recognize a problem on the network and fix that problem faster than a user can call to report a given problem.

Slow protocols, such as RIP and IGRP, can take up to minutes to converge when a problem occurs.

Fast protocols (OSPF, IS-IS, EIGRP) generally take less than 10 seconds to converge.

Proprietary and Open Standard Protocols

The important aspects to look for in routing protocols is speed of convergence and whether the protocol is classless (OSPF, IS-IS, and EIGRP). While OSPF and IS-IS are open standards (plays well with other vendors kit), EIGRP is Cisco proprietary (Cisco Only). Of the three protocols EIGRP is the easiest to configure and maintain but requires a pure Cisco environment to run.

Routing Protocol and the ECNM

The ECNM mentioned in previous posts can assist in showing where a particular routing protocol will run in the enterprise. Using information discussed above and using the ECNM the above diagram shows what the advanced routing protocols (EIGRP, OSPF, IS-IS) are best suited for when considering size of network, speed of convergence, VLSM, open or proprietary, and support staff knowledge needs.

The object (ideal) is to have a single routing protocol running throughout the enterprise (reality however is another story) where the enterprise edge will require BGP as the only EGP and at least one if not more of the IGPs within the enterprise boundaries depending on needs/requirements of end-points or design specifications.

In Summation

Older routing protocols (RIP, RIPv2 and IGRP) are slow because they send a full copy of their information periodically, these periodic transmissions act as both routing advertisement and keepalive message. In addition to being slow they consume a lot of bandwidth relative to their function (RIP every 30 seconds).

More modern routing protocols are faster because they separate the routing advertisements and the keepalive messages. Updates are only sent out when new networks need to be advertised or old networks need to be withdrawn; otherwise routers just need to verify that neighbours are still alive (EIGRP every 5 seconds).

RIP and IGRP

These are older distance vector routing protocols that are slow and classful. Some legacy systems (UNIX) expect to learn their default gateway by eavesdropping on RIP advertisements. If you deploy RIP use RIPv2 which is classless.

EIGRP

A modern distance vector routing protocol. It is classless and fast as well as being easy to configure and maintain. Some organizations refuse to implement proprietary standards though (EIGRP provides equivalent performance to OSPF but is easier to implement and maintain).

OSPF

OSPF is a modern classless and fast link-state routing protocol. OSPF has a steep learning curve and uses more processor time and memory than EIGRP. This is the open standard if an organization supports a heterogeneous mixture of routers or has a philosophical problem with proprietary standards.

IS-IS

This routing protocol was developed to compete with OSPF and the two are more similar than they are dissimilar. It is moderately difficult to find anyone who has experience working with IS-IS even if it is open, fast, and classless. There is still however some interest in IS-IS because it can be adapted to support MPLS and IPv6.

BGP

BGP is a routing protocol used between AS on the Internet and you will have to use it to connect your network to the Internet.

Resources:

Internetworking Technology Handbook Routing Basics

Internetworking Technology Handbook RIP

Internetworking Technology Handbook IGRP

Internetworking Technology Handbook OSPF

Internetworking Technology Handbook EIGRP

Notes and Notices:

Network Ninja

Archive for the 'EIGRP' Category

Enhanced Interior Gateway Routing Protocol – Scalable EIGRP – Single interface EIGRP

Enhanced Interior Gateway Routing Protocol – Configuring EIGRP

Enhanced Interior Gateway Routing Protocol – Tables

Enhanced Interior Gateway Routing Protocol – Introduction

BSCI Design Foundation – Routing Protocols

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