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:
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.