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 their neighbours and the networks 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.
Distance vector routing is 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 as the same as the one the receiving 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.
Variable Length Subnet Masks (VLSM) refer 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 VLSM and CIDR.
Interior and Exterior Gateway Protocols
Most protocols are “Interior Gateway”, meaning that they are designed to be run inside a network.
BGP on the other hand is an exterior gateway protocol (EGP) and is used for routing between autonomous systems (AS) on the Internet. 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. These are 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 the problem.
Slow protocols, such as RIP and IGRP, can take up to minutes to converge. 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 are speed and whether they are classless (OSPF, IS-IS, and EIGRP). While OSPF and IS-IS are open standards (plays well with other vendors), EIGRP is Cisco proprietary (Cisco Only). Of the three protocols EIGRP is the easiest to configure however but requires a pure Cisco environment to run.
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.
No comments:
Post a Comment