|W O R K S H O P|
Choosing an Interior Gateway Protocol
October 18, 1999
By Peter Morrissey
When you enable routing protocols, and their corresponding processes, you unleash a powerful, dynamic force that automates the population of routing tables on your network. But using any major IGP (Interior Gateway Protocol)--RIP/RIP2, OSPF or IGRP/ EIGRP--has trade-offs.
The first rule of thumb is to implement IGPs only where a single administrator has responsibility for the operation and performance of the network--otherwise, you can expect serious problems such as configuration errors bringing down the network or causing stability problems. In addition, when problems occur, you want to be able to fix them quickly and not waste time arguing about who caused them.
For networks where many administrators share responsibility, such as the Internet, consider an EGP (Exterior Gateway Protocol), like BGP4; we'll explore EGPs in an upcoming workshop.
If you have only one router, you may not need a routing protocol; it's necessary only when you have multiple routers that need to share information. Even then, if you have only a handful of networks, you can update the tables manually via static routes, though they don't scale very well--which brings us back to routing protocols.
Routing Information Protocol
In spite of the popular phrase, "RIP: May it RIP," RIP is well-tested, widely implemented and ideally suited for networks without redundant routes. RIP v.1 is still used to support most of Syracuse University's 10,000-node network. It does the job, and we have not suffered a major network outage in two years.
RIP is classified as a distance vector protocol, which means it uses distance, as measured in routing hops, to determine a packet's optimal path. Routers send out advertisements to one another every 30 seconds. Each router that receives the advertisement increases the hop count by one. If advertisements are received from multiple routers, the path to the router with the lowest hop count is the path chosen. Should the preferred route be unavailable, the route with the higher hop count is used as a backup.
With RIP (and other routing protocols), the routers on the network must go through a process to determine alternate paths when one path becomes unavailable. This process is called convergence. That RIP takes a long time to converge is a major problem. RIP is designed to wait until it has missed six updates, totaling 180 seconds, before it will consider the route unreachable. It then waits for the next advertisement of another available route before it updates the routing table with the new route. This means at least three minutes will pass before a backup route is used, certainly long enough for most users to notice a lag and for most applications to time out. Of course, this won't be a problem if you have only one route to any destination.
The other fundamental problem with RIP is that it ignores the speed of links involved when choosing a path. For example, if one path consisting of all Fast Ethernet links is one hop further away than a path that includes a 10-Mbps Ethernet link, the path with the slower 10-Mbps Ethernet link will be selected as optimal (see "IGP Routing Metrics," to the right).
The original version of RIP was unable to use VLSMs, barring you from slicing and dicing your address space to make the most efficient use of limited IP addresses. RIP 2 fixes this by advertising the subnet mask in use with every routing advertisement. Because the earlier version did not advertise the subnet mask, it had no way to convey the details of different-length masks for different networks or subnets.
Routing protocols should also prevent your packets from going in circles, or falling into routing loops, a problem that affects networks with redundant links. RIP assumes that if there are more than 15 routing hops from one end of a network to another, loops must be involved. Therefore when a route reaches 16 hops it considers it to be unavailable. Obviously, this limits RIP to networks in which no more than 15 routers need be traversed.
RIP's biggest problems involve larger networks with redundant paths. If your network does not have redundant paths, RIP should work just fine. It is an Internet standard implemented by just about every vendor that supports routing. RIP is implemented in most server operating systems and is very easy to configure and troubleshoot. If you have a larger network, or one with redundant paths, you should seriously consider one of the other routing protocol choices.
|PAGE: 1 I 2 I 3 I NEXT PAGE|