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Chapter 4 - Routing Protocols Used in TCP/IP

Cisco TCP/IP Routing Professional Reference
Chris Lewis
  Copyright © 1999 The McGraw-Hill Companies, Inc.

Exterior Gateway Routing Protocols
These protocols are designed to regulate what traffic can travel between different autonomous systems and protect each from any bugs in another AS. The mechanisms we will examine here are static routing, the Exterior Gateway Protocol, and the Border Gateway Protocol.
Static Routing
Usually an experienced network administrator will seek to minimize any manual configuration. In the case of exterior routing, this might be different, because static routing offers a number of advantages when routing between autonomous systems. These advantages can be summarized as follows:
  Complete flexibility over the advertisement of subnets and their next-hop routers.
  No routing protocol traffic travels over the link connecting autonomous systems.
  Because no routing protocol is operating over the inter-AS link, there is no possibility of a faulty router in one AS affecting the other AS.
The down sides are obvious. Static routes do not adapt to link failures, and manual configuration can be a headache to maintain. Despite these down sides, static routing often is a popular choice for connecting internetworks that do not "trust" one another. Let's say autonomous system 1 consists of network numbers 45.0.0.0 and 46.0.0.0, and autonomous system 2 consists of network numbers 47.0.0.0 and 48.0.0.0. This is illustrated in Fig. 4-21.
Figure 4-21: Internetwork used to discuss static routing
To complete static routing for connecting these two autonomous systems together, use the following commands:
RouterA(config)#ip route 47.0.0.0 255.0.0.0 80.4.4.5
RouterA(config)#ip route 48.0.0.0 255.0.0.0 80.4.4.5
RouterB(config)#ip route 45.0.0.0 255.0.0.0 80.4.4.4
RouterB(config)#ip route 46.0.0.0 255.0.0.0 80.4.4.4
This tells each autonomous system how to get to networks in the other autonomous system.
Exterior Gateway Protocol
The Exterior Gateway Protocol, or EGP, has three components, neighbor acquisition, neighbor reachability, and routing information. EGP was designed to add a measure of automation to the configuration of routes between different autonomous systems. EGP has been superseded by the Border Gateway Protocol, (BGP), so we will not spend too much time on it.
EGP Neighbor Acquisition.     We can use Fig. 4-21 to discuss how EGP works. Suppose router A has been configured to initiate a neighbor relationship with router B. This will initiate a neighbor acquisition packet to be sent from router A to router B. If router B has been configured to accept a neighbor relationship with A, this packet is accepted. The neighbor reachability mechanism is the hello packet, used to verify that a router that is accepted as a neighbor is still functioning. The routing information of EGP is similar to distance vector protocols, but it omits the metric for routes advertised. EGP was implemented like this because it was designed for the Internet, when it was assumed that there would be a core network with separate routing domains connected to this core by one router.
The major problem with using EGP in a more generalized network is that, because no use is made of metrics, if there is more than one path to a destination, packets can easily get caught in routing loops (Fig. 4-22).
Figure 4-22: A sample internetwork in which EGP does not work
If router R4 receives a packet for AS1, it will have two routes to AS2, one via R3 and one via R6. Because there are no metrics involved, the routes are equal in R4's determination, so it may choose either. Let's say it chooses to send the packet to R3. R3 also has two routes to AS1, one via R4 and one to R1. Again, these routes are equal in R3's consideration, so the packet could be returned to R4, where the cycle starts over again. Clearly, EGP has problems for a general environment, so I would recommend leaving it alone.
BGP: Border Gateway Protocol
BGP was introduced to improve upon EGP. One of BGP's main features is that it introduces a reliable transport protocol to ensure that route updates are received. BGP also implements a keep-alive mechanism, ensuring that BGP routers know if neighboring BGP routers fail. BGP does not transmit metrics with its route updates, but does transmit a path for each AS that lists the autonomous systems to be visited on the way to the destination AS. BGP thus avoids the circulating-packet problem of EGP.
BGP works on the principle of enforcing policies. A policy is manually configured and allows a BGP-enabled router to rate possible routes to other autonomous systems, selecting the best path.
Configuring BGP.We can use Fig. 4-22 to discuss how to configure BGP on a router. In this example, let's take router R6. To configure BGP we will perform the following:
  Define BGP as a routing process.
  Define the networks internal to this AS that are going to be advertised.
  Define the relationships that this router will have with its neighbors.
  Assign administrative weights to paths to control the path selection process.
This is a basic configuration for BGP. There are, however, many further configuration customizations that can be made. If you really want to get into complex BGP configuration, I would recommend talking to a Cisco Systems engineer who can help you with customizing BGP for your particular internetwork.
For a basic configuration, the following commands are entered into router R6:
Router6(config)#router bgp 3
Router6(config-router)#network 147.30.0.0
Router6(config-router)#network 150.1.0.0
Router6(config-router)#neighbor 147.30.1.1 remote-as 3
Router6(config-router)#neighbor 160.4.5.5 remote-as 2
The first line in this configuration defines BGP for autonomous system 3 on router 6. The next two lines define the network numbers internal to AS 3 that will be advertised via BGP. The fourth line defines an internal neighbor that is in the same AS. The BGP process on router 6 will now exchange information with a BGP process defined on R5. The fifth line defines the neighbor in a different AS with which router 6 will exchange information.
The effect of this configuration is that R6 will share information about networks 147.30.0.0 and 150.1.0.0 with the two specified routers via BGP updates.The last thing left to do in our basic configuration of BGP is to assign administrative weights to control the path selection process. In the following example, a weight of 40,000 is assigned to the path to router R4.
Router6(config-router)#neighbor 160.4.5.5 40000
This administrative weight can vary between 0 and 65535, with the default being 32768. The effect of increasing the weight to R4 is to make it less attractive when R6 is calculating which paths to use.

 


 
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