The Longest Match Routing Rule
Routing to any destination is always done on a longest match basis: A router that has to decide between two different length prefixes of the same network will always follow the longer mask. Suppose, for example, that a router has the following two entries in its routing table: 198.32.1.0/24 via path 1 198.32.0.0/16 via path 2 When attempting to deliver traffic to host 192.32.1.1, the router tries to match the destination that has the longest prefix and will deliver the traff..
Classful Routing: Summarization at Boundary Routers
A question arises from the preceding discussion: How does a RIP process interpret the subnet of a major network if it has no interfaces attached to that network? Without an interface on the class A, B, or C network of the destination, the router has no way of knowing the correct subnet mask to use and therefore no way of correctly identifying the subnet. The solution is simple: If a router has no direct attachments to the network, then it needs only ..
Network Fault Tolerance and MPLS-Based Recovery
To this point, discussion has focused on ways to protect service outages against control-plane failures. Although discussion of data-plane protection against network failures is not the main focus of this chapter, for the sake of completeness it would be useful to examine schemes that are commonly used to improve network survivability in the face of node and link failures. Generally network outages are caused by failures of network elements such as links and nodes. Network s..
When an OSPF network grows beyond a single area, you need to be aware of the role played by each OSPF router in a topology. Specifically, four OSPF router types exist: ■ Internal router: All the networks directly connected to an internal router belong to the same OSPF area. Therefore, an internal router has a single link-state database. ■ Area border router (ABR): An ABR connects to more than one OSPF area and therefore maintains multiple link-state databases (one for e..
You can use RIP to exchange customer routing information between the CE and PE routers at each site. The advantage to doing this is that any customer routes at one site can be automatically propagated to other customer sites. This requires a normal RIP configuration on the CE router: Router-CE-A2#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router-CE-A2(config)#router rip Router-CE-A2(config-router)#version 2 Router-CE-A2(config-r..
