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(config-router)# no auto-summary (config-router)# exit (config)# interface Ethernet 0/0 (config-if)# ip summary-address 17216640 2552552540

That's it Route aggregation really isn't hard, as long as you understand the binary behind it Now that you've learned about nearly every common configuration option that RIP has to offer, let's take a look at some common troubleshooting tips

Like with all other networking issues, the first step in solving any problem is to determine the root cause For this task, your show and debug commands should come in particularly handy For show commands, the hands down most useful commands at your disposal are show ip route and show running-config Show ip route shows you what is in your routing table (as described in 22), whereas show running-config verifies the RIP configuration You can also use the show ip protocols command to show details about the running routing protocols on the router, including timers and summarization details, as shown in the following example:

Gage# show ip protocols Routing Protocol is "rip" Sending updates every 30 seconds, next due in 25 seconds Invalid after 180 seconds, hold down 180, flushed after 240 Outgoing update filter list for all interfaces is Incoming update filter list for all interfaces is Redistributing: rip Default version control: send version 2, receive version 2 Interface Send Recv Triggered RIP Key-chain Ethernet0/0 2 2 Ethernet0/1 2 2 Ethernet1/0 2 2 Automatic network summarization is not in effect Address Summarization: 17216640/23 for Ethernet0/0

Show ip rip database is also useful for showing which networks are currently being advertised by RIP, as you can see in the following output:

CCNA2600C#sh ip rip database 10000/8 auto-summary 10110/24 directly connected, Ethernet0/0 16925400/16 auto-summary 16925400/16 [1] via 101114, 00:00:19, Ethernet0/0 1723100/16 auto-summary 1723100/16 [2] via 101114, 00:00:19, Ethernet0/0

The most useful debugging command is probably debug ip rip Once enabled, RIP debugging shows you all RIP packets sent from or received by the router, as well as the associated error messages and networks sent in the update If you run into problems with RIP, be on the watch for a few common issues:

VLSM is not supported in RIP 1 Remember, depending on the configuration, RIP 1 either summarizes VLSM subnets into one class-based network address or neglects to advertise the route entirely Autosummary is enabled by default in RIP 2 If you are using RIP 2, ensure that your network topology can be autosummarized, or disable autosummarization If authentication is enabled, all routers that will participate in the RIP network must use the same password(s) Non-Cisco routers may not support MD5 authentication Split horizon can cause problems on Frame Relay PVCs if multiple PVCs are mapped to the same interface Use subinterfaces with one PVC per subinterface to eliminate this problem Split horizon must be disabled for multipoint PVCs Make sure to include route filtering to prevent routing loops in multipoint topologies Ensure that timers on all routers in the topology match If they do not, routing updates can fail and routing loops can form Ensure that the timer values are relative to each other Remember, RIP is prone to pinhole congestion when load balancing If updates are not being sent out an interface, ensure that the correct network statements are entered and that the interface is not configured as a passive interface If you are using unicast updates, ensure that the IP addresses of the remote routers are entered correctly Ensure that the problem is due to an issue involving RIP and not a physical or basic configuration problem (ie, wrong IP address applied to an interface)

Again, remember that most errors are simply misconfigurations Although these tips do not cover every problem you are likely to see with RIP (a list of bugs in different IOS versions alone would take a full chapter), they are the most common causes of RIP problems

Summary

This chapter discussed RIP 1 and 2, and examined how to configure, optimize, and troubleshoot RIP in most environments Although an old protocol, RIP is still very widely used, so this information will likely come in handy in a variety of situations In addition, many of the conventions discussed in this chapter (like split horizon and timers) apply to nearly all routing protocols, and will be an important building block as you move into more complex routing protocols, beginning with IGRP in 24