RIP is the oldest routing protocol still in use today. It is available on many platforms and is not a protocol that will be deprecated anytime soon. It is a useful protocol for networks that are large enough for a routing protocol but not large enough for some of the more powerful and expensive protocols available. RIP even has a new RIPng version to handle IPv6 networks.
RIP Characteristics
• Hop count is routing metric.
• Broadcast messages every 30 seconds.
• Distance vector routing protocol.
• Hop counts greater than 15 are unreachable.
• Classful routing protocol.
• Administrative Distance is 120.
• Automatic route summarization.
RIP Message Header and Route Entry
In the past we have studied the Data Link layer header and trailer, the Network header, and the Transport Layer header. We will now inspect the RIP header, which is a Layer 7 service. The following is the byte breakdown.
• Command (0-7) [1 for a request, 2 for a reply]
• Version (8-15) [1 for RIPv1, 2 for RIPv2]
• Must be zero (16-31)
• Address Family Identifier (32-47) [set to 2 for IP or 0 for full routing table request]
• IP Address (48-79) [Network Address, Subnet, or Host Address]
• Metric (80-111) [number between 1-16]
One RIP message can contain up to 25 routes. Max size for a message is 512 bytes (not including IP or UDP headers).
RIP Startup
When RIP is enabled on a router the first message sent is a request for its neighbors routing tables, which it inspects and installs any better or unknown routes. It then sends a triggered update containing its routing table to its neighbors.
Configuring RIP
To enable any routing protocol on a Cisco router you must enter global configuration and enter the command,
Router(config)# router ripOnce you have entered the RIP configuration mode you list each network (without subnet masks) that is directly connected to the router you are configuring,
Router(config-router)#
Router(config-router)#network 192.168.1.0At this point RIP is enabled and configured. You should continue this process on all routers that will run RIP. It is a best practice to always have your routers up and running plus tested before configuring RIP. This practice will help you troubleshoot any RIP issues that arise during the initial RIP configuration.
Router(config-router)#network 192.168.2.0
To disable RIP globally enter,
Router(config)# no router ripTo remove a network enter,
Router(config-router)#no network 192.168.1.0
Troubleshooting Commands
• show running-config
• show ip protocols
• show ip route
• debug ip rip
Passive Interfaces
If you need to disable a specific interface from sending RIP updates you must use the passive interface command. It is important to disable RIP updates for security and bandwidth reasons when a specific interface is not required to have updates sent out. All routing protocols support passive interface. This command does not prevent the network from being broadcast, only the interface as a broadcast location.
Router(config-router)#passive-interface fa0/0
Broadcasting a Default Route
One issue we have not learned how to overcome is how to dynamically broadcast a default route (Gateway of Last Resort). Routing protocols have the ability to do this if told too. The command to issue needs to be issued only once on the router that will be the last point before the border. To configure you must issue a static route on this router to the appropriate outgoing interface.
Router(config)#ip route 0.0.0.0 0.0.0.0 s0Then enable RIP and enter the following command to broadcast the default route.
Router(config)#router ripImportant Note
Router(config-router)#default-information originate
Router(config-router)#exit
On assessment exams and certification exams a problem will be considered incorrect if you do not enter the classful address of a network when configuring RIP. For example, you must enter network 172.16.0.0 not network 172.16.1.0 even if this network is /24 because it is not classful.
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