Routing information protocol is a distance vector protocol that uses hop count as its metric. It is an interior gateway protocol (IGP) that’s means that it performs routing within a single autonomous system.
- It is Open Standard protocol.
- It is a class full protocol, so it uses full address classes, not subnets.
- Updates are through broadcast via 255.255.255.255.
- It doesn’t support route authentication.
- It doesn’t provide router summarization.
- Administrative distance is 120.
- Maximum hop count is 15 (16 hops indicates the network is unreachable).
- It uses UDP port 520.
- 25 routes per RIP message.
- Implements split horizon with poison reverse.
- Implements triggered updates.
- It is used for small networks.
- If RIP finds more than one link to the destination with the same hop count, it will automatically perform load balancing. RIP can perform load balancing for up to sixteen equal cost links (four by default).
Update Timer: (30 Sec)
Update timer specify how frequently routing- update messages should be sent.
Invalid Timer: (180 Sec)
Invalid timer specifies how long a router should wait before declaring a route invalid if it doesn’t receive a specific update about it.
Hold-Down Timer: (180 Sec)
Hold Down timer specifies the amount of time during which routing information is suppressed. This continues until either an update packet is received with a better metric or until the hold down timer expires.
Flush Timer: (240 Sec)
Flush timer specifies the time between a route becoming invalid and its removal from the routing table.
Split Horizon Rule:
Split- horizon is a method of preventing a routing loop in a network. Split horizon states that if a neighboring router sends a route to a router, the receiving router will not propagate the same route back to the advertising router on the same interface.
Split horizon rule takes two forms:
- Simple split horizon &
- Split horizon with poisoned reverse.
Route poisoning is a method to prevent a router from sending packets through a route that has become invalid within networks.
For example, when network 4 goes down, router C starts route poisoning by advertising the metric (hop count) of this network as 16, which indicates an unreachable network.
When a router detects that one of its connected routes has failed, the router will poison the route by assigning an infinite metric to it and advertising it to neighbors. When a neighbor receives a route with infinite metric, its break the split horizon rule and sends back to the originator the same poisoned route, that process is called a poison reverse.
For example, if router B receives a route poisoning of network 4 from router C then router B will send an update back to router C (which breaks the split horizon rule) with the same poisoned hop count of 16. This ensures all the routers in the domain receive the poisoned route update.
Notice that every router performs poison reverse when learning about a downed network. In the above example, router A also performs poison reverse when learning about the downed network from B.
Route Summarization is the process of combining two or more networks or subnets with a common Classless Inter-Domain Routing (CIDR) prefix.
Purpose of Route Summarization:
- Saving space in the routing table.
- Speeding up packet switching.
Router’s Route Selection Criteria:
Basically, there is a four-step process a router goes through when looking for the best route to use:
- If there are multiple routes to a destination, the route with the longest prefix length is used.
- If there are multiple routes to a destination and they have the same prefix length, the route with the lowest administrative distance is used.
- If there are multiple routes with the same prefix length and AD, the route with the lowest metric is used.
- If there are multiple routes with the same prefix length, AD, and metric, all of these routes will be used in load balancing as allowed by the protocol.