RIP Route Database

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By Ivan Pepelnjak

The RIP routing protocol uses an internal database to store routes received from RIP neighbors. This database is also used to generate outbound RIP updates. The RIP database is almost identical to the list of RIP routes in the main IP routing table, with the exception of the RIP summary routes which appear only in the RIP database and not in the main IP routing table. The RIP database can be inspected with the show ip rip database [network mask] command; the contents are almost identical to the show ip route rip command with the following exceptions:

  • RIP summary routes are not present in the IP routing table.
  • Directly connected subnets included in RIP routing process are inserted in the RIP database to ease generation of outbound updates.

Processing of inbound routing updates

A router running Cisco IOS follows these rules when processing routes in an inbound RIP update:

  • Inbound update filters are processed (global and per-interface distribute list).
  • IP prefix matching any summary address configured with the ip summary-address rip network mask on the incoming interface is ignored.
  • If the IP prefix already exists in the RIP database, the metric in the incoming route update is compared with the metric in RIP database. Only the best RIP routes are retained in the RIP database (the RIP database has no backup information like the EIGRP topology table).
  • If the RIP route cannot be inserted in the IP routing table (another route with a better administrative distance is already present in the IP routing table), it’s not stored in the RIP database.

Sample RIP database

A simple three-router RIP network shown in the following diagram was used to generate the printouts in this section:

Sample RIP network

Relevant parts of the R2 configuration are shown in the following listing:

hostname R2
interface Loopback0
 ip address
interface Serial1/1
 description Link to R1(ROUTER) S1/1
 ip address
interface Serial1/2
 description Link to R3(ROUTER) S1/1
 ip address
 ip summary-address rip
router rip version 2 network no auto-summary

The IP routing table on R2 contains only two RIP routes: the prefixes for loopback interfaces received from R1 and R3.

R2#show ip route rip is variably subnetted, 5 subnets, 2 masks
R [120/1] via, 00:00:05, Serial1/2
R [120/1] via, 00:00:05, Serial1/1

The RIP database on R2 contains all the local subnets (RIP is running on all interfaces due to network router configuration command) as well as the interface summary prefix

R2#show ip rip database    auto-summary    int-summary
    [1] via, 00:00:02, Serial1/1     directly connected, Loopback0
    [1] via, 00:00:02, Serial1/2    directly connected, Serial1/2    directly connected, Serial1/1

Additional Resources  

Interconnecting Interconnecting Cisco Network Devices Part 2 (ICND2) course:

Building Scalable Cisco Internetworks (BSCI) course:

Other links

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