Use the debug ip routing privileged EXEC command to display information on Routing Information Protocol (RIP) routing table updates and route-cache updates. The no form of this command disables debugging output.
debug ip routingSyntax Description
This command has no arguments or keywords.
Examples
The following is sample output from the debug ip routing command:
Router# debug ip routing
RT: add 172.25.168.0 255.255.255.0 via 172.24.76.30, igrp metric [100/3020]
RT: metric change to 172.25.168.0 via 172.24.76.30, igrp metric [100/3020]
new metric [100/2930]
IP: cache invalidation from 0x115248 0x1378A, new version 5736
RT: add 172.26.219.0 255.255.255.0 via 172.24.76.30, igrp metric [100/16200]
RT: metric change to 172.26.219.0 via 172.24.76.30, igrp metric [100/16200]
new metric [100/10816]
RT: delete route to 172.26.219.0 via 172.24.76.30, igrp metric [100/10816]
RT: no routes to 172.26.219.0, entering holddown
IP: cache invalidation from 0x115248 0x1378A, new version 5737
RT: 172.26.219.0 came out of holddown
RT: garbage collecting entry for 172.26.219.0
IP: cache invalidation from 0x115248 0x1378A, new version 5738
RT: add 172.26.219.0 255.255.255.0 via 172.24.76.30, igrp metric [100/10816]
RT: delete route to 172.26.219.0 via 172.24.76.30, igrp metric [100/10816]
RT: no routes to 172.26.219.0, entering holddown
IP: cache invalidation from 0x115248 0x1378A, new version 5739
RT: 172.26.219.0 came out of holddown
RT: garbage collecting entry for 172.26.219.0
IP: cache invalidation from 0x115248 0x1378A, new version 5740
RT: add 172.26.219.0 255.255.255.0 via 172.24.76.30, igrp metric [100/16200]
RT: metric change to 172.26.219.0 via 172.24.76.30, igrp metric [100/16200]
new metric [100/10816]
RT: delete route to 172.26.219.0 via 172.24.76.30, igrp metric [100/10816]
RT: no routes to 172.26.219.0, entering holddown
IP: cache invalidation from 0x115248 0x1378A, new version 5741
In the following lines, a newly created entry has been added to the IP routing table. The "metric change" indicates that this entry existed previously, but its metric changed and the change was reported by means of IGRP. The metric could also be reported via RIP, OSPF, or another IP routing protocol. The numbers inside the brackets report the administrative distance and the actual metric.
RT: add 172.25.168.0 255.255.255.0 via 172.24.76.30, igrp metric [100/3020]
RT: metric change to 172.25.168.0 via 172.24.76.30, igrp metric [100/3020]
new metric [100/2930]
IP: cache invalidation from 0x115248 0x1378A, new version 5736
"Cache invalidation" means that the fast switching cache was invalidated due to a routing table change. "New version" is the version number of the routing table. When the routing table changes, this number is incriminated. The hexadecimal numbers are internal numbers that vary from version to version and software load to software load.
In the following output, the "holddown" and "cache invalidation" lines are displayed. Most of the distance vector routing protocols use "holddown" to avoid typical problems like counting to infinity and routing loops. If you look at the output of show ip protocols you will see what the timer values are for "holddown" and "cache invalidation." "Cache invalidation" corresponds to "came out of holddown." "Delete route" is triggered when a better path comes along. It gets rid of the old inferior path.
RT: delete route to 172.26.219.0 via 172.24.76.30, igrp metric [100/10816] RT: no routes to 172.26.219.0, entering holddown IP: cache invalidation from 0x115248 0x1378A, new version 5737 RT: 172.26.219.0 came out of holddown