RIP Offset Lists
RIP offset lists allow RIP to add to a route's metric, either before sending an update, or for routes „ received in an update. The offset list refers to an ACL (standard, extended, or named) to match the routes; any matched routes have the specified offset, or extra metric, added to their metrics. Any routes not matched by the offset list are unchanged. The offset list also specifies which routing updates to examine by referring to a direction (in or out) and, optionally, an interface. If the interface is omitted from the command, all updates for the defined direction will be examined.
Example 8-7 shows R1, which offsets the metric by 13 for any routes in the range 172.31.208.0/21, for R1's updates sent out toward the branches (R3-R6). (The example uses the same network from Figure 8-2.) Similarly, R2 uses the same ACL matching logic, but R2 offsets the metric upon receipt of inbound routing updates on its LAN interface (fa0/0).
The routes in the 172.31.208.0/21 range are loopbacks on S1, advertised by S1 with metric 1; unchanged, R1 and R2 would have routes with a metric of 1 to these subnets, and the branch routers would have routes with a metric of 2. After configuring the offset lists, the branches will have a metric of 15 for these routes. As a result, the branch routers will not advertise these routes any farther, because the advertised metric would have to be 16—which of course is infinite in RIP.
Example 8-7 RIP Offset Lists ! R1 Config
! Note that the offset-list command is a subcommand of the router rip command. router rip version 2
offset-list 11 out 13 Serial0/0.3 offset-list 11 out 13 Serial0/0.4 offset-list 11 out 13 Serial0/0.6 network 172.31.0.0
access-list 11 permit 172.31.208.0 0.0.7.255
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
! R2 Config router rip version 2
offset-list 11 in 13 FastEthernet0/0 network 10.0.0.0 network 172.31.0.0
access-list 11 permit 172.31.208.0 0.0.7.255
! Next, R1 lists the three routes learned from S1 with metric 1. The offset list ! on R1 did not change the offset (metric) for inbound updates to R1. R1# show ip route ! lines omitted for brevity.
172.31.0.0/16 is variably subnetted, 13 subnets, 2 masks
R 172.31.211.0/24 [120/1] via 172.31.11.201, 00:00:28, FastEthernet0/0
R 172.31.213.0/24 [120/1] via 172.31.11.201, 00:00:28, FastEthernet0/0
R 172.31.212.0/24 [120/1] via 172.31.11.201, 00:00:28, FastEthernet0/0
! Next, R1s next update to R4, out interface s0/0.4, is shown. Without the ! offset list, R1 would advertise the listed routes with metric 2; in this ! case, it added 13 more to the metric, for a total metric of 15. R1# debug ip rip RIP protocol debugging is on R1#
Jan 11 16:51:02.659: RIP: sending v2 update to 224.0.0.9 via Serial0/0.4 (172.31.14.1) Jan 11 16:51:02.659: RIP: build update entries ! lines omitted for brevity
Jan 11 16:51:02.663: 172.31.211.0/24 via 0.0.0.0, metric 15, tag 0
Jan 11 16:51:02.663: 172.31.212.0/24 via 0.0.0.0, metric 15, tag 0
Jan 11 16:51:02.663: 172.31.213.0/24 via 0.0.0.0, metric 15, tag 0
! Next, R2 lists the three routes learned from S1 with metric 14, because the
! offset list on R2 processes incoming updates. The offset list dictated that
! R2 add 13 to the received metric, which was 1 for these three routes.
Example 8-7 RIP Offset Lists (Continued)
R2# show ip route
! lines omitted for brevity.
172.31.0.0/16 is variably subnetted, 13 subnets, 2 masks
R 172.31.211.0/24 [120/14] via 172.31.11.201, 00:00:28, FastEthernet0/0
R 172.31.213.0/24 [120/14] via 172.31.11.201, 00:00:28, FastEthernet0/0
R 172.31.212.0/24 [120/14] via 172.31.11.201, 00:00:28, FastEthernet0/0
R 172.31.211.0/24 [120/14] via 172.31.11.201, 00:00:28, FastEthernet0/0
R 172.31.213.0/24 [120/14] via 172.31.11.201, 00:00:28, FastEthernet0/0
R 172.31.212.0/24 [120/14] via 172.31.11.201, 00:00:28, FastEthernet0/0
|
Finally, R4 learned metric 15 routes from both R1 and R2 for these |
routes. R4 |
|
|
will not advertise these routes to other routers, as they would be |
infinite |
|
|
metric routes. Note that R4 puts both equal-metric routes into its |
routing |
|
|
table below. |
||
|
R4# show ip route 172.31.208.0 255.255.248.0 longer-prefixes |
||
|
! lines omitted for brevity |
||
|
172.31.0.0/16 is variably subnetted, 13 subnets, 2 masks |
||
|
R 172.31.211.0/24 [120/15] via 172.31.13.1, 00:00:13, Serial0/0 |
.1 |
|
|
[120/15] via 172.31.23.2, 00:00:09, Serial0/0 |
.2 |
|
|
! Lines omitted for brevity |
||
Continue reading here: Filtering with Distribute Lists and Prefix Lists
Was this article helpful?