Weird RIP metric behavior

September 10, 2008 at 2:34 pm | Posted in RIP, Routing | Leave a comment

RIP metric automatically gets increased by one when we do interface summarisation for routes redistributed from other routing protocols.

This is interesting observation. I have never noticed it before.

I now also notice that the metric gets increased by 1 when these two conditions must meet

– manual interface summarization
– this is summary route for routes that you redistribute from other protocols (e.g. OSPF, static)

This means, if you do manual summary for RIP routes, then the metric is equal to the minimum metric amongh more specific routes, and NOT the minimum metric + 1.

Also, when you enable Auto-Summary, the metric of summary-route for prefixes redistributed from other protocols does not get increased by 1 automatically.

This could well a bug!

In a real lab, if the task asks for a specific metric, and you can not make the metric for redistributed routes, and the summary route the same using interface summary command, then remember that you may meet the task requirement by leaving the AutoSummary ON – an option that we normally overlook.

Configuration & Observation

      OSPF         RIP
R1 --------- R3 ----------- R2
        S2/2   S2/3

R3#
interface Loopback0
 ip address 3.3.3.3 255.255.255.0

interface Serial2/2
 ip address 13.0.0.3 255.255.255.0

interface Serial2/3
 ip address 23.0.0.3 255.255.255.0
 ip summary-address rip 1.0.0.0 255.0.0.0
 ip summary-address rip 3.3.0.0 255.255.0.0
!
router ospf 1
 network 13.0.0.3 0.0.0.0 area 0
!
router rip
 version 2
 redistribute static metric 3
 redistribute ospf 1 metric 10
 network 3.0.0.0
 network 23.0.0.0
 no auto-summary
!
ip route 33.33.33.0 255.255.255.0 Null0
!

R3#sh ip rip data
1.0.0.0/8    auto-summary
1.0.0.0/8    int-summary
1.1.1.0/24    redistributed
    [10] via 1.1.1.1, 
3.0.0.0/8    auto-summary
3.3.0.0/16    int-summary
3.3.3.0/24    directly connected, Loopback0
11.0.0.0/8    auto-summary
11.11.11.11/32    redistributed
    [10] via 1.1.1.1, 
13.0.0.0/8    auto-summary
13.0.0.0/24    directly connected, Serial2/2
23.0.0.0/8    auto-summary
23.0.0.0/24    directly connected, Serial2/3
33.0.0.0/8    auto-summary
33.33.33.0/24    redistributed
    [3] via 0.0.0.0, 

R2#sh ip route | b /
R    1.0.0.0/8 [120/11] via 23.0.0.3, 00:00:16, Serial0/1
     33.0.0.0/24 is subnetted, 1 subnets
R       33.33.33.0 [120/3] via 23.0.0.3, 00:00:16, Serial0/1
     3.0.0.0/16 is subnetted, 1 subnets
R       3.3.0.0 [120/1] via 23.0.0.3, 00:00:16, Serial0/1
     23.0.0.0/24 is subnetted, 1 subnets
C       23.0.0.0 is directly connected, Serial0/1
     11.0.0.0/32 is subnetted, 1 subnets
R       11.11.11.11 [120/10] via 23.0.0.3, 00:00:16, Serial0/1
     13.0.0.0/24 is subnetted, 1 subnets
R       13.0.0.0 [120/10] via 23.0.0.3, 00:00:16, Serial0/1

R3#c
Enter configuration commands, one per line.  End with CNTL/Z.
R3(config)#router rip
R3(config-router)#auto-summary
R3(config-router)#
R3#
R3#

R3#sh ip rip data
1.0.0.0/8    auto-summary
1.0.0.0/8    int-summary
1.1.1.0/24    redistributed
    [10] via 1.1.1.1, 
3.0.0.0/8    auto-summary
3.3.0.0/16    int-summary
3.3.3.0/24    directly connected, Loopback0
11.0.0.0/8    auto-summary
11.11.11.11/32    redistributed
    [10] via 1.1.1.1, 
13.0.0.0/8    auto-summary
13.0.0.0/24    redistributed
    [10] via 0.0.0.0, 
23.0.0.0/8    auto-summary
23.0.0.0/24    directly connected, Serial2/3
33.0.0.0/8    auto-summary
33.33.33.0/24    redistributed
    [3] via 0.0.0.0, 

R2#sh ip route | b /
R    1.0.0.0/8 [120/10] via 23.0.0.3, 00:00:04, Serial0/1
R    33.0.0.0/8 [120/3] via 23.0.0.3, 00:00:04, Serial0/1
R    3.0.0.0/8 [120/1] via 23.0.0.3, 00:00:04, Serial0/1
     23.0.0.0/24 is subnetted, 1 subnets
C       23.0.0.0 is directly connected, Serial0/1
R    11.0.0.0/8 [120/10] via 23.0.0.3, 00:00:04, Serial0/1
R    13.0.0.0/8 [120/10] via 23.0.0.3, 00:00:04, Serial0/1
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