Hi all,
I think I posted about some of this before, but I think is an interesting
"feature" and I'd love to have your opinion on it.
The task is to change the admin distance of ospf prefixes for a specific ospf
neighbor.
According to cisco doc
http://www.cisco.com/en/US/docs/ios-xml/ios/iproute_ospf/command/ospf-a1.html#GUID-C4090D42-B8FD-40F5-8DDA-AE2FEAB3ADD0
this is simply done by using:
distance weight [ip-address wildcard-mask [access-list name]]
only to discover that it doesn't work. Doing a bit of search on the internet I
have found this thread on the leraningcisco site talking about a possible bug
CSCeh46993
https://learningnetwork.cisco.com/thread/8963
I run some test and the command behaves differently to its definition on the
doc, below are my findings, I would love for someone to give me some feedback
on this. Thanks.
R2#sh run | sec ospf
router ospf 1
router-id 136.10.2.2
log-adjacency-changes
passive-interface Loopback0
network 136.10.2.2 0.0.0.0 area 0
network 136.10.24.2 0.0.0.0 area 0
network 136.10.25.1 0.0.0.0 area 0
network 136.10.100.2 0.0.0.0 area 0
neighbor 136.10.100.6 cost 56
neighbor 136.10.100.5 cost 56
R2#sh ip ospf neigh
Neighbor ID Pri State Dead Time Address Interface
136.10.4.4 0 FULL/ - 00:00:31 136.10.24.4 Serial1/1.24
136.10.5.5 0 FULL/ - 00:00:35 136.10.25.2 Serial1/2
136.10.6.6 0 FULL/ - 00:01:57 136.10.100.6 Serial1/1.256
136.10.5.5 0 FULL/ - 00:01:39 136.10.100.5 Serial1/1.256
R2#sh ip ospf int bri
Interface PID Area IP Address/Mask Cost State Nbrs F/C
Lo0 1 0 136.10.2.2/24 1 LOOP 0/0
Se1/1.24 1 0 136.10.24.2/29 64 P2P 1/1
Se1/2 1 0 136.10.25.1/30 64 P2P 1/1
Se1/1.256 1 0 136.10.100.2/27 64 P2MP 2/2
R2#sh ip route ospf
136.10.0.0/16 is variably subnetted, 15 subnets, 6 masks
O 136.10.6.6/32 [110/57] via 136.10.100.6, 00:10:19, Serial1/1.256
O 136.10.5.5/32 [110/57] via 136.10.100.5, 00:10:19, Serial1/1.256
O 136.10.4.4/32 [110/65] via 136.10.24.4, 00:10:19, Serial1/1.24
R2#
Let's say I want to change the admin distance for routes received from
136.10.100.6, acccording to documentation I would do
R2#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R2(config)#router ospf 1
R2(config-router)#distance 109 136.10.100.6 0.0.0.0
Let's have a look:
R2(config-router)#do sh ip route ospf
136.10.0.0/16 is variably subnetted, 15 subnets, 6 masks
O 136.10.6.6/32 [110/57] via 136.10.100.6, 00:00:36, Serial1/1.256
O 136.10.5.5/32 [110/57] via 136.10.100.5, 00:00:36, Serial1/1.256
O 136.10.4.4/32 [110/65] via 136.10.24.4, 00:00:36, Serial1/1.24
nothing, admin distance is still 110
Look what happens now:
R2(config-router)#no distance 109 136.10.100.6 0.0.0.0
R2(config-router)#distance 109 136.10.100.6 255.255.255.0
R2(config-router)#do sh ip route ospf
136.10.0.0/16 is variably subnetted, 15 subnets, 6 masks
O 136.10.6.6/32 [109/57] via 136.10.100.6, 00:00:35, Serial1/1.256
O 136.10.5.5/32 [110/57] via 136.10.100.5, 00:00:35, Serial1/1.256
O 136.10.4.4/32 [110/65] via 136.10.24.4, 00:00:35, Serial1/1.24
Weird, the admin distance has changed for 136.10.100.6,let's carry on, let's
see where it breaks
R2(config-router)#no distance 109 136.10.100.6 255.255.255.0
R2(config-router)#distance 109 136.10.100.6 255.255.254.0
R2(config-router)#do sh ip route ospf
136.10.0.0/16 is variably subnetted, 15 subnets, 6 masks
O 136.10.6.6/32 [109/57] via 136.10.100.6, 00:00:02, Serial1/1.256
O 136.10.5.5/32 [110/57] via 136.10.100.5, 00:00:02, Serial1/1.256
O 136.10.4.4/32 [110/65] via 136.10.24.4, 00:00:02, Serial1/1.24
It works if the wild card mask is 255.255.254.0, let's try the third octect .252
R2(config-router)#no distance 109 136.10.100.6 255.255.254.0
R2(config-router)#distance 109 136.10.100.6 255.255.252.0
R2(config-router)#do sh ip route ospf
136.10.0.0/16 is variably subnetted, 15 subnets, 6 masks
O 136.10.6.6/32 [110/57] via 136.10.100.6, 00:00:03, Serial1/1.256
O 136.10.5.5/32 [110/57] via 136.10.100.5, 00:00:03, Serial1/1.256
O 136.10.4.4/32 [110/65] via 136.10.24.4, 00:00:03, Serial1/1.24
It doesn't. Don't ask, I have no idea why it does that, hope you can help
Fulvio
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