| date | ospf_tag | tags | title | url | |
|---|---|---|---|---|---|
2008-02-14 07:44:00 +0100 |
details |
|
Common Sense Prevails Over RFC 2328 |
/2008/02/common-sense-prevails-over-rfc-2328.html |
When trying to extract the OSPF route selection rules from RFC 2328, I've stumbled across a very weird rule (section 16.4.1): if an ASBR within a non-backbone area advertises an external route (or if the forwarding address is within the non-backbone area), it's preferred over external routes advertised by ASBRs in other areas regardless of its metric. I simply had to test this on Cisco IOS ... and found out that Cisco engineers prefer common sense to OSPF RFC.
I've built a sample network where two routers (10.0.0.11 in area 1, 10.0.0.3 in the backbone area) advertise the same external routes. The router in area 1 advertises the routes with a higher metric:
S2#ospfExternals
External OSPF routes for OSPF process ID 1
Prefix Cost Tag ASBR Forward addr
==================================================================
> 10.1.0.1/32 10 E1 1 10.0.0.3
10.1.0.1/32 2000 E1 1 10.0.0.11
> 10.1.0.2/32 5 E2 2 10.0.0.3
10.1.0.2/32 200 E2 2 10.0.0.11
You can use the show ip ospf route command to verify which ASBR is within the area. An abbreviated printout is included:
S2#show ip ospf route
Intra-area Router Path List
i 10.0.0.11 [64] via 10.0.2.17, Serial1/2, ASBR, Area 1, SPF 2
i 10.0.0.2 [64] via 10.0.2.13, Serial1/1, ABR, Area 1, SPF 2
i 10.0.0.1 [64] via 10.0.2.5, Serial1/0, ABR, Area 1, SPF 2
Inter-area Router Path List
I 10.0.0.3 [128] via 10.0.2.5, Serial1/0, ASBR, Area 1, SPF 2
As you can see, 10.0.0.11 is within area 1, whereas 10.0.0.3 is in another area. Still, IOS prefers external routes advertised by 10.0.0.3 due to their lower metric:
S2#show ip ospf route
External Route List
*> 10.1.0.1/32, Ext1, cost 138, tag 1
via 10.0.2.5, Serial1/0
*> 10.1.0.2/32, Ext2, cost 5, tag 2
via 10.0.2.5, Serial1/0