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> The interface used for the RID is the loopback (Lo0) interface on > R3. I was just testing your suggestion (applying “no compatible > rfc1583″), and SW4 still installs both routes towards 158.1.0.0/24. RID actually doesn’t come into it. I was forgetting that when the forwarding address is 0.0.0.0, the path selected is the closet to the ASBR, rather to the than the RID of the ASBR.
Both paths being installed in the RIB is explained by 16.4.1, just a bit I overlooked It appears to be because of the last line in the first paragraph below: ”
16.4.1. External path preferences
When multiple intra-AS paths are available to ASBRs/forwarding addresses, the following rules indicate which paths are preferred. These rules apply when the same ASBR is reachable through multiple areas, or when trying to decide which of several AS-external-LSAs should be preferred. In the former case the paths all terminate at the same ASBR, while in the latter the paths terminate at separate ASBRs/forwarding addresses. In either case, each path is represented by a separate routing table entry as defined in Section 11.
This section only applies when RFC1583Compatibility is set to “disabled”.
The path preference rules, stated from highest to lowest preference, are as follows. Note that as a result of these rules, there may still be multiple paths of the highest preference. In this case, the path to use must be determined based on cost, as described in Section 16.4.
o Intra-area paths using non-backbone areas are always the most preferred.
o The other paths, intra-area backbone paths and inter- area paths, are of equal preference. ”
So multiple routes should be installed in the routing table if there are multiple paths to the ASBR. Path preference will become an issue if ospf is not permitted to install all of those paths.
Given the requirements of the question which you mentioned you can use area 38 default-cost 10 on SW4 to stop the loop.
But just to complicate matters… I’ve just tried a simple setup (just with 3 ospf 12.2 routers) and the behaviour I saw seems a little different to the rfc: - If the cost to the forwarding address via area 0 is lower than the cost via area 38, then only the path via area 0 is installed in the routing table. - If the cost to the forwarding address via area 0 is equal to, or higher than, the cost via area 38, then both paths are installed in the routing table.
Paul.
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