Receiving the same external routes from multiple ABRs

[yjim0140]

Hello,

We have a Router R1 and an ASBR is in area 1. The router R1 has two connections to area 0 via different ABRs (A and B).

We have a Router R2 in area 2. The router R2 also has two connections to area 0 via two ABRs (A and B).

In this case, R2 receives two external routes via two different ABRs(A and B) but imported by the same ASBR. Router R2 also receives two ASBR-summary from two ABRs for the same ASBR.

The problem is that Router R2 selects the path to ASBR via B-C-A-R1 rather than via A, although the ospf cost from R2 to B is 4 while cost from R2 to A is 1.

I'm wondering this might comply RFC2328. In particular, what preference rule is applied when R2 receives the same ASBR-summary (but with different Adv. Router) from two ABRs? Even though RFC 2328 specifies the preference rules when the same external route is imported by multiple ASBRs, no preference rule is defined when the same ASBR-summary is advertised from multiple ABRs.

An ASBR-summary from an ABR with higher Router ID will be preferred?

Any response will be greatly appreciated.

o. Network configuration mentioned above.

ABR(A)

/ | \

/ | \

area1 / | \ area2

/ C \

ASBR--area1--R1 area0 R2

\ | /

area1 \ | /

\ | / area2

\ | /

ABR(B)

[Georg Pauwen]

Hello,

can you issue the following command on both ABRs in area 2:

sh ip ospf database external

and check for the field:

Forward Address: X.X.X.X

See if one of the two has a specific address in that field, while the other one has 0.0.0.0.

There is a document which describes the effects of the forwarding address on type 5 LSA Path Selection, I am not really sure if that applies to your setup:

The Effects of the Forwarding Address on Type 5 LSA Path Selection

http://www.cisco.com/en/US/partner/tech/tk365/tk480/technologies_tech_note09186a0080124c7d.shtml#how

Regards,

Georg

[yjim0140]

Actually two external routes have the identical forwarding address (0.0.0.0) since two was generated by the same ASBR-1. Thus the document mentioned in your reply does not apply to our case. I have attached the OSPF network configuraiton(PPT file). After reviewing our network configuration, please enlighten me on what might be a problem. In particular I wonder the OSPF area configuration is valid, since I understand that an ABR is able to be connected to one or more areas in addition to boackbone area(area 0), but that kind of configuration is seldom examplified in OSPF literatures.

Thanks in advance.

Yongjun.

[Harold Ritter]

From your diagram, it certainly seems like you should prefer the route via ABR-A -> R1.

Even if the link between R2 and ABR-A was down, you wouldn't go via ABR-C but rather via ABR-B -> R1.

The fact that you are using ABR-B -> ABR-C -> ABR-A -> R1 is the sign that something is wrong.

Can you post the output of a "sh ip ospf data asbr-summary" for ASBR-1 taken from R2 and also a "show ip ospf border" from the same router.

Thanks,