Nexus prefers inter area to ASBR for external route?

[chinkevi_2]

hi

i was wondering if anyone can contribute this behavior.

we have Nexus n7k running 5.2(4), it seems the OSPF picks the inter area path to ASBR rather than intra area path, though the cost is much lower with intra area path.

the topology:

r1 and r2, both area 0 routers and ASBR, receivs bgp route and redistributed into ospf as external type 1

r1 and r2 connect to r3 and r4 in other area.

when r1 primary link died, we would expect r1 to take r2 as next best hop via intra area path, as they are directly connected.

what we are seeing is the r3 path is selected, and i checked and did the cost equals to the cost of r1+r3+r2 redistributed cost.

thank you in advance.

[Rolf Fischer]

Hi,

that's really odd. Independently form Metric and AD, OSPF always prefers intra-area routes over inter-area (in the same process).

I'd start troubleshooting with "show ip ospf database external" and verifying reachability to the ASBR.

Best regards

Rolf

[paul driver]

Hello

Do you have any summarisation applied at tbe ABR?

can you post from router 1:

sh ip ospf database external x.x.x.x ( an external route advertised by the ASBR)

sh ip route x.x.x.x ( forwarding address inside th LSA of the above)

Also at present their is an ongoing ask the expert for OSPF - May be you should also query him?

https://supportforums.cisco.com/message/3970284#3970284

res

Paul

[chinkevi_2]

hi,

apparently, this symptom is due to change in RFC, from RFC 1583 to RFC 2328.

where:

- prefer non-backbone path to ASBR first

this would be not preferred if ASBR is in backbone, or if non-backbone path will route through firewalls.

there is option to make Nexus use RFC1583 with this command:

rfc1583compatibility

[Rolf Fischer]

chinkevi,

you're totally right - 5* for that!

This is someting to keep in mind when having IOS and NX-OS platforms in the same OSPF domain, because at the worst it could result in routing loops.

The default path selection criterias on IOS are still from RFC1583 whereas NX-OS uses the RFC 2328  specifications.

Thanks,

Rolf