topic RE: OSPF establishes adjacency, but no routes are exchanged in Network Architecture & Design

OSPF establishes adjacency, but no routes are exchanged

Erik_Auerswald — Sun, 30 Apr 2017 15:12:00 GMT

Hi,

a customer had a relatively common problem concerning OSPF, but did not find the solution searching the web. This post is supposed to help others find the information that is already out there.

OSPF was configured between two Extreme Networks switches, an S6 and an X460-G2. The OSPF adjacency formed and link state databases were synchronized. The output of show ip ospf neighbor (S-Series) respectively show ospf neighbor (EXOS) showed full adjacencies, but the X460-G2 did not learn any routes over OSPF (the output of show iproute origin ospf did not contain any routes).

A hint regarding the problem may be found in the show [ip] ospf neighbors output, because two OSPF routers on a broadcast link will fill the roles of DR and BDR, but two OSPF routers on a point-to-point link will both have the role DROTHER (or no role, but not DR/BDR).

The problem was that one switch was configured to use a point-to-point network type on the interface, but the other was left at the default of using a broadcast network type. In this case the adjacency forms, LSAs are exchanged, and the link state databases are synchronized, but for OSPF one of the routers seems to be connected to a different network than the other router, because the network types are different. Thus OSPF does not see a usable transit link between the two routers. Therefore, no routes can use that link to reach the next hop, and as a result no OSPF routes are learned.

The network type is not checked when forming OSPF adjacencies.

OSPF point-to-point network type can be configured as follows:
configure ospf add vlan NAME area AREA link-type point-to-point(EXOS)
interface vlan NUMBER
ip ospf network point-to-point(EOS)

References:

GTAC Knowledge article OSPF neighbor adjacency is "Full" but not adding routes into route table
The Hub thread Ospf is not filling route table
incompatible OSPF network types
Cisco troubleshooting TechNote Why Are Some OSPF Routes in the Database but Not in the Routing Table?

Thanks,
Erik

RE: OSPF establishes adjacency, but no routes are exchanged

StephenW — Wed, 03 May 2017 17:36:00 GMT

That can be a fun one to troubleshoot. I would probably pass right over that when it's in a full state. Thanks for all the detail.

RE: OSPF establishes adjacency, but no routes are exchanged

Erik_Auerswald — Wed, 03 May 2017 18:26:00 GMT

Yeah, first time I had to troubleshoot this was at 3 a.m. in the night... I was woken by the telephone and asked why an N-Series (firmware version 6) and a Cisco 4500-X form an OSPF adjacency, but do not use the link for any routes. Back then the N-Series did not support point-to-point interfaces.

RE: OSPF establishes adjacency, but no routes are exchanged

Drew_C — Thu, 04 May 2017 07:54:00 GMT

Thanks for sharing, Erik. I've made this post an Article, rather than an Idea. I also added some of the more relevant categories.

RE: OSPF establishes adjacency, but no routes are exchanged

Erik_Auerswald — Thu, 04 May 2017 07:54:00 GMT

Thanks, I could not select "article", thus I used "idea" as everything else seemed worse.

RE: OSPF establishes adjacency, but no routes are exchanged

Drew_C — Thu, 04 May 2017 07:54:00 GMT

It's an option only available from the management console used for moderation.

RE: OSPF establishes adjacency, but no routes are exchanged

M_Nees — Fri, 27 Mar 2020 20:23:00 GMT

Thanks for that useful explanation!