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OSPF Compatability and LLS

Recently during an upgrade of a Cisco router I ran into a strange problem where my OSPF neighbors that were working prior to the upgrade stopped working after the upgrade. I also noticed that the broken neighbors were only to non-Cisco devices, namely Nortel Contivity VPN devices.

I could see this by checking the neighbor status from sh ip ospf neighbor

router#sh ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface
10.0.0.1 200 FULL/BDR 00:00:33 10.0.0.1 FastEthernet0/0.1
10.0.0.3 100 INIT/DROTHER 00:00:36 10.0.0.3 FastEthernet0/0.1
10.0.0.5 100 INIT/DROTHER 00:00:31 10.0.0.5 FastEthernet0/0.1

Doing a bit a research, I found on Cisco’s website that by default Link-Local Signaling or LLS is enabled by default.

LLS allows for the extension of existing OSPF packets in order to provide additional bit space. The additional bit space enables greater information per packet exchange between OSPF neighbors. This functionality is used, for example, by the OSPF Nonstop Forwarding (NSF) Awareness feature that allows customer premises equipment (CPE) routers that are NSF-aware to help NSF-capable routers perform nonstop forwarding of packets.

When LLS is enabled at the router level, it is automatically enabled for all interfaces. The OSPF Per-Interface Link-Local Signaling feature allows you to selectively enable or disable LLS for a specific interface. You may want to disable LLS on a per-interface basis depending on your network design. For example, disabling LLS on an interface that is connected to a non-Cisco device that may be noncompliant with RFC 2328 can prevent problems with the forming of Open Shortest Path First (OSPF) neighbors in the network.

My OSPF topology is fairly small and the traffic is very light. So the need for non-stop forwarding isn’t as great. In my situation I just disabled LLS globally.

router ospf 1
router-id 10.0.0.2
no capability lls

As soon as I disable LLS capability – all my neighbors came right up!

router#sh ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface
10.0.0.1 200 FULL/DROTHER 00:00:33 10.0.0.1 FastEthernet0/0.1
10.0.0.3 100 FULL/DROTHER 00:00:36 10.0.0.3 FastEthernet0/0.1
10.0.0.5 100 FULL/BDR 00:00:31 10.0.0.5 FastEthernet0/0.1

Additional information can be found on Cisco’s website (a CCO login may be required)
http://www.cisco.com/en/US/docs/ios/12_0s/feature/guide/ospflls.html

About Joe

Senior Network Engineer, technology enthusiast, guitar and bass player. Joe Wilson is the creator of RouterFreak.com as well as other niche websites that can be found around on the Internets.

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2 comments

  1. Thanks saved my life. Only thing different i picked up was when i used the loopback addresses OSPF failed to work. i had to use the interface IP’s to get OSPF to work after disabling lls

  2. thanks, this has been of great help on the same problem i got between a cisco router and a nortel contivity.

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