Friday, May 30, 2008

An OSPF Issue

Cisco introduced an non-RFC-defined OSPF feature called "OSPF throttle timer". You can use CLI to define the SLA generation delay and SPF calculation delay.

The problem is that if there's an OSPF adjacency flapping shorter than 500ms (which is also the default value of OSPF SLA generation delay timer) or the SLA generation delay you configured, NO SLA will be generated therefore no SPF recalculation will be scheduled. On the other hand, the OSPF routes were removed from routing table due to next-hop unreachable. As a result, the OSPF routes were not injected back to the routing table after the flapping.

There are two known workarounds. One is to increase the carrier delay to ignore the brief link interruption. The trade-off will be some packet loss. The other one is to reduce the SLA throttle timer, this will allow the router to generate SLA as soon as there's a topology change and bypass the issue. The trade-off will be increased router CPU load, especially in a large-scale SP network.

The descriptions bove are shown in below picture. Click to zoom.

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