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MPLS TE tunnel autoroute announce metric in SPF computation

Hi, I have a doubt whether MPLS TE tunnel metric is taking into SPF computation when the tunnel has "autoroute announce" configured.

From the book "MPLS traffice enginnering" written by Osbourn, IGP SPF computation is always performed before tunnel metric is modified, I found this is only true if IGP is ISIS, but if IGP is OSPF, tunnel metric specified by "autoroute metric" will always be taken into SPF computation, a.k.a, if tunnel metric is configured to be less than underlying IGP metric, a suboptimal routing will always happen to destination routers that are in between tunnel head and tunnel tail.

Any idea why there is a inconsistent behavior between OSPF and ISIS SPF computation? or I missed anything?

1 REPLY
Cisco Employee

Re: MPLS TE tunnel autoroute announce metric in SPF computation

Hi,

You're right. There is a different behavior between OSPF and ISIS on how they handle the autoroute metric feature.

ISIS: TE tunnel metric is not taken into account during SPF computation.

OSPF: TE tunnel metric is taken into account during SPF computation.

So playing with this feature can change the SPT if your IGP is OSPF.

The difference seems coming from the SPF implementation of OSPF and ISIS

HTH

Laurent.

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