RFC 4105 and its impact on dynamic routing over inter-area TE

ulatif · ‎12-15-2009

Folks,

RFC 4105 states:

"The use of IGP shortcuts MUST be precluded when TE-LSP head-end and

tail-end LSRs do not reside in the same IGP area.  It MAY be used
when they reside in the same area.

The advertisement of an inter-area TE LSP as a link into the IGP, in
order to attract traffic to an LSP source, MUST be precluded when
TE-LSP head-end and tail-end LSRs do not reside in the same IGP area.
It MAY be used when they reside in the same area."



So how does one use dynamic methods (like FA and IGP shortcuts) to forward traffic over inter-area TEs ?
Static routes is obviously not scalable at all.

Looking for some best practices in this regard. I am happy with one-hop TEs but still have reservations
with regards to how (just using one Loopback on ABR) we can achieve this ? its probably not possible.

I read a new feature in Cisco talking about "autoroute destination" - is that more of a hack or workaround?

jcozzupoli · ‎12-17-2009

Hi Usman,

In this case, each ABR will maintain 2 databases, one for IGP routing and one for MPLS-TE topology. Although the routing databases are exchanged, the TE topologies are not and the head-end and tail-end being in different areas, are not aware of each other in the TE topology.

The way around this is to confiure the head-end and tail-end to have explicit-paths with the next-hop addresses as loose objects to each ABR and last hop to tail-end/head-end. As the head-end dosn't know the prefixes reachable by the tail-end, autoroute announce is not an option and the only way to push traffic down these tunnels is via a static route. Although this is not scalable to use static routing to push the traffic down the tunnel, for now it is the only way, as I have explained above.

I have configured this in a lab if you wish to see my conigurations.

HTH.

Thanks.

Joe.

ulatif · ‎12-20-2009

Joe,

Setting up an inter-area TE tunnel was a never a problem (loose-explicit paths).

I was talking more about how to dynamically route traffic over such tunnels using IGP shortcuts or FA.

It seems that this is possible - if the inter-area TE tunnel is upto the ABR because all vendors (including Cisco) have a way of injecting the MPLS TE RID as intra-area route into both areas.