Subject: Cascaded U Turns for IP Unicast FRR

From: "Abhay D.S" <[email-masked]>
Date: Sat, 13 Oct 2007 13:22:38 +0800

hi alia and alex,

If a neighbor cannot provide a loop free alternate,
then a consideration for U turn alternate is made for
the same neighbor.

Now this U Turn Alternate cannot find a neighbor which can provide
a LFA, then we can consider this neighbors -> Neighbor upstream.

If this upstream neighbor can support LFA, then how to consider
the forwarding from

Source -> U-Turn Neighbor -> U-Turns Nbrs Nbr -> U-Turns Nbrs Nbrs Nbrs{ DISTANCE not via Source (This Nbr, Destination) < SPFDist(This Nbr, Src) + SPFDist(Src,Dest) is TRUE at upstream
neighbor).

Source can determine this and sends packet to U turn Neighbor say X,  U turn neighbor X  knows that it cannot send the packet to its neighbor Y, since Y
cannot provide a Loop Free Alternate but Z , Y's Upstream neighbor can.

In my understanding X can be configured to consider more than one hop neighbors can provide LFA, in this case Y's Neighbors Z

At Y the primary next hop to reach destination is Z.

I think X must signal to Src about its availablity as a U Turn for a Neighbor explicitly for a particular prefix.

How do you think ?.

Thanks and Regards,
Abhay

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