Rerouting Around Failures
Page 546 7450 ESS MPLS Guide
Rerouting Around Failures
Every failure in the network can be protected against, except for the ingress and egress PEs. All
other constructs have protection available. These constructs are LDP-over-RSVP tunnel and ABR.
• LDP-over-RSVP Tunnel Protection on page 546
• ABR Protection on page 546
LDP-over-RSVP Tunnel Protection
An RSVP LSP can deal with a failure in two ways.
• If the LSP is a loosely routed LSP, then RSVP will find a new IGP path around the failure,
and traffic will follow this new path. This may involve some churn in the network if the
LSP comes down and then gets re-routed. The tunnel damping feature was implemented
on the LSP so that all the dependent protocols and applications do not flap unnecessarily.
• If the LSP is a CSPF-computed LSP with the fast reroute option enabled, then RSVP will
switch to the detour path very quickly. From that point, a new LSP will be attempted from
the head-end (global revertive). When the new LSP is in place, the traffic switches over to
the new LSP with make-before-break.
ABR Protection
If an ABR fails, then routing around the ABR requires that a new next-hop LDP-over-RSVP
tunnel be found to a backup ABR. If an ABR fails, then the T-LDP adjacency fails. Eventually, the
backup ABR becomes the new next hop (after SPF converges), and LDP learns of the new next-
hop and can reprogram the new path.
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