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Label Distribution Protocol

7210 SAS M, T, X, R6, Mxp MPLS Configuration Guide Page 237

Default FEC Resolution Procedure

When LDP tries to resolve a prefix received over a T-LDP session, it performs a lookup in the

Routing Table Manager (RTM). This lookup returns the next hop to the destination PE and the

advertising router (ABR or destination PE itself). If the next-hop router advertised the same FEC

over link-level LDP, LDP will prefer the LDP tunnel by default unless the user explicitly changed

the default preference using the system wide prefer-tunnel-in-tunnel command. If the LDP tunnel

becomes unavailable, LDP will select an LDP-over-RSVP tunnel if available.

When searching for an LDP-over-RSVP tunnel, LDP selects the advertising router(s) with best

route. If the advertising router matches the T-LDP peer, LDP then performs a second lookup for

the advertising router in the Tunnel Table Manager (TTM) which returns the user configured

RSVP LSP with the best metric. If there are more than one configured LSP with the best metric,

LDP selects the first available LSP.

If all user configured RSVP LSPs are down, no more action is taken. If the user did not configure

any LSPs under the T-LDP session, the lookup in TTM will return the first available RSVP LSP

which terminates on the advertising router with the lowest metric.

FEC Resolution Procedure When prefer-tunnel-in-tunnel is Enabled

When LDP tries to resolve a prefix received over a T-LDP session, it performs a lookup in the

Routing Table Manager (RTM). This lookup returns the next hop to the destination PE and the

advertising router (ABR or destination PE itself).

When searching for an LDP-over-RSVP tunnel, LDP selects the advertising router(s) with best

route. If the advertising router matches the targeted LDP peer, LDP then performs a second lookup

for the advertising router in the Tunnel Table Manager (TTM) which returns the user configured

RSVP LSP with the best metric. If there are more than one configured LSP with the best metric,

LDP selects the first available LSP.

If all user configured RSVP LSPs are down, then an LDP tunnel will be selected if available.

If the user did not configure any LSPs under the T-LDP session, a lookup in TTM will return the

first available RSVP LSP which terminates on the advertising router. If none are available, then an

LDP tunnel will be selected if available.

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