Alcatel-Lucent 7210 SAS M User Manual 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.
7210 SAS M, T, X, R6, Mxp MPLS Configuration Guide
Label Distribution Protocol
Page 237