Signaling and Operation
Page 544 7450 ESS MPLS Guide
Signaling and Operation
• LDP Label Distribution and FEC Resolution on page 544
• Default FEC Resolution Procedure on page 545
LDP Label Distribution and FEC Resolution
The user creates a targeted LDP (T-LDP) session to an ABR or the destination PE. This results in
LDP hellos being sent between the two routers. These messages are sent unlabeled over the IGP
path. Next, the user enables LDP tunneling on this T-LDP session and optionally specifies a list of
LSP names to associate with this T-LDP session. By default, all RSVP LSPs which terminate on
the T-LDP peer are candidates for LDP-over-RSVP tunnels. At this point in time, the LDP FECs
resolving to RSVP LSPs are added into the Tunnel Table Manager as tunnel-in-tunnel type.
Note that if LDP is running on regular interfaces also, then the prefixes LDP learns are going to be
distributed over both the T-LDP session as well as regular IGP interfaces. However, only /32 FEC
prefixes will be resolved over RSVP LSPs. The policy controls which prefixes go over the T-LDP
session, for example, only /32 prefixes, or a particular prefix range.
LDP-over-RSVP works with both OSPF and ISIS. These protocols include the advertising router
when adding an entry to the RTM. LDP-over-RSVP tunnels can be used as shortcuts for BGP
next-hop resolution.
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