Label Distribution Protocol
Page 242 7210 SAS M, T, X, R6, Mxp MPLS Configu-
ration Guide
• When it receives a label request message from a directly attached DoD peer for the prefix
of another directly attached DoD peer. In this case the DoD peers are trying to establish a
SDP among themselves.
• Trying to establish a SDP to a directly attached LDP DoD peer.
The 7210 also supports sending and receiving the Label Abort Request Message as described. This
message is used to abort an outstanding request for a label in case no response was received from
the peer within a finite amount of time.
LDP over RSVP and ECMP
NOTE: ECMP is not supported for LDP over RSVP.
ECMP for LDP over RSVP is not supported (also see ECMP Support for LDP on page 227). If
ECMP applies, all LSP endpoints found over the ECMP IGP path will be installed in the routing
table by the IGP for consideration by LDP. It is important to note that IGP costs to each endpoint
may differ because IGP selects the farthest endpoint per ECMP path.
LDP will choose the endpoint that is highest cost in the route entry and will do further tunnel
selection over those endpoints. If there are multiple endpoints with equal highest cost, then LDP
will consider all of them.
LDP Fast-Reroute for IS-IS and OSPF Prefixes
LDP Fast Re-Route (FRR) is a feature which allows the user to provide local protection for an
LDP FEC by pre-computing and downloading to IOM both a primary and a backup NHLFE for
this FEC.
The primary NHLFE corresponds to the label of the FEC received from the primary next-hop as
per standard LDP resolution of the FEC prefix in RTM. The backup NHLFE corresponds to the
label received for the same FEC from a Loop-Free Alternate (LFA) next-hop.
The LFA next-hop pre-computation by IGP is described in RFC 5286 – “Basic Specification for IP
Fast Reroute: Loop-Free Alternates”. LDP FRR relies on using the label-FEC binding received
from the LFA next-hop to forward traffic for a given prefix as soon as the primary next-hop is not
available. This means that a node resumes forwarding LDP packets to a destination prefix without
waiting for the routing convergence. The label-FEC binding is received from the loop-free
alternate next-hop ahead of time and is stored in the Label Information Base since LDP on the
router operates in the liberal retention mode.
This feature requires that IGP performs the Shortest Path First (SPF) computation of an LFA next-
hop, in addition to the primary next-hop, for all prefixes used by LDP to resolve FECs. IGP also
populates both routes in the Routing Table Manager (RTM).
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