Label Distribution Protocol
7210 SAS M, T, X, R6, Mxp MPLS Configuration Guide Page 227
ECMP Support for LDP
Note: LER ECMP is not supported on 7210 SAS.
ECMP support for LDP performs load balancing for LDP based LSPs by having multiple outgoing
next-hops for a given IP prefix on ingress and transit LSRs.
An LSR that has multiple equal cost paths to a given IP prefix can receive an LDP label mapping
for this prefix from each of the downstream next-hop peers. As the LDP implementation uses the
liberal label retention mode, it retains all the labels for an IP prefix received from multiple nexthop
peers.
Without ECMP support for LDP (7210 SAS does not support ECMP for LDP), only one of these
next-hop peers will be selected and installed in the forwarding plane. The algorithm used to
determine the next-hop peer to be selected involves looking up the route information obtained
from the RTM for this prefix and finding the first valid LDP next-hop peer (for example, the first
neighbor in the RTM entry from which a label mapping was received). If, for some reason, the
outgoing label to the installed next-hop is no longer valid, say the session to the peer is lost or the
peer withdraws the label, a new valid LDP next-hop peer will be selected out of the existing next-
hop peers and LDP will reprogram the forwarding plane to use the label sent by this peer.
With ECMP support, all the valid LDP next-hop peers, those that sent a label mapping for a given
IP prefix, will be installed in the forwarding plane. In transit LSR, an ingress label will be mapped
to the nexthops that are in the RTM and from which a valid mapping label has been received. The
forwarding plane will then use an internal hashing algorithm to determine how the traffic will be
distributed amongst these multiple next-hops, assigning each “flow” to a particular next-hop.
The hash algorithm at transit LSR are described in the LAG and ECMP Hashing section of the
7210 SAS-M,T,X,R6, Mxp Interface Configuration Guide.
Label Operations
If an LSR is the ingress for a given IP prefix, LDP programs a push operation for the prefix in the
forwarding engine. This creates an LSP ID to the Next Hop Label Forwarding Entry (NHLFE)
(LTN) mapping and an LDP tunnel entry in the forwarding plane. LDP will also inform the Tunnel
Table Manager (TTM) of this tunnel. Both the LTN entry and the tunnel entry will have a NHLFE
for the label mapping that the LSR received from each of its next-hop peers.
If the LSR is to behave as a transit for a given IP prefix, LDP will program a swap operation for
the prefix in the forwarding engine. This involves creating an Incoming Label Map (ILM) entry in
the forwarding plane. The ILM entry will have to map an incoming label to possibly multiple
NHLFEs. If an LSR is an egress for a given IP prefix, LDP will program a POP entry in the
forwarding engine. This too will result in an ILM entry being created in the forwarding plane but
with no NHLFEs.
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