Ecmp Support For Ldp; Label Operations - Alcatel-Lucent 7450 Manual

ECMP Support for LDP

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 next-
hop peers.
Without ECMP support 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 If, for some reason, the outgoing label to the installed next-hop is no longer valid,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 both cases, ingress LER and 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 LER and transit LSR are described in the LAG and ECMP Hashing section
of the SR OS Interface 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.
7450 ESS MPLS Guide
Label Distribution Protocol
Page 535