IP Router Configuration
Router Configuration Guide 59
ECMP Considerations
When ECMP is enabled and multiple equal-cost next-hops exit for the IGP route, the ingress
IOM or linecard will spray the packets for this route based on hashing routine currently
supported for IPv4 packets.
When the preferred RTM entry corresponds to an LDP shortcut route, spraying will be
performed across the multiple next-hops for the LDP FEC. The FEC next-hops can either be
direct link LDP neighbors or T-LDP neighbors reachable over RSVP LSPs in the case of
LDP-over-RSVP but not both. This is as per ECMP for LDP in existing implementation.
When the preferred RTM entry corresponds to a regular IP route, spraying will be performed
across regular IP next-hops for the prefix.
Spraying across regular IP next-hops and LDP-shortcut next-hops concurrently is not
supported.
Handling of Control Packets
All control plane packets will not see the LDP shortcut route entry in RTM with the exception
of the following control packets which will be forwarded over an LDP shortcut when enabled:
• A locally generated or in transit ICMP Ping and trace route of an IGP route. The
transit message appears as a user packet to the ingress LER node.
• A locally generated response to a received ICMP ping or trace route message.
All other control plane packets that require an RTM lookup and knowledge of which
destination is reachable over the LDP shortcut will continue to be forwarded over the IP next-
hop route in RTM.
Handling of Multicast Packets
Multicast packets cannot be forwarded or received from an LDP LSP. This is because there
is no support for the configuration of such an LSP as a tunnel interfaces in PIM. Only an
RSVP P2MP LSP is currently allowed.
If a multicast packet is received over the physical interface, the RPF check will not resolve to
the LDP shortcut as the LDP shortcut route in RTM is not made available to multicast
application.
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