Alcatel-Lucent 7710 SR OS Configuration Manual page 80

Assume the egress LER advertised a FEC for some /24 prefix using the fec-originate command. At
the ingress LER, LDP resolves the FEC by checking in RTM that an exact match exists for this
prefix. Once LDP activated the FEC, it programs the NHLFE in the egress data path and the LDP
tunnel information in the ingress data path tunnel table.
Next, LDP provides the shortcut route to RTM which will associate it with the same /24 prefix.
There will be two entries for this /24 prefix, the LDP shortcut next-hop and the regular IP next-
hop. The latter was used by LDP to validate and activate the FEC. RTM then resolves all user
prefixes which succeed a longest prefix match against the /24 route entry to use the LDP LSP.
Assume now the aggregate-prefix-match was enabled and that LDP found a /16 prefix in RTM to
activate the FEC for the /24 FEC prefix. In this case, RTM adds a new more specific route entry of
/24 and has the next-hop as the LDP LSP but it will still not have a specific /24 IP route entry.
RTM then resolves all user prefixes which succeed a longest prefix match against the /24 route
entry to use the LDP LSP while all other prefixes which succeed a longest prefix-match against the
/16 route entry will use the IP next-hop. LDP shortcut will also work when using RIP for routing.
LDP Shortcut Forwarding Plane
Once LDP activated a FEC for a given prefix and programmed RTM, it also programs the ingress
Tunnel Table in IOM with the LDP tunnel information.
When an IPv4 packet is received on an ingress network interface, a subscriber IES interface, or a
regular IES interface, the lookup of the packet by the ingress IOM will result in the packet being
sent labeled with the label stack corresponding to the NHLFE of the LDP LSP when the preferred
RTM entry corresponds to an LDP shortcut.
If the preferred RTM entry corresponds to an IP next-hop, the IPv4 packet is forwarded unlabelled.
The switching from the LDP shortcut next-hop to the regular IP next-hop when the LDP FEC
becomes unavailable depends on whether the next-hop is still available. If it is (for example, the
LDP FEC was withdrawn due to LDP control plane issues) the switchover should be faster. If the
next-hop determination requires IGP to re-converge, this will take longer. However no target is set.
The switching from a regular IP next-hop to an LDP shortcut next-hop will normally occur only
when both are available. However, the programming of the NHLFE by LDP and the programming
of the LDP tunnel information in the ingress IOM tunnel table are asynchronous. If Tunnel Table is
configured first, it is possible that traffic will be black holed for some time .
ECMP Considerations
When ECMP is enabled and multiple equal-cost next-hops exit for the IGP route, the ingress IOM
will spray the packets for this route based on hashing routine currently supported for IPv4 packets.
Page 80 7710 SR OS Router Configuration Guide