Weighted Load-Balancing over MPLS LSP
58 Router Configuration Guide
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-IGP Synchronization
See the SR OS MPLS Guide for details of the LDP-IGP Synchronization.
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 or on linecards 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 or linecard 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
unlabeled.
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the LDP FEC was withdrawn due to LDP control plane issues) the switchover should be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 or linecards tunnel table are
asynchronous. If Tunnel Table is configured first, it is possible that traffic will be black holed
for some time.
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