Weighted Load-Balancing over MPLS LSP
48 Router Configuration Guide
2. IGP computes the normalized weight for each prefix tunnel next-hop. The minimum
value of the normalized weight is 1 and the maximum if 64. IGP updates the route in
RTM with the set of tunnel next-hops and normalized weights. RTM downloads the
information to IOM for inclusion in the FIB.
3. The normalized weights of route tunnel next-hops are updated in the following cases:
→ When the main SPF is run following a trigger, e.g., network failure, and updates
a given route with a modified set of tunnel next-hops. This will trigger a route re-
download to the IOM and all users of RTM are notified.
→ The user adds or changes the weight of one or more LSPs. In this case, RTM will
perform a route download to IOM but other users of RTM should not be notified
since the route resolution did not change.
4. The weighted load balancing feature is only applied to a prefix when all the tunnel
next-hops in the ECMP set have the same endpoint. If an IGP prefix resolves in RTM
to a set of ECMP tunnel next-hops which do not terminate on the same endpoint, the
regular ECMP spraying is performed. If BGP performs BGP ECMP to a set of BGP
ECMP next-hops for a prefix [weighted-bgp-ecmp-prd], regular ECMP spraying is
performed towards a given BGP next-hop if the subset of its tunnel next-hops does
not terminate on the same endpoint.
5. Regular ECMP spraying is also applied if a prefix is resolved in RTM to an ECMP
set which consists of a mix of IP and tunnel next-hops.
6. This feature is not supported in the following contexts:
→ Packets of BGP prefix with the BGP next-hop resolved in TTM to RSVP LSP
(BGP shortcut).
→ CPM generated packets, including OAM packets, which are looked-up in RTM
and which are forwarded over tunnel next-hops. These will continue to be
forwarded using either regular ECMP or by selecting one next-hop from the set
as in existing implementation.
ECMP Considerations
The weight assigned to an LSP impacts only the forwarding decision, not the routing
decision. In other words, it does not change the selection of the set of ECMP tunnel next-hops
of a prefix when more next-hops exist than the value of the router ecmp option. This selection
continues to follow the algorithm used in the IGP shortcut feature.
Once the set of tunnel next-hops is selected, the LSP weight is used to modulate the amount
of packets forwarded over each next-hop.
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