This section describes the behavior of the weighted load-balancing feature for IGP, BGP, and static route prefixes resolved in RTM to IGP shortcuts.
When an IGP, BGP, or a static route prefix is resolved in RTM to a set of ECMP tunnel next-hops of type RSVP-TE, and the router level weighted-ecmp option is enabled, the ingress hash table for the next-hop selection is populated with a number of tunnel next-hop entries for each LSP equal to the normalized LSP weight value. All prefixes resolving to the same set of ECMP tunnel next-hops use the same table.
This feature performs the following:
MPLS populates the user-configured LSP weight in TTM. When the global command weighted-ecmp is enabled, and any LSP in the ECMP set of a prefix does not have a weight configured, the regular ECMP spraying for the prefix is performed.
IGP computes the normalized weight for each prefix tunnel next-hop. The minimum value of the normalized weight is 1 and the maximum is 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.
The normalized weights of route tunnel next-hops are updated in the following cases:
When the main SPF is run following a trigger, for example, network failure, and updates a route with a modified set of tunnel next-hops. This triggers 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 performs a route download to IOM, but other users of RTM are not notified because the route resolution did not change.
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 that 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 toward a BGP next-hop if the subset of its tunnel next-hops does not terminate on the same endpoint.
Regular ECMP spraying is also applied if a prefix is resolved in RTM to an ECMP set that consists of a mix of IP and tunnel next-hops.
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 are forwarded using either regular ECMP or by selecting one next-hop from the set.