For PIM-SM and PIM-SSM, therefore, the operation of ECMP multicast load splitting can only be guaranteed
when all downstream devices on a LAN are consistently configured Cisco devices.
ECMP Multicast Load Splitting and Reconvergence When Unicast Routing Changes
When unicast routing changes, all IP multicast routing states reconverge immediately based on the available
unicast routing information. Specifically, if one path goes down, the remaining paths reconverge immediately,
and if the path comes up again, multicast forwarding will subsequently reconverge to the same RPF paths
that were used before the path failed. Reconvergence occurs whether load splitting of IP multicast traffic over
ECMP is configured or not.
Use of BGP with ECMP Multicast Load Splitting
ECMP multicast load splitting works with RPF information learned through BGP in the same way as with
RPF information learned from other protocols: It chooses one path out of the multiple paths installed by the
protocol. The main difference with BGP is that it only installs a single path, by default. For example, when
a BGP speaker learns two identical external BGP (eBGP) paths for a prefix, it will choose the path with the
lowest device ID as the best path. The best path is then installed in the IP routing table. If BGP multipath
support is enabled and the eBGP paths are learned from the same neighboring AS, instead of picking the
single best path, BGP installs multiple paths in the IP routing table. By default, BGP will install only one path
to the IP routing table.
To leverage ECMP multicast load splitting for BGP learned prefixes, you must enable BGP multipath. Once
configured, when BGP installs the remote next-hop information, RPF lookups will execute recursively to find
the best next hop towards that BGP next hop (as in unicast). If for example there is only a single BGP path
for a given prefix, but there are two IGP paths to reach that BGP next hop, then multicast RPF will correctly
load split between the two different IGP paths.
Use of ECMP Multicast Load Splitting with Static Mroutes
If it is not possible to use an IGP to install equal cost routes for certain sources or RPs, static routes can be
configured to specify the equal-cost paths for load splitting. You cannot use static mroutes to configure
equal-cost paths because the software does not support the configuration of one static mroute per prefix. There
are some workarounds for this limitation using recursive route lookups but the workarounds cannot be applied
to equal-cost multipath routing.
For more information about configuring static mroutes, see the
Configuring Multiple Static Mroutes in
configuration note on the Cisco IOS IP multicast FTP site at
ftp://ftpeng.cisco.com/ipmulticast/config-notes/static-mroutes.txt.
Note
You can specify only static mroutes for equal-cost multipaths in IPv4 multicast; however, those static mroutes
would only apply to multicast, or you can specify that the equal-cost multipaths apply to both unicast and
multicast routing. In IPv6 multicast, there is no such restriction. Equal-cost multipath mroutes can be configured
for static IPv6 mroutes that apply to only unicast routing, only multicast routing, or both unicast and multicast
routing.
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3650 Switches)
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IP Multicast Optimization: IP Multicast Load Splitting across Equal-Cost Paths
Overview of ECMP Multicast Load Splitting
