Overview of ECMP Multicast Load Splitting
By default, ECMP multicast load splitting of IPv4 multicast traffic is disabled. ECMP multicast load splitting
can be enabled using the
ip multicast multipath
command.
ECMP Multicast Load Splitting Based on Source Address Using the S-Hash Algorithm
ECMP multicast load splitting traffic based on source address uses the S-hash algorithm, enabling the RPF
interface for each (*, G) or (S, G) state to be selected among the available equal-cost paths, depending on the
RPF address to which the state resolves. For an (S, G) state, the RPF address is the source address of the state;
for a (*, G) state, the RPF address is the address of the RP associated with the group address of the state.
When ECMP multicast load splitting based on source address is configured, multicast traffic for different
states can be received across more than just one of the equal-cost interfaces. The method applied by IPv4
multicast is quite similar in principle to the default per-flow load splitting in IPv4 CEF or the load splitting
used with Fast and Gigabit EtherChannels. This method of ECMP multicast load splitting, however, is subject
to polarization.
Related Topics
Enabling ECMP Multicast Load Splitting Based on Source Address, on page 325
Example Enabling ECMP Multicast Load Splitting Based on Source Address, on page 331
ECMP Multicast Load Splitting Based on Source and Group Address Using the Basic S-G-Hash
Algorithm
ECMP multicast load splitting based on source and group address uses a simple hash, referred to as the basic
S-G-hash algorithm, which is based on source and group address. The basic S-G-hash algorithm is predictable
because no randomization is used in coming up with the hash value. The S-G-hash mechanism, however, is
subject to polarization because for a given source and group, the same hash is always picked irrespective of
the device this hash is being calculated on.
The basic S-G-hash algorithm ignores bidir-PIM groups.
Note
Related Topics
Enabling ECMP Multicast Load Splitting Based on Source and Group Address, on page 328
Example Enabling ECMP Multicast Load Splitting Based on Source and Group Address, on page 332
Predictability As a By-Product of Using the S-Hash and Basic S-G-Hash Algorithms
The method used by ECMP multicast load splitting in IPv4 multicast allows for consistent load splitting in a
network where the same number of equal-cost paths are present in multiple places in a topology. If an RP
address or source addresses are calculated once to have flows split across N paths, then they will be split
across those N paths in the same way in all places in the topology. Consistent load splitting allows for
predictability, which, in turn, enables load splitting of IPv4 multicast traffic to be manually engineered.
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
