Ecmp Multicast Load Splitting Based On Source Group And Next-Hop Address - Cisco Catalyst 3650 Configuration Manual

IP Multicast Optimization: IP Multicast Load Splitting across Equal-Cost Paths

ECMP Multicast Load Splitting Based on Source Group and Next-Hop Address

Configuring ECMP multicast load splitting based on source, group, and next-hop address enables a more
complex hash, the next-hop-based S-G-hash algorithm, which is based on source, group, and next-hop address.
The next-hop-based S-G-hash algorithm is predictable because no randomization is used in calculating the
hash value. Unlike the S-hash and basic S-G-hash algorithms, the hash mechanism used by the next-hop-based
S-G-hash algorithm is not subject to polarization.
Note The next-hop-based S-G-hash algorithm in IPv4 multicast is the same algorithm used in IPv6 ECMP
multicast load splitting, which, in turn, utilizes the same hash function used for PIM-SM bootstrap device
(BSR).
The next-hop-based hash mechanism does not produce polarization and also maintains better RPF stability
when paths fail. These benefits come at the cost that the source or RP IP addresses cannot be used to reliably
predict and engineer the outcome of load splitting when the next-hop-based S-G-hash algorithm is used.
Because many customer networks have implemented equal-cost multipath topologies, the manual engineering
of load splitting, thus, is not a requirement in many cases. Rather, it is more of a requirement that the default
behavior of IP multicast be similar to IP unicast; that is, it is expected that IP multicast use multiple equal-cost
paths on a best-effort basis. Load splitting for IPv4 multicast, therefore, could not be enabled by default
because of the anomaly of polarization.
Load splitting for CEF unicast also uses a method that does not exhibit polarization and likewise cannot
Note
be used to predict the results of load splitting or engineer the outcome of load splitting.
The next-hop-based hash function avoids polarization because it introduces the actual next-hop IP address of
PIM neighbors into the calculation, so the hash results are different for each device, and in effect, there is no
problem of polarization. In addition to avoiding polarization, this hash mechanism also increases stability of
the RPF paths chosen in the face of path failures. Consider a device with four equal-cost paths and a large
number of states that are load split across these paths. Suppose that one of these paths fails, leaving only three
available paths. With the hash mechanism used by the polarizing hash mechanisms (the hash mechanism used
by the S-hash and basic S-G-hash algorithms), the RPF paths of all states would likely reconverge and thus
change between those three paths, especially those paths that were already using one of those three paths.
These states, therefore, may unnecessarily change their RPF interface and next-hop neighbor. This problem
exists simply because the chosen path is determined by taking the total number of paths available into
consideration by the algorithm, so once a path changes, the RPF selection for all states is subject to change
too. For the next-hop-based hash mechanism, only the states that were using the changed path for RPF would
need to reconverge onto one of the three remaining paths. The states that were already using one of those
paths would not change. If the fourth path came back up, the states that initially used it would immediately
reconverge back to that path without affecting the other states.
The next-hop-based S-G-hash algorithm ignores bidir-PIM groups.
Note
Related Topics
Enabling ECMP Multicast Load Splitting Based on Source Group and Next-Hop Address, on page 329
OL-29890-01
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3650 Switches)
Overview of ECMP Multicast Load Splitting
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