Information About Ip Multicast Load Splitting Across Equal-Cost Paths; Load Splitting Versus Load Balancing; Default Behavior For Ip Multicast When Multiple Equal-Cost Paths Exist - Cisco Catalyst 3850 series Configuration Manual

Information About IP Multicast Load Splitting across Equal-Cost Paths

Information About IP Multicast Load Splitting across Equal-Cost
Paths

Load Splitting Versus Load Balancing

Load splitting and load balancing are not the same. Load splitting provides a means to randomly distribute
(*, G) and (S, G) traffic streams across multiple equal-cost reverse path forwarding (RPF) paths, which does
not necessarily result in a balanced IP multicast traffic load on those equal-cost RPF paths. By randomly
distributing (*, G) and (S, G) traffic streams, the methods used for load splitting IP multicast traffic attempt
to distribute an equal amount of traffic flows on each of the available RPF paths not by counting the flows,
but, rather, by making a pseudorandom decision. These methods are collectively referred to as equal-cost
multipath (ECMP) multicast load splitting methods and result in better load-sharing in networks where there
are many traffic streams that utilize approximately the same amount of bandwidth.
If there are just a few (S, G) or (*, G) states flowing across a set of equal-cost links, the chance that they are
well balanced is quite low. To overcome this limitation, precalculated source addresses--for (S, G) states or
rendezvous point (RP) addresses for (*, G) states, can be used to achieve a reasonable form of load balancing.
This limitation applies equally to the per-flow load splitting in Cisco Express Forwarding (CEF) or with
EtherChannels: As long as there are only a few flows, those methods of load splitting will not result in good
load distribution without some form of manual engineering.

Default Behavior for IP Multicast When Multiple Equal-Cost Paths Exist

By default, for Protocol Independent Multicast sparse mode (PIM-SM), Source Specific Multicast (PIM-SSM),
bidirectional PIM (bidir-PIM), and PIM dense mode (PIM-DM) groups, if multiple equal-cost paths are
available, Reverse Path Forwarding (RPF) for IPv4 multicast traffic is based on the PIM neighbor with the
highest IP address. This method is referred to as the highest PIM neighbor behavior. This behavior is in
accordance with RFC 2362 for PIM-SM, but also applies to PIM-SSM, PIM-DM, and bidir-PIM.
The figure illustrates a sample topology that is used in this section to explain the default behavior for IP
multicast when multiple equal-cost paths exist.
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3.6E (Catalyst 3850 Switches)
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IP Multicast Optimization: IP Multicast Load Splitting across Equal-Cost Paths
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