Cisco Catalyst 3850 series Configuration Manual page 381
Ip multicast routing configuration guide
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Also See for Catalyst 3850 series:
- Configuration manual (58 pages) ,
- Hardware installation manual (146 pages)
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IP Multicast Optimization: IP Multicast Load Splitting across Equal-Cost Paths
Although the following illustration and example uses routers in the configuration, any device (router or
Note
switch) can be used.
Figure 19: ECMP Multicast Load Splitting and Assert Processing in PIM-DM and DF Election in Bidir-PIM
In the figure, Device 2 has two equal-cost paths to S1 and S2 and the RP addresses on Device 1. Both paths
are across Gigabit Ethernet interface 1/0/0: one path towards Device 3 and one path towards Device 4. For
PIM-SM and PIM-SSM (*, G) and (S, G) RPF selection, there is no difference in the behavior of Device 2
in this topology versus Device 2 in the topology illustrated in the figure. There is, however, a difference when
using PIM-DM or bidir-PIM.
If PIM-DM is used in the topology illustrated in the figure, Device 3 and Device 4 would start flooding traffic
for the states onto Gigabit Ethernet interface 1/0/0 and would use the PIM assert process to elect one device
among them to forward the traffic and to avoid traffic duplication. As both Device 3 and Device 4 would have
the same route cost, the device with the higher IP address on Gigabit Ethernet interface 1/0/0 would always
win the assert process. As a result, if PIM-DM is used in this topology, traffic would not be load split across
Device 3 and Device 4.
If bidir-PIM is used in the topology illustrated in the figure, a process called DF election would take place
between Device 2, Device 3, and Device 4 on Gigabit Ethernet interface 1/0/0. The process of DF election
would elect one device for each RP to forward traffic across Gigabit Ethernet interface 1/0/0 for any groups
using that particular RP, based on the device with the highest IP address configured for that interface. Even
if multiple RPs are used (for example one for G1 and another one for G2), the DF election for those RPs
would always be won by the device that has the higher IP address configured on Gigabit Ethernet interface
1/0/0 (either Device 3 or Device 4 in this topology). The election rules used for DF election are virtually the
same as the election rules used for the PIM assert process, only the protocol mechanisms to negotiate them
are more refined for DF election (in order to return the results more expediently). As a result, when bidir-PIM
is used in this topology, load splitting would always occur across Gigabit Ethernet interface 1/0/0.
The reason that ECMP multicast load splitting does influence the RPF selection but not the assert process in
PIM-DM or DF election in bidir-PIM is because both the assert process and DF election are cooperative
processes that need to be implemented consistently between participating devices. Changing them would
require some form of protocol change that would also need to be agreed upon by the participating devices.
RPF selection is a purely device local policy and, thus, can be enabled or disabled without protocol changes
individually on each device.
OL-32598-01
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3.6E (Catalyst 3850 Switches)
Overview of ECMP Multicast Load Splitting
357
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