DR failure with IP multicast routing
If the actual DR fails, the proxy DR becomes the DR for all VLANs. It turns all the bridge entries into route entries for downstream VLANs. Multicast traffic is resumed without waiting for a new DR election nor waiting for the SPT to be computed, as it is already prebuilt.
When the VSX peer returns, it becomes a proxy DR to avoid any unnecessary traffic routing switchover and the associated impact. The fresh VSX peer will not pre-empt the DR role. It is achieved by sending the hello packet with a maximum DR priority value.
Keep the PIM timer hello interval to the default value of 30 seconds, as it no longer can improve VSX PIM fast-failover.
Consider configuring the
pim-sparse dr-priorityin a way that each VSX node has an equal VLAN load distribution, regarding the actual DR role. This configuration helps to further minimize the impact during fast-failover.
If there is an ISL failure, there might be multicast duplication to downstream links for 5 seconds (max), before the secondary node shuts is downstream links of VSX LAGs. When the ISL is back up, the secondary switch stays as the proxy DR.
