Symptom
Traffic with a link-local multicast destination of 224.0.0.251 is being routed by 3650/3850 switches with the presence of PIM, and mrouter port. This traffic should be not routed, under any circumstance, unless there is an mDNS/Bonjour Gateway defined.
We can see that this traffic is egressing the switch, possibly in multiple VLANs, with the source MAC Address of the SVI on the switch, and the TTL is decremented by one.
Conditions
+ SVI with PIM configured
+ Traffic will flood out of designated mrouter interface
+ You may also experience extremely high volumes of multicast traffic on these interfaces as this behavior can created a loop
Workaround
+ Remove PIM from interfaces if they are not needed
or
+ Create an ACL on the SVI to block traffic destined to this multicast group address.
