I have a single multicast source (Server A) sending to destinations and, the source being directly connected to Switch A on VLAN 100.

Switch A is sending dozens of multicasts from multiple VLANs to Router B.

Router B is connected to Switch A on VLAN 200. There is also a client connected to VLAN 200 on switch A directly.

For the two multicasts 226 and 227, multiple days were spent diagnosing the issue until 226 just started to "work". No changes had been made.

227 continues to not work, despite being requested from the same device that requested 226 and eventually received it.

Full mroute entries for 226 (the working one):

(*,, 1d21h/00:03:22, RP, flags: S
  Incoming interface: Null, RPF nbr
  Outgoing interface list:
    Vlan200, Forward/Sparse, 00:00:07/00:03:22

(,, 1d21h/00:01:50, flags: T
  Incoming interface: Vlan100, RPF nbr
  Outgoing interface list:
    Vlan200, Forward/Sparse, 00:00:07/00:03:22

full mroute entries for 227 (Not working, coming from same source and requested by same client)

(*,, 7w0d/00:02:59, RP, flags: SJC
  Incoming interface: Null, RPF nbr
  Outgoing interface list:
    Vlan200, Forward/Sparse, 00:00:30/00:02:59
    Vlan100, Forward/Sparse, 00:33:28/00:02:20

Troubleshooting steps taken:

  • Verified that the source is definitely sending multicast out correct interface
  • Verified that I could join from a client directly connected to VLAN 100
  • Added a port to Switch A in VLAN 200, and I could not join from a client connected to that port
  • A new multicast coming from server A on VLAN 101 works fine.

In Summary:

Server A (VLAN 100) -> -> (VLAN 200) client B: Didn't work for two days, then started to work with no changes made

Server A (VLAN 100) -> -> (VLAN 200) client B: Started testing after 226 started to work inexplicably. This one still doesn't work

Server A (VLAN 101) -> -> (VLAN 200) client B: Worked right away

The configuration for each SVI (100, 101, 200) is identical regarding PIM configuration:

 ip pim dr-priority 100
 ip pim sparse-mode

What could cause this behavior?

  • You must have the router interfaces join IGMP for each group. IGMP is between the host and the router to tell the router that a host is interested in that multicast traffic. PIM is for routing multicast traffic between routers, and Sparse Mode requires an RP.
    – Ron Maupin
    May 10, 2022 at 16:40

1 Answer 1


Multicasts are confined to a single VLAN unless you enable multicast routing on your router, and multicast routing is very different than unicast routing. Remember that VLANs logically break a switch into multiple, unconnected, virtual switches, and it requires a router to route traffic between VLANs.

A unicast packet has a single destination to which the packet can be deterministically routed, but multicast packets are sent to a group address, so multicast routing is about where to not send multicast packets. Multicast packets are only sent to where they are requested, using IGMP to tell the local router that a host wishes to receive packets destined to a particular multicast group. Your router needs to be configured for IGMP on the interface. You may also need a multicast routing protocol, e.g. PIM, to send multicast packets between routers.

Modern switches can also snoop on IGMP between the hosts and router to further limit where multicasts are sent. That can present a problem when you connect two switches. This question has answers about that problem.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.