Stackwise behaviour with multicast traffic -traffic traversing the SVL

[Octavio Alfageme]

Hello everyone,

I've been trying to find information regarding how Cat9500s in Stackwise architecture manage multicast traffic, but I couldn't find anything to confirm the behaviour that I see in the lab. I'd be grateful if anybody in the community might help me to find the explanation.

If you take a look at figure 1, it's a typical multicast flow: a multicast source connected to a layer 2 switch with IGMP snooping enabled that forwards all the multicast traffic on the considered VLAN to a L3 router whose interface sends PIM hellos towards the switch, so this port-channel interface is classified as mrouter by the IGMP snooping function. So far so good.

Now, let's consider figure 2 where we have an Stackwise architecture: two Cat9500s switches (primary and standby); the multicast source is single-homed to the standby chassis and the L3 router is dual-homed to both chassis. In this case, the multicast traffic traverses the Stackwise Virtual Link and is sent towards the L3 router only over the port-channel interfaces connected to the active chassis. Is this behaviour correct? It's as if the primary chassis needed to process all the multicast flows sent from the single-homed source. We expected that multicast traffic wouldn't traverse the SVL and would directly proceed towards the L3 router over the port-channel links connected to the standby chassis.

Finally, figure 3 is a more typical configuration where the multicast source is dual-homed to both chassis. In this case, there is no multicast traffic traversing the SVL between both chassis and multicast traffic takes the shortest path towards the layer 3 router.

I'd be grateful if you might explain the reason why multicast traffic flow in figure 2 is what we must expect (traffic flowing on the SVL towards the primary switch) and how it fits with figure 3 behaviour, where there is no multicast traffic on the SVL.

Thanks in advance

Kind regards

Octavio

[Reza Sharifi]

Hi,

For figure-2, the behavior is correct since the device is single homed to one of the 9500 switches. Here is some more info:

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9500/software/release/16-9/configuration_guide/ha/b_169_ha_9500_cg/configuring_cisco_stackwise_virtual.html

Cisco StackWise Virtual transmits data traffic over a StackWise Virtual link under the following circumstances:

• Layer 2 traffic flooded over a VLAN (even for dual-homed links).

• Packets processed by software on the Cisco StackWise Virtual active switch where the ingress interface is on the Cisco StackWise Virtual standby switch.

• The packet destination is on the peer switch, as described in the following examples:

  • Traffic within a VLAN where the known destination interface is on the peer switch.

  • Traffic that is replicated for a multicast group and the multicast receivers are on the peer switch.

  • The known unicast destination MAC address is on the peer switch.

  • The packet is a MAC notification frame destined for a port on the peer switch.HTH

[Octavio Alfageme]

Thank you very much for your answer and the information you add, Mr. Sharifi. Please, let me go a little further. If you take a look at the figure 2, destination router is dual-homed to both chassis, so I don't know if the case you highlight fully applies to justify traffic on the SVL.

However, my doubt if the restriction that applies among the ones you mention is the following:

• Packets processed by software on the Cisco StackWise Virtual active switch where the ingress interface is on the Cisco StackWise Virtual standby switch.

Might be a requirement for the IGMP snooping to work that the primary switch processes the traffic? Please, keep in mind that we are talking about multicast flows. However, to be honest, if this was the case, I wouldn't know how to justify the behaviour on figure 3, as there would be multicast flows that will never traverse the primary switch.

Thank you very much for your help

Regards

Octavio

[DavidMcClain9266]

The (active) switch is the one that is administering the control plane only. Both switches actively forward data traffic. The multicast traffic always flows through the SVL to the primary (Control Plane Only) switch.