Multicast in Layer 3 and Layer 2

[dianawinsky]

It's my first time configuring multicast on a switch. Do I need to configure both Layer 3 and Layer 2 for multicast on a single VLAN that is in DHCP? Should I enable IP multicast routing on Layer 3 and PIM sparse-mode on that VLAN? Also, do I need to enable IGMP snooping on Layer 2?

[Joseph W. Doherty]

The multicast would be just between hosts within the same L2 domain (e.g. VLAN)?

If so, it should work fine unless the switch(es) are IGMP snooping capable and it's enabled (which is usually desired).

If there's active IGMP snooping, either you'll need to deactivate it or insure you have an IGMP querier within the L2 domain.  The latter might be (and usually) provided by a router (activate PIM on gateway interface) or a feature on an IGMP snooping switch.

[Giuseppe Larosa]

Hello @dianawinsky ,

in modern switches IGMP snooping the L2 feature is enabled by default.

As already noted for IGMP snooping to work someone has to send out IGMP queries at regular intervals.

This can be achieved in two ways:

a) enabling ip multicast routing

+ ip pim sparse-dense mode under the SVI L3 VLAN interface

b) by using the IGMP querier function on the VLAN.

IGMP querier is not compatible with multiple PIM devices connected to the VLAN because it is stupid and it is not able to take part in IGMP querier election performed by PIM speaking devices ( lowest IP address is the IGMP querier, highest IP address is the PIM Designated Router for the subnet)

Sparse mode requires a RP to be configured, usually this is done using a loopback interface.

Hope to help

Giuseppe

[Joseph W. Doherty]

BTW, what Giuseppe describes is 100% correct, although he is making some (reasonable) assumptions.

However not all switches support IGMP snooping.  But if they do, as he notes, it's usually enabled by default.

Multicast routing, routing, has its own considerations when doing multicast between L2 domains.  If you had multiple subnets within the same L2 domain (unusual for host subnets), additional multicast routing considerations don't apply.

Not all switches are L3, but some L2 switches might support IGMP snooping.

Routers can also fulfill roll of an IGMP querier.

[paul driver]

Hello
if all mc traffic is within  a single vlan then you do NOT require any pim multicast routing
Also if your mc traffic is traversing L2 switches and you have igmp snooping enabled then you need to append a snooping querier on those other switches uplink ports otherwise snooping will negate connection to mc source on the switch where it is attached is to.

[Joseph W. Doherty]

"if all mc traffic is within  a single vlan then you do NOT require any pim multicast routing"

I recall enabling PIM, on a L3 interface, within the L2 domains provides an IGMP querier even if multicast routing is unneeded, disagree?  If not, likely a better choice as it positions network for multicast routing, disagree?

"Also if your mc traffic is traversing L2 switches and you have igmp snooping enabled then you need to append a snooping querier on those other switches uplink ports otherwise snooping will negate connection to mc source on the switch where it is attached is to."

Are you sure?  I recall only one querier is required per L2 domain, but I also vaguely recall some possible issue with multiple switches and a switch performing queries; something about active quering switch treating downstream switch as single host(?).  Could you expand on this?

[paul driver]

Hello @Joseph W. Doherty 
If there is an additional L2 switch in the mix (igmp snooping enabled) with a mc receiver., my understanding here is the switch will register the receivers request in its snoop table but mc will fail on this 2nd switch due to the fact the snooping will negate that request on its interconnect between the two switches,  the source switch wont be aware of any mc request coming in on that interconnect, but if a snoop queirer is enabled on the second switch this will allow the source switch to create an mrouter port on that interconnect port as such mc will then flow successfully to the 2nd switch

As regards enabling pim I agree mate , just another way of creating the mrouter ports, however you do need a L3 SVI enable interface, something a L2 domain may not have.

[Joseph W. Doherty]

As I wrote before, I did vaguely recall there was some consideration when using multiple switches and IGMP snooping.

Found this Tech Note Fix Multicast Traffic Issues in Same VLAN on Catalyst Switches, which describes the problem and 5 possible solutions.

Option 2 is the one germane to what you're suggesting.  Unfortunately, the Tech Note doesn't make it explicitly clear whether every IGMP snooping switch, if using a switch querier, needs to activate its own querier.  My reading/understanding of the Tech Note, and my understanding of IGMP snooping is, only one querier is needed within the L2 domain (the equivalent of only needing a single multicast routing port).

[paul driver]

Hello @Joseph W. Doherty 
I was recalling from the past, when I had to POC L2 mc for implementation.
This document your shared referencing scenario 1/2 does explain it in a much clearer way to what I was trying to say.
Even I though found recent in switching software ive never had the need to specify any querier with/without snooping enabled for L2 MC, but maybe that was because they were L3 interfaces as the sources.
Anyway in this post Its was the snooping that I was mindful of hence my suggestion as similarly to that document about enabling a querier -

In anycase thanks for that document, I shall keep it for reference 

[dianawinsky]

In addition to my post, I am using catalyst 1300 for access switches and catalyst 9500 as my core switch. I just don't understand what configuration I should follow since I've seen so many multicast configuration guides, and I'm not sure which one to follow. By the way, I'm using VLAN 10 with a network of 172.16.10.0/24 for my multicast setup of tv boxes

[Joseph W. Doherty]

All multicast traffic will ONLY be on VLAN 10?

If so, is there a L3 interface, on VLAN 10 to reach other subnets?

If so, minimally all you may need to do is enable PIM on the gateway interface.

[dianawinsky]

Multicast traffic is only in VLAN 10, while other VLANs in my setup are not.

There's an L3 interface or SVI interface.

If I enable PIM on the gateway interface in L3 or the core switch, is there no need to configure in the access switch?

Am I correct only PIM should I enable? How about multicast routing?

Here's what I've configure in L3:

interface vlan 10

ip address 172.16.10.1 255.255.255.0

ip pim sparse-mode

[Joseph W. Doherty]

That may be enough for just VLAN 10.  Likely you don't need to configure access switch too.

Multicast routing would apply if you wish to distribute multicast across VLANs.  Do you?

[dianawinsky]

So there is no need to configure the igmp snooping in the access switch or no configuration of multicast in access switch?

If the VLAN for IT needs to reach VLAN 10's multicast, do I need to enable multicast routing?

[Giuseppe Larosa]

Hello @dianawinsky ,

>> If the VLAN for IT needs to reach VLAN 10's multicast, do I need to enable multicast routing?

yes you need multicast routing enabled and ip pim sparse-dense mode under both SVI interfaces.

You can check the status of igmp snooping on catalyst 1300 access layer switches.

see

https://www.cisco.com/c/en/us/td/docs/switches/campus-lan-switches-access/Catalyst-1200-and-1300-Switches/Admin-Guide/catalyst-1300-admin-guide/get-to-know-your-switch.html#Cisco_Concept.dita_256e233d-a6f8-4a89-ad2c-5f69163d26af

There is a multicast section here:

https://www.cisco.com/c/en/us/td/docs/switches/campus-lan-switches-access/Catalyst-1200-and-1300-Switches/Admin-Guide/catalyst-1300-admin-guide/multicast.html

IGMP Snooping

To support selective IPv4 Multicast forwarding, bridge Multicast filtering must be enabled (in Multicast Properties). The IGMP Snooping must be enabled globally and for each relevant VLAN in the IGMP Snooping page.

So you need to perform some configuration actions in GUI advanced mode . I was wrong they do not have IGMP snooping enabled by default.

Hope to help

Giuseppe