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Multicast over WAN link & Layer-2 vs Layer-3, IN RE VLANs, PIM or GRE

Level 1
Level 1

Not being familiar with all things multicast. maybe some simple responses are in order as I try to immerse myself in these subjects.

If there is a restriction not permitting multicast (which is sort of layer 2, nor are any other layer-2 tunneling protocols) over the WAN links. What are the possible solutions?

1.) GRE: As I understand GRE, this is a layer-3 tunnel.  Possibly this is the only solution needed?

2.) VLAN Routing: Also, it seems than unique VLANs on each side of the WAN would necessitate VLAN routing (also layer 3) between them, but does this automatically raise MULTICAST to layer-3 while it traverses the WAN between VLANs as well?

3.) PIM: It seems that PIM is also layer-3 but it raises the same question as #2.  Does the actual multicast traffic get raised to a "layer-3" protocol as it traverses the WAN when using PIM?  Is PIM plus GRE the only option in this WAN case? 

What if PIM, being a multicast 'protocol' is also not permitted over the WAN?  Then GRE would 'hide' it, and it would work in the  two local LANs the WAN connects?

Are there other solutions besides these above, (that in my ignorance, may or may not be solutions at all)?

it seems that multicast is in a gray area between layers 2 and 3, due to it's unique IP to MAC address mapping. (yes? no?)

Possibly some combination of the above technologies is the solution?

Thank you in advance for all inputs/replies, they are appreciated.



5 Replies 5

Thomas Schmitt
Level 1
Level 1


you should specify what kind of "WAN" do you mean, some WAN solutions support multicast and other don't.

But from design perspective, there are just use cases, where multicast traffic over WAN is required. Most often you want multicast in LAN and then you can interconnect multiple multicast domains.

Assumed, you have m-cast design for LAN, then you can use MSDP to interconnect different domains.

There is a multicast solution for MPLS networks, take a look on MVPN or MDT

In case you are designing multicast solution from scratch, consider also usage of other multicast routing protocols (BIER)

The MCast design configuration for the LAN has not been solidified. The hardware is likely a combination of c8300 and c9300 and c9200.  The WAN is the WAN, with an undetermined Cisco router (black-box) at the edge of the WAN that will not support MCast. My 'edge' will likely be the c8300. There will be no layer-2 tunneling over the WAN. So I assume that precludes MPLS/MVPN or similar.

Cisco Bit Indexed Explicit Replication (BIER)...  Thank you for the suggestion here.  It seems quite obscure, even on the Cisco site.  I can find almost nothing about it.

Level 1
Level 1

...considering my original post's WAN limitations, I am starting to think that the only solution is to use GRE.  It is the only layer-3 option that I am aware of, that might be agnostic to this WAN and its traffic restrictions.

Thomas Schmitt
Level 1
Level 1

I think you have to understand something about multicast - it is ip multicast, so layer 3. at Layer 2 the mcast ip is mapped to MAC address. Some WAN solutions forward Multicast, other don’t.
also it doesn’t matter if use different VLANs or not, you can’t „rise“ mcast  level at all.

PIM is used to distribute Mcast routes, you can extend your PIM domain with gre tunnel or you can use MPBGP to exchange mroutes over WAN

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