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New Member

Multicast static routing at layer 2 (help)

We have a 4507r that routes multicast at layer 2 because of our particular setup. We are sending multicasts of video streams to remote locations and to do the transport we are using a product called a ITU and it only transmits and doesnt have 2 way communication. It uses the rx as a tx on one side to be able to do 2 gbit instead of 1 but in one way only. So since we would never get a reply from a pear, then any multicast routing protocol would fail since nobody will ever send a join message. So the 4507r recieves a bunch of multicast streams that it then forwards to the ITU that then brings that to a 3750 a few km down the road which then sends it to devices who then forwards them on the cable plant there. That in a nut shell is what it does.

layer 2 multicast by default is broadcasted in the vlan. so to fix this issue in the 4507r I used the mac addess-table static command to manually point my multicast in the right direction. It looks like this:

mac address-table static 0100.5e02.020a vlan 100 interface gi3/2 gi3/4 Gi3/40

What that does is:

when I see which when connverted to layer 2 it becomes 0100.5e02.020a in vlan 100 send to interfaces gi3/2 gi3/4 Gi3/40

This works fantastic in the 4507r. The problems I encountered was with the 3750 a few km down the road. now this guy receieves only and never will send me any traffic since the ITU the is rx only. So it recieves about 720mb of multicast then when I try to direct the traffic properly like I did in the 4507r in the 3750 it detects that the input traffic from the ITU interface is not braodcast but multicast because in a show interface I see example Received 3706088870 broadcasts (3706051178 multicasts) but then when I tell it to send it only to this and that interfaces in the vlan, the switch ignores my command and broadcasts the traffic in the whole vlan which is causing me to over subscribe on my devices connected to it. They cant recieve 720mb of data

Problem number 2. when I turn on port number 2 on the ITU, the switch detects a loop and disables the interface. So I cant bring it up.

So here is the details of my switches ios version and switch models:


Cisco IOS Software, Catalyst 4500 L3 Switch Software (cat4500-IPBASEK9-M), Version 12.2(50)SG2, RELEASE SOFTWARE (fc2)

on 2 Supervisor IV



model is WS-C3750G-12S-S

basic configs and show int

3750 rx of the multicast interface g1/0/10

30 second input rate 719275000 bits/sec, 65823 packets/sec

30 second output rate 0 bits/sec, 0 packets/sec

123965173158 packets input, 169333824008372 bytes, 0 no buffer

Received 3706088870 broadcasts (3706051178 multicasts)

and then the tx to the devices

g1/0/1 example

30 second input rate 0 bits/sec, 0 packets/sec

30 second output rate 718667000 bits/sec, 65960 packets/sec

12597 packets input, 806208 bytes, 0 no buffer

Received 12597 broadcasts (0 multicasts)


30 second input rate 0 bits/sec, 0 packets/sec

30 second output rate 717076000 bits/sec, 65813 packets/sec


30 second input rate 0 bits/sec, 0 packets/sec

30 second output rate 718183000 bits/sec, 65915 packets/sec

So we see that it recieves 720mb from the ITU which is connected in port g1/0/10 in multicast then broadcasts to all interfaces in the vlan 100 in that switch that 720mb

the config looks like this:

ip multicast-routing distributed

no ip igmp snooping vlan 100

...output removed..

mac address-table static 0100.5e02.0211 vlan 100 interface GigabitEthernet1/0/1

mac address-table static 0100.5e02.0216 vlan 100 interface GigabitEthernet1/0/3

4507r is configured same way and works great

So again the goal of the ITU is to save fibers hehe since it will use both fiber to TX instead of having a tx and rx. We only need traffic in one way.

So thats my pickle. Thank you in advance for any advise.


  • LAN Switching and Routing
Cisco Employee

Re: Multicast static routing at layer 2 (help)

Hello Jason,

Well, that's a complicated description :) But you have a complicated situation, no doubt.

I just wonder here: You have the IGMP snooping disabled on the VLAN 100 on your 3750 switch. Why? If the subscribers are directly connected to it then the 3750 should be able to hear the IGMP joins and direct the multicast traffic appropriately to the subscribing ports only.

By the way, you do not need to have the multicast routing activated on your switches. What you are doing is only Layer2 switching of multicasts, not routing. I suggest turning it off by "no ip multicast-routing distributed".

Best regards,


New Member

Re: Multicast static routing at layer 2 (help)

well i turned off igmp snooping since nobody will ever send a join. and no one is connected to the 3750. in the 3750 which recieved multicasts from the 4507 I have a bunch of SEM devices which are devices which listens to a multicast then formats that for a cable network and shoots it in the network for setup boxes to pick up and display on the clients TV.

As for turning off multicast routing i guess would do no harm because indeed im doing layer 2 multicast. But why is the 3750 not doing the layer 2 multicast and instead broadcasts it in all the VLAN?

and thanks for the reply.


Re: Multicast static routing at layer 2 (help)

Hi Jason,

Problem number 1:

Here are some outlines: when the 3750 recieve the Multicast traffic from the ITU, then you could prevent the unnecessary ports on vlan 100 from recieving those multicast packet by applying the following command on it:

interface name

switchport block multicast

Problem number 2: to which port the second ITU port connected to? Is it to another port in the 4507?

If so, then the ITU would act as normal bridge that forwards BPDUs , Multicast and broadcast, and this is the correct behaviour, what you need to do is try to have layer-2 Ether channel (Port channel) between 2 ports on the 4507 and 2 ports on the 3750. This would allow you to have STP treat both ports as one single port and prevent it from blocking one.

Hope it helps,


New Member

Re: Multicast static routing at layer 2 (help)

Thanks for my solution to problem number 2 Mohamed.

This will most probably work. Now back to problem number 1, The 4507 is in one location recieving

the streams then it actually dispatches those stream to the right SEMS(the devices that will then

send that on the cable network) there are a few sems in the 4507 to feed the cable plant where

the 4507 is but then the ITU serve as transport to also feed another location with the exact

streams. So they forward that and then it comes out in a 3750 in another location. That 3750

has 3 SEMS connected to it plus the ITU in RX to reieve those streams to then forward them to

the apropriate SEMS. Then they forward that onto the cable plant where the 3750 is.

My problem is the 3750 is not dispatching these stream properly. It broadcasts the multicast to

every other ports in the vlan. so all the 3 sems recieves the same multicast stream instead of

the one only for them. thats my main issue atm and Im over subscibing my SMES because this 3750

isnt multicasting like i want it too hehe.