Could anyone help me with Cisco 4948 config? I have to route multicast traffic from one Vlan to another. Multicast source and multicast destination are IGMP-enabled devices, there are no PIM-devices in the network. Can this task be solved with such hardware?

1 Answer 1


The 4948 is a very capable platform and can definitely route IP multicast between subnets. Take a look at this configuration guide to get yourself started. It's fairly comprehensive and you'll want to focus on the section about PIM with a static RP ("Configuring a Single Static RP"). You'll generally be doing the following:

  1. Globally enable multicast routing ("ip multicast routing").
  2. Set up an RP - "ip pim rp x.y.z.q" If you don't have an RP in your network just set up a loopback interface on your switch and use its address.
  3. On each L3 interface (SVI, routed switchport) you want to support multicast routing enter "ip pim sparse-mode"

That's pretty much it. There are some other considerations if you want to route the multicast to hosts on other switches or if you want to support SSM but this should get you started with basic multicast routing.

  • Now i am thinking about filtering multicast traffic. As I understand, multicast routing works in both ways if muticast routing is globally enabled. Is it a common practice to filter groups with ip access-lists or is there some other filtering tool more suitable for multicast routing?
    – user32907
    Dec 21, 2016 at 12:24
  • There are a number of means of filtering/limiting multicast traffic depending on what you're trying to accomplish. One common method is administrative scoping, which allows a given range to be set for strictly local to the LAN, another as going no further than the local site, no further than the company, no further than a country and then global (the number of ranges is arbitrary - just an example). If you're using IGMPv3 and SSM each join can have an explicit lists of permitted and rejected sources. Describe what you'd like to achieve and we can suggests options.
    – rnxrx
    Dec 21, 2016 at 15:50
  • My topology consists of thee segments connected to one Cisco switch. Vlan100, vlan101 aggregate m-cast sources (DVB-S tuners, IGMPv2, groups 239.131.x.x), Cisco has to route this traffic to vlan200, which has only receivers connected to it. 1st scenario: m-cast traffic forwarding only in one direction from vlan1xx to vlan200, any group originated from vlan200 should not pass to vlan1xx. 2nd scenario: specific group forwarding from vlan300 ( to vlan100. Of course I'm not waiting for detailed solution. I'll be glad to get just an advice which tool is more suitable in my case.
    – user32907
    Dec 22, 2016 at 16:01
  • So the first question would be whether filtering is really needed? Unless something explicitly sends a join request from a given VLAN (or even port) then traffic won't be flooded. If the concern is security then using IGMPv2 your options may just be a simple ACL - deny any IP to the group (or range) in question applied outbound to the SVI's to be secured.
    – rnxrx
    Dec 23, 2016 at 2:10

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