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Any router will automatically join at least two multicast groups on each interface. How do I see that on a Cisco router?

I know that they all join these two groups: all multicast nodes and all routers.

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  • The answer is show ip interface type/number. I need write a script that shows what multcast groups an interface joined..neah that works if a routing protocol is active...still looking for an answer – MiniMe Jun 18 '16 at 19:06
  • You know that all the interfaces listen to those multicast groups. They don't really join them in the sense of IGMP/PIM/multicast routing. Real joining tells a multicast router that you wish to join the group. I'm not clear on why you are needing to see that information since it is guaranteed if IPv4 is enabled. You will see the multicast group(s) for a routing protocol which uses multicast (not all do), but that is not something that is guaranteed to be on an interface. – Ron Maupin♦ Jun 18 '16 at 19:29
  • I knew that a router will listen to these group addresses and I was also looking for the same info (command line) for IPv6. – MiniMe Jun 18 '16 at 21:07
  • That's easy: show ipv6 interface type/number. IPv6 uses a lot of multicast addresses because it doesn't have broadcast. – Ron Maupin♦ Jun 18 '16 at 21:15
  • Also, understand that IPv6 multicast addresses have flags and scopes, some combinations are invalid, and some combinations are required. – Ron Maupin♦ Jun 18 '16 at 21:18
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I knew that a router will listen to these group addresses and I was also looking for the same info (command line) for IPv6.

That's easy: show ipv6 interfaces <type/number>. IPv6 may use a lot of multicast addresses because it doesn't have broadcast.

For example:

Router1#sh ipv6 interface
GigabitEthernet0/0 is administratively up, line protocol is up
  IPv6 is enabled, link-local address is FE80::1 
  Global unicast address(es):
    2001:DB8:ABCD:EF::2, subnet is 2001:DB8:ABCD:EF::/64 
  Joined group address(es):
    FF02::1
    FF02::2
    FF02::1:FF00:2
  MTU is 1500 bytes
...
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  • should I be able to see these joined groups as MAC groups? Because when I enter "show mac-address-table", I can't see the correspondant MAC groups. – AhmedWas Nov 15 '18 at 9:17
  • Broadcast and multicast (anything with the I/G bit set in the address) are not tied to a single interface, and are not used as source addresses, so those addresses are not going to be in the switch MAC address table. Some switches use IGMP snooping, and there will be a table for those multicast addresses, not including things like link-local multicast addresses. – Ron Maupin♦ Nov 15 '18 at 16:57

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