Multicast packets in unicast network

Answered Question
Nov 5th, 2009

Hello,

What will happen if multicast packet comes to router which doesn't have multicast enabled?

1) Multicast sender sends packet with its own unicast source address and destination multicast group address (this is correct?)

1a) Will router automaticly drop packet

1b) Will it treat it as a broadcast address?

2) From receiver to sender only goes unicast packets with unicast source and destination address , is this correct?

If IGMP join message comes to router that doesn't have multicast enabled will it:

2a) drop or

2b) broadcast

this packet?

3) And on layer 2 network that doesn't have multicast enabled, multicast packets/frames will be treated as broadcast, because sender will automaticlly send frame with multicast MAC destination address. Is this correct?

regards,

A.

I have this problem too.
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Correct Answer by Giuseppe Larosa about 7 years 2 months ago

Hello Antonio,

a non multicast enabled router uses link local multicast addresses like 224.0.0.5 and 224.0.0.6 for OSPF but it cannot route multicast packets and ignores IGMP reports.

1) drop

2) drop

3) here some caution: if no multicast capable router is present and IGMP snooping is enabled multicast traffic is blocked.

some switches platforms allow to configure one switch as the igmp snooping querier and it will perform the IGMP queries like it was a multicast enabled router.

alternatively to this, IGMP snooping has to be disabled.

this can be done on a per vlan basis

Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Thu, 11/05/2009 - 04:51

Hello Antonio,

a non multicast enabled router uses link local multicast addresses like 224.0.0.5 and 224.0.0.6 for OSPF but it cannot route multicast packets and ignores IGMP reports.

1) drop

2) drop

3) here some caution: if no multicast capable router is present and IGMP snooping is enabled multicast traffic is blocked.

some switches platforms allow to configure one switch as the igmp snooping querier and it will perform the IGMP queries like it was a multicast enabled router.

alternatively to this, IGMP snooping has to be disabled.

this can be done on a per vlan basis

Hope to help

Giuseppe

Joseph W. Doherty Thu, 11/05/2009 - 05:39

Just to expand a little on what Giuseppe describes for #3. . .

Switches without some kind of mutlicast control switches (or hubs) will forward multicast much like broadcast (as also mentioned in the OP). Also much like broacast, without routing support, they will not normally pass through a router.

Unlike broadcast, although host NICs will see all multicast packets (again assuming they are not suppressed by a switch), the host NIC will ignore multicast packets it's not interested in (much like unicast seen on hub port but without the host's address). (NB: IP mulitcast maps into blocks of Ethernet multicast, so Ethernet NICs can accept some muliticast they don't want [also again much like broadcast].)

CriscoSystems Thu, 11/05/2009 - 15:08

A switch will just flood a multicast frame, since the mcast address was never anyone's source it won't have an entry in its ARP table; right?

Or, does the switch notice the 0100.5E in the MAC address, determine from that that it's multicast, and drop it?

Giuseppe Larosa Fri, 11/06/2009 - 06:35

Hello Stuey,

without IGMP snooping that is a forwarding optimization tecnique that tries to build a list of L2 ports for each multicast group (to save BW), multicast frames are treated as broadcast.

Antonio's question about a network without multicast routers leads to this note because IGMP snooping is enabled by default on modern cisco switches.

so possible issues arise in a network without multicast routers but with IGMP snooping enabled on switches.

Note:

this can be seen as another reason why IGMP queries made by routers are necessary that you had asked in another thread.

Hope to help

Giuseppe

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