Multicast

INTERNETWORK EXPERT .ORG

** IP Multicast NOTES **

IGMP v2 MRT field = Message Response Time.  Set in a group specific query by the router.

MRT tells the hosts the time interval to provide Membership Reports.  The MRT can be set differently per multicast group.  If you have a large number of hosts joining different groups you can use MRT varations to limits simultaneous bursts of IGMP from the hosts and spread it out per group.

IGMP general group query messages (sent by the router to hosts) contains 0.0.0.0 and is sent by default every 125 seconds.

IGMP queries are sent to 224.0.0.1 (the all multicast hosts address)

IGMP Membership Reports are sent by the hosts on a LAN segment, reporting to the router which multicast groups they are listening for.

The main advantage of IGMPv2 or v1 is faster leaves.  IGMPv2 hosts send leave messages, followed by the router sending a Group Specific query, both of which are not possible with IGMPv1.

IGMP Leave messages are sent to the “All multicast routers” address 224.0.0.2, MAC 01-00-5E-00-00-02

IGMP snooping helps switches control distribution of multicast traffic on ports where multicast hosts are connected, but it does not help switches control distribution of multicast traffic on ports where multicast routers are connected.

RGMP optimizes multicast on router ports.

RGMP is not compatible with CGMP.

CGMP is silently disabled when RGMP is turned on, and vice versa.

When a switch receives an RGMP hello (224.0.0.25) it stops forwarding all multicast traffic on that port.

When a switch receives an RGMP Join it forwards multicast traffic for the group specified in the join message.

When a switch receives an RGMP Leave it stop forwarding multicast traffic the group specified.

When a switch receives an RGMP Bye it starts forwarding all multicast traffic on that router port.

Dense mode multicast routing protocols start by flooding multicast traffic out all interfaces away from the source, and wait for downstream routers to prune themselves off (ask not to receive the traffic)

Dense mode protocols:  DVMRP, PIM-DM, MOSPF

Sparse mode multicast routing waits for downstream routers to ask for the traffic (Join) before sending the traffic out an interface.

When scoping multicast with TTL scoping, the TTL value of the packet must be equal to or greater than the configured TTL threshold of the interface to forwarded.

The default TTL threshold is 0.

Administrative scoping is when you apply and ACL that blocks private range mcast 239.0.0.0 – 239.255.255.255 from entering or exiting an interface.