Saturday, December 18, 2010

concise Multicast notes CCNA/CCNP/CCIP/CCIE


Multicast :

What is IP Multicast :
 IP multicast is a bandwidth-conserving technology that reduces traffic by simultaneously delivering a single stream of information to potentially thousands of corporate recipients and homes. Applications that take advantage of multicast include video conferencing, corporate communications, distance learning, and distribution of software, stock quotes, and news.
IP multicast delivers application source traffic to multiple receivers without burdening the source or the receivers while using a minimum of network bandwidth. Multicast packets are replicated in the network at the point where paths diverge by Cisco routers enabled with Protocol Independent Multicast (PIM) and other supporting multicast protocols, resulting in the most efficient delivery of data to multiple receivers.

Reverse Path Forwarding (RPF)

PIM uses the unicast routing information to create a distribution tree along the reverse path from the receivers towards the source. The multicast routers then forward packets along the distribution tree from the source to the receivers. RPF is a key concept in multicast forwarding. It enables routers to correctly forward multicast traffic down the distribution tree. RPF makes use of the existing unicast routing table to determine the upstream and downstream neighbors. A router will forward a multicast packet only if it is received on the upstream interface. This RPF check helps to guarantee that the distribution tree will be loop-free.

Multicast operation :

 sparse mode :  shared tree concept . explicit join , no traffic unless you ask for it
employs rendezvous point (RP) to process join request .uses pull technology.

Dense mode : source tree concept .implicit join , all traffic unless you say you want it, uses push technology , flood and prune behavior .

Sparse-Dense : sparse for a group with an RP , dense for all other .


If sparse mode assign a RP ...

to configure Statically  RP use : ip pim rp-address  command
to dynamically assign RP use :  Auto-rp ( cisco proprietary )
                                                   bootstrap router(BSR) pim version 2 (industriy standard )


S,G  :   S  meaning source  ,G meaning group

*.G  : don’t  care about the source if there is an * meaning any source

show ip pim neighbor   command will  multicast neighbor router

shortest path tree  : DR sends uni cast to RP

shared path tree : request goes to RP and come back

multicast uses UDP port


some well-known multicast address :

224.0.0.1  all multicast systems on the subnet

224.0.0.2 all multicast router on the subnet

global multicast address range 224.0.1.0 -238.255.255.255

private range 239.0.0.0  -  239.255.255.255

IGMP internet group management protocol , client use this protocol to join
a multicast group . three version available v1, v2, v3 .

using v2 client can use leave message  to router , so router can drop the multicast flow.

using v3 client can specify by a join message to a multicast enable router which multicast stream they want to listen to meaning the source, by v2 it was not possible .


Cisco group management protocol (CGMP ) router tells to switch about multicast Mac address.

IGMP snooping : IGMP snooping allows switch to see IGMP query message , join message , leave message ,

What is multicast assert message :

with in multicast router if there is two path to a multicast source , client will use one path based on unicast routing table , when that route goes down ,the other router takes over and when the failed router come back online it will send an assert message to replace the backup router .

The assert election criteria are as follow in decreasing order of priority:

1- Administrative distance to the source S (10.10.10.1)

2- Cost of the route to S (10.10.10.1)

3- Highest multicast interface IP address.

When multiple PIM routers are connected to a LAN subnet, they send Assert messages
to determine which router will be the forwarder of the multicast traffic on the LAN.
Both PIMDM and PIM-SM routing protocols use assert messages.

A PIM-DM router sends Prune and Graft messages based on the demand for multicast
group traffic. If nobody wants the group traffic, the PIM-DM router sends a Prune
message to its upstream router. If somebody requests group traffic and the router is not
receiving the traffic from its upstream router, it sends a Graft message to its upstream router.

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