Fundimental of multicast

In order to make multicast work we need to study the following 3 areas:

1, multicast address (ip and mac)
2, IGMP protocal (to make host join or leave a multicast group)
3, IP mulicast routing protocal (PIM is mostly used)

1, Multicast address

IP multicast address - are all multicast address, especially The All Hosts multicast group addresses all hosts on the same network segment. The All Routers multicast group addresses all routers on the same network segment. This address is used in the Distance Vector Multicast Routing Protocol (DVMRP) to address multicast routers. The Open Shortest Path First (OSPF) All OSPF Routers address is used to send Hello packets to all OSPF routers on a network segment. The OSPF All Designated Routers “”(DR)”” address is used to send OSPF routing information to designated routers on a network segment. The Routing Information Protocol (RIP) version 2 group address is used to send routing information to all RIP2-aware routers on a network segment. The Enhanced Interior Gateway Routing Protocol (EIGRP) group address is used to send routing information to all EIGRP routers on a network segment. Protocol Independent Multicast (PIM) Version 2 Virtual Router Redundancy Protocol (VRRP)–21 IS-IS over IP Internet Group Management Protocol (IGMP) version 3[2] Hot Standby Router Protocol version 2 (HSRPv2) / Gateway Load Balancing Protocol (GLBP)

MAC multicast address range of 01-00-5E-00-00-00 to 01-00-5E-7F-FF-FF
The limitation of number of MAC multicast address require that MAC to IP map for multicast is 32/2^5 to 1 maping. especially, the last 23 bits of IP or MAC address will be match.


IGMP will be used to make host able to join and leave multicast group
IGMP v1: to types of message : membership query and membership report
especially, router will send out membership query towards multicast ip And host will send membership report towards the mulicast ip that host want to join.
Once router receives membership report from host via interface x, router will add interface x into the mroute table. Mroute table item can be timeout after, for example, 3 mins, if host does not want to join multicast group any longer host will stop sending membership report.The router will keep forwarding multicast traffic to the hosts until the timer expires.
IGMP general queries are destined to the address (all systems on a subnet). And IGMPv1 has not leave report.

IGMP v2:
Host is able to send membership leave message instead of letting multicast timeout in the router. Memembership leave message will be sent to general multicast ip
When hostx send membership leave message to router, router will issue a specific membership query message towards the interface where hostx sent leave message. This specific memebership query message will be sent towards the specific multicast address instead of generic address.
when there is multiple routers in one network segment, there will be one router with lowest ip selected as active router to send membership query message.
IGMPv2 leave-group messages are destined to the address (all devices on a subnet).

IGMPv3 enhances the leave process by introducing the capability for a host to stop receiving traffic from a particular group, source, or channel in IGMP by including or excluding sources, groups, or channels in IGMPv3 membership reports.
IGMPv3 membership reports are destined to the address; all IGMPv3-capable multicast devices must listen to this address.

IGMP snooping:
This feature will be used in switches, so switches will snoop IGMP traffic between hosts and routers, maintain a mulicast table (CAM table), each item in the table will contain multicast dest MAC adress and interfaces that should get traffic wich that multicast dest MAC.
On the switch, all multicast traffic that is IGMP traffic will be sent to core of switches to process, realtime mulicast traffic (not IGMP traffic) will not be forwarded to switch core, this will reduce the burden of switches core

In the scenario where there is no Router, in order to get multicast work, we can
1, configure one of the svi interface on the switch to send member query message
2, configure svi interface with pim sparse mode
3, use broadcast instead of multicast by disable igmp snoopint
4, configure static mrouter port
5, configure Static multicast entry

3, IP multicast routing protocol

There are 2 mode of multicast dense mode and sparse mode
dense mode:
mulitcast traffic will send to all of the interfaces of the router except for the interface where the traffic come in. If the other routers do not want the traffic (not get membership report message),routers need send ‘no this mutlicast traffic’ back to the orignal router

There are a number of dense mode routing protocols:
DVMRP (Distance Vector Multicast Routing Protocol)
MOSPF (Multicast OSPF)
PIM Dense Mode ( This is mostly used)

PIM Dense mode will do RPF check (reverse path forwarding). router will check the source ip of the multicast traffic, if the source ip A is supposed to come in from portX on the router, the incoming traffic that comes from portY with source ip A will be dropped. The source path check is using unicast routing table, in the case that the reverse path for the source is different with that in the unicast routing table, a static mroute can be configured. RPF check will check static mroute table first, then go to unicast routing table.

PIM sparse mode:
Dense mode floods multicast traffic until a router asks you to stop.
Sparse mode sends multicast traffic only when a router requests
In sparse mode, there will be a RP (Rendezvous Point), router who receives IGMP membership report message will send PIM join message to RP. Routers will find RP by static configuration or by dynamic learning. If no RP configured in the router which only enabled PIM spars mode, the multicast will not function.

For those routers who are not interested in multicast traffic and filter out all multicast traffic on the router.

There are 2 protocols that can be used for dynamic RP configuration:Auto RP and BSR(Bootstrap router). Auto RP is Cisco proprietary protocol and BSR is standard procotol.

When there are more than one RP configured for a group, MSDP (multicast Discovery protocol) will be used to setup tcp session between those RPs, so that they can share the joined member information with each other.

PIM assert mechanism

PIM forwarder vs PIM Designated Router vs Designated forwarder

The DR is the router that will forward the PIM join message from the receiver to the RP (rendezvous point).

These Assert Messages are used to elect the PIM forwarder using the following three rules:

  1. The router generating an Assert with the lowest Administrative distance is elected the forwarder.  The AD would only differ if the routes to R5 where from different routing protocols.  If the Administrative Distances are the same then we move to step 2.
  2. The best unicast routing metric is used to break a tie if the Administrative Distances are the same.  The combination of AD and the unicast routing metric is referred to as a “tuple”. If metrics are the same them we move on to step 3.
  3. The device with the highest IP Address will be elected as the PIM Forwarder.

The PIM forwarder’s job is simply to forward multicast traffic onto a shared segment. It used PIM assert message to select forwarder.

Designated forwarder is the only router on the segment that is allowed to send multicast traffic towards the RP. This is used to solve loop problem in PIM bidirectional multicast where there is no RPF check.

The DF will be elected using the following mechanism:

  • The router will the lowest metric to the RP will be the DF.
  • If the metric is equal then the router with the highest IP address will become the DF.

4, MLD and MLD snooping

According to wikipedia “Multicast Listener Discovery (MLD) is a component of the Internet Protocol Version 6 (IPv6) suite. MLD is used by IPv6 routers for discovering multicast listeners on a directly attached link, much like Internet Group Management Protocol (IGMP) is used in IPv4. The protocol is embedded in ICMPv6 instead of using a separate protocol. MLDv1 is similar to IGMPv2 and MLDv2 similar to IGMPv3. The protocol is described in RFC 3810 which has been updated by RFC 4604.”

MLD uses three types of messages – Query, Report, and Done. Report message is like the join message in IGMP v2. The Done message is like the Leave message in IGMP version 2.It indicates a host no longer wants to receive the multicast transmission. This triggers a Query to check for any more receivers on the segment.

MLD snooping is for IPv6 multicast, like IGMP snooping for IPv4 multicast traffic.

Configuring IPv6 multicast-routing with the global configuration command ipv6 multicast-routing, automatically configures Protocol Independent Multicast (PIM) an all active interfaces. This also includes the automatic configuration of MLD. Here are verifications:

5,Bidirectional RP

Bidirectional RP can be useful for application like video conference where multicast traffic need to be sent downward and upward.

bidirectional has no RPF check. There is a different solution to prevent loops, we will use a DF (Designated Forwarder). This designated forwarder is the only router on the segment that is allowed to send multicast traffic towards the RP. When there is only 1 router per segment that forwards multicast traffic there will be no loops.

The DF will be elected using the following mechanism:

  • The router will the lowest metric to the RP will be the DF.
  • If the metric is equal then the router with the highest IP address will become the DF.

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