IGMP, SSM, Bidirectional PIM and MSDP

What it is

These four multicast tools handle how receivers join, how trees are shaped and how rendezvous points share information.

IGMP is how a host tells its local router that it wants a group. Source specific multicast (SSM) and bidirectional PIM are two variations of PIM that change how trees are built. The Multicast Source Discovery Protocol (MSDP) links rendezvous points so that they know about each other's sources.

How it works

Each tool solves one problem that basic PIM sparse mode leaves open, and the examples use IOS XE.

IGMP

The Internet Group Management Protocol runs between hosts and their local router. It lets a host join and leave groups.

A router sends general queries to 224.0.0.1 to ask which groups are in use, and hosts answer with reports. If several routers share a segment, one is elected as the querier, and it is the one with the lowest IP address. IGMPv2 added a leave message so that the router can stop forwarding quickly.

IGMPv3 lets a host name the sources it wants, or the sources it does not want. This is what makes SSM possible.

R1(config)# interface GigabitEthernet0/0
R1(config-if)# ip igmp version 3
R1# show ip igmp groups
R1# show ip igmp interface GigabitEthernet0/0

On a switch, IGMP snooping listens to the reports and sends the group traffic only to the ports that need it. Without it, a switch floods multicast to every port in the VLAN.

Source specific multicast

In any source multicast, a receiver joins a group and receives from any source. SSM lets the receiver join a specific source and group pair, written (S, G). The default SSM range is 232.0.0.0/8.

SSM has clear benefits.

• No RP is needed, because there is no shared tree.
• Traffic from unwanted sources is not delivered, even if they use the same group.
• The trees are simpler and the design is easier to secure.

Receivers normally use IGMPv3 so that they can name the source. The routers need SSM turned on.

R1(config)# ip pim ssm default
R1(config)# interface GigabitEthernet0/0
R1(config-if)# ip pim sparse-mode

The default keyword uses the 232.0.0.0/8 range.

Bidirectional PIM

Bidirectional PIM (bidir) is a form of sparse mode that uses only a shared tree. Traffic flows both up toward the RP and down to the receivers on the same tree. There are no source trees and no register messages, so routers keep only (*, G) state.

This suits applications with many sources and many receivers, since the state does not grow with the number of sources. A designated forwarder on each link decides which router sends traffic up toward the RP.

R1(config)# ip pim rp-address 192.168.0.1 bidir

The bidir keyword marks the range that the RP serves as bidirectional. All routers need to agree on this, and the platform must support the feature.

MSDP

MSDP connects rendezvous points. It lets RPs in different PIM domains, or several RPs in one domain, learn which sources are active.

An RP sends a Source Active (SA) message to its MSDP peers when it learns about a new source. MSDP peers use a TCP connection on port 639.

R1(config)# ip msdp peer 10.2.2.2 connect-source Loopback0
R1# show ip msdp peer
R1# show ip msdp sa-cache

MSDP is often paired with anycast RP. Several RPs use the same loopback address, and each router reaches the nearest one. The RPs use MSDP to share the sources they know about, so any RP can serve any receiver.

Comparing the options