Describe

3.3.d Describe multicast protocols, such as RPF check, PIM SM, IGMP v2/v3, SSM, bidir, and MSDP

5 min read ENCOR 350-401 v1.2

Aligned to Cisco's 350-401 ENCOR v1.2 exam topics.

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Multicast is one-to-many traffic. Instead of sending one copy of a packet to every receiver, the source sends traffic to a multicast group address. The network replicates packets only where receivers exist.

The hard part is not the definition. The hard part is remembering that multicast forwarding is built around trees, receiver interest, and reverse-path checks.

Multicast roles

Role	Meaning
Source	Sends traffic to a multicast group.
Receiver	Wants traffic for a multicast group.
Group	Multicast destination address, usually in 224.0.0.0/4 for IPv4.
First-hop router	Router near the source.
Last-hop router	Router near receivers.
Rendezvous Point	Shared meeting point used by PIM Sparse Mode.

For labs, do not use 224.0.0.x as an arbitrary application multicast group. 224.0.0.0/24 is the local-network control block and many addresses in it are reserved for routing and control protocols. Use administratively scoped 239.0.0.0/8 for generic PIM-SM labs, or 232.0.0.0/8 when practicing SSM.

Source S -> PIM register -> RP -> (*,G) shared tree -> Receivers · IGMP join toward RP

SSM skips the RP because receivers join a specific (S,G) from the start.

IGMP: hosts tell routers they want traffic

Internet Group Management Protocol runs between hosts and local routers.

IGMPv2 lets hosts join and leave multicast groups.
IGMPv3 adds source-specific joins, which are important for SSM.

On a LAN, the receiver sends an IGMP membership report. The last-hop router learns that someone on that interface wants group traffic.

Verify:

show ip igmp groups
show ip igmp interface

PIM: routers build multicast trees

Protocol Independent Multicast does not discover unicast routes by itself. It uses the unicast routing table to make multicast forwarding decisions.

Common modes:

Mode	Purpose
PIM Sparse Mode	Assumes receivers are sparse. Uses an RP for shared tree setup.
PIM SSM	Source Specific Multicast. Receivers ask for a specific source and group.
Bidirectional PIM	Efficient many-to-many multicast with shared bidirectional trees.

Enable multicast routing and PIM on participating interfaces:

ip multicast-routing
!
interface GigabitEthernet0/0
 ip pim sparse-mode
!
interface GigabitEthernet0/1
 ip pim sparse-mode

Verify:

show ip pim neighbor
show ip mroute
show ip rpf <source-ip>

RPF check

Reverse Path Forwarding is the safety check that prevents multicast loops.

When a router receives multicast traffic from source S, it checks the unicast routing table and asks:

Would I use the interface this packet arrived on to reach source S?

If yes, the RPF check passes. If no, the packet is dropped.

This is one of the most important multicast troubleshooting ideas.

Verify:

show ip rpf 10.1.1.10
show ip route 10.1.1.10
show ip mroute

If RPF fails, fix unicast routing or use an appropriate multicast routing feature. Do not guess at PIM first.

PIM Sparse Mode with an RP

Sparse Mode uses a Rendezvous Point. Receivers join toward the RP. Sources register with the RP. The network can then build multicast forwarding state.

Simple static RP example:

ip multicast-routing
ip pim rp-address 10.255.255.1
!
interface gi0/0
 ip pim sparse-mode
!
interface gi0/1
 ip pim sparse-mode

Every router in the PIM domain must agree on the RP for the group range.

Verify:

show ip pim rp mapping
show ip pim neighbor
show ip mroute

SSM

Source Specific Multicast uses a receiver request that includes both source and group: (S,G).

Typical IPv4 SSM group range is 232.0.0.0/8.

SSM does not need an RP because the receiver already names the source.

Basic configuration:

ip multicast-routing
ip pim ssm default
!
interface gi0/0
 ip pim sparse-mode

Verify:

show ip mroute 232.1.1.1
show ip igmp groups

IGMPv3 is important because hosts need to express source-specific interest.

Bidirectional PIM

Bidirectional PIM is used for many-to-many multicast where multiple sources and receivers share a group. Traffic flows on a shared bidirectional tree rooted at the RP. It can reduce state in certain designs.

You do not need deep configuration for ENCOR, but know the concept:

PIM SM commonly builds source-specific shortest-path trees after initial RP use.
Bidir PIM keeps traffic on a shared bidirectional tree.
Bidir is useful when many sources send to the same groups.

MSDP

Multicast Source Discovery Protocol lets RPs in different PIM domains share information about active sources.

Key idea:

An RP learns about a local source.
It sends Source Active messages to MSDP peers.
Remote RPs can learn that the source exists.

MSDP is commonly associated with interdomain or multi-RP multicast designs for IPv4 PIM Sparse Mode.

Verify conceptually with commands like:

show ip msdp peer
show ip msdp sa-cache

Lab: PIM Sparse Mode with static RP

Topology

Source
R1
R2
R3
Receiver
RP Lo0

RP loopback: 10.255.255.1. Source: 10.1.1.10. Group: 239.1.1.1.

The 239.1.1.1 choice is intentional: it avoids the local-control 224.0.0.0/24 range and keeps the lab in administratively scoped IPv4 multicast space.

Task 1: Enable multicast and PIM

On all routers:

ip multicast-routing
ip pim rp-address 10.255.255.1

On interfaces in the multicast path:

interface gi0/0
 ip pim sparse-mode

Task 2: Verify PIM neighbors

show ip pim neighbor
show ip pim rp mapping

Task 3: Join from receiver LAN

On a router interface or test receiver, create group interest. Depending on lab platform, you might use an IGMP join feature or a test host application.

Check last-hop router:

show ip igmp groups
show ip mroute 239.1.1.1

Task 4: Send multicast traffic

Start traffic from the source to 239.1.1.1. Then verify:

show ip mroute 239.1.1.1
show ip rpf 10.1.1.10

You should see multicast forwarding state and a passing RPF path toward the source.

Troubleshooting order

Does unicast routing work to the source and RP?
Are PIM neighbors formed on router-to-router links?
Do all routers agree on the RP?
Does the last-hop router see IGMP group membership?
Does RPF pass for the source?
Is the multicast route in show ip mroute?
Is traffic actually being generated by the source?

Exam traps

PIM depends on the unicast routing table.
RPF checks the path back to the source, not the path toward receivers.
IGMP is host-to-router; PIM is router-to-router.
PIM Sparse Mode needs an RP unless using SSM.
SSM uses (S,G) and avoids RP dependence.
MSDP shares active source information between RPs.

Quick checklist

Is multicast routing enabled?
Are PIM interfaces enabled and neighbors up?
Is the RP known and consistent?
Is IGMP membership present on receiver LANs?
Does RPF pass toward the source?
Does show ip mroute contain the expected (*,G) or (S,G) state?