How SSM help with Type-3 route (IMET)

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SCENARIO — 3 LEAF FABRIC


   Spine1 (PIM-SSM)
     /      |       \
Leaf1    Leaf2    Leaf3
(VTEP1) (VTEP2)  (VTEP3)


Participating VTEPs in VNI 10010

Leaf1 → YES

Leaf2 → YES

Leaf3 → NO (this leaf does not have VLAN 10/VNI 10010)

Assigned multicast group:

G = 232.1.1.10


Step 1 — Each VTEP advertises IMET Route-Type 3

Leaf1 advertises:

IMET: VNI 10010, mcast 232.1.1.10, VTEP=10.1.1.1

Leaf2 advertises:

IMET: VNI 10010, mcast 232.1.1.10, VTEP=10.1.1.2

Leaf3 DOES NOT advertise IMET for VNI 10010

because it has no VLAN/VNI for it.


Step 2 — BGP delivers IMET routes to all VTEPs

All VTEPs (Leaf1/Leaf2/Leaf3) receive:

2 IMET RT-3 routes(one from Leaf1, one from Leaf2)

BUT receiving IMET ≠ joining multicast.


Step 3 — How each leaf behaves

1. Leaf1

Has VLAN10 → YES participates

Receives IMET from Leaf2

→ Joins (S=10.1.1.2, G=232.1.1.10)

2. Leaf2

Has VLAN10 → YES participates

Receives IMET from Leaf1

→ Joins (S=10.1.1.1, G=232.1.1.10)

3. Leaf3

Does NOT have VLAN10 → does NOT participate

Receives IMET from Leaf1 & Leaf2 (just stored in BGP table)

→ No IGMP Join

→ No PIM state

→ No multicast traffic delivered


 BIG RULE (IMPORTANT)

A VTEP only joins multicast for the VNIs defined under its NVE interface.

Config example:

interface nve1

  member vni 10010 mcast-group 232.1.1.10


If Leaf3 does NOT have this line:

It never sends IGMP joins

It never processes BUM of VNI 10010

It is NOT in the flood list

Even though IMET routes appear in BGP, they are ignored for flood list construction.


Step 4 — PIM-SSM State on Spine1

Output of show ip mroute (PIM-SSM only)

Spine1# show ip mroute 232.1.1.10

(10.1.1.1, 232.1.1.10), uptime 00:01:02, flags: S

  Incoming interface: Eth1/1 (toward Leaf1)

  Outgoing interface list:

    Eth1/2 (to Leaf2)

(10.1.1.2, 232.1.1.10), uptime 00:00:58, flags: S

  Incoming interface: Eth1/2 (toward Leaf2)

  Outgoing interface list:

    Eth1/1 (to Leaf1)


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