cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
403
Views
5
Helpful
3
Replies

can this one-armed router cause multicast failure?

ewang
Level 1
Level 1

pls look at diagram attached.

Router A and B are branch routers. B is PIM DR on the LAN becuase it has higher IP 10.36.200.3. There is only sparse-mode running and RP is WAN2 across the WAN links.

1. Normally, multicast tree is built from router B to WAN2(RP)

2. But if link B goes down, router B must build tree from B, A, WAN1, core-rtr1 then WAN2

3. however would this ever work? because when link B goes down, router B is one-armed with only interface F1/0 connected. But I was told interface F1/0 can't be incoming and outgoing interfaces at the same time. As a result, the tree can never be set up.

is it true?

3. if true, does it mean this design is flawed for multicast?

4. remedy?

a backup link between router A and B via interface F1/1?

Thus router B can build tree via interface F1/1 which also becomes incoming int while F1/0 is outgoing interface

3 Replies 3

Harold Ritter
Cisco Employee
Cisco Employee

This design is perfectly valid even without the additional link between router A and B (F1/1 to F1/1).

If you loose link B, router B remains the DR and sends a PIM join towards router A as it receives the IGMP report message from the receiver. Router A puts F1/0 in the OIList for the specific group and therefore the receiver gets the multicast stream directly from Router A. As you stated router B doesn't put F1/0 in its OIList as this interface is also the incoming interface (rpf interface).

Hope this helps,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

"router B doesn't put F1/0 in its OIList as this interface is also the incoming interface (rpf interface)"

1. does it imply router B would "refuse" to build the multicast tree or not?

2. in short, should this design work for multicast when link B goes down?

1. Router B doesn't need to build the mcast tree since it is not in the forwarding path. It simply needs to send the PIM join to router A when it gets the IGMP report message from the mcast listener (aka mcast receiver).

2. As I already stated, this design is perfectly valid even when link B goes down.

Hope this helps,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Getting Started

Find answers to your questions by entering keywords or phrases in the Search bar above. New here? Use these resources to familiarize yourself with the community:

Innovations in Cisco Full Stack Observability - A new webinar from Cisco