PIM-SM - Encap & Decap tunnel

[Andreseo90]

Hello everyone,

I have a question regarding the automatically created tunnels for encapsulating and decapsulating PIM registers. Tunnel 0 is automatically established between the First Hop Router (FHR) and the Rendezvous Point (RP), particularly when I configure the RP statically.

However, when I statically configure the RP on the RP router, both tunnel 0 and tunnel 1 are created. Up to this point, there is nothing unusual, but when I check the configuration of tunnel 0, both the source and destination IPs are that of the RP.

RTR-lab#show derived-config int tu 0
Building configuration...

Derived configuration : 189 bytes
!
interface Tunnel0
description Pim Register Tunnel (Encap) for RP 10.0.X.Y
ip unnumbered Loopback1 --> 10.0.X.Y
tunnel source Loopback1 --> 10.0.X.Y
tunnel destination 10.0.X.Y

My questions are: Is this tunnel automatically created by the OS code as it does not differentiate between an RP router and an FHR?  Will this tunnel never be used since this router is the RP itself?

Tunnel 1 on the other hand is a Lo0 interface that does not have PIM enabled. Why is this?

RTR-lab#show derived-config int tu 1
Building configuration...

Derived configuration : 189 bytes
!
interface Tunnel1
description Pim Register Tunnel (Decap) for RP 10.0.X.Y
ip unnumbered Loopback0
tunnel source Loopback0
tunnel destination 10.0.X.Y

Thanks,

[MHM Cisco World]

Yes this tunnel is automatic add when config RP of SM PIM

***Use command in below guide to check tunnel.

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipmulti_pim/configuration/xe-16/imc-pim-xe-16-book/imc-auto-mlt-tun.html

MHM

[Andreseo90]

Hello,

Thanks for your answer.

Yes, but what intrigues me is that the encapsulation tunnel is also created on the router that assumes the role of RP, with both the source and destination interfaces having the same IP. I would like to understand why a tunnel is created towards itself.

Thanks, 

[paul driver]

Hello
Even though the logical tunnel comes up it wont be used for any encapsulation or routing of traffic, because the source/destination addressing are the same, in fact it should come up even without any ip address defined on the tunnel 

[Andreseo90]

Are you suggesting that the RP (Rendezvous Point) creates this tunnel simply because it is required to do so, and not necessarily because it serves a practical purpose? My understanding is that it is created as part of the IOS code, but I couldn't find any specific mention of this in the documentation. Thanks 

[paul driver]

Hello

---

@Andreseo90 wrote:

the RP router has its loopback interface, which is also the RP's IP, as both source and destination in Tunnel0. It seems odd to see it as both source and destination. Why is it created if it has no utility? Because  it is part of the IOS code? Besides Tunnel0, the RP also has Tunnel1 for decapsulation.

TBH I am not sure what your asking, you show a tunnel with the same SIP/DIP and mention MC RP, even though the tunnel has no pim applied to it.  

As to why the rtr you mention has a tunnel interface with the same SIP/DIP, I cannot answer, My understanding is no MC configuration I am aware of automatically creates logical tunnel interfaces in this manner, As for pim-sm RP'S these are either specified either statically or automatically  (BSRs for example)

[MHM Cisco World]

this in router you config RP in it? 
check other router in multicast domain and it need to point to your RP Lo as destination

MHM

[Andreseo90]

All routers that are part of the multicast network have Tunnel0 (Encapsulation) with the RP's IP as the destination and the nearest physical interface with active PIM to the RP as the source. However, the RP router has its loopback interface, which is also the RP's IP, as both source and destination in Tunnel0. It seems odd to see it as both source and destination. Why is it created if it has no utility? Because  it is part of the IOS code? Besides Tunnel0, the RP also has Tunnel1 for decapsulation.

Thanks

[MHM Cisco World]

It normal I think

All other routers have tunnels back to RP' and router that have RP have tunnel if it have any interface with ip igmp enable (i.e. it also have some multicast reciever).

MHM

[Andreseo90]

The tunnel is established between the FHR and the RP, so it wouldn't make much sense for this tunnel to also be created on the RP towards itself. Does it create it automatically even though it is never used, just because it's in the code?

[paul driver]

Hello
Not sure what you are asking here, your OP shows Only a creation of a GRE tunnel it does not show any PIM configuration just an interface description it?

[Andreseo90]

Hello,

The PIM configuration works fine; what intrigues me is that on the RP, the encapsulation tunnel is created with the same source and destination IP. The encapsulation and decapsulation tunnels are automatically created when I configure the RP address.

Thanks