How to check the physical interface(s) used by a tunnel interface

[morris0117]

Greetings,

In my guess, since tunnel interface is a logical interface, it should match one or more physical interfaces. But I tried several commands on a IOS-XE ASR1006 router and there's no output of anyone showing the composition or matching info of this tunnel interface. (please refer to the image attached)

Is my understanding wrong or how can I know which physical interface(s) is used by this tunnel interface?

Thank you very much.

[M02@rt37]

Hello @morris0117 

Please share your image.

[morris0117]

Sorry, I don't know why the image wasn't uploaded. Now it's attached.

[Torbjørn]

The output of "show interface tunnel (number)" will give you the tunnel source interface(where your tunnel traffic logically originates). Which interface will be used for ingress/egress is however determined by regular routing, you will need to inspect your route table to find the entry for the tunnel destination address to see which interface will be used. Alternatively you can use "show ip cef (address)" to find this information.

[morris0117]

Hi Torbjørn,

The attached image below is how I imagine a tunnel. I think what you're talking about is for example I can check R2's tunnel interface info on R1 with a ip address.
But what I want know is how to show which physical interface is assigned as a "tunnel interface", either on R1 or R2. Also thank you for mentioning "tunnel source interface", it confuses me too. On our router, I see the source interface is loopback 0 instead of a physical interface... Is it similar to the concept of L2MPLS VC using the loopback address as the xconnect destination?

Sorry for making it more complex. What I can see is "tunnel 128 is connecting to the customer", but I want to know the real port connecting to the customer.

Thank you all.

[MHM Cisco World]

your topolgy is explain why you confuse 

you have tunnel and you use LO as tunnel source and you have two path to tunnel destination
you want to see which path (which egress interface) the tunnel use 

show ip route <tunnel destination> longest <<- this give you which path router select 

NOTE:- if both path same metric the router use load balance 
MHM 

[morris0117]

Hi MHM,

Sorry, I'm not asking about the path or destination of a tunnel. I'd like to know the physical interface being used by this tunnel.

What I can see from "show run interface tunnel128" is:
ip vrf forwarding xxx
ip address 172.x.x.17 255.255.255.248
ip nhrp map multicast dynamic
ip nhrp network-id xxx
tunnel source Loopback0
tunnel mode gre multipoint
tunnel key xxx
tunnel protection ipsec profile X shared

According to internal reference, the customer router is using 172.x.x.18/29.
So it's configured as a normal interface running vrf and ipsec... but how can I know what the physical interface is?

Thank you.

[MHM Cisco World]

Friend

R1 use loopback as tunnel source 

We need to see which physical interface R1 use to reach R2 LO (which is same as R1 tunnel destination)

We use 

Show ip route <tunnel destination> longest

This give use the egress interface R1 use to reach R2 LO

MHM