Enabling cdp on GRE tunnel..

Unanswered Question
Apr 13th, 2007


I am issuing the command "cdp enable" on tunnel interface of routers. On some of the routers its accepting the command and I can see it in the "sh run" but on some of the routers its giving the error "% CDP is not supported on this interface, or for this encapsulation". I have checked the encapsulation on every gre tunnel interface and its "tunnel" no matter cdp can be enabled or no. Pls help me to know why am I getting this error. Thanks.

I have this problem too.
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Paolo Bevilacqua Sat, 04/14/2007 - 02:57

Sorry, you can't have CDP on GRE.

CDP is not IP hence cannot be carried by GRE.

Hope this helps, if so please rate post!

Edison Ortiz Sat, 04/14/2007 - 15:38

CDP only runs over Layer2 interfaces, in other words, physical interfaces.

A GRE tunnel is a logical interface hence the lack of support for this feature/service.

minumathur Sun, 04/15/2007 - 13:34


i agree with your point, i have check up in 7200 serial router , in tunnel configuration, cdp enable is there , but unfortunely my envirement is live , so i could not try too much , but atleast i can say in 7200 series router, this feature is available, may be other router do not have this feature.

i hope this will help , please rate this post.


Paolo Bevilacqua Sun, 04/15/2007 - 14:24

The fact that you can configure cdp under a tunnel interface does not mean it will work, for the reasons explained above.

In IOS there are many commands that are configurable, but without effect and no warning is given.

Richard Burts Sun, 04/15/2007 - 16:12


Your logic sounds convincing and used to be true but is in fact not correct any longer. It is a release dependent behavior. I have a lot of routers running 12.3T code and 12.4 code that are successfully running CDP over GRE tunnels. I am confident that if Syed will look at the routers where it works (is accepted) and the ones where it does not work (is not accepted) that he will find differences in the versions of code.

I speak from direct personal experience when I say that in at least some versions of code that CDP does work over GRE tunnels.



royalblues Mon, 04/16/2007 - 01:18

Rick is right.

I did a quick test to find this out.

The CDP does not show any information when i run a GRE tunnel between a ISR and a 2621XM router. There is no support for the smae

MS_VTR_PUN_R01(config)#int tunnel 3


MS_VTR_PUN_R01(config-if)#cdp en

MS_VTR_PUN_R01(config-if)#cdp enable

% CDP is not supported on this interface, or for this encapsulation

But when i tried the same between 2 ISR 2811 routers it does work

MS_M6_MPLS_TATA#sh cdp neighbors

Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge

S - Switch, H - Host, I - IGMP, r - Repeater

Device ID Local Intrfce Holdtme Capability Platform Port ID

M5 Tunnel0 176 R S I 2811 Tunnel0

Interface are shown as tunnel 0


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Paolo Bevilacqua Mon, 04/16/2007 - 02:38

Good to know, so it appears this feature has been added at some point in IOS. Certainly it does not depend by the hardware.

Thanks to those that have pointed out and tried.

royalblues Mon, 04/16/2007 - 06:47

Thanks harold.

Ya i got to know that only when tried configuring it


Harold Ritter Mon, 04/16/2007 - 06:34

Before CSCec01500, configuring CDP on a tunnel interface would lead to the following message:

RouterC(config-if)#cdp enable

% CDP is not supported on this interface, or for this encapsulation

Hope this helps,


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