What is the voip-null0 interface?

[David_Mitchell]

Hi all,

I have just started in a new company and they have WhatsUp Gold monitoring their network.

We have recieved an email alert for a router voip-null0 interface going down for a couple of minutes and then back up again!

I have searched the web but I can't see what the interface is or does....I assume it has something to do with the routers voice capability but I am not sure what or why we would be monitoring it as an interface.

Any light shed on this topic would be greatly appreciated as always

Thanks

David

[Andrey Chertolyas · Accepted solution]

Hi David,

According to the book "TcL Scripting for Cisco IOS" "VoIP-Null0 is purely cosmetic and will be enabled when Cisco Express Forwarding (CEF) is turned on".

Hope it helps.

Kind regards,
Andrew C.

[David_Mitchell]

Thanks for the swift response Andrew, much appreciated

[Carlo Poggiarelli]

Hi David

voip-null0 is a cosmetic interface and it's automatically enabled  when ip cef is enabled.

HTH

regards

Carlo

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[David_Mitchell]

Thanks for the swift response Carlo, much appreciated

[Ayodeji Okanlawon]

David,

Null0 interface is the graveyard for any packet sent to it. It is a virtual interface and it is where all packets sent to it  meets a horrible death! . Any traffic sent to this interface never comes back alive..To understand it better you can refer to this doc and thread

https://supportforums.cisco.com/docs/DOC-5714

https://learningnetwork.cisco.com/thread/3985

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[David_Mitchell]

Thanks for the comments Aokanlawon,

I understand the concept of a standard Null 0 interface, but my question was referring to the voip-null0 interface which i couldn't see the point of .  I have however had a read of the links you posted and gave myself a little refresher so that was nice

Now I know the voip-null0 interface is cosmetic I am going to remove it as a monitored interface within WhatsUp Gold.  However I still don't know how it failed to respond to icmp ping if it's logical and the router and all other interfaces remained up!

As I understand it this is a one off

Thanks again,

David

[Andrey Chertolyas]

Most likely it wasn't an ICMP-ping issue but SNMP-get issue ) I mean that your monitoring system probably collects the snmp-status of your gateway and reacts on the loss of the voip-null0 interface (due to CEF off/on for example) like you've mentioned.

Kind regards,
Andrew C.