Disable HDLC

Unanswered Question
Feb 27th, 2009

We have a cisco router in another location that has both ARPA and HDLC entries. We don't use HDLC and we don't need it at all. How do I get rid of these sessions and disable them from reoccurring.

They are only coming in on serial0/0 in the interface, there is nothing to do with HDLC anywhere. I went into that interface and used the "no encapsulation HDLC" command. This didn't solve the problem.

I would like to avoid blocking specific IPs/MACs. Is there a way to completely disable HDLC on the router itself.

Any Help is appreciated.

I have this problem too.
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Giuseppe Larosa Fri, 02/27/2009 - 09:22

Hello Ross,

HDLC is a point-to-point serial encapsulation that can be used on a serial interface. Actually Cisco implements a proprietary version that includes a protocol field.

What is unclear is where you see mixed ARPA entries and HDLC entries

and you speak of IP/MACs.

You probably have some form of bridging over the WAN serial interface I don't see other reason to learn MAC addresses on a serial interface.

So probably if this is not needed anymore you need to remove commands like

bridge-group 1

under the serial interface configuration

But be careful if bridging over serial was configured was needed in the past.

Hope to help


grungerokker13 Fri, 02/27/2009 - 11:23

Sorry I left that part out. I found those entries in with the show arp command.

Giuseppe Larosa Fri, 02/27/2009 - 11:49

Hello Ross,

so you mean that in a sh arp you see entries with encapsulation HDLC ?

I've never seen this

May I ask you to post a filtered version of the configuration of your router ?

What plaform ? IOS version ?

I may be missing something but you can get better help if you provide us more details

Hope to help


CriscoSystems Fri, 02/27/2009 - 13:12

Also I don't believe "no encapsulation hdlc" is the exact command you want to be using here; it's best to specifically enable a different form of serial encaps (be it frame, PPP, or whichever).

Cisco's proprietary HDLC is the default encapsulation type for their serial interfaces, so if you use a "no encapsulation {anything}" command, the interface will default back to HDLC encapsulation, no matter which encaps type you were trying to disable.


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