[cisco-voip] Cisco 8851 not failing over to backup circuit...

[Charles Goldsmith]

It may not just be MTU, we had issues with MSS a few years ago with
IPSEC/GRE tunnels and SSL certs.  it was causing fragmentation and SSL
was complaining.

ip tcp adjust-mss 1340 resolved it, that had a bit of buffer room
built in, but it worked, and we applied that to all of our tunnel
interfaces that were encrypted.

Maybe try that, and increase it until it breaks, if it does resolve it?

On Fri, Apr 24, 2015 at 3:18 PM, Jonathan Charles <jonvoip at gmail.com> wrote:
> Cranked the MTU to 1500, no change, dropped it down to 1100, no change...
> they will not register over the backup link... we have confirmed full
> connectivity over this link...
>
>
> Jonathan
>
> On Fri, Apr 24, 2015 at 11:22 AM, Chris Ward (chrward) <chrward at cisco.com>
> wrote:
>>
>> VPN registration issues usually point to MTU issues. Or at least packet or
>> fragments due to MTU issues. I suspect there is a different in packet size
>> during the registration of these two devices or capabilities that affects
>> packet size.
>>
>>
>>
>> When the primary link is down, you could run some ping tests while setting
>> the ping size to 1X00 and setting the DF bit as well, this will help you
>> find the max size packet with overhead that can fit over the tunnel.
>> Typically VPN tunnels take at least 80 bytes of overhead, so the largest MTU
>> I would expect you could fit over the tunnel would be 1420.
>>
>>
>>
>> I would try and adjust your tunnel MTU down to 1400 or even 1300 just as a
>> test to see if it helps. (In my demo setups with EZVPN tunnels, I can only
>> use 1350 max) Also, are your VPN endpoints able to fragment packets or clear
>> DF bits so that they can fragment large packets? If you can clear df-bit at
>> the interface, that may help move some of the larger packets through IF they
>> have the DF-bit set.
>>
>>
>>
>> +Chris
>>
>> TME - Unity Connection and MediaSense
>>
>>
>>
>> From: cisco-voip [mailto:cisco-voip-bounces at puck.nether.net] On Behalf Of
>> Jonathan Charles
>> Sent: Friday, April 24, 2015 11:44 AM
>> To: Charles Goldsmith
>> Cc: cisco-voip at puck.nether.net
>> Subject: Re: [cisco-voip] Cisco 8851 not failing over to backup circuit...
>>
>>
>>
>> MTU was set to 1440, we set it to Auto, no change...
>>
>>
>>
>>
>>
>>
>>
>> Jonathan
>>
>>
>>
>> On Thu, Apr 23, 2015 at 10:13 PM, Charles Goldsmith <wokka at justfamily.org>
>> wrote:
>>
>> What's your MTU over the backup VPN?  I've seen odd issues on some
>> networks with different providers and MTU and fragmenting packets
>> always caused issues until the MSS was set.
>>
>> I'm not sure why this would affect the 8851's, but we've noticed some
>> other oddities with the 8851's.  For instance, computers with intel
>> nic's behind the phone have issues after we apply config, and we
>> narrowed it down to intel gigabit master slave mode setting on the
>> driver, at least, setting that to slave instead of auto resolves the
>> problem.  Otherwise, you have to reboot the phone a couple of times to
>> get consistent connection through the 8851.  Phones are connected to a
>> 2960 with a basic config, nothing out of the ordinary.
>>
>>
>> On Thu, Apr 23, 2015 at 6:35 PM, Jonathan Charles <jonvoip at gmail.com>
>> wrote:
>> > We have CUCM 8.6.2 with Cisco 8851, Cisco 8831 phones at a remote
>> > location;
>> > they are connected over MPLS and a Peplink Balance VPN as a backup.
>> >
>> > When we yank the MPLS, the 8831 registers with CUCM and works fine....
>> > the
>> > 8851s do NOT.
>> >
>> > Any reason the 8851 would act differently?
>> >
>> >
>> >
>> >
>> > Jonathan
>> >
>>
>> > _______________________________________________
>> > cisco-voip mailing list
>> > cisco-voip at puck.nether.net
>> > https://puck.nether.net/mailman/listinfo/cisco-voip
>> >
>>
>>
>
>