[cisco-voip] New install 11.5 phones won't register or the subscriber

[Wes Sisk (wsisk)]

I’ll add a hard learned lesson here:

Example:
Subscriber sends a frame >1500 bytes. IF the frame makes it through the network it will by dropped by the NIC on the publisher if it exceeds the MTU configured on the publisher. You will not see the frame with ‘utils network capture…’ on the publisher as that dumps frames from the processor/linux kernel and not from the NIC. The frame is dropped at the NIC before it reaches the kernel.

-Wes

On Oct 12, 2017, at 2:11 PM, Dave Goodwin <Dave.Goodwin at december.net<mailto:Dave.Goodwin at december.net>> wrote:

While I haven't tried to test this specifically, my understanding is that a network transport that fully supports jumbo frames (e.g. 9000 bytes) will cause no issues with hosts configured with 1500 byte MTU trying to communicate. I thought the issues can happen when the configured MTU in the two hosts communicating exceeds the effective transport MTU anywhere in the path between the hosts.

On Thu, Oct 12, 2017 at 1:48 PM, Mike King <me at mpking.com<mailto:me at mpking.com>> wrote:


On Wed, Oct 11, 2017 at 4:10 AM, Ryan Huff <ryanhuff at outlook.com<mailto:ryanhuff at outlook.com>> wrote:

- Make sure you’re accounting for any non-standard MTU between the publisher and subscriber nodes. By default, CUCM will use 1500 if you don’t change it (which is the typical network standard). For example; If you have jumbo frame support in the network path between the two nodes and you set 1500 MTU on the publisher and subscriber, the subscriber installation will typically “fail node connectivity validation” with no visible clue as to why. This is because this is the first point in the subscriber installation where there is enough back/fourth TCP with the publisher to cause the TCP window to burst, which causes the TCP retransmission error. Since it happens at this point, CUCM assumes it’s because the publisher can’t verify the subscriber.



Hey Ryan,  Do you have links for that?  I find the behavior very odd, and not the way I understood standard MTU over a Jumbo enabled network to function.   Or is this just a Cisco Caveat type deal?

Mike

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