[j-nsp] Jumbo frames / mismatch MTU

[Matthew Crocker]

I set the interface MTU to 9192 and the family inet mtu to 1586 and everything is working.

I don’t think I’ll need to worry about the 1600 byte limitation for the TLS devices as the only thing that will be greater than 1500 bytes is the SAN replication traffic going between two ACX and will be encapsulated in MPLS. ‘family mpls’ on the two AXCs is 9166.   Clearly if machines on the other side of the TLS link need to talk to the SAN they will have to do so on a different SAN interface that is set to 1500

Currently the ACX 2200 are acting as the border between the QFX/TLS WAN and the local LAN in each building.  I haven’t enabled jumbo frames on the LAN side of the ACX yet as I need coordinate with the LAN manager during a maintenance window.

From: juniper-nsp <juniper-nsp-bounces at puck.nether.net> on behalf of Chuck Anderson <cra at alum.wpi.edu>
Date: Friday, April 23, 2021 at 4:32 PM
To: juniper-nsp at puck.nether.net <juniper-nsp at puck.nether.net>
Subject: Re: [j-nsp] Jumbo frames / mismatch MTU
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On Fri, Apr 23, 2021 at 01:23:32PM +0000, Matthew Crocker wrote:
> The SRX devices are limited to an MTU of 1600 due to the TLS carrier they are using to connect back to the  QFX.
>
> I need to support 9K frames from one ACX to another over this network.  The QFX is configured for MTU of 9192 on all interfaces.  When I configure a couple ACXs with 9192 MTU the OSPF & LDP sessions go away.
>
> I can ping ACX to ACX with 9k packets just fine.


> Everything is working.  If I ‘set mtu 9192’ everything breaks

Just set the IP MTU in addition to the physical L2 "mtu".  That way
you don't have to care about calculating any possible differences
caused by encapsulation overhead:

set interface ... family inet mtu 9000

But how are you planning on overcoming the 1600 limitation on the TLS carrier?
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