T2603: PPPoE-server change default min-mtu value 1280

[sever-sever]

Change Summary

Minimum acceptable MTU. If client will try to negotiate less then specified MTU then it will be NAKed or disconnected if rejects greater MTU.

Change 'min-mtu' from 1492 to 1280

https://docs.accel-ppp.org/en/latest/configuration/ppp.html

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe): Change default min-mtu value

Related Task(s)

• https://phabricator.vyos.net/T2603

Component(s) name

pppoe-server

Proposed changes

How to test

set service pppoe-server authentication local-users username user1 password 'user1'
set service pppoe-server authentication mode 'local'
set service pppoe-server client-ip-pool start '192.0.2.10'
set service pppoe-server client-ip-pool stop '192.0.2.100'
set service pppoe-server gateway-address '192.0.2.1'
set service pppoe-server interface eth1

Expected min-mtu 1280

vyos@r14# cat /run/accel-pppd/pppoe.conf | grep min-mtu -B 5
[ppp]
verbose=1
check-ip=1
ccp=0
unit-preallocate=0
min-mtu=1280

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly