VLAN Interfaces Have MTU 1500

[tahmed-dev]

Description
VLAN interfaces on host, have default MTU1500. Attempt to reconfigure the MTU to 9100 (Jambo frames). Subsequently, radv does throw a warning message when it add a link MTU:

Aug 10 06:38:14.914860 <sonic-hostname> WARNING radv#radvd[44]: AdvLinkMTU for Vlan2820 (9100) must be zero or between 1280 and 1500  

Steps to reproduce the issue:

1. Reboot into SONiC device with Vlans configured (ToRRouter)
2. Observe Vlan MTU using sudo ifconfig Vlan<x>
3. Observe Bridge MTU using sudo ifconfig Bridge

Describe the results you received:
Vlan MTU is set to 1500
Bridge MTU is set to 1500

Describe the results you expected:
Vlan MTU 9100 in order to be inline with member ports MTU

[tahmed-dev]

@rlhui, This issue has been fixed via PR:1393

[tahmed-dev]

This issue was opened to track updates into 201911. it is resolved as per above comment. assigning to @rlhui

[prsunny]

This is a limitation with 4.9 kernel (201911). Hence the fix is not taken to 201911 but only available in master (4.19 kernel). Please find the notes here - sonic-net/sonic-swss#1393 (comment)