How to Do S5700 Switch MTU Setting on L3 Interfaces?

Abstract

This topics will elaborate on how to do Huawei S5700 switch MTU setting on L3 interfaces. We noticed that while testing S5720-HI switch with V200R007C00SPC500 version on board, we configured L3 interface from L2 by using undo portswitch command.
And applied MTU command.

[S5720-HI-ACC1-GigabitEthernet0/0/36]di th 
# 
interface GigabitEthernet0/0/36 
undo portswitch 
MTU3000 
ip address 10.10.100.1 255.255.255.0 
ospf enable 1 area 0.0.0.0 

But command display interface shows us that:

<S5720-HI-ACC1>dis int g 0/0/36 
GigabitEthernet0/0/36 current state : UP 
Line protocol current state : UP 
Last line protocol up time : 2015-07-14 09:34:23 

Description: 
Route Port,The Maximum Frame Length is 9216 

The Maximum Frame Length is 9216. 9216 is constant and independent of MTU setting parameter.

Maximum Frame Length is not MTU, which is set by MTU command.

Real MTU(ip) can be checked by protocol commands.

Display ospf brief

Interface: 10.10.100.1 (GigabitEthernet0/0/36) 
Cost: 1       State: DR        Type: Broadcast    MTU: 3000  
Priority: 1 
Designated Router: 10.10.100.1 
Backup Designated Router: 0.0.0.0 
Timers: Hello 10 , Dead 40 , Poll  120 , Retransmit 5 , Transmit Delay 1

But if we use real L3 interface, on AR router for example, or vlanif interface:

interface GigabitEthernet0/0/2 
MTU9202 
ip address 10.10.100.2 255.255.255.0 
ospf enable 1 area 0.0.0.0 
# 
return

[AR3260-GigabitEthernet0/0/2]dis int g 0/0/2 
GigabitEthernet0/0/2 current state : UP 
Line protocol current state : UP 
Last line protocol up time : 2015-07-14 14:46:02 
Description:HUAWEI, AR Series, GigabitEthernet0/0/2 Interface 
Route Port,The Maximum Transmit Unit is 9202 

We set MTU9202 and this is shown by display interface command.

Solution

Take care when we set and verify MTU parameter on ex L3 interfaces (undo portswitch) at S5700 devices. By displaying interface command it is still shown L2 Maximum Frame Length.

END

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