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Eth-trunk MTU vs physical interface MTU

Publication Date:  2012-09-10 Views:  129 Downloads:  0
Issue Description
The customer is using the following devices in the backbone network:

Huawei Versatile Routing Platform Software
VRP (R) software, Version 5.70 (NE40E&80E V600R001C00SPC800)
Copyright (C) 2000-2009 Huawei Technologies Co., Ltd.
HUAWEI NE40E

They build an Eth-trunk interface formed by 4 physical interafce of 1G between 2 NE40E.
In the first phase the Eth-trunk MTU was set to 1500 bytes.

In the second phase the customer has changed the MTU of Eth-trunk to 1600 bytes. After that they had checked the MTU on Eth-trunk and on physical interfaces belongind to Eth-trunk. They have discovered that on Eth-trunk the MTU was 1600 bytes but on physical interfaces the MTU remains 1500 bytes.

Questions: Which MTU is going to be used, the 1600 bytes MTU from Eth-trunk or the MTU from physical interfaces which was 1500 bytes?

If you will try to manually modify the MTU of physical interface to match the MTU of Eth-trunk the system will reject the command.


For example, we have the bellow output (the example is related to change the MTU from 1500 to 1560 and I have used in the laboratory just 3 links):

<TRUNK1>dis trunkmembership eth-trunk 1
Trunk ID: 1
used status: VALID
TYPE: ethernet
Working Mode : Normal
Working State: Normal
Number Of Ports in Trunk = 3
Number Of UP Ports in Trunk = 3
operate status: up
 
Interface Ethernet1/0/0, valid, selected, operate up, weight=1,
standby interface NULL
 
Interface Ethernet1/0/1, valid, selected, operate up, weight=1,
standby interface NULL
 
Interface Ethernet1/0/3, valid, selected, operate up, weight=1,
standby interface NULL
 
[TRUNK1]dis int Eth-Trunk 1
Eth-Trunk1 current state : UP
Line protocol current state : UP
Last line protocol up time : 2012-06-13 18:25:10
Description:HUAWEI, Quidway Series, Eth-Trunk1 Interface
Route Port,Hash arithmetic : According to flow,The Maximum Transmit Unit is 1500
Internet Address is 10.10.10.1/24
-----------------------------------------------------
PortName                Status              Weight
-----------------------------------------------------
Ethernet1/0/0           UP                  1
Ethernet1/0/1           UP                  1
Ethernet1/0/3           UP                  1
-----------------------------------------------------
The Number of Ports in Trunk : 3
The Number of UP Ports in Trunk : 3
 
[TRUNK1]int Eth-Trunk 1
[TRUNK1-Eth-Trunk1]mtu 1560
Warning: Please shutdown and then undo shutdown the interface to make changes take effect.
[TRUNK1-Eth-Trunk1]shut
[TRUNK1-Eth-Trunk1]
Jun 13 2012 18:26:47 TRUNK1 %%01IFNET/4/LINKNO_STATE(l): The line protocol on the interface Eth-Trunk1 has entered the DOWN state.
[TRUNK1-Eth-Trunk1]undo shut
Jun 13 2012 18:26:51 TRUNK1 %%01IFNET/4/LINKNO_STATE(l): The line protocol on the interface Eth-Trunk1 has entered the UP state.
 [TRUNK1-Eth-Trunk1]dis int Eth-Trunk 1
Eth-Trunk1 current state : UP
Line protocol current state : UP
Last line protocol up time : 2012-06-13 18:26:51
Description:HUAWEI, Quidway Series, Eth-Trunk1 Interface
Route Port,Hash arithmetic : According to flow, The Maximum Transmit Unit is 1560
Internet Address is 10.10.10.1/24
-----------------------------------------------------
PortName                Status              Weight
-----------------------------------------------------
Ethernet1/0/0           UP                  1
Ethernet1/0/1           UP                  1
Ethernet1/0/3           UP                  1
-----------------------------------------------------
The Number of Ports in Trunk : 3
The Number of UP Ports in Trunk : 3
 
[TRUNK1-Eth-Trunk1]dis int Ethernet1/0/0
Ethernet1/0/0 current state : UP
Line protocol current state : UP
Description:HUAWEI, Quidway Series, Ethernet1/0/0 Interface
Route Port, The Maximum Transmit Unit is 1500    -> is normal? Why is not changed to 1560?

Bellow you will find the troubleshooting method to find what MTU is used by the system in the forwarding process.



None

Handling Process
1. Make the configuration in laboratory and on customer network
2. Perform ping test with packets less than MTU configured on Eth-trunk and greater that MTU of Eth-trunk interface using don't fragment function conclude which MTU is used.
3. Send out the conclusions of the tests


Root Cause
the 1600 bytes MTU from Eth-trunk and the MTU from physical interfaces which was 1500 bytes which will be used. 
Solution
I have performed a test as follow:
 
MTU on Eth-trunk (the physical interface also 1500 bytes MTU) = 1500:
If we are using ping with payload 1472 bytes and don't fragment option set everything is ok (1472 + 28 = 1500). If we are using 1473 it doesn’t not work, because the MTU of 1500 bytes is exceeded and the packet is not fragmented.
 
MTU on Eth-trunk (on the physical interface remains 1500 bytes) = 1600 bytes:
 
Ping with 1550 is successful (1550+ 28 = 1578 < 1600). Ping with 1580 (1580 + 28 = 1606 > 1600) there isn’t any echo reply, so the ping is not working between routers.
The customer confirmed that using ping with 1500 bytes works, this means 1500+28=1528>1500. So we can conclude that the router is working as expected.

From the above test we can conclude that even is on physical interface the MTU is still 1500 bytes the real MTU used by the system in the process of forwarding the data is the MTU set up on the Eth-trunk.

Tacking into account the above test we can conclude that the MTU used for forwarding is the MTU of the ETH-TRUNK interface.


Suggestions
  • If the MTU is changed on Eth-trunk from 1500 to 1600 on the physical interface belonging to Eth-trunk it will be displayed 1500 and not 1600. We can confirm that the MTU taken into account when forwarding the traffic using the Eth-trunk interface is 1600 and not 1500.


END