Hi there, everyone!
This post explains what is the difference between the packet size commands on Huawei and Cisco routers. Please find more details on the topic below.
ISSUE DESCRIPTION
When facing a problem regarding the IP MTU on the Huawei AR2220E or any Huawei AR router and there is no problem on another type/model of router (ie. Cisco), you need to know that there are some differences in the packet size commands used on both sides.
Below you will find an explanation of the 'symptoms'.
NOTE:
The actual value of the MTU is 1500, but in this case the MTU cannot exceed the 1472 value. As a result, some applications were not working, like e-mails with attachments etc.
The router is used as the Internet gateway and the Internet feed (Ethernet) is plugged in it.
If I am trying with a Cisco router, the ping works with the 1500 MTU size, so the circuit/provider allows until the 1500 size (to assure the integrity of the internet line).
Please see below an example of the ICMP requests performed on the Huawei router:
[HUAWEI-AR]ping -s 1472 8.8.8.8
PING 8.8.8.8: 1472 data bytes, press CTRL_C to break
Reply from 8.8.8.8: bytes=64 Sequence=1 ttl=44 time=63 ms
Reply from 8.8.8.8: bytes=64 Sequence=2 ttl=44 time=66 ms
Reply from 8.8.8.8: bytes=64 Sequence=3 ttl=44 time=52 ms
Reply from 8.8.8.8: bytes=64 Sequence=4 ttl=44 time=52 ms
Reply from 8.8.8.8: bytes=64 Sequence=5 ttl=44 time=58 ms
--- 8.8.8.8 ping statistics ---
5 packet(s) transmitted
5 packet(s) received
0.00% packet loss
round-trip min/avg/max = 52/58/66 ms
[HUAWEI-AR]ping -s 1473 8.8.8.8
PING 8.8.8.8: 1473 data bytes, press CTRL_C to break
Request time out
Request time out
Request time out
Request time out
Request time out
--- 8.8.8.8 ping statistics ---
5 packet(s) transmitted
0 packet(s) received
100.00% packet loss
HANDLING PROCESS
In the case of Huawei routers, the MTU size it is composed as follows: '1468 bytes IP data + 20 bytes IP header + 8 bytes ICMP header + 4 bytes VLAN/Subinterface = 1500 bytes'.
In most cases, the 4 bytes for VLAN/Subinterface are free. Therefore, they are used for the payload. It results in 1472 bytes for IP data.
Packets that exceed the value of 1472 will be fragmented.
Please see the attachmet named 'packet size explanation' for your clarification with the following statement in my tests.
Considering the explanation attached, let’s try to get a network capture:
1. Set this rule on the router:
Acl 3000
Rule 1 permit icmp
2. The command for network capture:
capture-packet interface gigabitethernet 0/0/0 acl 3000 destination terminal packet-len 1518
ping -a x.x.x.x -s 1518 8.8.8.8
Please don’t forget the following!
The packet capture configuration is not saved in the configuration file and becomes invalid as soon as the packet capture is complete, so please log the session before running this command.
In fact, for this capture you must open two simultaneous sessions - one for the packet capture and another one to start the ping command.
ROOT CAUSE
Please see the attachement named with the entire capture analyzed.
The command used was on the Cisco device and the capture results:
Router#ping 8.8.8.8 si 1500
Router#ping 8.8.8.8 si 1518
The below capture revealed the fact that the Cisco device fragmented every packet which is above 1500 bytes.
The command used on the Huawei AR router was:
[HUAWEI-AR]ping -s 1518 8.8.8.8
The below capture revealed the fact that the AR fragments and sends the packets, but somewhere along the way something unexpected happens and the Huawei AR does not receive back the replay message - (no response found!):
SOLUTION
From the above printout we can conclude that the problem does not reside on the AR side; there are no faults from the configuration or behavior.
I found out that there was a different result in the commands performed on the Cisco and Huawei routers.
On CISCO routers: Parameter size represents datagram-size: this option enables you to specify the total size of the ping packet (including headers) in bytes that will be sent.
On Huawei routers: Parameter “-s packetsize” on ping command: Specifies the length of an ICMP Echo Request message, excluding the IP header and ICMP header.
This would be all on what is the difference between the packet size commands on Huawei and Cisco routers. Thanks for reading this post!
