1. The impact of MTU
The MTU differences between devices of different vendors often cause problems on the routing plane and forwarding plane.
(1) The common problem on the forwarding plane is that the interface receives an IP packet no fragmentation flag (DF flag is set to 1), If the MTU does not match, the interface discards the packet.
(2) The common problem on the routing plane is that the protocol interconnection is abnormal. For example: OSPF interface MTU check, ISIS hello packet size, and BGP path MTU.
a. By default, Huawei devices do not check the MTU. However other vendors' devices, such as Cisco, check the MTU by default. If the MTU does not match, an error occurs when DBD packets are exchanged. As a result, the neighbor relationship cannot be established.
b. As for the impact of MTU on IS-IS, I saw an article in the forum written in detail.
https://forum.huawei.com/enterprise/en/isis-mtu/thread/563763-861
c. Impact of MTU on BGP path
The smallest MTU on the communication path is called the path MTU. The path MTU varies with the selected path and therefore may change. In addition, the path MTU in one direction may be inconsistent with that in the reverse direction. Path MTU auto discovery allows BGP to discover the smallest MTU value on a path so that BGP messages are transmitted based on the path MTU. This function improves transmission efficiency and BGP performance. TCP encapsulates IP packets based on the path MTU when transmitting BGP messages.
After the PATH MTU function is enabled for BGP, TCP sends non-fragmented packets and triggers the intermediate device to send TooBig errors. However, the intermediate device must support the TooBig error response so that the TCP can learn the MTU of the intermediate device. If the intermediate device does not support this function, the TCP link will be disconnected. For example, The switch does not support the MTU. If a switch exists in the BGP network, the TCP packet is discarded when it enters the switching node, and the BGP update packet and keepalive packet cannot be exchanged.
2. MTU Difference Between Vendors
Different vendors have different understandings of the MTU. The MTU configured for some vendors' Ethernet ports is IP MTU, while that of some vendors is Layer2 MTU (IP MTU+ Layer2 header or IP MTU+ Layer2 header+Layer2 CRC).
The following are some device examples:
No. | Vendor | device | Meaning of MTU |
1 | CISCO | CRS-1 | IP MTU + 14 |
2 | CISCO | C7600/GSR | IP MTU |
3 | JUNIFER | M/T series | IP MTU + 14 |
4 | JUNIFER | ERX/E320 | IP MTU + 18 |
5 | ALU | 7750SR | IP MTU + 14 |
6 | REDBACK | SE800 | IP MTU |
7 | HUAWEI | NE40E/80E | IP MTU |
3. Impact of MTU on Actual Work
(1) Pay attention to the MTU in the interconnection between devices of different vendors.
(2) During the replacement, check whether the original device is responsible for packet fragmentation of certain services. If no, change the MTU based on the MTU difference of different vendors to ensure that the protocol interconnection is successful.If packet fragmentation is required, check the packet fragmentation mechanism of each vendor. Arbitrary cutover may cause service interruption.
(3) When designing a network, ensure that the MTU of the core device is greater than the MTU of the access device, so that packet fragmentation is completed on the service gateway or host side.
I hope to see your supplement, thank you!
