What is the MTU set on the Ethernet interface that connects the NE5000E/NE80E/NE40E with the CRS1 of Manufacturer C?
The CRS1 is a TB router launched by Manufacturer C in 2007. The software system and default configurations of the CRS1 are greatly different from those of the legacy routers such as 120xx, 100xx, 76xx, and 65xx. Special attention should be paid to the change of the default Ethernet interface MTU. The default MTU on all Ethernet interfaces of the routers (including 120xx, 100xx, 76xx, and 65xx) of the company is 1500 bytes, which include only the payload of packets. The default MTU on all Ethernet interfaces of the CRS1 is 1514 bytes, which include an Ethernet frame header and packet payload. The default MTU on the Ethernet interfaces of Huawei routers is 1500 bytes, which include only the packet payload.
1. The IS-IS protocol is run on the Ethernet interface between the CRS1 and a Huawei router and Small-hello is not configured, the interface sends a Hello packet according to the MTU. The default MTU 1514 on Ethernet interfaces of the CRS1 corresponds to the MTU 1500 on Ethernet interfaces of Huawei devices. Any change to the MTUs should be based on 1514 and 1500. Otherwise, the ISIS neighbors will never be Up. For example, if the MTU on Ethernet interfaces of the CRS1 is changed to 1518, then the MTU on Huawei devices needs to be changed to 1504 accordingly.
2. If the CRS1 and Huawei devices are interconnected through Ethernet interfaces in an MPLS VPN, the Ethernet interface MTU on the CRS1 must be greater than or equal to 1522. Otherwise, large packets cannot be fragmented and service failures may occur.