1. MTU and lsp-length
By default, the MTU of an interface is 1500 bytes, and the lsp-length (length of the LSPs) that are originated and received by the current IS-IS system is 1497 bytes.
This is because in the broadcast network, according to the 802.3 link protocol encapsulation format, there is a 3-byte local link control (LLC) field in the data. The value of this field is FEFE03. Therefore, in IS-IS, the actually transmitted data length is 1497 bytes, which is three bytes smaller than the physical MTU.
You can change the MTU of an interface, For example：
interface GigabitEthernet0/0/1
mtu 1400
However, after the MTU changed, you may find that the device does not send IS-IS packets. This is related to the lsp-length. For example, if the MTU is changed to 1400, the value of lsp-length is still the default 1497. As the MTU value is smaller than the value of lsp-length, the device will not send the IS-IS packets.
The lsp-length must meet the following conditions:
On an Ethernet interface, MTU >= lsp-length + 3
On a P2P interface, MTU >= lsp-length
In fact, a warning will be generated when configuring lsp-length on the device.
In addition, when configuring LSP-length, ensure that the length of originated LSP (or the forwarded LSP) must be smaller than or equal to that of the received LSP, or it is not able to configure successfully.
2. MTU check
If the MTUs of the two ends are different:
Broadcast interface: If isis small-hello or isis circuit-type p2p is not configured, the neighbor relationship cannot be established. In fact, a broadcast interface pads the Hello packets based on the MTU. If the MTUs are different, one end cannot receive the Hello packet.
P2P interface: If the MTU of each end >= lsp-length originate, the neighbor relationship can be established. Because the P2P interface pads the Hello packets based on the lsp-length. If the MTU of an end is smaller than lsp-length of any end, the Hello packet cannot be received or sent.
Broadcast interface simulates P2P interface: Similar to the P2P interface. But the MTU of each end must >= lsp-length originate of any end + 3.
