The device and its version are as follows:
NE40 VRP (R) software, Version NE40NE80-VRP5.30-0236.01
NE40 and enterprise C's 17 serial router are connected through the CPOS interface. The physical interface is E1. PPP is used for encapsulation on Layer 2 interface. After PPP negotiation is completed, LCP and NCP negotiation are Up. However, the directly connected address cannot be pinged and the network of the peer device cannot be accessed.
There is no abnormal alarm. When the interface information is displayed, MTU negotiation value is 0, that is, the negotiated MTU is 0 bytes.
Possible reasons are as follows:
1. The quality of the physical link is faulty. As a result, there is delay and packet loss.
2. Timeslot division and interface logic are faulty.
3. Some security policies are configured on the device to filter the traffic.
4. The protocol processed by NE40 and enterprise C's device is different.
When MTU is 1500, enterprise C's device does not negotiate the MTU.
1. Use the tester. The physical interface and the physical link are normal.
2. Run the dis cu int Serial3/0/0/8:0 command to check the configuration of NE40. It is normal.
mtu 1500 (default configuration)
ip address 18.104.22.168 255.255.255.252
3. The interface of enterprise C's device is normal.
4. Run the dis interface Serial3/0/0/8:0 command.
Serial3/0/0/8:0 current state: up
Line protocol current state: up
AU-4 1, TUG-3 2, TUG-2 3, E1 1, ChannelGroup 0, Channel ID 1
Description: "rt-bb-ura1-01 -> cl-rt-ura-uniube-01 (0497509-URA) Link-2"
Internet address is 22.214.171.124/30
The configured MTU is 1500 bytes, and the negotiated MTU is 0 bytes
TimeSlotList include 1-8,the BandWidth is 512 Kbits
The Max HQOS BandWidth is 448 Kbits
Link-protocol is PPP, loopback not set, clock source master, urpf disabled
MRU: 1500 bytes, Negotiate Timeout set 3 seconds
Peer MRU: not negotiate, Magic number: 50342202
Peer IP address is 126.96.36.199/32
NCP: IPCP Opened, MPLSCP Stopped
Input: 94 Lcps, 0 Ipcps, 0 Ip6cps, 0 Mplscps, 0 Osicps, 0 Invalids
Output:52 Lcps, 0 Ipcps, 0 Ip6cps, 42 Mplscps, 0 Osicps, 0 Invalids
Hold time is 10 (sec)
Input: 52 Timerhold Packets
Output:52 Timerhold Packets
Last 5 minutes input rate 36 bytes/sec, 0 packets/sec
Last 5 minutes output rate 11 bytes/sec, 0 packets/sec
Input: 376 packets, 19566 bytes
0 errors, 0 CRC, 0 giants
Output:161 packets, 6068 bytes, 0 underruns
0 CRC, 0 aborted sequences, 0 giants
The information is found: and the negotiated MTU is 0 bytes
MTU negotiation may fail.
When MTU is 1500, enterprise C's device does not negotiate the MTU. The device does not send MTU negotiation packet and receive the negotiation packet from the peer device. However, NE40 negotiates MTU in any situation. The initial value of the negotiation is 0. The subsequent negotiation is based on the previous one. If the negotiation fails, take the last negotiation.
After the device is Up, if the MTU of NE40 and enterprise C's device is 1500, enterprise C's device does not negotiate the MTU, but NE40 negotiates the MTU. NE40 sends negotiation packets to enterprise C's device, while enterprise C's device does not receive them. In this manner, NE40 cannot receive the negotiation packet of enterprise C's device and the result of NE40 is 0. Enterprise C's device remains 1500. When the MTU is 0, NE40 does not communicate.
If the MTU of enterprise C's device is changed as 1400, enterprise C's device starts to negotiate the MTU. NE40 also negotiates the MTU. The MTU is 1400. The link is reachable.
When the MTU is 1400, if the MTU of enterprise C's device is changed as 1500 and the MTU is not negotiated, NE40 fails to negotiate and the previously negotiated MTU 1400 is not changed. Check the negotiation result. The MTU of enterprise C's device is 1500, while that of NE40 is still 1400. The communication is not affected once the MTU is not 0.