Branch 1 of a company uses Huawei NE40E-X3s and is connected to the HQ through E1 lines.
According to the customer feedback, the ping to the HQ address fails and the physical status and protocol status of the local serial interface are both up. No abnormal alarms are generated on the NE40E-X3s.
1. Networking environment and simulated configuration:
Branch 1 (Serial2/0/0)--------(Serial7/0/0) HQ (Serial7/0/1)-----(Serial3/0/0) branch 2
Simulate HQ, and configure the IP address 184.108.40.206 to connect to branch 1 and the IP address 220.127.116.11 to connect to branch 2.
Simulate branch 1, and configure the IP address 18.104.22.168 to connect to the HQ.
Simulate branch 2, and configure the IP address 22.214.171.124 to connect to the HQ.
2. Test result:
Ping branch 1 and branch 2 from HQ. The ping succeeds.
Ping HQ from branch 1. The ping fails.
Ping HQ from branch 2. The ping succeeds.
Mirror traffic on Serial 7/0/0 connecting the HQ and branch 1. The packet header information shows that the HQ does not send ICMP packets as a reply.
Mirror traffic on Serial 7/0/1 connecting the HQ and branch 2. Packets can be exchanged properly.
3. Cause analysis:
(1) Ping the HQ (126.96.36.199) from branch 1. The source IP address of the ICMP echo request packet is 188.8.131.52 and the destination IP address is 184.108.40.206.
(2) The PPP addresses of both branch 1 and branch 2 are 220.127.116.11. Therefore, equal-cost route are generated on the HQ.
(3) The packet header information shows that the HQ sends ICMP echo replay packets to branch 2.
(4) In this test, AR2240 is used to simulate the HQ, and the equal-cost routes are forwarded based on flows. Therefore, the ping on the HQ (18.104.22.168) from branch 1 fails but the ping on the HQ (22.214.171.124) from branch 2 succeeds.
(5) If the HQ device forwards equal-cost traffic based on flows, the ping to the HQ from branch 1 may fail for a while and succeed for a while.
(6) The IP addresses of both ends of a PPP link are on different network segments and the interfaces can go up after negotiation. Therefore, the HQ generates equal-cost routes although the IP address of branch 2 is incorrect.
When the network of branch 2 is adjusted, the IP address of branch 1 is used, causing equal-cost routes on the NE40E-X3.
Reconfigure an IP address for branch 2.
The service restoration takes a lot of efforts because the troubleshooting is performed on branches. Due to the special characteristics of PPP, you are advised to manually shut down the unwanted serial interface on the HQ. This prevents the problem caused by malicious configuration or misoperation.