Network Topology
Devices and Versions: S5560X-EI R6526
Networking: SW1 and SW2 form an M-LAG system, and they are connected to SW3 using ports 1/0/3 and 1/0/4 respectively without configuring M-LAG interface:
Problem Description
SW1 and SW2 form an M-LAG system. After configuring, it was found that the G1/0/4 port on SW3 connected to SW2 is in discarding state and G1/0/4 is disputed:
Process Analysis
STP state is abnormal. We first check the STP configuration of several devices and find that the STP configurations of SW1 and SW2 are not exactly the same:
Because SW1 is the root bridge and SW2 is configured as the backup root bridge, the priorities carried by the BPDUs sent by the two devices are different:
After receiving the BPDU from SW1, port 1/0/3 on SW3 is calculated as the root port, and port 1/0/4 is calculated as the designated port. When SW3 sends the BPDU packet carrying the root bridge information to SW2, SW2 does not process it because the bridge MAC of SW2 and SW1 is the same, which is the system MAC. SW2 still considers itself as the root bridge and sends the BPDU packet out. Port 1/0/4 on SW3 receives the BPDU sent by SW2, and its priority is lower (4096), triggering a dispute, which causes the fault phenomenon at the site:
Solution
Configure the STP configurations on SW1 and SW2 to be the same according to the specifications.
When configuring in the live network, many engineers unconsciously designate the primary device as the root bridge and the backup device as the backup root bridge, which is incorrect. Please remember to keep the STP configurations of the two devices in the M-LAG system consistent.