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VRRP Traffic Flowed Along an Unexpected Path Because the Root Bridge Was Not Properly Selected

Publication Date:  2013-10-26 Views:  46 Downloads:  1
Issue Description

Two NE40Es and two S6500s formed a square-shaped network. VRRP was deployed on VLANIF100 of the NE40Es, and NE40E-1 served as the VRRP master.

Ports connecting NE40Es and S56s using VLAN trunks and could forward packets with the VLAN ID being 100.

The customer reported that the traffic on the inside network flowed along the path S56-1 > S56-2 > NE40E-2 > NE40-1 instead of the expected S56-1 > S56-2 > NE40E-1 (VRRP master).

For details about the network topology, see the attachment.
Handling Process

To address the issue, Huawei performed the following operations and observed the following information:

1. Checked whether any hardware fault on S56-1 or NE40E-1 occurred, for example, the network cable between them was not properly connected.

The status of ports connecting S56-1 and NE40E-1 was normal. Therefore, this problem was not caused by a physical-layer fault.

2. Checked the MAC address table on S56-1. The virtual MAC address of the VRRP, which was supposed to be learned from port 2, was learned from port 1. This indicated that port 2 on S56-1 or port 2 on NE40-1 was blocked.

According to experience, if STP was enabled on a Layer 2 square-shaped network, a port must be blocked.

3. Checked the STP status of the four NEs and found that NE40E-2 was the root bridge, port 1 on NE40E-1 was a designated port (forwarding) and port 2 on NE40E-1 was a non-designated port (blocking).

Therefore, this problem was caused because the root bridge was not properly selected and a port on the VRRP master was blocked.
Root Cause
A port on the VRRP master was blocked because the root bridge was not properly selected.
Solution
Specified NE40E-1 as the root bridge.
Suggestions
On a similar square-shape network, ensure that the VRRP master is also the root bridge so that all its ports are in the forwarding state.

END