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[All About Switches] Some Eth-Trunk Interfaces Are Down and Some Are Up Due to Incorrect Interface Connections After LACP Negotiation

Latest reply: Mar 9, 2018 05:53:11 1747 1 0 0 0

Involved Products and Versions

All products and versions


As shown in Figure 1-1, SwitchA is connected to DeviceA and DeviceB through Eth-Trunk5 and Eth-Trunk6 respectively. The member interfaces of Eth-Trunk5 are GE1/0/18 and GE2/0/18, and the member interfaces of Eth-Trunk6 are GE1/0/10 and GE2/0/10.

Figure 1-1 Networking diagram for the fault that some Eth-Trunk interfaces are Down and some Eth-Trunk interfaces are Up due to incorrect interface connections after LACP negotiation



Fault Symptom

When the Eth-Trunk link is adjusted, Eth-Trunk5 is Up and Eth-Trunk6 is Down. Before the adjustment, both Eth-Trunk5 and Eth-Trunk6 are Up.

Cause Analysis

When the four member interfaces are connected in the following sequence, the problem occurs:

1.       In Eth-Trunk5, the member interface GE1/0/18 is connected to DeviceA, and the member interface GE2/0/18 is not connected. In this case, Eth-Trunk5 is Up.

In Eth-Trunk6, the member interface GE1/0/10 is connected to the peer DeviceB, and the member interface GE2/0/10 is not connected. In this case, Eth-Trunk6 is Up.

Both Eth-Trunk5 and Eth-Trunk6 are correctly connected. Therefore, both of them are Up.

2.       When the member interface GE2/0/10 of Eth-Trunk6 is connected to DeviceA, a conflict occurs. In Eth-Trunk5 and Eth-Trunk6, the Eth-Trunk5 member interface with a smaller value of trunkid is selected. Therefore, the member interface GE2/0/10 is not selected. However, the other member interface GE1/0/10 is selected, which does not cause the entire Eth-Trunk6 to go Down.

When the member interface GE2/0/18 of Eth-Trunk5 is connected to DeviceB, Eth-Trunk6 is also connected to DeviceB. When this conflict occurs, the member interface of Eth-Trunk5 is selected in Eth-Trunk5 and Eth-Trunk6. The member interface GE2/0/10 of Eth-Trunk6 is not selected, and the entire Eth-Trunk6 goes Down.

The GE2/0/18 member interface of Eth-Trunk5 further compares information with the other member interface GE1/0/18 and finds that they are not connected to the same device. Therefore, the interface GE2/0/18 is not selected. However, a member interface of Eth-Trunk5 is already selected. Therefore, Eth-Trunk5 is not Down.

Troubleshooting Procedure

                          Step 1     Connect the cables correctly. The fault is rectified.


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Created Mar 9, 2018 05:53:11

Good documentation
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