Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

Loop in network


I have setup the network as shown in the attached topology.

The two Barracuda Load Balancers have been deployed in bridge mode meaning that they are each configured with only one physical IP address.

They have been configured to operate in an Active-Standby mode whereby the WAN port of each Load Balancer is connected to Switch A and the LAN port of each Load balancer is connected to Switch B. The Load balancers communicate and monitor each other by sharing a secret cluster password.

In this setup all traffic going to/coming from the servers need to pass through the Load Balancers.The network on both sides of the Load Balancers (WAN and LAN ports) have the same IP addressing which is

The setup works fine but at some times, a loop occurs in the network and this brings the network completely down where no device can be reached.

This usually occurs when plugging in the second Load Balancer or when modifying the configuration of the Active Load Balancer.

Please find attached the topology, configurations and errors generated.

I have used spanning-tree portfast on Switch A ports to lessen the time it takes for the Backup Load Balancer to discover that the Active Load Balancer is no longer operating correctly.




Re: Loop in network


Could you please check to which device do MAC addresses 0017.5401.03e0 and 0017.5401.192e belong to?

Could you please post the output of "show spanning-tree vlan 22" command?


Hall of Fame Super Blue

Re: Loop in network


I can't read visio files so this may not be relevant but just to check.

You have the Barracuda's in bridge mode using the same IP subnet. Are you using 2 vlans with the same IP subnet ie. you are not using the same vlan on both "sides" ?


New Member

Re: Loop in network

Dear Jon/Jorge,

In fact, the network on both sides of the Barracuda's is on the same IP subnet ( and vlan (vlan 22).

The MAC addresses 0017.5401.03e0 and 0017.5401.192e are for each Barracuda.

Please find attached the topology.



Re: Loop in network

The Barracuda's are most likely not participating in STP and will probably not forward STP BPDU's.

Also, I expect them to communicate with eachother (for redundancy) using some multicast-based protocol.

Because this is forwarded by the switches you are seeing the source addresses loop over their ingress / egress ports.

This is most likely an unsupported topology. Please check the manual for the Barracuda's to verify this.

You might be able to select a different mechanism for the redundancy checking.



CreatePlease login to create content