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P4300 ALB (adaptive load balancing)

Menlo Park
Occasional Visitor

P4300 ALB (adaptive load balancing)

I believe the P4300 ALB (adaptive load balancing) is designed and or supposed to work differently than what we are currently experiencing in our testing.

We have a P4300 SAN connected to two Cisco 2960 switches. Each switch is connected to the network core (4506) through a trunk connection. Each NIC on each SAN is connected to one Cisco switch. So there are two SAN nodes and two switches. All VLAN operations appear to be working

We when power down one switch both SAN nodes remain connected (This is good), however when we unplug the switch trunk port connection one of the SAN nodes becomes unreachable (This is bad). Now the interesting thing is when we disconnect the other trunk port on the other switch the other SAN node becomes unreachable while the other remains active.

Again , if one of the switches or Ethernet ports gets turned off the failover seems to work fine. Disconnecting the trunk seems to produce a strange result

Is this behavior expected ? or could this be a design or configuration issue.

4 REPLIES
Bryan McMullan
Trusted Contributor

Re: P4300 ALB (adaptive load balancing)

I have much the same setup. Two stacks of 3750's connected to 2 6500 cores and everything works correctly. But we have cross connects on the stacks (as well as cores).

My initial guess is you have a spanning tree configuration issue. IIRC, ALB sends from both ports but receives on one. It sounds like the SAN doesn't realize your link is down when you break the trunk link and since there is no cross connect, the traffic destined there is dropped. I would wager if you downed the switch completely, everything would work.

I can validate that our setup works correctly as I just did IOS updates on the stacks a couple of weekends ago and everything stayed up as expected.
Menlo Park
Occasional Visitor

Re: P4300 ALB (adaptive load balancing)

Bryan,

Thanks for the fresh perspective. It turned out to be a spanning tree issue. We had portfast enabled on the iSCSi vlan interfaces. Disabling portfast inconjuction with a trunk cross connect between switches, resolved the issue.

Now when we unplug the trunk connection to the core both storage nodes stay up.

cheers

-Danny
Bryan McMullan
Trusted Contributor

Re: P4300 ALB (adaptive load balancing)

Excellent! Glad I could help.
Menlo Park
Occasional Visitor

Re: P4300 ALB (adaptive load balancing)

Thanks for the fresh perspective. It turned out to be a spanning tree issue. We had portfast enabled on the iSCSi vlan interfaces. Disabling portfast inconjuction with a trunk cross connect between switches, resolved the issue.

Now when we unplug the trunk connection to the core both storage nodes stay up.