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Problem with mac address flapping

All,

I have an overly redundant (I think) switch layout, and I want to get everyone's opinion on if my proposed will be any better.

I'm getting a ton of flapping mac addresses between our core and edge switches. After further investigation, I noticed that the ports that the addresses are flapping to are disabled in spanning-tree.

The original thought on the current switch layout was that if any switch goes out, we're still okay. While that's true, I'm wondering if it's generating too much traffic between the switches. In the diagram, the Dell switches are the edge switches and the Cisco is our core (3750). I want to redesign this the best way possible. I don't know if having one link to the core from the edge is going to create a bottle neck at the switch though. It's going through gb ports on both sides.

Here's about half of what I see every night from the 3750:

Host 0006.5bfd.d4fb in vlan 1 is flapping between port Gi1/0/28 and port Gi3/0/21

Host 001d.7e77.4d0e in vlan 1 is flapping between port Gi1/0/27 and port Gi1/0/28

Host 00c0.ee6e.66b6 in vlan 1 is flapping between port Gi4/0/5 and port Gi1/0/28

Host 0011.43df.c184 in vlan 1 is flapping between port Gi1/0/28 and port Gi4/0/5

Host 0011.43f0.0d8f in vlan 1 is flapping between port Gi1/0/28 and port Gi4/0/5

Host 0011.43df.c183 in vlan 1 is flapping between port Gi1/0/28 and port Gi4/0/5

Host 0014.2216.75c1 in vlan 1 is flapping between port Gi1/0/28 and port Gi4/0/5

Host 0014.2216.75c0 in vlan 1 is flapping between port Gi1/0/28 and port Gi4/0/5

Host 0012.3f07.d66a in vlan 1 is flapping between port Gi1/0/28 and port Gi4/0/5

Host 0014.2218.a213 in vlan 1 is flapping between port Gi1/0/28 and port Gi4/0/5

Host 0014.2218.a214 in vlan 1 is flapping between port Gi1/0/28 and port Gi4/0/5

Host 0012.3fff.a46d in vlan 1 is flapping between port Gi1/0/28 and port Gi3/0/5

Host 0000.7482.6bd6 in vlan 1 is flapping between port Gi1/0/28 and port Gi4/0/5

Host 0012.3fff.a461 in vlan 1 is flapping between port Gi1/0/28 and port Gi4/0/1

Host 0021.9b3a.1ed7 in vlan 1 is flapping between port Gi1/0/28 and port Gi1/0/27

Host 4200.3c81.ff42 in vlan 1 is flapping between port Gi1/0/28 and port Gi1/0/18

Host 0011.4358.6408 in vlan 1 is flapping between port Gi1/0/28 and port Gi2/0/4

Host 000b.dbe6.b10d in vlan 1 is flapping between port Gi1/0/28 and port Gi3/0/6

Host 0050.56a4.4e3a in vlan 1 is flapping between port Gi1/0/28 and port Gi2/0/24

Host 4200.3c81.ff42 in vlan 1 is flapping between port Gi1/0/18 and port Gi1/0/28

Thanks!

--John

HTH, John *** Please rate all useful posts ***
7 REPLIES

Re: Problem with mac address flapping

Oh, and just to be clear on one part, the line that states:

Host 001d.7e77.4d0e in vlan 1 is flapping between port Gi1/0/27 and port Gi1/0/28

is from a Dell edge to the same dell edge switch. It's coming in on one port, but going out on another from what I can see.

Spanning tree has that port on the Dell disabled.

Thanks!

HTH, John *** Please rate all useful posts ***
Community Member

Re: Problem with mac address flapping

I was getting those mac flappings and the Cisco Tech told me there was a hardware problem but I was never able to identify the problem and they just stopped. I think that the problem was STP but as I said they stopped.

I would do channels from the edge switch stacks to the main stack. one channel for each edge switch stack is all that you should need. I have two stacks and the stackes are connected via one channel.

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_40_se/configuration/guide/swethchl.html

Re: Problem with mac address flapping

So you're suggestion is to create an etherchannel from the Dell to the Cisco? I don't know if the Dells support etherchannels among multiple switches do they? That would be the optimal solution I would think because then I have my redundancy and speed, but one mac address on the port channel.

--John

HTH, John *** Please rate all useful posts ***
Community Member

Re: Problem with mac address flapping

I forgot you mentioned that edge stack was dell. I guess you will need to check compatibility issues. But I know that cisco works with two different channel protocols.

Here is what the documentation says about the standard one: The LACP is defined in IEEE 802.3ad and enables Cisco switches to manage Ethernet channels between switches that conform to the IEEE 802.3ad protocol. LACP facilitates the automatic creation of EtherChannels by exchanging LACP packets between Ethernet ports.

So just find out if dells does LACP.

Re: Problem with mac address flapping

Let's assume that it doesn't. What would be the next suggestion? Is the physical design correct?

Thanks,

John

HTH, John *** Please rate all useful posts ***
Community Member

Re: Problem with mac address flapping

If Dell does not support channel config then I would modify your proposed config to provide at least one more trunk connection from each dell stack connected to the cisco stack so that you have backup trunk. Because you have single point of failure in the proposed config.

Re: Problem with mac address flapping

Should I still see flapping after doing this? I'm not really sure how the flapping is occurring in the first place. The Dell switches are located on separate floors, yet the core switch is seeing a mac address flap from one Dell to another.

Thanks for the responses John; I appreciate it.

--John

HTH, John *** Please rate all useful posts ***
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