[c-nsp] switch mac address learning

[Jon Lewis]

I've got the following network right now as part of a transition in which 
we'll be replacing switch1 (a 2924xl) with switch2 (a 3550).

6509a-L3-FE-port---|switch1|---L3-FE-port-6509b
|                      |                   |
|                      |                   |
L3-GE            FE crossover            L3-GE
|                      |                   |
|                      |                   |
-------------------|switch2|----------------

Each 6509 has a layer 3 port in a /27 talking through switch1 to each 
other and a bunch of routers hanging off switch1.  To test the new fiber 
runs for the 6509/3550 connections, I brought up layer 3 gigE ports on the 
6509s using a /30 with the intent of pinging one 6509 from the other. 
That works, but while doing it, I noticed an unreasonably large amount of 
traffic flowing between switch1 and switch2 and %RTD-1-ADDR_FLAP: messages 
on switch1.

For some reason, switch1 is learning about the 6509 GigE interface mac 
addresses via the FE connections it has to them.  Switch2 then learns 
these mac addresses from switch1, and AFAICT, is then sending a portion of 
the traffic I expected it to switch from g0/1 to g0/2 through switch1.

The explanations I found for %RTD-1-ADDR_FLAP: all talk about switch port 
loops...and though my current setup looks like a loop, I would have 
thought making all the involved 6509 interfaces layer 3 ports would have 
avoided this.  Is there a proper way to do this setup other than shutting 
off the 6509-2924 FEs?

What I planned to do after testing was make the 6509/2924 connections 
SVI's instead of L3 ports, then bring up the 6509/3550 GigE interfaces as 
switchports in those same SVIs.  Then move routers one at a time from 
switch1 to switch2, and eventually shut down switch1 once there are no 
devices left on it (other than the 6509s).

----------------------------------------------------------------------
  Jon Lewis                   |  I route
  Senior Network Engineer     |  therefore you are
  Atlantic Net                |
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