CAM Propagation Question

[slaquer]

PC-->sw1-->sw2-->CORE<--SW3<--SW4

If a new PC plugs into sw1 with a static IP NOT in the correct VLAN (can't do any layer 3 communication) will it's MAC show up in the CAM tables of each switch (sw1 - sw4), just sw1 or what? I know this all happens on layer 2, but it the PC can't do layer 3 forcing the other switches to ARP its IP, will the other switches still get the MAC of the PC? If so, how does this work? Does the CAM table auto-propagate even when no layer3 happens?

 

[imbadatthis]

cam table is local/unique in each switch so it doesn't propagate...

as for the MAC showing up in the switch, i reckon it will be in switches that are participating in the same vlan as the port you have your pc1 connected to .

ie: if its vlan 20, assuming sw1 is smart, then it sends it to all switches that are participating in vlan 20 to see if any of them can reply back to pc1's request.



We must go always forward, not backward
always up, not down and always twirling twirling towards infinity.

 

[slaquer]

I guess propagation was the wrong term, though I know for a fact the MAC will show up on the CORE CAM table (I tested this myself yesterday) in the exact situation listed above - this was counter to what I was expecting. I thought like you described - CAM is local to the switch and unless another switch has had to associate a MAC to an IP, it wouldn't even know the remote PC existed. Obviously each switch only shows that MAC on the trunk port until you hit that device's local switch, but how does the CORE get the info if no layer-3 traffic is happening?

 

[SweetRevelation]

A broadcast on the lan will hit all ports on the lan, as that frame travels through the switched network the switches will add the MAC to their cam tables associating it with the port that they recieved the broadcast from.

CAM and ARP are different.

 

[Quadratic]

The switch will just see an ARP request as a broadcast, like any other. It doesn't care what IP subnet the computer considers itself to be on. It sees the destination and source MAC only, it will forward the broadcast out all ports on the same vlan (not necessarily same subnet, remember these aren't the same thing and aren't even necessarily mapped one to one) while adding the source Mac to the CAM table. Every switch within that vlan will do the same thing. While they're often interchangeable terms, do not confuse vlans with subnets.

CCNP, CCDP, CCIP