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How exactly does STP traverse the network?

Supposed I have three Cisco switches connected to each other like so:

Switch 1 Switch 2 Switch 3

And I'm running STP on all ports as per default settings. Do STP packets traverse the network or do they only need to go to the next switch? For example, in my diagram, for STP to work from Switch 1 to Switch 3, do STP packets need to go directly from Switch 1 to Switch 3, or do they go from Switch 1 to the STP processor on Switch 2, then from Switch 2 to Switch 3?

What would happen if switch 2 was configured to only forward ethernet packets of types ARP and IPv4 and drop everything else (but could still send/receive all packets to/from itself)? Also what would happen if STP was disabled on Switch 2 and someone also directly connected Switch 1 and Switch 3?

I've looked at STP with a packet sniffer - it looks like they are sent from a mac address unique to the port the packet sniffer is connected to and sent to a special mac address for STP. Is this correct?

I'm simplified my example to try to understand how things work - in reality Switch 1 and Switch 3 would be VLANs and Switch 2 would be a Linux-based firewalling bridge. I had some hardware problems with either one of my switches or a device plugged into it which as far as I can tell caused two of my VLANs to be bridged on that switch as well as on the Linux bridge. I did not have STP enabled on the Linux bridge as I didn't think I need it, but when that happened I had address flaps occur on about half my switches and some of my users lost network connectivity for a couple minutes.

Thanks a lot for any help, Josh

Reply to
Josh T
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spanning tree goes from switch to switch (with possibly multiple spanning trees if you use vlans)

depends on implementation - some will just "eat" spanning tree packets, some pass them thru.

most dangerous thing in a spanning tree network is a link which eats spanning tree packets within a loop - you usually get a very efficient broadcast packet generator.

next common problem is a 1 way link - this has a similar effect if it is in the right place within the current spanning tree.

it can be (and usually is on modern hardware), but it doesnt have to be. some older bridges and switches had a single MAC.

Also, you may find devices with a different MAC source per VLAN, or per VLAN / per port....

sounds like a loop, or some variation.

if you have a sniffer look for duplicate packets with broadcast addresses.

if you switch is

Reply to
stephen

Thanks - this one statement and some docs on the Cisco site helped me figure it out.

Josh

Reply to
Josh T

Josh T wrote: [stp questions]

IMO, Clark and Kennedy's Cisco LAN Switching explains STP better than

*ANY* other book. And I'm comparing books like Interconnections, The Switch Book, and all the other industry bibles.
Reply to
Hansang Bae

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