Spanning Tree question

nevolved

Do any frames HAVE to go through the root bridge?

The following are some of the scenarios where they don't:

If on a given switch (not root) you have 2 devices on the same access vlan traffic between them will stay on the switch

2 switches that are both downstream from the root on the same path will not be forced to send frames through the root on devices with the same vlan

For Router-on-a-Stick frames would go through the root every time in a well designed network (because the router would probably be directly connected to the switch), but that still doesn't mean that they would HAVE to ...for instance if the router was connected to a non root switch



So, for some reason I'm remembering reading somewhere that some frames have to go through the root bridge, am I just going crazy or blurring Router-on-a-Stick design in with Spanning Tree? It's been close to a year since taking the CCNA, which is probably why I'm not remembering correctly. I did read through my books and on the Internet prior to posting.


Thanks,(and yes I know this is rambling)

roinuj2

Hey nevolved,

Yes you are blurring router on a stick with STP,

A router on a stick is related to VLANS and Inter-VLAN routing which is a layer 3 operation - STP is a layer 2 protocol, the two don't care about each other.

A router being on a non-root switch doenst mean other switches / clients will disobey the STP rule and not go to root.

mikej412

nevolved wrote: »

for some reason I'm remembering reading somewhere that some frames have to go through the root bridge

Topology Change Notification (TCN) BPDUs are propagated up to the root bridge, which then sets the Topology Change flag in its BPDUs -- then all the other switches learn from that they should quickly age out their forwarding tables.

nevolved

Ahhh thanks. That must be what was bothering me.