About STP

[yuanguitao]

I am learning STP recent days, and an issue came to me. if a root bridege has been elected and the stp topology is stable. while an switch with higher BID join the LAN, and what would happen? if the election would be held again and the new one would be the root bridge? thank you for your answers. big guys.

[Peter Paluch · Accepted solution]

Hello Wei,

In STP, it is the bridge with the lowest BID that will win the root elections. So if a new switch with a higher BID is connected, nothing happens with respect to the current root bridge elections - the root bridge will stay the same.

If a bridge with a lower BID than the current root's BID is connected, however, this new switch will become the new root bridge.

Best regards,

Peter

[yuanguitao]

Thank you for your answer. i think i made a mistake, i should write " the switch with higer priority instead of BID". Anyway, i got the answer.

[InayathUlla Sharieff]

WIEYOO,

In STP always it would be lowest --Just you remember this it would easy your understanding on STP :-)

At any given point of time who ever been configured as lowest BID(Priority & Mac address ) usually we configure the priority to be lower than any other switch that would become the root bridge ,

Regards

Inayath.

[johnlloyd_13]

Hi Peter,

Sorry if this isn't the right thread, but I was doing an STP lab just recently.

Brief diagram is like this:

871
|
C3560
/ \
C2950_C2960

So I've made the famous triangular switch formation/topology. To my surprise, root bridge was the 871 'coz of its lower MAC address/BID.

Is it possible for a router with built-in switch or perhaps a router with a switch WIC (haven't labbed yet) could win BID election? Sorry again I don't have the config/show output to share coz it's in my other PC. Thanks in advance!

Sent from Cisco Technical Support iPad App

[Aninda Chatterjee]

Hi John,

If Peter doesn't mind, I'll take a stab at this.

The 871 does seem to run spanning-tree on its switchports. This implies that the device is actively participating in spanning-tree and willl have a priority + mac-address set that it will use for root election. If it's priority or mac-address (in case of the need of a tie breaker) is the lowest in your layer 2 switched network, then yes, it will end up becoming the root bridge.

Because you have a single link that connects the 871 to the switch, you are not seeing the real affects of spanning-tree. If you were to add another link between the two or create a possible loop topology which involves the 871, you will see the loop being broken by spanning-tree.

Regards,

Aninda

[Peter Paluch]

Hello Aninda,

f Peter doesn't mind, I'll take a stab at this. 

By all means! You are always cordially welcome to join at any time.

There is almost nothing to add to Aninda's perfect answer. Indeed, the 871 must be running STP on its switchports, otherwise it would not participate in STP at all. And yes, as soon as a router with a switching ports runs STP, it is - from the viewpoint of STP - indistinguishable from a switch. I do not have hands-on experiences with 871 but I know that if EHWIC-SW modules are used in ISR or ISR G2 routers (such as EHWIC-4ESW), they run STP just like a normal switch.

Best regards,

Peter

[johnlloyd_13]

Thanks everyone for answering and have a great weekend! +5 for both

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[Joseph W. Doherty]

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Posting

As the other posters have surmised, the 871 does support STP on its LAN Ethernet ports.  (I recall it might only support "original" PVST without Cisco enhancements.)

When defining a STP topology, good practice would be to set the root's (and secondary root) priority to insure you get the root where you desire in your topology.  There are also other practices, and STP commands, to help insure STP operates as you desire and avoids switches from "hijacking" your root.

[Syed Aman Hussain]

In STP, it is the bridge with the lowest  BID that will win the root elections. So if a new switch with a higher  BID is connected, nothing happens with respect to the current root  bridge elections - the root bridge will stay the same.

If a bridge with a lower BID than the current root's BID is connected, however, this new switch will become the new root bridge.

correct answer

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