• # Bridge Priority

This question is in regards to Spanning Tree Protocol. If one switch has a bridge priority of 32768.1111.833d.1111 and the other switch has a bridge priority of 32768.1111.8339.1111, is 833d selected as the root bridge? Is having a letter as opposed to a number in its MAC address make a switch the rood bridge or a bridge with a higher priority? This is one of the questions that stumped me in ICND2.

• @Ponciana-S-Pajela The letters are just HEX so the place holders are as follows (A=10 B=11 C=12 D=13 E=14 F=15)

in your example 32768.1111.8339.111 should be the bridge because 8339 is lower than 833d

8339 in decimal is 33593
833d in decimal is 33597

remember the lower the number the higher the priority.

The way I remember is by keeping in mind the havoc an older switch can cause on a network that is not set up properly. Because the older switch has a lower MAC address (MAC addresses increase as more switches get manufactured) the older switch will win an STP election. The problem is the older switch is not as fast as newer switches with higher MACs. So the network comes to a halt when everyone tries to use the older less capable switch.

• In STP, when it comes to root election:
the Lowest RID (Root ID= Priority+ MAC) will be the root bridge.

So in your case, the Priority Default to 32768, and what you're left with is the ability to deal with the MAC address in Hex format. So you would have to determine the lower of the 1111.8339.1111 and the 1111.833d.1111. In this instance the 8339 would have been the lower of the two MAC addresses.

If you would have been told that one was 32768.1111.8339.1111 and 28672.1111.833d.1111, then the one with the 833d would have becomes the root bridge.

So it's a combination of both. @Daniel-Espinal make a good point. Normally in STP if your network is stable then it doesn't really matter if the older or newer switch is the root bridge. BUT, if you're constantly adding new switches or changing the network, you would want your newer more powerful switches to become the root bridge. The way to do that, easily is to influence the Priority number.

Cordially,
Ronnie Wong
Edutainer, ITProTV

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