Cisco switch and spanning tree

[just1moretime]

I am trying to learn spanning tree. Using Cisco Packet
Tracer with 3 switches and 2 hubs for a basic example.
All switch settings default.

Sw0 G1/0 + Sw1 G1/0 + Sw2 G1/0 all to hub 0
Sw0 G1/1 + Sw1 G1/1 + Sw2 G1/1 all to hub 1

According to my calculations, the wrong switch comes up the root bridge.

Here is my data:
S0 G1/0-MAC-0001.C99B.7C01 TO HUB 0 PORT 0
S0 G1/1-MAC-0001.C99B.7C02 TO HUB 1 PORT 0
S1-G1/0-MAC-0000.0C07.BB01 TO HUB 0 PORT 1
S1-G1/1-MAC-0000.0C07.BB02 TO HUB 1 PORT 1
S2-G1/1-MAC 0060.3E04.3701 TO HUB 0 PORT 2
S2 G1/2-MAC-0060.3E04.3702 TO HUB 1 PORT 2

IN PACKET TRACER SW 0 COMES UP ROOT BRIDGE.
USING SHOW SPANNING TREE cmd, ROOT MAC IS 0010.1130.98DC. HOW DID THIS NUMBER COME ABOUT?

MY thoughts/miscalculations:

I DONT UNDERSTAND. ACCORDING TO MY CALCULATIONS, SW 1 SHOULD
BE THE ROOT BECAUSE LOWEST MAC.

AM I FIGURING BRIDGE ID CORRECTLY?

S1 G1/0 0000.0C07.BBO1 = 201,833,217 201,833,217+32768= 210,865,985 BRIDGE ID SW 1

SO G1/0 0001.C99B.7C01 = 7,677,377,537
7,677,377,537+32768=7,677,410,305 BRIDGE ID SW 0 x

S2 G1/0 0060.3E04.3701 = 413,357,324,033
413,357,324,033 + 32768 = 413,357,356,801 BRIDGE ID SW 2

THANKS FOR YOUR HELP.

 

[just1moretime]

Cisco switches use vlan 1 mac by dflt.

 

[VinceWhirlwind]

You should design your spanning-tree schema and force your root bridge by assigning it a priority.

 

[Quadratic]

Never leave your root bridge placement up to the MAC tie-breaker. You want your network pathing to be predictable, you want network convergence in failure scenarios to be predictable, and you want to be able to effectively load balance or optimize uplink usage by manipulating STP blocking states. Bottom line, protect your root bridge.

CCNP, CCDP, CCIP

 

[Quadratic]

Also, for your question, "show spanning" will give you the bridge priority. I'm guessing SW0 has a priority configured.


CCNP, CCDP, CCIP