Quick Spanning Tree question

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Hi peeps,

Have wee quick question for you, just going over my theory for spanning tree (just when you think u'v got it, u second guess it and get it wrong!...well i do lol)

below is a lab setup;


a quick assumption:
SWA = MAC address is aaaa.aaaa.aaaa.aaaa
SWB = MAC address is bbb.bbbb.bbbb.bbbb
SWC = MAC address is cccc.cccc.cccc.cccc

now am i correct in assuming that the port that will set to the role of blocking with be FA0/1 on SWB, the thing that keeps throwing me is i assume that the because SWC has the highest MAC (which will be reflected the in BID) this means the blocking port is automatically on SWC - now i believe the BID (Bridge ID and Priority) is only needed for the election purpose of who is root bridge and this root bridge will instigate the bpdu's. ... because now that election is over we are now looking at the quickest way to the root bridge (via cost)

So my answer that FA0/1 on SWB will be blocked (set to a role of alternative) is correct, that right?

 

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found this lovely article

has a similar lab setup:

this shows all the same cost paths to the root (where as mine was different costs (link speeds) but the snippet found here:



Note

In each determination process discussed so far, two or more links might have identical Root Path Costs. This results in a tie condition, unless other factors are considered. All tie-breaking STP decisions are based on the following sequence of four conditions:

1. Lowest Root Bridge ID
2. Lowest Root Path Cost to Root Bridge
3. Lowest Sender Bridge ID
4. Lowest Sender Port ID

Clarifies my thinking, so once the election is over, port states need to be assigned and as you can see step 2 states: Lowest Root Path Cost to Root Bridge so my answer is correct

 

Thought i got this, but im confused again, here is my home lab;



now the blocking port is on switch2 fa0/12 , now i would have thought that MAC address from SW3 was higher so that would have been blocked?

output from SW2:
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 0001.96a0.dac0
Cost 19
Port 1 (FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0009.43ed.a4c0
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

Interface Port ID Designated Port ID
Name Prio.Nbr Cost Sts Cost Bridge ID Prio.Nbr
---------------- -------- --------- --- --------- -------------------- --------
Fa0/1 128.1 19 FWD 0 24577 0001.96a0.dac0 128.13
Fa0/12 128.12 19 BLK 19 32768 00d0.0600.1c00 128.25

P.S the only switch i changed the priorty for was SW1 - this was changed to 24577

anyone?

 

Hello,

The question is strange indeed, and your thinking is right, and that port should not have been blocked,

To work out why that was blocked, I will need the ‘’ show spanning tree command issued on all 3 switches, and output sent to me please.

Also the decision making process has 5 steps

Five-Step decision Sequence
Step 1 - Lowest BID
Step 2 - Lowest Path Cost to Root Bridge
Step 3 - Lowest Sender BID
Step 4 - Lowest Port Priority
Step 5 - Lowest Port ID

Thanks
Manee

 

Thanks Manee,

OKay output from Switch1 (trimmed down to only ports that are being used for trunking, as IOS is old and the output is miles long!)

Switch1:
S1-Switch-C2924M#show spanning-tree

Spanning tree 1 is executing the IEEE compatible Spanning Tree protocol
Bridge Identifier has priority 24577, address 0001.96a0.dac0
Configured hello time 2, max age 20, forward delay 15
We are the root of the spanning tree
Topology change flag not set, detected flag not set, changes 1
Times: hold 1, topology change 35, notification 2
hello 2, max age 20, forward delay 15
Timers: hello 1, topology change 0, notification 0


Interface Fa0/7 (port 19) in Spanning tree 1 is FORWARDING
Port path cost 100, Port priority 128
Designated root has priority 24577, address 0001.96a0.dac0
Designated bridge has priority 24577, address 0001.96a0.dac0
Designated port is 19, path cost 0
Timers: message age 0, forward delay 0, hold 0
BPDU: sent 0, received 0


Interface Fa0/11 (port 24) in Spanning tree 1 is FORWARDING
Port path cost 19, Port priority 128
Designated root has priority 24577, address 0001.96a0.dac0
Designated bridge has priority 24577, address 0001.96a0.dac0
Designated port is 24, path cost 0
Timers: message age 0, forward delay 0, hold 0
BPDU: sent 0, received 6

Switch2:
S2-Switch-2950#show spanning-tree

VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 0001.96a0.dac0
Cost 19
Port 1 (FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0009.43ed.a4c0
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

Interface Port ID Designated Port ID
Name Prio.Nbr Cost Sts Cost Bridge ID Prio.Nbr
---------------- -------- --------- --- --------- -------------------- --------
Fa0/1 128.1 19 FWD 0 24577 0001.96a0.dac0 128.13
Fa0/12 128.12 19 BKN* 19 32768 00d0.0600.1c00 128.25


Switch3: (trimmed down to only ports that are being used for trunking, as IOS is old and the output is miles long!)

3-Switch2924#show spanning-tree

Spanning tree 1 is executing the IEEE compatible Spanning Tree protocol
Bridge Identifier has priority 32768, address 00d0.0600.1c00
Configured hello time 2, max age 20, forward delay 15
Current root has priority 24577, address 0001.96a0.dac0
Root port is 13, cost of root path is 19
Topology change flag not set, detected flag not set, changes 1
Times: hold 1, topology change 35, notification 2
hello 2, max age 20, forward delay 15
Timers: hello 0, topology change 0, notification 0

Interface Fa0/1 (port 13) in Spanning tree 1 is FORWARDING
Port path cost 19, Port priority 128
Designated root has priority 24577, address 0001.96a0.dac0
Designated bridge has priority 24577, address 0001.96a0.dac0
Designated port is 24, path cost 0
Timers: message age 1, forward delay 0, hold 0
BPDU: sent 3, received 246

Interface Fa0/12 (port 25) in Spanning tree 1 is FORWARDING
Port path cost 19, Port priority 128
Designated root has priority 24577, address 0001.96a0.dac0
Designated bridge has priority 32768, address 00d0.0600.1c00
Designated port is 25, path cost 19
Timers: message age 0, forward delay 0, hold 0
BPDU: sent 0, received 9


I hope this helps, i still cant see the issue ....

 

done some more diggin on switch 2:


S2-Switch-2950#show spanning-tree detail

Port 12 (FastEthernet0/12) of VLAN0001 is broken (Port Type Inconsistent) <--- mmmm whats this
Port path cost 19, Port priority 128, Port Identifier 128.12.
Designated root has priority 24577, address 0001.96a0.dac0
Designated bridge has priority 32768, address 00d0.0600.1c00
Designated port id is 128.25, designated path cost 19
Timers: message age 2, forward delay 14, hold 0
Number of transitions to forwarding state: 0
BPDU: sent 13, received 1189



S2-Switch-2950#show int switchport

Name: Fa0/12
Switchport: Enabled
Administrative Mode: static access < ---ah forgot to set to trunk!
Operational Mode: static access
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: native
Negotiation of Trunking: Off
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Trunking VLANs Enabled: ALL
Pruning VLANs Enabled: 2-1001

sure enough;

S2-Switch-2950#show run inter fa0/12
Building configuration...

Current configuration : 73 bytes
!
interface FastEthernet0/12
switchport mode access
no ip address
end

changed this saved configure, reloaded switches;

S2-Switch-2950#show spanning-tree

VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 0001.96a0.dac0
Cost 19
Port 1 (FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0009.43ed.a4c0
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 15

Interface Port ID Designated Port ID
Name Prio.Nbr Cost Sts Cost Bridge ID Prio.Nbr
---------------- -------- --------- --- --------- -------------------- --------
Fa0/1 128.1 19 FWD 0 24577 0001.96a0.dac0 128.13
Fa0/12 128.12 19 BLK 19 32768 00d0.0600.1c00 128.25

Still blocking Fa0/12 .............mmmmmm i just cant see why!

 

The reason is the bridge priority:

From your earlier post, above, note that the root bridge has a priority of 24577 (which you set yourself), SW2 has a priority of 32769, but SW3 has a priority of 32768. So SW3 becomes the designated bridge for the SW2 - SW3 segment.

The real question is why has SW2 and SW3 have different priorities - that is because SW2 uses extended system id while SW1 and SW3 do not, presumably because SW2 is a 2950 and the SW1 and SW3 are 2924's.

Spice_Weasel

 

As always Spice_Weasel you have hit the nail on the head ....

Many thanks!

 

Thanks, glad to help, good questions like this are a great way to learn the practical side of networking

Spice_Weasel