Task 1.18 STP Root Bridge Election
ndiayemalick
✭✭
Hello,
I am a little confused about the solution given in the task.
The task states to:
- Configure SW1 as the STP Root for all active Vlans
- If SW1 goes down SW4 should take over as the STP Root for all active Vlans
The way I approched the task is to set SW1 as the primary root and SW4 as the secondary root by issuing the "spanning-tree vlan 9,79,7,67,146,43,5,58,8,22,10 root primary" on SW1 and "spanning-tree vlan 9,79,7,67,146,43,5,58,8,22,10 root secondary" on SW2. The task states for all active vlans.
In the solution, SW1 is configured with a priority of 0 for all active vlans which would make SW1 as the root bridge. On SW4, the priority for the vlan has been set to 4096 which is less than the default priority (32768). Would not this command make SW4 less likely of being the root bridge than SW2 & SW3 which has their priority set to the default?
Best Regards
Comments
Check your config and see what priority is configured on SW1 and SW4, and you will see what the command set it to. When you ran the command 'spanning-tree vlan 9,79,7,67,146,43,5,58,8,22,10 root primary' this is actually a macro that checks the priorities of all the switches and adjusts it down accordingly. It would have done the same on SW4 but given a priority higher than that set on SW1 but lower than that of SW2 and SW3.
Isn't the default priority 32768 ? If so, then SW2 or SW3 would be elected the root bridge instead of SW4 which has a higher priority of 4096 .
No - the lowest priority wins, so you set SW1 to 0 (so this is the
root) and SW4 to 4096 (root when SW1 is gone), and SW2 and SW3 are set
to the default of 32768
http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/12.2_44_se/configuration/guide/swstp.html#wp1169930
ndiayemalick wrote:
The problem is that the terminology is confusing. When you say a
"higher" priority value for STP, it actually means a lower numerical
value. The value 0 is the highest priority. 32768 is the default, and
while numerically higher, is a "lower" priority value. Look at the
result of the "spanning-tree vlan 1 root primary" command in the
running config and you'll see what I mean.
HTH,
Brian McGahan, CCIE #8593 (R&S/SP/Security)
[email protected]
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
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ccielab wrote:
Hello Brian,
my question is by setting the priority on SW1 to 0, it will become the root bridge. By setting the priority of SW4 to 4096 and leaving the default priority on SW2 & SW3; if SW1 goes down, it would not make SW4 the new root bridge? Correct me if I am wrong please.
Because the task ask to configure SW1 as the root bridge, and SW4 as the new root bridge should SW1 go down.
0 > 4096 > 32768
SW1 - priority 0 <-- root bridge
SW4 - priority 4096 <--- backup root bridge
SW2 - priority 32768 <--- tie for 3rd choice
SW3 - priority 32768 <--- tie for 3rd choice
Brian McGahan, CCIE #8593 (R&S/SP/Security)
[email protected]
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987 x 705
Outside US: 775-826-4344 x 705
Online Community: http://www.IEOC.com
CCIE Blog: http://blog.internetworkexpert.com
ndiayemalick wrote: