Re: Root Port issue

HI Marc,

The root bridge the core switch 6509-A is at equal distance from
2960. I you try to draw the topology diagram with the below mention points it
will give you the picture. the root bridge 6509-A is 2 hops away from from the
2960 so ideally f0/1 shoud be selected as the root port in 2960 because of
lower port ID but instead f0/2 is getting selected as the root port.

2960
(f0/1)------> 3750-A (G0/1) 3750-A (T1/49)-----> 6509-A (Root Bridge)
       
                                      3750-A (T1/50)-----> 6509-B
2960 (f0/2)------> 3750-B (G0/1) 3750-B (T1/49)-----> 6509-A (Root Bridge)

 
                                             3750-B (T1/50)-----> 6509-B

On
Wednesday, November 6, 2013 10:02 AM, marc edwards <renorider_at_gmail.com>
wrote:
 
Don't forget it is the position of the STP root bridge that will
determine which ports are elected root ports on down stream switch. Which one
of the 3750's is acting as root bridge? Which port on 2960 is closest to root
bridge?

On Tuesday, November 5, 2013, ramesh Kumar wrote:

Hello All,
>
>I
have topology like this, I see a problem that the right port is
>not getting
selected as the Root Port
>
>2960 (f0/1)------> 3750-A (G0/1)
>2960
>(f0/2)------> 3750-B (G0/1)
>
>checking at 2960 i should see f0/1 as RP for
any
>Vlan but strangely f0/2 is showing to be the RP despite having higher
port ID
>than f0/1. Any idea is it because of having fe/ge connectivity?
>
>
>2960#sh
>spann vlan 136
>
>MST1
> Spanning tree enabled protocol mstp
>
Root ID
> Priority 8193
> Address 8478.ac21.b2c1
>
   Cost
> 202000
> Port 2 (FastEthernet0/2)
>
   Hello
>Time 2 sec Max Age 20 sec Forward Delay 15 sec
>
> Bridge ID
 Priority
> 61441 (priority 61440 sys-id-ext 1)
> Address
d0c7.899b.6400
> Hello Time 2 sec Max Age 20 sec Forward Delay
15 sec
>Interface Role Sts Cost Prio.Nbr Type
>------------------- ----
>--- --------- --------
--------------------------------
>Fa0/1
>Altn BLK 200000 128.1 P2p
>Fa0/2 Root FWD 200000 128.2
> P2p
>
>config on 2960
>interface FastEthernet0/1
> switchport trunk allowed
>vlan 136,166,176
> switchport mode trunk
> duplex full
> speed 100
>end
>
>config on
>3750-A
>interface GigabitEthernet0/1
> switchport trunk allowed vlan 136,166,176
> switchport mode trunk
> speed 100
> duplex full
>end
>
>config on 3750-B
>interface
>GigabitEthernet0/1
> switchport trunk allowed vlan 136,166,176
> switchport mode
>trunk
> speed 100
> duplex full
>end
>
>MST config
>
>3750-A#sh spann mst config
>Name
> []
>Revision 0 Instances
configured 2
>
>Instance Vlans mapped
>--------
> ---------------------------------------------------------------------
>0
>
none
>1 1-4094
>-----------------------------------------------------------------------------
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-- 
Marc Edwards
CCIE #38259
Blogs and organic groups at http://www.ccie.net