STP Root Bridge pointing to the Server

[mahesh18]

                   Hi Everyone,

Trying to understand STP root bridge  in a network

Switch A  is SVI  for VLAN 527  and is running PVST  mode.

sh spanning-tree summary

Switch is in pvst mode

Root bridge for: none

sh spanning-tree vlan 527

VLAN0527
  Spanning tree enabled protocol ieee
  Root ID    Priority    4123
             Address     000d.28bf.5000
             Cost        12
             Port        8 (GigabitEthernet1/0/8)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    33295  (priority 32768 sys-id-ext 527)
             Address     001b.90e7.3e00
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/0/2             Desg FWD 19        128.2    P2p Edge
Gi1/0/8             Root FWD 4         128.8    P2p

Now mac 000d.28bf.5000 should be of root bridge right and port gi1/0/8 should take us to root bridge switch right?

But here int gi1/0/8 goes to server  and mac 000d.28bf.5000 goes to switch B.

Switch B shows

       sh spanning-tree vlan 527

VLAN0527
  Spanning tree enabled protocol ieee
  Root ID    Priority    4123
             Address     000d.28bf.5000
             Cost        16
             Port        333 (GigabitEthernet6/13)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    4623   (priority 4096 sys-id-ext 527)
             Address     000d.28bf.5000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi5/33              Desg FWD 4         128.289  P2p Edge
Gi6/13              Root FWD 4         128.333  P2p

MAC --- 000d.28bf.5000 belongs to switch B

Switch B  shows Root port going back to switch A.

Need to know which device is root bridge here Switch B  as it has same MAC address but Switch A interface going to server

Thanks

Mahesh

[glen.grant]

which ports are your connecting ports between the switches?

[mahesh18]

Hi glen,

Switches A and B are connected via

Switch A port    Gig 1/0/25        Goes to Switch B  port--  Gig 6/13

sh span vlan 527  on switch A  shows

Gi1/0/25            Desg FWD 4         128.25   P2p

thanks

Mahesh

[ALIAOF_]

What do you get when you do:

show spanning-tree root address

show spanning-tree root port

[mahesh18]

Hi,

From switch B

sh span     root address

VLAN0527         000d.28bf.5000

VLAN0527         GigabitEthernet6/13

Switch A

VLAN0527         000d.28bf.5000

VLAN0527         GigabitEthernet1/0/8

thanks

Mahesh

[milan.kulik]

Hi,

what about the server mentioned?

Isn't it connected by a second NIC to Switch B within another VLAN?

And bridging enabled between its NICs?

HTH,

Milan

[mahesh18]

Hi Milan,

I have to check on that if server is connected with second NIC  to switch B or not?

will get back to you on that.

When you say bridging enabled between server NIC can you please tell me what does it mean ?

Thanks

Mahesh

[milan.kulik]

Hi Mahesh,

my idea is following:

I can imagine the server being connected to Switch A (access port within VLAN 527) and also to Switch B by a second NIC (access port within a different VLAN - VLAN23, e.g.).

And as you know, some server (even PC) Operating Systems are capable of bridging between particular NICs - creating a bridge/switch effectively.

So let's imagine Switch B being configured as root for VLAN23 too with bridge priority 4096.

With sys-id-ext 23 added for VLAN ID 23, the STP BPDU sent on the VLAN 23 port would include MAC Address     000d.28bf.5000 (=Bridge ID) and  Priority    4119   (priority 4096 sys-id-ext 23).

The server with bridging activated would forward the BPDU out of the second NIC and Switch A would receive it (on an access port assigned to VLAN 527).

Adding port cost 4 for a Gigabit port would create the root priority 4123 visible on your Switch A.

(And note this is a better value than the original BPDU sent from Switch B within VLAN 527.)

As the same MAC address is used as a switch ID for all VLANs, you would see the same Bridge IDs for root on both switches.

So in fact you would see one common STP root for both VLANs 527 and 23 interconnected through the server.

(The calculations might not be accurate but the principle reamins valid.)

HTH,

Milan

[acampbell]

Hi,

Just to add that the root bridge MAC claims 000d.28bf.5000

MAC addresses starting with 000d28 are Cisco devices.

Can you confirm that there is NO other device

between SWITCH A -  g1/0/8

and SWITCH B - g6/13

May be use sh cdp nei to confirm

Regards,
Alex.
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