02-26-2013 07:59 AM - edited 03-10-2019 12:21 PM
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
02-26-2013 10:19 AM
which ports are your connecting ports between the switches?
02-26-2013 10:35 AM
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
02-26-2013 11:32 AM
What do you get when you do:
show spanning-tree root address
show spanning-tree root port
02-26-2013 11:35 AM
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
02-26-2013 12:44 PM
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
02-26-2013 01:09 PM
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
02-26-2013 01:41 PM
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
02-26-2013 05:47 PM
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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