Wednesday, February 12, 2014

STP Review:

Lessons Learned.


You can verify which switch is the Root with the command:

# sh spanning=tree root

This will show the Per-VLAN STP instance. The Root priority, the MAC ID of the root Switch for that instance . the end to end cost to get to the root switch and the timers the particular root bridge is advertising.

NOTE: if you modify the root bridge timers – this will affect the entire spanning-tree. Setting the timers locally will not affect your network.

 
To modify STP you also change the Root port or designated ports role to change the election process.

The root port is the interface that has the lowest total cost value in order to reach the root bridge. The switch will look at all BPDU’s coming in plus the local link cost.


Ex: If my link cost is 10 and the switch receives a BPDU with a cost of 10, that means the total root path cost will be 20.

Basically adding the cost on a hop-by-hop basic. The one with the lowest cost will be used as the root port.

If there’s multiple interfaces and there’s an equal cost value. Then we need to look at the lowest upstream bridge ID, then the lowest upstream port ID. PORT ID will only because if there’s two or more link’s to the upstream switch.

 
This can be change by manually changing the port Cost or bandwidth. You can also lower the bridge ID to change the priority.

 

# sh spanning-tree detail will show all local interfaces and info received in from the upstream switches.

 
Output from above command:

Interface           Role Sts Cost      Prio.Nbr Type

------------------- ---- --- --------- -------- --------------------------------

Gi0/1               Altn BLK 4             128.49   P2p

Gi0/2               Root FWD 4         128.50   P2p


Note: This only shows the local interface cost not the total root path cost. This also shows’ the port ID (128) Note: the Port ID doe not correlate to the actual port #.

To show what’s actually going on you need to show the detail. EX: sh spanning tree vlan 10 detail

VLAN0010 is executing the rstp compatible Spanning Tree protocol

  Bridge Identifier has priority 32768, sysid 10, address 000b.5fcc.1a80

  Configured hello time 2, max age 20, forward delay 15, transmit hold-count 6

  Current root has priority 24586, address 001c.586c.5840

  Root port is 49 (GigabitEthernet0/1), cost of root path is 4

  Topology change flag not set, detected flag not set

  Number of topology changes 6572 last change occurred 1d02h ago

          from GigabitEthernet0/1

  Times:  hold 1, topology change 35, notification 2

          hello 2, max age 20, forward delay 15

  Timers: hello 0, topology change 0, notification 0, aging 300

 

 Port 49 (GigabitEthernet0/1) of VLAN0010 is root forwarding

   Port path cost 4, Port priority 128, Port Identifier 128.49.

   Designated root has priority 24586, address 001c.586c.5840

   Designated bridge has priority 24586, address 001c.586c.5840

   Designated port id is 128.259, designated path cost 0

   Timers: message age 15, forward delay 0, hold 0

   Number of transitions to forwarding state: 1

   Link type is point-to-point by default

   BPDU: sent 28, received 4390693

 

 Port 50 (GigabitEthernet0/2) of VLAN0010 is alternate blocking

   Port path cost 4, Port priority 128, Port Identifier 128.50.

   Designated root has priority 24586, address 001c.586c.5840

   Designated bridge has priority 28682, address 001c.586c.57c0

   Designated port id is 128.202, designated path cost 3

   Timers: message age 16, forward delay 0, hold 0

   Number of transitions to forwarding state: 2

   Link type is point-to-point by default

   BPDU: sent 3309, received 4387300

 

From this output you need to figure what is the total end-to end cost. In this instance the total cost for GI0/1 is 4. The designated path cost for GI1/2 is 7 - Port path cost 4, plus designated path cost 3.

If the local switch port is Blocking, that means on the other side of that link is the designated port. Likewise the other side of the local Root port is other switches designated port because designated is always facing downstream.

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