Spanning Tree Protocol

BPDU

Configuration BPDU

BPDU Field	Length in Octets
Protocol Identifier	2
Protocol Version	1
BPDU Type	1
Flags	1
Root Bridge ID	8
Root Path Cost	4
Sending Bridge ID	8
Sending Port ID	2
Message Age	2
Max Age	2
Hello Time	2
Forward Delay	2

Topology Change Notification BPDU

BPDU Field	Length in Octets
Protocol Identifier	2
Protocol Version	1
BPDU Type	1

Protocol Version

0 for legacy STP
2 for RSTP

Flags

BPDU Flag Field	Length in Bits
Topology Change Acknowledgment	1
Agreement	1
Forwarding	1
Learning	1
Port Role	2
Proposal	1
Topology Change	1

Message Age

Serves as a TTL to prevent a BPDU being forwarded in a loop which can cause all the ports in a forwarding state and not expired.

BPDU is assumed to be expired when MaxAge - MessageAge less than 0. MessageAge increases before being transmited to other bridge.

Port Cost

Port Speed	Pre-802.1D-1998	802.1D-1998(PVST, RSTP)	802.1D-2004(MSPT)
10 Mbps	100	100	2000000
100 Mbps	10	19	200000
1 Gbps	1	4	20000
10 Gbps	1	2	2000

The following command can be used to change which cost standard to used.

(config)# spanning-tree pathcost method [short | long]

Example

IOU2(config)#do sh spanning-tree vlan 1

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     aabb.cc00.0200
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     aabb.cc00.0200
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  15  sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Desg FWD 100       128.1    Shr
Et0/1               Desg FWD 100       128.2    Shr
Et0/2               Desg FWD 100       128.3    Shr
Et0/3               Desg FWD 100       128.4    Shr
Et1/0               Desg FWD 100       128.5    Shr
Et1/1               Desg FWD 100       128.6    Shr
Et1/2               Desg FWD 100       128.7    Shr
Et1/3               Desg FWD 100       128.8    Shr
Et2/0               Desg FWD 100       128.9    Shr
Et2/1               Desg FWD 100       128.10   Shr
Et2/2               Desg FWD 100       128.11   Shr
Et2/3               Desg FWD 100       128.12   Shr
Et3/0               Desg FWD 100       128.13   Shr
Et3/1               Desg FWD 100       128.14   Shr
Et3/2               Desg FWD 100       128.15   Shr
Et3/3               Desg FWD 100       128.16   Shr

IOU2(config)#spanning-tree pathcost method ?
  long   Use 32 bit based values for default port path costs
  short  Use 16 bit based values for default port path costs

IOU2(config)#spanning-tree pathcost method long
IOU2(config)#do sh spanning-tree vlan 1

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     aabb.cc00.0200
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     aabb.cc00.0200
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Desg FWD 2000000   128.1    Shr
Et0/1               Desg FWD 2000000   128.2    Shr
Et0/2               Desg FWD 2000000   128.3    Shr
Et0/3               Desg FWD 2000000   128.4    Shr
Et1/0               Desg FWD 2000000   128.5    Shr
Et1/1               Desg FWD 2000000   128.6    Shr
Et1/2               Desg FWD 2000000   128.7    Shr
Et1/3               Desg FWD 2000000   128.8    Shr
Et2/0               Desg FWD 2000000   128.9    Shr
Et2/1               Desg FWD 2000000   128.10   Shr
Et2/2               Desg FWD 2000000   128.11   Shr
Et2/3               Desg FWD 2000000   128.12   Shr
Et3/0               Desg FWD 2000000   128.13   Shr
Et3/1               Desg FWD 2000000   128.14   Shr
Et3/2               Desg FWD 2000000   128.15   Shr
Et3/3               Desg FWD 2000000   128.16   Shr

Topology Change Event

A Topology Change Notification BPDU is received by a Designated Port of a switch
A port moves to the Forwarding state and the switch has at least one Designated Port (meaning that it is not a standalone switch with just a Root Port connected to an upstream switch and no other connected ports)
A port moves from Learning or Forwarding to Blocking
A switch becomes the root switch

Advanced Feature

Portfast

When an interface receives BPDU, it disables portfast.

IOU1(config-if)#spanning-tree portfast [trunk]

UplinkFast

Uplinkfast assumes the switch is not the root bridge, so it increases its bridge-id by increasing its priority, and all ports have their STP cost increased by 3000.

When uplinkfast changes a port to forwarding, it floods dummy packets with the MAC address in its CAM table to let the upstream devices know where to forward the upcoming frames.

Switch(config)# spanning-tree uplinkfast

Switch# sh spanning-tree uplinkfast

BackboneFast

Switch(config)# spanning-tree backbonefast

BPDU Filter

Switch(config)# spanning-tree portfast bpdufilter default

Enables the BPDU filter on the portfast interfaces. This prevents switch from sending BPDUs but receiving BPDU is allowed. A few BPDUs will be sent as a safety mechanism as well.

Switch(config-if)# spanning-tree bpdufilter enable

BPDU Guard

(config)# spanning-tree bpduguard enable

Loop Guard

(config)# spanning-tree loopguard default

(config-if)# spanning-tree guard loop

UDLD

(config)# udld enable

This will only activate UDLD for fiber links.

STP EtherChannel Misconfig Guard

This mechanism makes an assumption that if multiple ports are correctly bundled into a Port-channel at the neighbor side, all BPDUs received over links in this Port-channel must have the same source MAC address in their Ethernet header, as the Port-channel interface inherits the MAC address of one of its physical member ports. The EtherChannel Misconfig Guard is active by default and can be deactivated using the no spanning-tree etherchannel guard misconfig global configuration command.

STP