Spanning-tree Interoperability Switch Settings

Spanning tree is used to avoid network loop in layer 2 switch

Common Spanning tree characteristics :

• Long forwarding time (50 seconds)

• Blocking (20 seconds -> max-age)
• Listening (15 seconds -> forward delay)
• Learning (15 seconds -> forward delay) -> building mac table
• Forwarding -> up/up
• Disabled

Spanning tree timers :

• Blocking - Listening 20 seconds
• Listening - Learning 15 seconds
• Learning - Forwarding 15 seconds

For faster forwarding state when you connect to workstation/pc, you can use spanning tree portfast.

Spanning tree portfast characteristics :

• Connecting only to workstation, not switch or there will be a loop network
• Decreasing forwarding time to zero
• No TCN (topology change notification)

If you connect switch to a portfast port, there is opportunity for network loop. Although it will no happen because of the stp algorithm. But in the first time, the network will be overwhelmed by the broadcast storm.

Common spanning-tree result :

W1#show spanning-tree

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    24577
             Address     0019.f5d5.4256
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    24577  (priority 24576 sys-id-ext 1)
             Address     0019.f5d5.4256
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- ---------------------------
Fa0/1               Desg FWD 19        128.3    P2p 

Global Config (per port access) 

interface fa0/0 

shutdown 

interfacen fa0/0

spanning-tree portfast

int fa0/0

no shutdown

SW1#show spanning-tree

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    24577
             Address     0019.f5d5.4256
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    24577  (priority 24576 sys-id-ext 1)
             Address     0019.f5d5.4256
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- ---------------------------
Fa0/1               Desg FWD 19        128.3    P2p Edge 

There is identifier of edge in spanning-tree type. The port switches to forwarding directly without dealing listening, learning state.


Configuring Spanning Tree for all access ports (GLOBAL)

Global Config
spanning-tree portfast default

SW1#show spanning-tree summary
Switch is in rapid-pvst mode
Root bridge for: VLAN0001
Extended system ID           is enabled
Portfast Default             is enabled
PortFast BPDU Guard Default  is disabled
Portfast BPDU Filter Default is disabled
Loopguard Default            is disabled
EtherChannel misconfig guard is enabled
UplinkFast                   is disabled
BackboneFast                 is disabled
Configured Pathcost method used is short

Name                   Blocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ----------
VLAN0001                     0         0        0          1          1
---------------------- -------- --------- -------- ---------- ----------
1 vlan                       0         0        0          1          1

Spanning-tree Portfast & Spanning-tree Portfast Trunk Difference

Access port,
spanning-tree portfast and spanning-tree portfast trunk make the port as  PortFast-enabled (edge port). That port will be Forwarding state after coming up.

Trunk Port
spanning-tree portfast has no effect on trunk ports. For Trunk port if you want immediately to make the trunk in forwarding state,use spanning-tree porfast trunk command. But this command must not be used when connecting to switch. Because it will no detect looping in your network. This command only suitable when connecting to Layer 3 devices for examples routers (with encapsulation vlans).


Spanning-tree Rapid (802.1w) RW

The forward delay is not used (synchronization process)
Faster,Max age timer is used (6 seconds -> three missed BPDUs in a row)

RSTP bridge port roles:

1. Root port – The closest port to the root bridge (forwarding) cost 
2. Designated port – A forwarding port 
3. Alternate port – The best alternate path to the root bridge. .
4. Backup port – Two Ports/more connected to a hub. The port will be in fwd in case one of them in trouble
5. Disabled port

Spanning-tree States :

1. Discarding
2. Learning
3. Forwarding

Spanning-tree Compatibility
RSTP - STP (backward to STP in that port boundary)
PVRSTP - RSTP (compatible using vlan 1 instance with switch in rstp mode, other instances will be tunneled without connecting to switch running rstp mode)
MSTP - RSTP, RPVST+,PVST+ ->compatible with instance 0 in mst -> common spanning tree (CST).

RSTP,RPVST+, PVST+ see MST region as a single switch.

MSTP - MSTP pre standard (MISTP) -> must set spanning-tree mst pre-standard in mst switch

Table 17-2 PVST+, MSTP, and Rapid-PVST+ Interoperability

	PVST+	MSTP	Rapid PVST+
PVST+	Yes	Yes (with restrictions)	Yes (reverts to PVST+)
MSTP	Yes (with restrictions)	Yes	Yes (reverts to PVST+)
Rapid PVST+	Yes (reverts to PVST+)	Yes (reverts to PVST+)	Yes

MSTP instance 0 is the IST/CST region inside MST.

MSTP instance 0 as the CST region outside of MST.

The MST to PVST+ interaction replicates the BPDU from IST for each vlan. Simulating PVST+ neighbour.