INE R&S ATC012 - STP Timers, STP PortFast

STP Timers

• Timers effect the transition between port states
– Set only on the Root Bridge
• Hello
– How often configuration BPDUs are sent
– Defaults to 2 seconds
• MaxAge
– How long to wait in blocking state without hearing a BPDU
– Defaults to 20 seconds
• Forward Delay
– How long to wait in each the listening and learning phases
– Defaults to 15 seconds

Changing STP Timers

• Configuration
– spanning-tree vlan [vlan]hello-time
– spanning-tree vlan [vlan] forward-time
– spanning-tree vlan [vlan] max-age
• Verification
– show spanning-tree vlan [vlan]

Advanced STP Features

• PortFast
– Edge ports shouldn’t be subject to Forwarding Delay
– Also effects TCN generation

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CSP,PVST,PVST+都是由Root Switch送BPDU.....STP Timer也都由Root Switch控制
RSTP hop by hop Switch送BPDU

BLK--->LIS--->LRN---FWD
        20s      15s          15s

正常SW接上PC
LIS--->LRN---FWD
     15s          15s

當SW針對特定Port下spanning-tree portfast時, 此Port的UP/DOWN就不會產生TCN通知ROOT
避免所有的SW被Root通知Topology Change Notice....Flush CAM Table.......
Portfast --->     LIS-------jump to--------->FWD

我們可以用指令確認
# debug spanning-tree events

# mac address-table aging-time 300

Global Command for portfast
# spanning-tree portfast default

如果接PC/Server具有Trunk能力...但沒有送BPDU封包時....

int fa0/3
 description Connected to VMware ESXi
 spanning-tree portfast trunk
 switchport trunk encapsulation dot1q
 switchport mode trunk