STP Basics

 Wissenswertes:

  • Lower is always better ( Priority, Cost, Root Election, MAC Adresse etc..)
  • 0 ist die tiefste Priority und 32768 die höchste (Standard)
  • Lower is always better ( Priority, Cost, Root Election, MAC Adresse etc..)
  • Konfigurierbare Priorities: 0, 4096, 8192, 12288, 16384….32768
  • Jeder Switch hat nur einen einzigen Root Port
  • Ein Switch der nur Designated Ports hat, ist per Definition Root Bridge
  • Eine Root Bridge hat eine Blocking Ports.

Die wichtigstens Path Costs:

  •  10 Mbit/s = 100
  • 100 Mbit/s = 19
  • 1000 Mbit/s = 4
  • 10 Gbit/s = 2

 Zur Berechnung der Pfadkosten wird die Cost sämtliche Links zusammen gezählt:

 

Aus Sicht eines Non-Root Switches:

CAT1#show spanning-tree root 

                                        Root    Hello Max Fwd
Vlan                   Root ID          Cost    Time  Age Dly  Root Port
---------------- -------------------- --------- ----- --- ---  ------------
VLAN0001             1 001b.54a5.6c00        12    2   20  15  Po1             
VLAN0005             5 20  15  Po1             
VLAN0010            10 001b.54a5.6c00        12    2   20  15  Po1             
VLAN0011            11 001b.54a5.6c00        12    2   20  15  Po1             
VLAN0012            12 001b.54a5.6c00        12    2   20  15  Po1             
VLAN0013            13 001b.54a5.6c00        12    2   20  15  Po1             
VLAN0014            14 001b.54a5.6c00        12    2   20  15  Po1             
VLAN0015            15 001b.54a5.6c00        12    2   20  15  Po1             
VLAN0016            16 001b.54a5.6c00        12    2   20  15  Po1             
VLAN0017            17 001b.54a5.6c00        12    2   20  15  Po1   

CAT1#show spanning-tree vlan 10

VLAN0010
 Spanning tree enabled protocol ieee
 Root ID Priority 10
 Address 001b.54a5.6c00
 Cost 12
 )
 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

 Bridge ID Priority 4106 (priority 4096 sys-id-ext 10)
 Address 0014.a978.b980
 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
 Aging Time 300 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/2 Desg FWD 4 128.2 P2p Edge 
Fa0/22 Desg FWD 19 128.24 P2p 
Po1 Root FWD 12 128.56 P2p             

Aus Sicht einer Root Bridge:

CAT2#show spanning-tree root port 
VLAN0001         This bridge is root
VLAN0005         This bridge is root
VLAN0010         This bridge is root
VLAN0011         This bridge is root
VLAN0012         This bridge is root
VLAN0013         This bridge is root
VLAN0014         This bridge is root
VLAN0015         This bridge is root
VLAN0016         This bridge is root
VLAN0017         This bridge is root

CAT2#show spanning-tree vlan 10

VLAN0010
 Spanning tree enabled protocol ieee
 Root ID Priority 10
 Address 001b.54a5.6c00
 This bridge is the root
 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

 Bridge ID Priority 10 (priority 0 sys-id-ext 1b.54a5.6c00
 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
 Aging Time 300 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/1 Desg FWD 4 128.1 P2p Edge 
Fa0/15 Desg FWD 19 128.17 P2p Edge 
Fa0/16 Desg FWD 19 128.18 P2p Edge 
Fa0/20 Desg FWD 19 128.22 P2p 
Po1 Desg FWD 12 128.56 P2p 

Manipulation der Pfade:

1) via Costs
2) via Port Priority

CAT1(config-if)#int fa0/22

CAT1(config-if)#spanning-tree ?
  bpdufilter     Don't send or receive BPDUs on this interface
  bpduguard      Don't accept BPDUs on this interface
  cost           Change an interface's spanning tree port path cost
  guard          Change an interface's spanning tree guard mode
  link-type      Specify a link type for spanning tree protocol use
  mst            Multiple spanning tree
  port-priority  Change an interface's spanning tree port priority
  portfast       Enable an interface to move directly to forwarding on link up
  stack-port     Enable stack port
  vlan           VLAN Switch Spanning Tree


CAT1(config-if)#spanning-tree port-priority ?
  <0-240>  port priority in increments of 16


CAT1(config-if)#spanning-tree port-priority 32 
CAT1(config-if)#

 

 fortsetzung folg….

 

Samuel Heinrich
Senior Network Engineer at Selution AG (Switzerland)
Arbeitet in Raum Basel (Switzerland) als Senior Network Engineer mit über 10 Jahren Erfahrung im Bereich Netzwerk und Telekommunikation.

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