why access port shows amber light and trunk port green while in blocking state

Answered Question
Mar 14th, 2010
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Hi all,


My question is  i have 2 2950t switches and when i do this


1> connect  port from switch A  to switch B  with crossover cable and config them as

config t

switchport mode access

spanning tree portfast


then Switch A  port has Amber light while it is in blocking state


2>connect  port from switch A  to switch B  with crossover cable and config them as trunk port only


then Switch A port has green light and it is in  blocking state.


My question is why blocking port has green light  instead of Amber


thanks

mahesh

Correct Answer by Jon Marshall about 7 years 2 weeks ago

Mahesh


It's what i was saying. Once you configure fa0/8 as a trunk link on vlan 1 it is still blocking but for vlan 30 -


VLAN0030
  Spanning tree enabled protocol rstp
  Root ID    Priority    32768
             Address     0003.6b7b.5a01
             Cost        22
             Port        65 (Port-channel1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32798  (priority 32768 sys-id-ext 30)
             Address     000b.bece.bbc0
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 1000

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/8            Desg FWD 19        128.8    P2p**********************************
Po1              Root FWD 3         128.65   P2p


You can see that fa0/8 is forwarding for vlan 30. This is why the port is green when you configure it as a trunk because it blocks on vlan 1 but it forwards on vlan 30 so the port shows as forwarding. Remember that each vlan runs it's own STP instance.


Jon

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Jon Marshall Sun, 03/14/2010 - 00:41
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mahesh18 wrote:


Hi all,


My question is  i have 2 2950t switches and when i do this


1> connect  port from switch A  to switch B  with crossover cable and config them as

config t

switchport mode access

spanning tree portfast


then Switch A  port has Amber light while it is in blocking state


2>connect  port from switch A  to switch B  with crossover cable and config them as trunk port only


then Switch A port has green light and it is in  blocking state.


My question is why blocking port has green light  instead of Amber


thanks

mahesh


Mahesh


If the connection is blocking presumably the switches are already connected with an existing link ?


As for the amber/green lights. Remember that STP is per vlan on Cisco switches so when it is configured as an access port it could well block on that vlan. With the port configured as a trunk port the same vlan may be blocked but other vlans may well be forwarding. Can you check whether all the vlans allowed on the trunk are blocked.


Jon

mahesh18 Sun, 03/14/2010 - 00:56
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Hi jon


Here is info from switch A


950T#sh spanning-tree

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    32768
             Address     0003.6b7b.5a00
             Cost        22
             Port        65 (Port-channel1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     000b.bece.bbc0
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 1000

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/8            Altn BLK 19        128.8    P2p**********************
Fa0/9            Desg FWD 19        128.9    Edge P2p
Fa0/16           Desg FWD 19        128.16   Edge P2p
Fa0/19           Desg FWD 19        128.19   Edge P2p
Po1              Root FWD 3         128.65   P2p


VLAN0030
  Spanning tree enabled protocol rstp
  Root ID    Priority    32768
             Address     0003.6b7b.5a01
             Cost        22
             Port        65 (Port-channel1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32798  (priority 32768 sys-id-ext 30)
             Address     000b.bece.bbc0
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 1000

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Po1              Root FWD 3         128.65   P2p


Here is config of port fa0/8 port from switch A


2950T#sh  run int fa0/8
Building configuration...

Current configuration : 144 bytes
!
interface FastEthernet0/8
description Connection to 2950T2 Switch with cross over wire.
switchport mode access
spanning-tree portfast
end


Please explain me in more detail what you mention


thanks

Jon Marshall Sun, 03/14/2010 - 06:45
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Mahesh


Can you configure fa0/8 as a trunk and then post the output of "sh spanning-tree" ?


Jon

mahesh18 Sun, 03/14/2010 - 15:18
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Hi Jon,


On switch A which has amer light for port fa0/8  i config that as trunk


2950T#sh run int fa0/8
Building configuration...

Current configuration : 119 bytes
!
interface FastEthernet0/8
description Connection to 2950T2 Switch with cross over wire.
switchport mode trunk
end

2950T#sh spann
2950T#sh spanning-tree

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    32768
             Address     0003.6b7b.5a00
             Cost        22
             Port        65 (Port-channel1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     000b.bece.bbc0
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 1000

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/8            Altn BLK 19        128.8    P2p**********************************************
Fa0/9            Desg FWD 19        128.9    Edge P2p
Fa0/16           Desg FWD 19        128.16   Edge P2p
Fa0/19           Desg FWD 19        128.19   Edge P2p
Po1              Root FWD 3         128.65   P2p


VLAN0030
  Spanning tree enabled protocol rstp
  Root ID    Priority    32768
             Address     0003.6b7b.5a01
             Cost        22
             Port        65 (Port-channel1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32798  (priority 32768 sys-id-ext 30)
             Address     000b.bece.bbc0
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 1000

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/8            Desg FWD 19        128.8    P2p**********************************
Po1              Root FWD 3         128.65   P2p


Now the port fa0/8 on switch A  has green light and other nei switch B  port fa0/8 has also green light.



Switch B  port fa0/8 config and spanning tree output



950T2#                     sh  run int fa0/8
Building configuration...

Current configuration : 143 bytes
!
interface FastEthernet0/8
description Connection to 2950T Switch with cross over wire.
switchport mode access
spanning-tree portfast
end

2950T2#sh spann
2950T2#sh spanning-tree

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    32768
             Address     0003.6b7b.5a00
             Cost        19
             Port        15 (FastEthernet0/15)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     000d.65de.ff40
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 1000

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/8            Desg FWD 19        128.8    P2p
Fa0/15           Root FWD 19        128.15   P2p Peer(STP)
Fa0/16           Altn BLK 19        128.16   P2p Peer(STP)
Po1              Desg FWD 3         128.65   P2p


VLAN0030
  Spanning tree enabled protocol rstp
  Root ID    Priority    32768
             Address     0003.6b7b.5a01
             Cost        19
             Port        15 (FastEthernet0/15)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32798  (priority 32768 sys-id-ext 30)
             Address     000d.65de.ff40
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 1000

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/15           Root FWD 19        128.15   P2p Peer(STP)
Fa0/16           Altn BLK 19        128.16   P2p Peer(STP)
Po1              Desg FWD 3         128.65   P2p



Thanks

Correct Answer
Jon Marshall Sun, 03/14/2010 - 16:21
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Mahesh


It's what i was saying. Once you configure fa0/8 as a trunk link on vlan 1 it is still blocking but for vlan 30 -


VLAN0030
  Spanning tree enabled protocol rstp
  Root ID    Priority    32768
             Address     0003.6b7b.5a01
             Cost        22
             Port        65 (Port-channel1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32798  (priority 32768 sys-id-ext 30)
             Address     000b.bece.bbc0
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 1000

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/8            Desg FWD 19        128.8    P2p**********************************
Po1              Root FWD 3         128.65   P2p


You can see that fa0/8 is forwarding for vlan 30. This is why the port is green when you configure it as a trunk because it blocks on vlan 1 but it forwards on vlan 30 so the port shows as forwarding. Remember that each vlan runs it's own STP instance.


Jon

mahesh18 Sun, 03/14/2010 - 19:00
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Hi jon,


thanks once again for your explanation.

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