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Beginner

Spanning_Tree_Protocol

Hi ,

what are the port roles of the switch in the following scenario if it is connecting to some down stream switches and currently acting as a root bridge?




root.png




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Hall of Fame Cisco Employee

Re: Spanning_Tree_Protocol

Hi Bharad,

what are the port roles on the root bridge if two ports are inter connected to each other?

One of the ports is Designated Forwarding, the other is Backup Discarding.

Best regards,

Peter

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7 REPLIES 7
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Rising star

Spanning_Tree_Protocol

What do you mean by self loop? Do you have two ports on that switch connected to each other?

Daniel Dib
CCIE #37149

Daniel Dib
CCIE #37149
CCDE #20160011

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Rising star

Spanning_Tree_Protocol

Are you asking what the port states will be when you have an active loop in the network?

I would hope you would be using BPDGuard to prevent something like this...

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VIP Mentor

Re: Spanning_Tree_Protocol

Hello

If you have a switch (1) which is the root and 2 other switches (2 & 3 )connected to the root switch

Then the port roles will be:

Switch 1) all interconnected links designated forwarding
Switch 2 & 3 ) interconnected link to switch 1 - root ports -forwarding
Switch 2 & 3 ) will negotiate their interconnected links between each other and one port will be the designated port for this lan segment and the other will become a Blocked port as its neither a root or designated port

Res
Paul



Sent from Cisco Technical Support iPad App



kind regards
Paul

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Beginner

Re: Spanning_Tree_Protocol

Hi All,

what are the port roles on the root bridge if two ports are inter connected to each other?

Regards,

Bharad.

Highlighted
Hall of Fame Cisco Employee

Re: Spanning_Tree_Protocol

Hi Bharad,

what are the port roles on the root bridge if two ports are inter connected to each other?

One of the ports is Designated Forwarding, the other is Backup Discarding.

Best regards,

Peter

View solution in original post

Highlighted
Beginner

Re: Spanning_Tree_Protocol

Hello,

if your scenario is looping f1 and f2 of Root Bridge, then

In case of pvst/rapid-pvst/mst

     F1 --> will be the desiginated forwarding

     F2 --> will be in backup bloacking

Reson for the above selection is, Root Bridge priority is tie / Root Bridge id is tie / and port path cost is tie / port priority is tie but the port identifier is different (Port identifier is the combination of port priority + the port number (this changes depends on the hardware)). please find the below example

Port 1 (GigabitEthernet1/0/1) of VLAN0001 is designated forwarding

   Port path cost 4, Port priority 128, Port Identifier 128.1.

   Designated root has priority 40961, address b4a4.e3f4.b000

   Designated bridge has priority 40961, address b4a4.e3f4.b000

   Designated port id is 128.1, designated path cost 0

   Timers: message age 0, forward delay 0, hold 0

   Number of transitions to forwarding state: 1

   Link type is point-to-point by default

   BPDU: sent 19, received 1

Port 2 (GigabitEthernet1/0/2) of VLAN0001 is backup blocking

   Port path cost 4, Port priority 128, Port Identifier 128.2.

   Designated root has priority 40961, address b4a4.e3f4.b000

   Designated bridge has priority 40961, address b4a4.e3f4.b000

   Designated port id is 128.1, designated path cost 0

   Timers: message age 16, forward delay 0, hold 0

   Number of transitions to forwarding state: 0

   Link type is point-to-point by default

   BPDU: sent 1, received 19

Regards

Azif

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Beginner

Re: Spanning_Tree_Protocol

Hi All,

Thanks Alot for you help. 

Regards,

Bharad.

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