STP - Determining Root port selection when lowest sender port id is the same.

[James14]

Hi All,

When determining the root port for a non-root switch, how does STP decide which interface to select when the sender port id is the same?

In this case, SW1 is the root.

[SW1]----[Hub]====[SW2]

Output on SW2:

Port 2 (GigabitEthernet0/1) of VLAN0001 is root forwarding
Port path cost 4, Port priority 128, Port Identifier 128.2.
Designated root has priority 32769, address [mac]
Designated bridge has priority 32769, address [mac]
Designated port id is 128.4, designated path cost 0

Port 4 (GigabitEthernet0/3) of VLAN0001 is alternate blocking
Port path cost 4, Port priority 128, Port Identifier 128.4.
Designated root has priority 32769, address [mac]
Designated bridge has priority 32769, address [mac]
Designated port id is 128.4, designated path cost 0

Thanks

[Reza Sharifi · Accepted solution]

Hi,

The lower port id determines that. So, in your case GigabitEthernet0/1 has lower port id than GigabitEthernet0/3.

HTH