Root Port Selection
typesh
Member Posts: 168
in CCNA & CCENT
Hey guys. I needed some help in determining the Root Port.
I understand how the election process works for the most part, however I am getting caught up on the method a non-root switch uses to select it's RP.
In Odom's ICND2 Guide on Page 101 it outlines the steps for how a switch will select it's root port. Here is what it says about a tie:
"The lowest cost found is the cost to reach the root, and the outgoing interface is that switch's RP. If the cost ties, use the port priority tiebreaker, and if that ties, use the lowest port number tiebreaker"
Okay, I get that. I labbed it up using 2 switches connected with redundant links. Then I modified the port priorities and saw the RP change.
Here is what it says in Lammle's book on Page 507:
"If multiple links have the same cost, the bridge with the lower advertising Bridge ID is used. Since multiple links can be from the same device, the lowest port number will be used."
Here is my question:
Regarding the topology I have inserted, which method will Switch D use to select it's RP?
SwitchD is receiving equal cost BPDUs to reach the Root from Switch B and also from Switch C. So SwitchD need to determine it's root port.
Based on the description in Odom's book, SwitchD will listen to the advertised costs from the upstream switches and notice a tie. Then it will look at the port priorities for Fa0/16 and Fa0/17 to see which one is lower. If those port priorities tie, then it will choose Fa0/16.
Based on the description from Lammle's book, Swtich D will use Fa0/17 as it's RP since it will base it's choice who has the lower BID when comparing all upstream switches.
I know if two non-root switches advertise the same cost onto their common segment, the one with the lower BID will become DP on that segment.
Could anyone please clarify?
Thanks!
I understand how the election process works for the most part, however I am getting caught up on the method a non-root switch uses to select it's RP.
In Odom's ICND2 Guide on Page 101 it outlines the steps for how a switch will select it's root port. Here is what it says about a tie:
"The lowest cost found is the cost to reach the root, and the outgoing interface is that switch's RP. If the cost ties, use the port priority tiebreaker, and if that ties, use the lowest port number tiebreaker"
Okay, I get that. I labbed it up using 2 switches connected with redundant links. Then I modified the port priorities and saw the RP change.
Here is what it says in Lammle's book on Page 507:
"If multiple links have the same cost, the bridge with the lower advertising Bridge ID is used. Since multiple links can be from the same device, the lowest port number will be used."
Here is my question:
Regarding the topology I have inserted, which method will Switch D use to select it's RP?
SwitchD is receiving equal cost BPDUs to reach the Root from Switch B and also from Switch C. So SwitchD need to determine it's root port.
Based on the description in Odom's book, SwitchD will listen to the advertised costs from the upstream switches and notice a tie. Then it will look at the port priorities for Fa0/16 and Fa0/17 to see which one is lower. If those port priorities tie, then it will choose Fa0/16.
Based on the description from Lammle's book, Swtich D will use Fa0/17 as it's RP since it will base it's choice who has the lower BID when comparing all upstream switches.
I know if two non-root switches advertise the same cost onto their common segment, the one with the lower BID will become DP on that segment.
Could anyone please clarify?
Thanks!
Comments
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alan2308
Member Posts: 1,854 ■■■■■■■■□□
Lammle is right in this case (though I suspect that the two examples you're looking at aren't quite the same). It will be the one connecting to the switch with the lower BID.
Only if thats a tie, meaning both ports connect to the same switch, then the lower port number is the final tie breaker. -
typesh
Member Posts: 168
Lammle is right in this case (though I suspect that the two examples you're looking at aren't quite the same). It will be the one connecting to the switch with the lower BID.
Only if thats a tie, meaning both ports connect to the same switch, then the lower port number is the final tie breaker.
Thanks alan.
Okay, so in my diagram, Switch D will pick the upstream switch with the lower BID (Switch
. However, in 2-switch scenario with redundant links, the BID cannot be used since the BID being received by the non-root switch will be the same on all ports. Therefore, the non-root switch will have to check port priorities, and if those tie, use the lower interface number.
Correct?
