Spanning Tree Tie-Breakers

[at@ps]

Hey Folks,

I know very well the tiebreakers to elect Root Port and Designated Port, but I believe that the use of these ties differs between electing Root Port and Designated Port.

Tiebreakers as follow:

1. Lower Sender Bridge ID
2. Lower Sender Port ID
3. Lower Local Port ID

The issue I have, that the use of the word "sender bridge", sometimes it refers to the switch itself that has the port we are arguing about; and other times it refers to the neighbour switch - NOT the actual switch that has the port we are arguing about.

I need to know the correct use of these ties when we elect RP, and DP as well!

Please, clarify this point to me.

[MHM Cisco World]

Assume SW have TWO link to TWO different SW lead to Root SW 
here the tie break is the TWO different SW MAC 

assume SW have TWO link to Same SW lead to Root SW
here the tie break is the TWO link Sender and receiver Port ID

MHM

[balaji.bandi]

Its all depends how your topology and STP you using here.

But high level most want to achieve the same goal small change depends on the STP you using :

there is good example illustrate how that process take place :

https://www.9tut.com/stp-root-port-election-tutorial

complete detailed guide from cisco :

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4000/8-2glx/configuration/guide/spantree.html#wp1174063

[M02@rt37]

Hello at@ps 

First thing, in context of STP,  whether it's RSTP or the classic IEEE 802.1D STP, the tiebreakers you mentioned (Lower Sender Bridge ID, Lower Sender Port ID, Lower Local Port ID) play different roles in the election of the RP and the DP. 

RP Tiebreakers

Lower Sender Bridge ID - this tiebreaker compares the Bridge ID of the sending switch (neighbor) with the potential Root Ports on the local switch. The switch with the lower Bridge ID on the other end of the link is favored.

Lower Sender Port ID - if the Sender Bridge IDs are the same, the Sender Port ID is considered. This is the port ID of the port on the neighboring switch.

Lower Local Port ID - if the Sender Bridge IDs and Sender Port IDs are still tied, the Local Port ID is considered. This is the port ID of the local switch (the switch you are currently configuring).

Designated Port Tiebreakers

Lower Sender Bridge ID - like in the Root Port tiebreakers, the Sender Bridge ID is used to determine the Designated Port. The switch with the lower Bridge ID on the other end of the link is favored.

Lower Sender Port ID - if  the Sender Bridge IDs are the same, the Sender Port ID is considered. This is the port ID of the port on the neighboring switch.

Lower Local Port ID - if the Sender Bridge IDs and Sender Port IDs are still tied, the Local Port ID is considered. This is the port ID of the local switch (the switch you are currently configuring).

Tebreakers are used in a consistent manner for both RP and DP elections. They are designed to provide a clear and deterministic way to choose the best port based on various criteria, with the Sender Bridge ID referring to the switch on the other end of the link. The ties are broken in favor of the switch or port with the lower ID in each category.

[at@ps]

Hey M02@rt37 ,

thanks for your replay.

as you mentioned that "play different roles in the election of the RP and the DP.", but as I read the description you've provided that both of them play the same role!!

As I got from you, that for both ports we consider the neighbour switch.

Please, Correct me if I'm wrong!

[M02@rt37]

at@ps 

To correct any confusion: for the RP election, "sender bridge" is the neighboring switch, and for the DP election, "sender bridge" is the local switch where the election is happening.

RP Election:

Sender Bridge ID: Refers to the Bridge ID of the neighboring switch.

Sender Port ID: Refers to the Port ID of the neighboring switch.

In the RP election, "sender bridge" indeed refers to the neighboring switch.

DP Election:

Sender Bridge ID: Refers to the Bridge ID of the local switch (the one on which the election is taking place).

Sender Port iD: Refers to the Port ID of the local switch.

In the DP election, "sender bridge" refers to the local switch, not the neighboring switch.

[MHM Cisco World]

Root port is elect first and SW check MAC of neighbor and select rp' if tie then it use port number and priority.

Each SW have only one rp other are dp or blk.

Now sw elect rp and start forward bpdu it receive to other SW via remaining ports

How sw decided this port will be dp or blk? The sw check it bpdu with bpdu it receive from neighbor' if it bpdu superior (it mac address is lower than it neighbor or port number or priority) the SW make port dp if it inferior it make it blk.

So always mac of neighbor is check

MHM

[MHM Cisco World]

SW0 who is king of this STP domain the Root Bridge, all it port MUST be DP(FWD)

NOW SW1 have two link to same Root Bridge SW (neighbour) and it need to elect which one will be RP, it elect e0/2 as RP (FWD) 
other as I mention must be DP or BLK 
since King Root bridge have lowest MAC, the port from SW1 to SW0 is BLK (remaining one must be BLK, since it BPDU is superior from SW0)

for SW2 it have direct connect to Root bridge it port must RP 
other Port must be DP or BLK 

but since it receive superior from SW1 then it (SW2) two port will be BLK and SW1 two port will be DP.

this Lab is simple but explain all point about the STP, I hope so 
thanks 
MHM 

[M02@rt37]

@MHM Cisco World , at@ps 

When deciding whether a port should become a DP or a Blocking Port (BLK), the key criteria include comparing the received BPDU information.

First, BPDU comparison. The switch compares the received BPDU with the one it generates. It looks at factors such as MAC address, port number, and priority in the BPDU.

If the received BPDU is superior (i.e., lower MAC address, lower port number, or higher priority), the local switch designates its port as a Designated Port (DP).

If the received BPDU is inferior, indicating a better path from the perspective of the neighboring switch, the local switch keeps its port in a Blocking state (BLK).

Ports designated as DP are allowed to forward user data. Ports in BLK state are blocking, preventing loops by not forwarding user data.....this process helps in loop avoidance, as the switches collectively determine the best paths to the Root Bridge, preventing loops in the network.

So, indeed, the MAC address (along with other parameters) of the neighbor is crucial in determining whether a port becomes a DP or remains in a BLK state after the Root Port election.

[Gopinath_Pigili]

Hello at@ps,

Use the below link....You can find the similar discussion...hope, this one also might be helpful...

https://community.cisco.com/t5/switching/stp-tie-breakers/td-p/2200595

Best regards
******* If This Helps, Please Rate *******

[Richard Pidcock]

I haven't seen any mention of Spanning Tree Path cost....see below directly from CCNP and CCIE Enterprise Core ENCOR 350-401 Official Cert Guide

After the switches have identified the root bridge, they must determine their root port (RP).
The root bridge continues to advertise configuration BPDUs out all of its ports. The switch
compares the BPDU information to identify the RP. The RP is selected using the following
logic (where the next criterion is used in the event of a tie):
1. The interface associated to lowest path cost is more preferred.
2. The interface associated to the lowest system priority of the advertising switch is
preferred next.
3. The interface associated to the lowest system MAC address of the advertising switch is
preferred next.
4. When multiple links are associated to the same switch, the lowest port priority from
the advertising switch is preferred.
5. When multiple links are associated to the same switch, the lower port number from
the advertising switch is preferred.