Editing Spanning Tree Protocol (section)

=== Tiebreakers ===
[[Image:Spanning tree protocol at work 4.svg|thumb|right|250px|Path tie: The least-cost path to the root from network segment ''e'' goes through bridge 92. Therefore, the designated port for network segment ''e'' is the port that connects bridge 92 to network segment ''e''.]]

;Root ports
:When multiple paths from a bridge are least-cost paths, the chosen path uses the neighbor bridge with the lower bridge ID. The root port is thus the one connecting to the bridge with the lowest bridge ID. For example, in the figures, if switch 4 were connected to [[network segment]] d instead of segment f, there would be two paths of length 2 to the root, one path going through bridge 24 and the other through bridge 92. Because there are two least-cost paths, the lower bridge ID (24) would be used as the tiebreaker in choosing which path to use.
;Paths
:When more than one bridge on a segment leads to a least-cost path to the root, the bridge with the lower bridge ID is used to forward messages to the root. The port attaching that bridge to the network segment is the ''designated port'' for the segment. In the figures, there are two least-cost paths from network segment d to the root, one going through bridge 24 and the other through bridge 92. The lower bridge ID is 24, so the tiebreaker dictates that the designated port is the port through which network segment d is connected to bridge 24. If bridge IDs were equal, then the bridge with the lowest MAC address would have the designated port. In either case, the loser sets the port as being blocked.
;Designated ports
:When the root bridge has more than one port on a single network segment, the bridge ID is effectively tied, as are all root path costs (all equal zero). The port on that network segment with the lowest port ID becomes the designated port. It is put into forwarding mode while all other ports on the root bridge on that same network segment become non-designated ports and are put into blocking mode.<ref>802.1d-1998 section 8.3.1: The designated port for each LAN is the bridge port for which the value of the root path cost is the lowest: if two or more ports have the same value of root path cost, then first the bridge identifier of their bridges, and their port identifiers are used as tie breakers.</ref> Not all bridge manufacturers follow this rule, instead making all root bridge ports designated ports, and putting them all in forwarding mode.{{citation needed|date=September 2020}}
;Final tiebreaker
:In some cases, there may still be a tie, as when the root bridge has multiple active ports on the same network segment (see above) with equally low root path costs and bridge IDs, or, in other cases, multiple bridges are connected by multiple cables and multiple ports. In each case, a single bridge may have multiple candidates for its root port. In these cases, candidates for the root port have already received BPDUs offering equally-low (i.e. the "best") root path costs and equally-low (i.e. the "best") bridge IDs, and the final tiebreaker goes to the port that received the lowest (i.e. the "best") port priority ID, or port ID.<ref>802.1d-1998 section 8.3.2 b) A Bridge that receives a Configuration BPDU on what it decides is its Root Port conveying better information (i.e. highest priority Root Identifier, lowest Root Path Cost, highest priority transmitting Bridge and Port), passes that information on to all the LANs for which it believes itself to be the Designated Bridge.</ref>