<P> 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 diagram on the right 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 tie breaker 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 . </P> <P> The final tie - breaker . In some cases, there may still be a tie, as when the root bridge has multiple active ports on the same LAN segment (see above, "Breaking ties for designated ports") 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 tie breaker goes to the port that received the lowest (i.e. the "best") port priority ID, or port ID . </P> <P> The above rules describe one way of determining what spanning tree will be computed by the algorithm, but the rules as written require knowledge of the entire network . The bridges have to determine the root bridge and compute the port roles (root, designated, or blocked) with only the information that they have . To ensure that each bridge has enough information, the bridges use special data frames called Bridge Protocol Data Units (BPDUs) to exchange information about bridge IDs and root path costs . </P> <P> A bridge sends a BPDU frame using the unique MAC address of the port itself as a source address, and a destination address of the STP multicast address 01: 80: C2: 00: 00: 00 . </P>

Which are the port roles in stp protocol