---

The designated bridge and designated ports

---

• Designated Bridges
  • Designated bridge:

    Designated bridge = a bridge that provide the shortest connection (path) to the root bridge for some LAN segment

  • Example:

    The designated bridges of LANs 1, 2, 3 and 4:

    Note:

    The root bridge is also: the designated bridge          for LANS 1 and 4

    ---

    ---

    ---

  • Multiple shortest paths:

    If there are multiple shortest paths from a LAN to the root bridge, then The designated bridge for a LAN = the bridge with the smallest ID

  • Example:

    LAN 5 has multiple shortest paths to the root bridge: The designated bridge for LAN 5 is bridge 1 which has the smallest ID:

    ---

    ---

    ---

  • Notice that:

    Every LAN (segment) has a unique designated bridge Example:

  ---

  ---

  ---

  ---

• Designated ports
  • Designated port:

    Designated port = the port of a designated bridge that connects to the LAN (on the shortest path to the root bridge)

    ---

    ---

    Example:

    Previously, we have identifid the designated bridges for each LAN: The designated port are marked by Status = D in the following figure:

  ---

  ---

  ---

  ---

• The spanning tree constructed by the IEEE 802.1D algorithm
  • Consider now the logical network consisting of:

    All the LAN (segments), The root bridge and all designated bridges The root ports and all designated ports

    This is that logical network:

    ---

    ---

  • Notice that:

    Contain all LANS The logical network is a tree !!!