Connecting Devices, Transparent Bridges & Spanning Tree Protocol (STP)

To interconnect local area networks, hardware devices operate across distinct layers of the protocol stack:

1. Repeaters & Hubs (Layer 1 - Physical): Receive electrical/optical signals and regenerate (amplify) bit streams onto all other ports. They have no memory, do not inspect frame headers, and create a single shared Collision Domain and a single Broadcast Domain.

2. Bridges & Layer 2 Switches (Layer 2 - Data Link): Multi-port bridges that maintain an internal forwarding database (CAM table). They inspect 48-bit MAC addresses, filter traffic to prevent collisions from spreading, and isolate each port into an independent Collision Domain. However, standard Layer 2 switches forward broadcast frames (e.g. ARP requests) to all ports, leaving a single Broadcast Domain.

3. Routers (Layer 3 - Network): Examine 32-bit/128-bit logical IP addresses to route packets between disparate networks. Routers block Layer 2 broadcasts by default, isolating both Collision Domains and Broadcast Domains per interface.

A transparent bridge operates plug-and-play without requiring manual network configuration. It learns network topology dynamically through backward learning:

Algorithm Execution on Arrival of Frame (Source = S, Destination = D) on Port P:

1. Inspection & Learning: The bridge inspects Source MAC S. It writes or refreshes the entry in its Filtering/Forwarding Table: (MAC = S, Port = P, TTL = 300s).

2. Destination Lookup & Forwarding Decision:

• Case A (Same Segment): If Destination D is in the table and mapped to Port P, the bridge drops (filters) the frame because the destination is already on the source segment.

• Case B (Known Different Segment): If Destination D is in the table and mapped to Port Q (where Q != P), the bridge forwards the frame exclusively onto Port Q.

• Case C (Unknown Destination or Broadcast): If Destination D is NOT in the table, or if D is the broadcast MAC FF:FF:FF:FF:FF:FF, the bridge floods the frame to ALL active ports EXCEPT the incoming port P.

Redundant physical links between switches are essential for fault tolerance. However, redundant links create Layer 2 Physical Loops, causing:

• Broadcast Storms: Broadcast frames circulate infinitely, saturating network bandwidth.

• Multiple Frame Copies: Unicast frames arrive repeatedly at destination hosts.

• CAM Table Instability: Switch MAC learning tables thrash continuously as frames arrive from oscillating ports.

STP Algorithm (Radia Perlman):

1. Elect Root Bridge: Switches exchange Bridge Protocol Data Units (BPDUs). The switch with the lowest numerical Bridge ID (Priority + MAC) becomes the Root Bridge.

2. Elect Root Ports (RP): On every non-root switch, the port with the lowest cumulative path cost to the Root Bridge is designated as the Root Port (exactly 1 RP per non-root switch).

3. Elect Designated Ports (DP): On every physical link/segment, the switch port providing the lowest path cost to the Root Bridge is elected as the Designated Port (all ports on the Root Bridge are DPs).

4. Block Alternate Ports: All remaining ports are placed into the Blocking/Discarding state, breaking all loops while maintaining hot standby backup paths.