Cisco Switching

LAN SWITCHES

A LAN switch has three primary functions:

1. Address Learning – maintains a table (CAM – Content Addressable Memory) table of addresses and which port they can be reached on.

2. Forward/filter decision – forwards frames only out of the relevant port.

3. Loop avoidance - STP

Broadcast frames are forwarded out of all ports. Because ethernet hosts can all transmit at the same time this can lead to collisions thus slowing down the network considerably.


Transmitting Frames Through a Switch Store-and-Forward – switch copies the entire frame into its buffer and computes the CRC. Frame is discarded if there is an error. High latency.

Cut-through – reads only the destination address (first 6 bytes after preamble), looks up address and forwards frame. Lower latency. Fragment free – switch reads first 64 bytes before forwarding the frame. Collisions normally occur within the first 64 bytes.

Spanning Tree Protocol (STP) IEEE 802.1d

STP is a link management protocol that provides path redundancy whilst preventing undesirable loops in the network. For communication to work correctly on an ethernet network there can only be one path between two destinations. STP uses Bridge Protocol

Data Units (BPDU) received by all switches to determine the spanning-tree topology. A port on a switch is either in forwarding or blocking state. Forwarding ports provide the lowest cost path to the root bridge, a port will remain in blocking state from start up if spanning tree determines there is a better path.

Rapid Spanning Tree Protocol (RSTP) IEEE 802.1w

Spanning tree takes up to 50 seconds to converge to a stable network whereas RSTP takes 2 seconds. RSTP port roles are root port, designated port, backup port, alternate port and disabled. Most implementations of RSTP use PVST+, Per VLAN Spanning Tree+, here multiple instances of Spanning Tree are running so the load on the CPU is higher but we can load share over the links.

To enable RSTP for each VLAN in our switched network we use the following command:

Switch(config)#spanning-tree mode rapid-pvst

Bridging / Switching

Bridges are primarily software based and have one spanning-tree instance per bridge.

Normally 16 ports per bridge. LAN Switches are primarily hardware based. Many spanning-tree instances per switch and up to 100 ports.

Virtual LAN (VLAN)

A VLAN is a switched network that consists of logically segmented communities without regard to physical location. Each port on a switch can belong to a VLAN. VLAN ports share broadcasts. A router is needed to route traffic between VLANs because layer 2

devices do not use IP addresses. Reduces admin costs, tighter security and better control of broadcasts