Notes on Catalyst Switches - I

Three primary switch operating modes are   -   Store and Forward,Cut Through   -   which has lowest latency (delay) and Fragment Free   -   which reads 64 bytes before starting to forward the frame.

Switch learns the address of a device using entries in its mac address table.Source address of incoming frames are stored in the mac address table. If the table is empty, as when the switch is just turned on, then the frames are forwarded through all the ports other than the one through which it recieved the frame. This process is known as flooding. Once the mac address table is filled, the switch resorts to frame filtering.

Broadcast address in ethernet is FFFF.FFFF.FFFF. A multicast address usually starts with 01.

Three main functions of a switch are as follows :

• Address learning function


• Forwarding / Filtering


• Loop avoidance   -   Accomplished by using Spanning Tree Protocol

Purpose and functions of Spanning Tree Protocol (STP)

• Eliminate Broadcast Storms


• Eliminate duplicate non-broadcast frame transmissions


• Eliminate mac address table database instability


• Eliminate multiple loops in switched networks.

Spanning Tree Protocols supported by catalyst switches are 802.1d and 802.1w (Rapid STP).

Spanning tree port states for 802.1d are listening, learning, forwarding and blocking.

Forward delay   -   Time it takes for the switch port to transition from listening state to learning state or learning state to forwarding state.

Key concepts used by STP are Bridge ID and Path costs. Bridge ID is a 8 byte hex number whose first 2 bytes is the priority number (32,768 or 0x8000) and the rest 6 bytes is the mac address of the switch. The Bridge ID of a neighboring switch is learned from the BPDU (Bridge Protocol Data Units) exchanged between switches (every 2 secs).

Spanning Tree Protocol   -  

• Elects a root bridge   -   a switch with lowest bridge ID.


• For each non root bridge, there is one root port.


• On each segment, there is one designated port.

Using STP you can determine the following for a switched network   -   Root bridge, Root Port, Designated Port, Blocking, Forwarding.

Default settings for Catalyst Switches   -  

• ip address   -   0.0.0.0


• CDP   -   enabled


• 10/100 port   -   Autonegotiate duplex mode


• STP   -   enabled


• Console password   -   none