Switch Vlan Configuration


S#vlan database

S(vlan)#vlan <vlan number> name <vlan name>††††††††††† (vlan number range 1-1005)


S(config)#int VLAN 1†††††††††††††††††††††††††††††††††††††††††††††††††††††††† (allocate ip for management)

S(config-if)#ip address <ip addr> <subnet mask>††††††††


Allocates ports to vlans

S(config)#int f 0/1

S(config-if)#Switchport Mode Access

S(config-if)#Switchport Access vlan <vlan number>


Allocate port for a trunk

S(config)#int f0/4

S(config-if)#switchport mode trunk

S(config-if)#switchport trunk encapsulation [ isl | dot1q ] †††††††††† (donít specify isl|dot1q on 2950ís)


Configure the VTP

S#vlan database

S(vlan)#vtp v2-mode

S(vlan)#vtp domain <name>

S(vlan)#vtp [ client | server | transparent ]


Configure the Switch for management access

S(config)#int VLAN 1

S(config-if)#ip address <ip addr> <subnet mask>†††††††† (ip allocated to switch)


S(config)#ip default-gateway <ip addr>††††††††††††††††††††††††† (ip of adjacent router subif)


Router show and debug commands

show vlan [brief]

show spanning-tree

debug sw-vlan packets

show vtp [ status | counters ]


Switching Types



Store and Forward

Complete Frame is received before forwarding

Most Latency, but best error checking

Cut Through

(Also called fast forward

Only Destination Address read before forwarding

Least Latency, and no error checking

Fragment Free

Reads first 64bytes of data read before forwarding

Reasonable Latency and some error checking. Decides on error if < 64bytes.


Note: Bridges are Store and Forward by default. Switches are Fast Forward (cut through) by default.


Note: Adaptive Cut Through is Cut Through, but if errors detected, then it transitions to Store and Forward.