Virtual LANs and Trunking

Things to remember from INTRO:

• A VLAN is essentially a broadcast domain

• L2 switches forward frames between devices in the same VLAN

• L3 switch or router needed to communicate between VLANS

• ISL is proprietary, fully encapsulates original frame (26-byte header, 4-byte trailer, 12-bit VLAN ID)

• 802.1q adds 4-byte header after destination MAC address, CRC needs to recalculate

• Both support separate instance of STP per VLAN

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ISL supports multiple spanning trees using Per-VLAN Spanning Tree (PVST+).

-> Cisco’s PVST+ allows multiple STP instances over 802.1q trunks

802.1S: IEEE’s new specification that adds to 802.1q specification, allowing multiple spanning trees.

Native VLAN: 802.1q defines one VLAN on each trunk (by default, VLAN1), does not encapsulate frames. When receiving switch receives frame not encapsulated, assumes it is native VLAN.

-> ISL does not use a concept like this, all frame have an ISL header

Function	ISL	802.1Q
Standards body	Cisco-proprietary	IEEE
Encapsulate original frame	Yes	No
Allows multiple spanning trees	Yes (PVST+)	Yes (PVST+ or 802.1S)
Uses a native VLAN	No	Yes

VLAN Trunking Protocol (VTP): Cisco proprietary protocol (L2) that broadcasts switch VLAN configuration information. Configure settings on one switch, and all other switches learn the VLAN settings dynamically.

1. Creation of switch called the VTP Server.

2. Scale to large sizes by reducing need for manual configuration.

3. Advertises every 5 minutes, or whenever change occurs

Broadcasts Include:

• Configuration Revision Number (each time switch modifies config, increment by 1). When switch receives broadcast with larger revision number, updates config.
• VLAN names/numbers
• Info about which switches have ports assigned to which VLAN

VTP Operates in 3 Modes

• Server Mode (switch acts as server) -> create, modify, delete VLAN configuration, stored in NVRAM

• Client Mode -> Configuration not stored at all, can’t modify/create/delete

• Transparent Mode -> Avoid’s using VTP to exchange configuration info, to be transparent = ignore VTP broadcasts (they still forward to other switches, however).
  • Changes to VLAN can be made, but only saved for that switch.

Function	Server	Client	Transparent
Originates VTP advertisements	Yes	No	No
Processes received advertisements and synchronizes VLAN configuration information with other switches	Yes	Yes	No
Forwards VTP advertisements received in a trunk	Yes	Yes	Yes
Saves VLAN configuration in NVRAM	Yes	No	Yes
Can create, modify, or delete VLANs using configuration commands	Yes	No	Yes

VTP Pruning: Since switches usually don’t have interfaces for every VLAN in the network, bandwidth is usually wasted in broadcasting those updates. VTP pruning allows switches to prevent broadcasts and unknown unicasts from going to switches with no interfaces in those VLANs.

• Broadcast is only flooded toward switches with ports in a given VLAN

VLAN Trunking and Configuration

• VTP is enabled by default

• Trunking negotiation is attempted on all ports by default

Command	Command Description
vlan database	Exec command to enter VLAN config mode
vtp {domain | password | pruning | v2 mode}	Defines VTP parameters in VLAN config mode
vlan vlan id [name vlan name]	Creates/names a VLAN
switchport mode {access | dynamic {auto | desirable} | trunk}	Interface subcommand that configures the interface for trunking.
switchport trunk {{allowed vlan vlan-list} | {native vlan vlan-id} | {pruning vlan vlan-list}}	Refines the list of allowed VLANs, defines the 802.1Q native VLAN, and limits the range of VLANs for which pruning can occur.
switchport access vlan vlan-id	Interface subcommand that statically configures the interface into that one VLAN.
show interfaces [interface-id | vlan vlan-id] [switchport | trunk]	Displays trunk status.
show vlan [brief | id vlan-id | name vlan-name | summary]	EXEC command that lists information about the VLAN.
show vlan [vlan]	Displays VLAN information.
show vtp status	Lists VTP configuration and status information.
show spanning-tree vlan vlan-id	EXEC command that lists information about the spanning tree for a particular VLAN.

*To configure multiple interfaces simultaneously for same VLAN, use these steps:

vlan database

vlan 1 name test1

exit (apply update/increments revision number – if abort used, no changes saved)

config t

interface range [fastEthernet 0/1 – 5]

switchport mode access (trunking negotiations disabled, ports are access ports)

switchport access vlan 1

exit

show vlan brief (to verify updates, VLAN name, status, and ports)

-or-

show vlan id 1

** Note that vlan 1 is configured by default **

Option	Description	Action
access	Disables port trunk mode and does not even attempt to form a trunk on the interface.	Does not trunk.
trunk	Configures the port in permanent trunk mode.	Always tries to trunk.
dynamic desirable	The port negotiates to a trunk port if the connected device is in the trunk, dynamic desirable, or dynamic auto state. Otherwise, the port becomes a nontrunk port.	Trunks to switches set to the trunk, dynamic desirable, or dynamic auto state.
dynamic auto	Lets a port become a trunk only if the connected device is in the dynamic desirable or trunk state.	Trunks to switches set to the trunk dynamic desirable