VLAN Trunking protoco ( VTP )

A switch can belong to only one VTP domain, in addition to sharing VLAN information with other switches in the domain. Switches in diffeent VTP domain, do not share VTP information.

Switches in a VTP domain advertise several attributes to their domain neighbors. Each advertisement contains information about the VTP management domain, VTP revision number, known VLANs, and specific VLAN parameters. When VLAN is added to a switch in management domain, other switches are notified of new VLAN through VTP advertisements. In this way, all switches in a domain can prepare to receive traffic on their trunk ports using the new VLAN.



VTP Modes-Every switch is server mode by default. Server mode is  the default VTP mode and allows VLANs to be created and deleted 

VTP modes determines how the switch can process and advertises VTP information. you can use following modes.

Server Mode-In this mode, VTP servers have full control over VLAN creation and modification for their domains.

Client Mode- VTP clients do not allow the administrator to create, change, or delete any VLANs. Instead, they listen to VTP advertisementss from other switches and modify their VLAN configurations accordingly. this is also called passive lstion mode. received information is always forwarded on it's truck link to neighboring switches in the domain.

Transparent Mode-VTP transparent sw3itches do not participate in VTP. it can create and delet VLANS that are lcoal only to itself. these VLAN changes are not propagated to any other switch .
It does not advertise it's own VLAN configuration and, also switch does not synchronize it's VLAN database with received advertisement


New Switch Adding in the existing infrastructure  or VTP domain-

Since by default, every switch operates in VTP server mode, it hears a VTP summary advertisement on a trunk port from any other switch, it automatically learns the VTP domain name, VLANs, and configuration revision numbers This makes it easy to bring up a new switch in an existing VTP domain.

You just need to verify all these information by below commands
Switch # show vtp status


Configuratin a VTP management domain for newly setup - 

If a switch is the first one on the network, the management domain must be created.

switch(config)# vtp domain <domain name>

Dynamic Trunking Protocol ( DTP )

DTP is used to negotiate forming a trunk between two cisco devices. DTP is a Cisco proprietor protocol that is automatically enabled on Catalyst 2960 and Catalyst 3560 Series switches 

The default switchport mode for newer Cisco switch Ethernet interfaces is dynamic auto

Note that if two Cisco switches are left to the common default setting of auto, a trunk will never form. switchport mode dynamic desirable: Makes the interface actively attempt to convert the link to a trunk link. please refer  the DTP modes below carefully 

DTP modes:

1. switchport mode access -  Just to add a computer on the switch 

2. switchport mode dynamic auto - Makes the interface able to convert the link into a trunk link, when someone ask/request this port, then trunking will be negotiated. 

3. switchport mode dynamic desirable -  Makes the interface actively attempt to convert the link into a trunk link. it " asks" the far end switch to bring up a trunk

4. switchport mode trunk - Puts the interface into permanent trunking modes and negotiates to convert the neighboring link into a trunk link. The interface becomes a trunk interface even if the neighboring interface is not a trunk interface. For this mode there will be no such condition, rather it accepts whatever the mode neighbor is.

5. switchport nonegotiate -  Prevents the interface from generating DTP frames. you can use this command only when the interface switchport mode is access or trunk.you must manually configure the neighboring interface as a trunk interface to establish a trunk link.

                                 Please refer the below table for quick understanding

                           sh interface gigbitether 2/1 trunkc to check the trunking status                                  

Key Points -

DTP frames are sent out every 30 seconds to ensure their neighboring switch port informed of the links’s mode

·     Switch port truck will will be the always first and best choice making both switch connected with swichport mode truck manually, so that both of them will never ask for negotiation.

When should I make truck between two switches - A trunk link can be negotiated between two switches only at following conditions.

  If both switches belonging to the same VTP domain

  If one switch has not been defined VTP domain, between two of them

  If both switches have not been defined VTP domain

  If two switches are in different VTP domains, though trunking can be possible, by typing 

  switchport mode trunk and switchport nonegotiate on both of the switches.

When should I disable DTP -

  

Whenever you made permanent trunk link with switchport mode trunk, you should also disable DTP completely so that these frame are not changed, hence no negotiation is possible until this configured removed.

You should disable DTP negotiation if a switch has a trunk link connected to a non-trunking router or firewall interface.



Note - Please refer another page for VTP explanation 

VLANs and Trucking

By default, a VLAN is single broadcast domain, every connected devices sees every broadcast packet which is transmitted.that means all devices connected to that VLAN receive broadcasts sent by any other VLAN members. However , devices connected to a different VLAN will not receive those same broadcasts.

A port or group of ports on a single switch can assigned to many VLANs. Even though two devices are connected to the same switch, traffic will not pass between them if they are connected to ports on different VLANs.  To perform this, you could use either a layer 3 device to route packets or an external layer 2 device to bridge packets between the two VLANs.

We have two types of VLAN configurations:

1). Static VLAN -  port based configuration

2). Dynamic VLAN - mac address based configuration.

3). Negotiate( the default)- The encapsulation is negotiated to select either ISL or IEEE 802.1Q, whichever both ends of the trunk support. if both ends support both types, ISL is favored.

By default, all switch ports assigned to VLAN 1 , are set to be a VLAN type of Ethernet, and have a maximum  transmission unit ( MTU ) of 1500 byes.

Static VLAN configuration -

1. Creating a VLANs

2. Adding switch port into VLANs

3. VTP configuration

1. Creating a VLANs -


step 1. VLAN must be created on  a switch, range could be 2 - 1002
  
            Switch ( config ) # vlan 100

step 2. Assigning a name to VLAN 

           Switch ( config - vlan ) # name sales

step 3 . Removing a VLAN

            Switch ( config ) # no vlan 100


2. Adding switch port into VLANs

step 1.  Switch# Interface Fastethernet 0/1

step 2.  config-switch # switchport - [ you will understand about this command in below paragraph ]

step 2.  config-switch #  switchport mode access

step 3.  config-switch # switchport mode vlan 100


3. VTP configuration

Before you begin VTP configuration, you must understand what is switchport command actually do?By default, every Catalyst IOS switch port is ready for Layer 3 operations, something that doesn't co-exit with Layer 2 VLANs. a switch port must be in layer 2 mode before it can support a trunk.a switchport command makes switch port put into layer 2 mode, and also you must learn following stages 

1.VLAN Identification and encapsulation methods

2.DTP modes

3.Native VLAN

Please go through separate links for DTP modes and Native VLAN explanation.

1.VLAN Identification and encapsulation methods -

a. ISL ( Inter Switch Link) -
       In this ISL method, VLANs are tagged and encapsulating in each frame using cisco ISL protocol.     It is a cisco propitiatory

b. 802.1Q -
    In this 802.1Q mthod, VLANs are tagged by tagged and encapsulating in each frame using IEEE.802.1Q standard protocol. The only exception is the native VLAN, which is sent normally and is not tagged. It is a open standard.

switch # interface fasthethernet 0/1

switch (config-if)# switchport

switch (config-if)#switchport trunk encapsulation isl/dot1q/negotiate   

switch (config-if)# switchport mode trunk/dynamic desirable/auto

switch (config-if)#switchport trunk native vlan 100

switch (config-if)#switchport trunk allowed vlan  vlan list/all/add/except/remove

switch (config-if)#switchport trunk allowed vlan 100-102, 104-105

switch (config-if)#switchport trunk allowed vlan remove 103