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VLAN Trunking Protocol

VLAN Trunking Protocol

VLAN Trunking Protocol (VTP) is a Cisco proprietary protocol that propagates the definition of Virtual Local Area Networks (VLAN) on the whole local area network. To do this, VTP carries VLAN information to all the switches in a VTP domain. VTP advertisements can be sent over ISL, 802.1Q, IEEE 802.10 and LANE trunks. VTP is available on most of the Cisco Catalyst Family products. Using VTP, each Catalyst Family Switch advertises the following on its trunk ports:

  1. Management Domain
  2. Configuration Revision Number
  3. Known VLANs and Their Specific Parameter

One concept in VTP is that larger scale networks may need to be limited in terms of which switches will act as the VLAN servers. VTP offers various options for recovery after a crash or for efficiently serving up redundant network traffic.

VLAN Trunking Protocol
VLAN Trunking Protocol

In the image above, each switch has two VLANs. On the first switch, VLAN A and VLAN B are sent through a single port (trunked) to the router and through another port to the second switch. VLAN C and VLAN D are trunked from the second switch to the first switch, and through the first switch to the router. This trunk can carry traffic from all four VLANs. The trunk link from the first switch to the router can also carry all four VLANs. In fact, this one connection to the router allows the router to appear on all four VLANs, as if it had four different physical ports connected to the switch.

The VLANs can communicate with each other via the trunking connection between the two switches using the router. For example, data from a computer on VLAN A that needs to get to a computer on VLAN B (or VLAN C or VLAN D) must travel from the switch to the router and back again to the switch. Because of the transparent bridging algorithm and trunking, both PCs and the router think that they are on the same physical segment!

Example without and with VTP
Example without and with VTP

On Cisco Devices, VTP (VLAN Trunking Protocol) maintains VLAN configuration consistency across the entire network. VTP uses Layer 2 trunk frames to manage the addition, deletion, and renaming of VLANs on a network-wide basis from a centralized switch in the VTP server mode. VTP is responsible for synchronizing VLAN information within a VTP domain and reduces the need to configure the same VLAN information on each switch.

VTP minimizes the possible configuration inconsistencies that arise when changes are made. These inconsistencies can result in security violations, because VLANs can cross connect when duplicate names are used. They also could become internally disconnected when they are mapped from one LAN type to another, for example, Ethernet to ATM LANE ELANs or FDDI 802.10 VLANs. VTP provides a mapping scheme that enables seamless trunking within a network employing mixed-media technologies.

Currently there are three version of VLAN Trunking Protocol (VTP). The functions of VTP Version 1 and VTP Version 2 are almost similar. The support for Token Ring VLANs is there with VTP V2.

According to Cisco VTP V3 documentation, VLAN Trunking Protocol (VTP) Version 3 introduces the concept of transferring an opaque database in situations where VTP version 1 and VTP version 2 interacted with the VLAN process directly. VTP version 3 includes support for the MST mapping table.

These are the enhancements made on VLAN Trunk Protocol (VTP) V3:

  1. Protection from unintended database overrides during insertion of new switches.
  2. Support for VLAN numbers up to 4096.
  3. Support for interaction with VTP Version 1 and VTP Version 2.
  4. Support for a structured and secure VLAN environment (Private VLAN, or PVLAN).
  5. Option of clear text or hidden password protection.
  6. Configuration option on a per port base instead of only a global scheme.
  7. Optimized resource handling and more efficient transfer of information.


VTP provides the following benefits:

  1. VLAN configuration consistency across the network.
  2. Mapping scheme that allows a VLAN to be trunked over mixed media.
  3. Accurate tracking and monitoring of VLANs.
  4. Dynamic reporting of added VLANs across the network.
  5. Plug-and-play configuration when adding new VLANs.


As beneficial as VTP can be, it does have disadvantages that are normally related to the spanning tree protocol (STP) as a bridging loop propagating throughout the network can occur. Cisco switches run an instance of STP for each VLAN, and since VTP propagates VLANs across the campus LAN, VTP effectively creates more opportunities for a bridging loop to occur.

Before creating VLANs on the switch that will propagate via VTP, a VTP domain must first be set up. A VTP domain for a network is a set of all contiguously trunked switches with the same VTP domain name. All switches in the same management domain share their VLAN information with each other, and a switch can participate in only one VTP management domain. Switches in different domains do not share VTP information.

Another, even greater concern with VTP is the issue known colloquially as the “VTP Bomb”. When a new switch is added to the network, by default it is configured with no VTP domain name or password, but in VTP server mode. Since a new switch has a VTP version of 0, it will accept any larger version number as newer and add that VLAN information to its configuration as long as the other switches have the same VTP domain and password. However, if you were to accidentally connect a switch to the network with the correct VTP domain name and password but a higher VTP version number than what the network currently has, then the entire network would adopt the VLAN configuration of the new switch – likely bringing down your entire network, or at least that VTP domain.

Dynamic Trunking Protocol

The Dynamic Trunking Protocol (DTP) is a proprietary networking protocol developed by Cisco Systems for the purpose of negotiating trunking on a link between two VLAN-aware switches, and for negotiating the type of trunking encapsulation to be used. It works on the Layer 2 of the OSI model. VLAN trunks formed using DTP may utilize either IEEE 802.1Q or Cisco ISL trunking protocols.

DTP should not be confused with VTP, as they serve different purposes. VTP communicates VLAN existence information between switches. DTP aids with trunk port establishment. Neither protocol transmits the data frames that trunks carry.

Switch Port Modes

The following switch port mode settings exist:

  1. Access: Puts the LAN port into permanent nontrunking mode and negotiates to convert the link into a nontrunk link. The LAN port becomes a nontrunk port even if the neighboring LAN port does not agree to the change.
  2. Trunk: Puts the LAN port into permanent trunking mode and negotiates to convert the link into a trunk link. The LAN port becomes a trunk port even if the neighboring port does not agree to the change.
  3. Dynamic Auto: Makes the LAN port willing to convert the link to a trunk link. The LAN port becomes a trunk port if the neighboring LAN port is set to trunk or desirable mode.
  4. Dynamic Desirable: Makes the LAN port actively attempt to convert the link to a trunk link. The LAN port becomes a trunk port if the neighboring LAN port is set to trunk, desirable, or auto mode. This is the default mode for all LAN ports.
  5. Nonegotiate: Puts the LAN port into permanent trunking mode but prevents the port from generating DTP frames. You must configure the neighboring port manually as a trunk port to establish a trunk link.

VTP Advertisement Messages

Three types of VLAN Trunking Protocol (VTP) advertisement messages are:

  1. Client Advertisement Request: A client advertisement request message is a VTP message which a client generates for VLAN information to a server. Servers respond with both summary and subset advertisements.
  2. Summary Advertisement: Summary advertisements are sent out every 300 seconds (5 minutes) by default or when a configuration change occurs, which is the summarized VLAN information.
  3. Subset Advertisement: Subset advertisements are sent when a configuration change takes place on the server switch. Subset advertisements are VLAN specific and contain details about each VLAN.

VTP Protocol

Cisco Inter-Switch Link (ISL)

Cisco Inter-Switch Link (ISL) is a Cisco Systems proprietary protocol that maintains VLAN information in Ethernet frames as traffic flows between switches and routers, or switches and switches.

ISL is Cisco’s VLAN Encapsulation protocol and is supported only on some Cisco equipment over Fast and Gigabit Ethernet links. It is offered as an option to the IEEE 802.1Q standard, a widely used VLAN tagging protocol, although the use of ISL for new sites is deprecated by Cisco. In the case of ISL the tag is external to the Ethernet frame, which effectively is the same as encapsulating the Ethernet frame, whereas with IEEE 802.1Q the tag is internal. This is a key advantage for IEEE 802.1Q as it means tagged frames can be sent over standard Ethernet links.

The size of an Ethernet encapsulated ISL frame can be expected to start from 94 bytes and increase up to 1548 bytes because of the overhead (additional fields) the protocol creates via encapsulation. ISL adds a 26-byte header (containing a 15-bit VLAN identifier) and a 4-byte CRC trailer to the frame. ISL functions at the Data-Link layer of the OSI model. ISL is used to maintain redundant links.

Another related Cisco protocol, Dynamic Inter-Switch Link Protocol (DISL) simplifies the creation of an ISL trunk from two interconnected Fast Ethernet devices. Fast EtherChannel technology enables aggregation of two full-duplex Fast Ethernet links for high-capacity backbone connections. DISL minimizes VLAN trunk configuration procedures because only one end of a link needs to be configured as a trunk.

IEEE 802.1Q

IEEE 802.1Q is the networking standard that supports virtual LANs (VLANs) on an Ethernet network. The standard defines a system of tagging for Ethernet frames and the accompanying procedures to be used by bridges and switches in handling such frames. The standard also contains provisions for a quality of service prioritization scheme commonly known as IEEE 802.1p and defines the Generic Attribute Registration Protocol.

Portions of the network which are VLAN-aware (i.e., IEEE 802.1Q conformant) can include VLAN tags. When a frame enters the VLAN-aware portion of the network, a tag is added to represent the VLAN membership of the frame’s port or the port/protocol combination, depending on whether port-based or port-and-protocol-based VLAN classification is being used. Each frame must be distinguishable as being within exactly one VLAN. A frame in the VLAN-aware portion of the network that does not contain a VLAN tag is assumed to be flowing on the native (or default) VLAN.

The standard was developed by IEEE 802.1, a working group of the IEEE 802 standards committee, and continues to be actively revised with notable revisions including IEEE 802.1ak, IEEE 802.1Qat and IEEE 802.1Qay.

Frame Format

Insertion of 802.1Q tag in an Ethernet frame
Insertion of 802.1Q tag in an Ethernet frame

802.1Q does not encapsulate the original frame. Instead, for Ethernet frames, it adds a 32-bit field between the source MAC address and the EtherType/length fields of the original frame, leaving the minimum frame size unchanged at 64 bytes (octets) and extending the maximum frame size from 1,518 bytes to 1,522 bytes (for the payload a 42-octet minimum applies when 802.1Q is present; when absent, a 46-octet minimum applies. IEEE 802.3-2005 Clause 3.5). Two bytes are used for the tag protocol identifier (TPID), the other two bytes for tag control information (TCI). The TCI field is further divided into PCP, DEI, and VID.

16 bits 3 bits 1 bit 12 bits

Tag Protocol Identifier (TPID)

A 16-bit field set to a value of 0x8100 in order to identify the frame as an IEEE 802.1Q-tagged frame. This field is located at the same position as the EtherType/length field in untagged frames, and is thus used to distinguish the frame from untagged frames.

Tag control information (TCI)

  • Priority Code Point (PCP): A 3-bit field which refers to the IEEE 802.1p class of service and maps to the frame priority level. Values in order of priority are: 1 (background), 0 (best effort), 2 (excellent effort), 3 (critical application), …, 7 (network control). These values can be used to prioritize different classes of traffic (voice, video, data, etc.).
  • Drop eligible indicator (DEI): a 1-bit field. (formerly CFI[note 1][2]) May be used separately or in conjunction with PCP to indicate frames eligible to be dropped in the presence of congestion.
  • VLAN Identifier (VID): a 12-bit field specifying the VLAN to which the frame belongs. The hexadecimal values of 0x000 and 0xFFF are reserved. All other values may be used as VLAN identifiers, allowing up to 4,094 VLANs. The reserved value 0x000 indicates that the frame does not belong to any VLAN; in this case, the 802.1Q tag specifies only a priority and is referred to as a priority tag. On bridges, VLAN 1 (the default VLAN ID) is often reserved for a management VLAN; this is vendor-specific.

For frames using IEEE 802.2/SNAP encapsulation with an OUI field of 00-00-00 (so that the protocol ID field in the SNAP header is an EtherType), as would be the case on LANs other than Ethernet, the EtherType value in the SNAP header is set to 0x8100 and the aforementioned extra 4 bytes are appended after the SNAP header.

Because inserting the VLAN tag changes the frame, 802.1Q encapsulation forces a recalculation of the original frame check sequence field in the Ethernet trailer.

The IEEE 802.3ac standard increased the maximum Ethernet frame size from 1518 bytes to 1522 bytes to accommodate the four-byte VLAN tag. Some network devices that do not support the larger frame size will process the frame successfully but may report them as a “baby giant” anomalies.

Double Tagging

With the IEEE standard 802.1ad, double-tagging can be useful for Internet service providers, allowing them to use VLANs internally while mixing traffic from clients that are already VLAN-tagged. The outer (next to source MAC and representing ISP VLAN) S-TAG (service tag) comes first, followed by the inner C-TAG (customer tag). In such cases, 802.1ad specifies a TPID of 0x88a8 for service-provider outer S-TAG.

Insertion of 802.1ad double tag in an Ethernet Frame
Insertion of 802.1ad double tag in an Ethernet Frame

Multiple VLAN Registration Protocol

IEEE 802.1Q defines the Multiple VLAN Registration Protocol (MVRP), an application of the Multiple Registration Protocol, allowing bridges to negotiate the set of VLANs to be used over a specific link.

MVRP replaced the slower GARP VLAN Registration Protocol (GVRP) in 2007 with the IEEE 802.1ak-2007 amendment.

IEEE 802.10

IEEE 802.10 is a former standard for security functions that could be used in both local area networks and metropolitan area networks based on IEEE 802 protocols.

802.10 specifies security association management and key management, as well as access control, data confidentiality and data integrity.

The IEEE 802.10 standards were withdrawn in January 2004 and this working group of the IEEE 802 is not currently active. Security for wireless networks was standardized in 802.11i.

The Cisco Inter-Switch Link (ISL) protocol for supporting VLANs on Ethernet and similar LAN technologies was based on IEEE 802.10; in this application 802.10 has largely been replaced by IEEE 802.1Q.

The standard being developed has 8 parts:

  1. Model, including security management
  2. Secure Data Exchange (SDE) protocol
  3. Key Management
  4. – has now been incorporated in ‘a’ –
  5. SDE Over Ethernet 2.0
  6. SDE Sublayer Management
  7. SDE Security Labels
  8. SDE PICS Conformance

Parts b, e, f, g, and h are incorporated in IEEE Standard 802.10-1998.

VLAN Trunking Protocol (VTP) Modes

A network switch, which is participating in VLAN Trunking Protocol (VTP), can have three different modes.

VTP Modes
VTP Modes

Server Mode

Server Mode is the default VTP mode for all Catalyst switches. At least one server is required in a VTP domain to propagate VLAN information within the VTP domain. We can create, add, or delete VLANs of a VTP domain in a Switch which is in VTP Server mode and change VLAN information in a VTP Server. The changes made in a switch in server mode are advertised to the entire VTP domain.

Client Mode

Client Mode switches listen to VTP advertisements from other switches and modify their VLAN configurations accordingly. A network switch in VTP client mode requires a server switch to inform it about the VLAN changes. We CANNOT create, add, or delete VLANs in a VTP client.

Transparent Mode

Transparent Mode switches do not participate in the VTP domain, but VTP transparent mode switches can receive and forward VTP advertisements through the configured trunk links.

VTP Pruning

Pruning is a feature in Cisco switches, which stops VLAN update information traffic from being sent down trunk links if the updates are not needed. If the VLAN traffic is needed later, VLAN Trunking Protocol (VTP) will dynamically add the VLAN back to the trunk link.

In normal operation a switch needs to flood broadcast frames, multicast frames, or unicast frames where the destination MAC address is unknown to all its ports. If the neighbouring switch doesn’t have any active ports in the source VLAN, this broadcast is unnecessary and excessive unwanted traffic may create problems on the network.

VLAN Trunking Protocol (VTP) pruning helps in increasing the available bandwidth by reducing unnecessary flooded traffic. Broadcast frames, multicast frames, or unicast frames where the destination MAC address is unknown are forwarded over a trunk link only if the switch on the receiving end of the trunk link has ports in the source VLAN.


VTP Topology
VTP Topology

3512xl#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
3512xl(config)#int vlan 1
3512xl(config-if)#ip address
3512xl(config)#ip default-gateway

3512xl#vlan database
3512xl(vlan)#vtp transparent

Setting device to VTP TRANSPARENT mode

3512xl(vlan)#vlan 2
VLAN 2 added:
Name: VLAN0002
APPLY completed.

Enable trunking on the interface fastEthernet 0/1

3512xl#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
3512xl(config)#int fastEthernet 0/1
3512xl(config-if)#switchport mode trunk

Enter the trunking encapsulation as either isl

3512xl(config-if)#switchport trunk encapsulation isl

or as dot1q

3512xl(config-if)#switchport trunk encapsulation dot1q

Allow all VLANs on the trunk.

3512xl(config-if)#switchport trunk allowed vlan all

3512xl(config)#int fastEthernet 0/2
3512xl(config-if)#switchport access vlan 2
3512xl(config-if)#spanning-tree portfast

3512xl(config)#int fastEthernet 0/3
3512xl(config-if)#spanning-tree portfast

c2600#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
c2600(config)#int fastEthernet 0/0
c2600(config-if)#no shut
c2600(config)#int fastEthernet 0/0.1
c2600(config-subif)#ip address

Enter the trunking encapsulation as either isl

c2600(config-subif)#encapsulation isl 1

or as dot1q

c2600(config-subif)#encapsulation dot1Q 1 ?
native Make this is native vlan

c2600(config-subif)#encapsulation dot1Q 1 native

c2600(config)#int fastEthernet 0/0.2
c2600(config-subif)#ip address
c2600(config-subif)#encapsulation isl 2

or as dot1q

c2600(config-subif)#encapsulation dot1Q 2

Debug and show Commands

On the Catalyst 2900XL/3500XL/2940/2950/2970 switch, use the following commands:

  • show int {FastEthernet | GigabitEthernet} switchport
  • show vlan
  • show vtp status

On the Cisco 2600 router, use the following commands:

  • show vlan
  • show interface


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