CCIE R&S Written Overview: VLANs/Trunking/VTP

VLANs/Trunking/VTP

VLAN Numbering

• VLAN membership defined by number

• 12-bit field (0-4095)

– 0 & 4095 reserved per 802.1Q standard

• Normal VLANs 1-1005

– 1 – Default Ethernet VLAN

– 1002/1004 – Default FDDI VLANs

– 1003/1005 – Default Token Ring VLANs

• Extended VLANs 1006-4094

VLAN Trunks

• Traffic sent over a trunk link receives special trunking encapsulation

– Normal Ethernet header does not have a field for VLAN number

– ISL or 802.1Q headers are added to include this information

Trunking Encapsulations

• Both ISL and 802.1Q accomplish the same goal of encoding VLAN number in frame header to separate traffic

• The key differences are…

– ISL

• Cisco proprietary

• 30-byte encapsulation for all frames

– 26-byte header

– 4-byte trailer (FCS)

• Does not modify original frame

802.1Q

• IEEE standard

• 4-byte tag except for “native” VLAN

• Modifies original frame

– See Inter-Switch Link and IEEE 802.1Q Frame Format for more info

DTP – Dynamic Trunking Protocol

• Dynamic switchports automatically choose whether to run in access or trunking mode

• Runs Dynamic Trunking Protocol (DTP) to negotiate, in order…

– ISL trunk

– 802.1Q trunk

– Access port

• Configured as switchport mode dynamic [auto|desirable]

• Disabled with switchport nonegotiate or switchport mode access

Auto vs Desirable

  • Auto will not initiate but will accept
  • Desirable will initiate a negotiation and also accept one

VTP Overview

• VLAN Trunk Protocol

• Cisco proprietary

• Used to dynamically…

– Advertise addition, removal, modification of VLAN properties

• Number, name, etc.

– Negotiate trunking allowed lists

• “VTP Pruning”

• Does not affect actual VLAN assignments

– Still manually needed with switchport access vlan [vlan]

How VTP Works

• VTP Domain

– To exchange information, switches must belong to the same

domain

• VTP Mode

– Controls who can advertise new/modified information

– Modes are…

• Server

• Client

• Transparent

• VTP Revision Number

– Sequence number to ensure consistent databases

– Higher revision indicates newer database

VTP Domains

• VTP domain name controls which devices can exchange VTP advertisements

• VTP domain does not define broadcast domain

– Switches in different VTP domains that share same VLAN numbers hosts’ are still in the same broadcast domain

• Configured as vtp domain [name]

• Defaults to null value

– Switch inherits VTP domain name of first advertisement it hears

VTP Pruning

• Broadcasts and unknown unicast/multicast frame are flooded everywhere in the broadcast domain

– Includes trunk links

• Editing allowed list limits this flooding, but large administrative overhead

• VTP pruning automates this procedure

– Switches advertise what VLANs they need

– All other VLANs are pruned (removed) off the trunk link

• Does not work for transparent mode

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