VLAN Trunking protocol

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VLAN Trunking protocol

• CCNA Exploration Semester 3
• Chapter 4

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Topics

• The role of VLAN Trunking Protocol (VTP)
• Operation of VTP
• Configure VTP on switches

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Semester 3
4
Purpose of VTP

• You can create VLANs on a switch.
• What if you have the same VLANs on 10 linked
  switches? Or 100 linked switches?
• Do you have to create the VLANs on every switch
  and allow them on each trunk?
• VTP helps.
• But you still have to assign access ports to
  VLANs on each switch.

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VTP domain

• Group of layer 2 switches sharing VLAN data.
• Ends at router or layer 3 switch.
• Switch can be linked but not part of domain.
• Each switch can belong to only one domain.
• Domain is defined by its name.
• Proprietary to Cisco, so all switches in domain
  must be Cisco switches.

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VTP server

• One switch acts as server.
• Create VLANs on this switch.
• Information saved in vlan.dat.
• Server sends VLAN information to client switches
  over active trunk links.
• Add, delete, rename VLANs on server.
• Default mode of switch is server.

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VTP client

• Client receives VLAN information from server.
• Client switches then have the same VLANs as the
  server.
• Client does not save VLAN information. It is held
  only in RAM and lost if switch is powered off.

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VTP modes

• Server default mode. Sends VLAN information to
  other switches.
• Client receives VLAN information and forwards
  it to other switches.
• Transparent forward VTP traffic but do not
  originate or use it. They can have their own
  VLANs, not shared with other switches.

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VTP defaults

• Version 1. (Versions 2 and 3 also exist.)
• VTP domain name is not set.
• VTP mode server
• One active VLAN, VLAN 1
• Configuration revision number 0
• Any switches added to a domain should be in the
  default condition or they may send unwanted
  information to other switches.

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Show vtp status

• VTP Version
• Maximum VLANs Supported
• Number of Existing VLANs
• VTP Operating Mode- server, client, or
  transparent.
• VTP Domain Name
• VTP Pruning Mode
• VTP V2 Mode (disabled by default)
• VTP Traps Generation
• MD5 Digest (checksum of VTP configuration)
• Configuration Last Modified

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VTP advertisements

• Consist of VTP header and VTP message
• Encapsulated inside Ethernet frame with tag to
  pass over trunk link.
• Destination MAC address is multicast address
  01-00-0C-CC-CC-CC

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VTP message header

• Domain name
• Domain name length
• Version - VTP 1 or VTP 2, on Cisco 2960 switch.
• Configuration revision number
• Other fields, depending on type of message.

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Configuration revision number

• 32-bit number.
• Default value is 0.
• It is incremented each time a VLAN is added or
  removed.
• Reset to 0 is domain name changes.
• Switch uses it to see if information is more
  recent that what it already holds.

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Summary advertisement

• Sent immediately after a change is made, with
  updated revision number.
• Sent every 5 minutes by servers and clients to
  check on current VTP configuration revision
  number.
• Contains the VTP domain name, the current
  revision number, and other VTP configuration
  details.

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VTP Summary advertisement

• A switch receives a summary advertisement.
• Compares domain name to its own domain name.
• If name is different, the switch ignores the
  packet.
• If the name is the same, the switch compares the
  revision number to its own revision number.
• Number higher or equal, ignored the packet.
• Number lower, sends an advertisement request.

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Request Advertisement

• Sent by client to server if
• The VTP domain name has been changed
• The switch receives a summary advertisement with
  a higher configuration revision number than its
  own
• A subset advertisement message is missed for some
  reason
• The switch has been reset

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Subset advertisement

• Contains VLAN information. Several may be needed
  if there is a lot of information.
• Sent by server in response to a request or after
  
• Creating or deleting a VLAN
• Suspending or activating a VLAN
• Changing the name of a VLAN
• Changing the MTU of a VLAN

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Subset advertisement
Version Code Seq-number Domain name length
Management domain name (padded to 32 bytes) Management domain name (padded to 32 bytes) Management domain name (padded to 32 bytes) Management domain name (padded to 32 bytes)
Configuration revision number Configuration revision number Configuration revision number Configuration revision number
VLAN info field 1 VLAN info field 1 VLAN info field 1 VLAN info field 1
VLAN info field 2 VLAN info field 2 VLAN info field 2 VLAN info field 2
Etc. Etc. Etc. Etc.
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Subset advertisement VLAN info
VLAN-Info VLAN-Info VLAN-Info VLAN-Info
Info length Status VLAN-Type VLAN-name Len
ISL VLAN ID ISL VLAN ID MTU size MTU size
802.10 index 802.10 index 802.10 index 802.10 index
VLAN name (padded to multiple of 4 bytes) VLAN name (padded to multiple of 4 bytes) VLAN name (padded to multiple of 4 bytes) VLAN name (padded to multiple of 4 bytes)
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Pros and cons

• Small network dont bother with VTP.
• Big network good for consistency and easier to
  make changes.
• Server switches need lots of flash memory,
  clients do not.
• Redundancy dont have everything on one server
  switch.
• Problems from large domains.
• Extreme care when adding a switch.

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Why transparent?

• Put a switch in transparent mode if it has local
  VLANs that are not on other switches.
• The other switches do not need to know about them.

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VTP pruning

• Disabled by default
• Enable it on one server in domain.
• Stops VLAN traffic from being sent on links that
  do not lead to devices on that VLAN.
• Cuts down on traffic on trunk links.

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Domain names and passwords

• Configure a domain name on the first server
  switch. The other switches will learn it.
• If you configure it on other switches, check that
  it is exactly the same. It is case sensitive.
• If you use a password then it must be exactly the
  same on all switches.

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Versions

• VTP version 1 is used by default on Catalyst
  switches, but they can use version 2.
• If you configure version 2 on one switch then the
  other switches should learn the new version and
  change to it.
• If a switch is not capable of running version 2
  then it will not exchange advertisements.

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Configure VTP

• Configure VTP with a domain name before creating
  VLANs on the server.
• Existing VLANs are removed when you start to
  configure VTP.
• Check that links are trunk links.
• Configure client switches to client mode.

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Commands on server

• SW1(config)vtp domain cisco1
• SW1(config)vtp password cisco
• (Password is optional)
• Server mode is default, but if it was changed
• SW1(config)vtp mode server
• Version 1 is default, but command is
• SW1(config)vtp version 1

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Commands on server

• Create VLANs
• Check that link is a trunk.
• Check VTP operation
• SW1 show vtp status
• Assign switch ports to VLANs.

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Client configuration

• SW2(config)vtp mode client
• Check that link is a trunk.
• Check VTP operation
• SW2 show vtp status
• Assign switch ports to VLANs.

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Things to check

• VTP Version. It needs to be the same on all
  switches in the domain.
• Domain name. Is it exactly the same on all
  switches?
• VTP Password if any. Is it exactly the same on
  all switches?
• Check that there is at least one server. Better
  to have at least two.
• If you recently added a new switch, had its
  revision number been set to 0?

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• The End