CCIE RS – Written – L2 – Implement and Troubleshoot Trunking

Implement and Troubleshoot Trunking


Trunking

  • Allows devices to send traffic for multiple vlans across a single link
  • Trunking protocols must match on both ends

ISL

  • Cisco Proprietary
  • Supports normal and extended vlans
  • Encapsulates original frame
    • 26 byte header + trailer for FCS
  • No concept of native vlan

802.1Q

  • Open Standard (IEEE)
  • Supports normal and extended vlans
  • Inserts tag into original frame
    • 4 byte header
    • Frame tagging
  • Concept of native vlan
    • Traffic in native vlan is untagged
    • All received untagged frames are sent over native vlan
    • Must match on both ends of trunk
    • Detection for mismatch
      • Cisco Switches – Proprietary extension in PVST+ and Rapid PVST+
      • CDP Neighbor can detect and report (syslog)
    • Default native vlan is 1
      • Best practice is to change to prevent vlan hopping attacks

DTP – Dynamic Trunking Protocol

  • Dynamically learn if device on other end wants to perform trunking and negotiate what protocol to use
  • Modes
    • Dynamic Auto – negotiate automatically
      • Prefers to be access port
    • Dynamic Desirable
      • Prefers to be a trunk port
      • Highest priority
      • Will choose ISL is both sides support
    • Different switch models have different default behaviors
    • DTP and VTP are independent
      • DTP carries the VTP domain name
  • Reduce number of vlans on trunk
    • Vlans must be configured on switch before being considered active
    • Administratively configure or use VTP pruning
      • Switchport trunk allowed [add | remove ]
  • Config
    • Switchport mode [trunk | dynanmic desirable / auto | access
    • Switchport nonegotiate
      • Port no longer sends DTP messages
    • Switchport trunk encapsulation [isl | dot1q]

Q in Q Tunneling

  • VLANs traditionally do not extend past WAN boundary
  • Standard 802.1ad – Provider Bridges
  • Allow SP to preserve 802.1q vlan tag across WAN service
  • CDP and VTP can be configured to pass transparently over Q-in-Q
  • New tagging concept
    • C-Tag – Original customer tagged frame
    • S-Tag – Additional tag added to C-Tag frame at ingress of SP
      • Tag is removed at egress of SP, preserving original C-Tag
      • Different customers get different S-Tags
  • Configuration
    • Change MTU to 1504 for extra header size

Manual VLAN Pruning

Manually specify which vlans are allowed across a trunk link


VTP – VLAN Trunking Protocol

  • Advertises vlan config information to neighboring switches
  • Allows vlan config to be done from 1 switch in the domain (dynamic)
  • Advertises VLAN ID, name and type
  • Does not forward which ports are in the vlan
  • Sends updates out all active trunk links by default
    • Without domain name switches do not send any VTP updates
  • VTP starts working when there is an active trunk link and domain name
  • Configuration
    • Vtp domain [name]
    • Show vtp status
    • Options
      • Domain – sets domain name
        • Switch can only belogn to one domain
      • Password – sets password to prevent unauthorized switches from joining domain
      • Mode [server | client | transparent]
      • Version [1 | 2 | 3]
        • Can configure on server switch and will sync with other switches
        • V3 must be manually configured and have domain name configured
      • Pruning
        • Prevent flooding per-VLAN to switches that do not have the VLAN configured
        • Only applied to normal VLAN range
      • Interface
        • Sets interface whos IP is used to ID the switch
        • Default is to use IP of lowest numbered SVI

VTPv1

Version 1 is default

  • Supports only normal vlan range

Message Types (v1 and v2)

  • Summary advertise
    • Originated by VTP server and client every 5 minutes and after each modification to vlan database
    • Carries domain name, revision number, ID of updater, timestamp, MD5, password & number of subset advertise
    • Does not carry vlan database contents
  • Subset Advertise
    • Originated by server and client after modification of vlan database
    • Carries full vlan database contents
  • Advertise Request
    • Request neighbors complete vlan database or part of it
    • Request sent when switch enters client mode or client switch is restarted
  • Join
    • Every 6 seconds if VTP pruning is active

Update Process (v1 and v2)

  • Begins when a modification is made on a vlan
  • VTP server will increment revision number by 1 and advertise entire vlan database with new revision number
  • Revision number allow switches to know when vlan database changes have occurred
    • Switches replace their vlan database when a larger revision shows up

VTP Modes

  • Server –
    • Default with no domain name
    • Updates to not occur until a domain name is configured
  • Client
    • Will assume first received domain name
    • Default domain is NULL
  • Transparent / Off
    • Prevents switches from listening to other switches VTP updates
  • Protect VTP domain – VTP password
    • Summary adv – carries MD5 hash computed over the vlan database contents and VTP password if configured

VTPv2

Enhanced version of v1

  • Supports token ring concentrator relay function and bridge relay function
    • Not used in ethernet based networks
  • Supports unknown TLV (Type Length Value)
    • V1 would drop unrecognized TLV’s which would stop propagation to other switches
  • Optimized vlan database consistency checking
    • Implementation optimization
    • Skips consistency check if change was received by a VTP message

VTPv3

Introduced to IOS release 12.2(52)SE

  • New server roles – Addresses problem of inadvertent rewrites to the vlan database
    • Primary – Only 1 at a time
      • Can modify VTP domain contents
      • Only switch who’s vlan database is propagated through domain
      • Database will be shared if switch agrees on domain name and ID of primary server (MAC address)
    • Secondary –
      • Cannot make changes,
      • but can be promoted
      • All other server switches in domain are secondary
    • Primary role is a runtime state. Does not get saved into configuration
    • Server role helps prevent unwanted changes to VTP domain
  • Password storage and usage improved
    • Encrypted and cannot be displayed in plaintext
    • Promotion of secondary required entering password in plaintext
    • String is carried encrypted to different switches
  • Capable of distributing full range of vlans and private vlans
    • No longer need to be in transparent mode when using extended vlan range and private vlans
    • Pruning only applied to normal vlan range
  • Supports VTP off
    • Switch does not participate in VTP and drops all VTP messages
  • Distributes content of MST region config
  • Client switches cannot make changes to domain or be promoted
  • Both Secondary and Client store copy of primaries vlan database and share with neighboring server and client if they agree on the primaries ID
  • Secondary or client servers with higher revision number can overwrite vlan database, but must match on domain name, primary ID and password
  • Conflicts
    • Server or client in domain having different primary server IDs
    • Conflciting switches do not sync the vlan database even if all other parameters match
  • Promotion to primary sever done in exec mode
    • Vtp primary
  • Cannot reset revision number to 0 by switching mode to transparent
    • Only will be reset by modifying the domain name or configuring password
  • Switches that cannot run v3, the port will revert to v2
    • V3 to v1 is not supported

VTP Pruning

Reduces unnecessary flooded traffic such as broadcasts, multicast and unicast packet

  • Disabled by default
  • Enabling on server enables for entire domain
  • Vlans 2 – 1000 are eligible for pruning
  • Cmd: vtp pruning

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