CCNA 3 Week 9
VLAN Trunking
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Origins
Dates back to radio and telephone
Trunk carries multiple channels over a single link
Single link between switches carrying multiple VLANS
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Lan Trunking
Trunking saves ports and reduces wiring between switches
Removes complexity from physical hardware
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Trunking Operation
Need a method to identify which frames are destined for each subnet/VLAN
– Filtering– Tagging
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IEEE 802.1Q
Switches need to be able to determine which VLAN frames from a trunk should be passed to
Cisco offers Propriatary ISL or 802.1Q
802.1Q protocol includes VLAN tagging as part of layer 2 frames
Standard protocol used by many manufacturers
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Frame Tagging
Frames have a VLAN id inserted into Frame header
Understood and read by switches
VLAN identifier removed before traffic leaves trunked/backbone links
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Setup Trunking
Commands to enable trunkingSw1(config)# interface fastethernet0/1Sw1(config-if)# switchport mode trunkSw1(config-if)# switchport trunk encapsulation dot1qSw1(config-if)# end
Naturally need to enable trunking at each end of a trunked link
VLAN Trunking Protocol
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History of VTP
Protocol created by Cisco
Need consistency in VLAN numbering to allow trunking across large network
Either have to manage each switch individually
Manage centrally to ensure consistency
Otherwise risk cross-connection or failure to pass on frames
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VTP Concepts
VTP uses layer 2 trunk frames to Add, Delete and Rename VLANs
VTP messages encapsulated in ISL or 802.1q
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VTP Messages
Contain the following– VTP protocol version - Either version 1 or 2 – VTP message type - Indicates one of four types of
messages – Management domain name length - Indicates the size
of the name that follows – Management domain name - Name configured for the
management domain
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VTP Modes
VLAN Server– Can create/modify/delete vlans for entire domain– Save VLAN config in NVRAM– Send VTP messages to all trunk ports
VLAN Client– Cannot make changes– Record information and forward VTP messages
Transparent Mode– Forward Messages but ignore content– Can create local VLANs
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VTP Implementation
Each switch advertises multicasts via trunk ports– its management domain– configuration revision number– the VLANs that it knows about
Clients request info at bootup, servers respond
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Summary Advertisements
Sent every five minutes with current VTP revision number
If greater that the receiver's revision number, an advertisement request is sent
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Subset Advertisements
Subset advertisements contain detailed information about VLANs
– VTP version type– Domain name – Configuration revision number.
Certain actions can trigger subset advertisements:– VLAN creation or deletion – VLAN suspension or activation – VLAN name change – VLAN maximum transmission unit (MTU) change
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Advertisement Contents
Management domain name - Advertisements with different names are ignored.
Configuration revision number - The higher number indicates a more recent configuration.
Message Digest 5 (MD5) - MD5 is the key that is sent with the VTP when a password has been assigned. If the key does not match, the update is ignored.
Updater identity - The updater identity is the identity of the switch that sends the VTP summary advertisement.
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VTP Configuration
Determine the version number of VTP that will be utilized.
Is a new management domain needed
To join an existing domain, name and password are needed
Choose a VTP mode for the switch– V1 and V2 not interoperable
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VTP Commands
Switch#vlan database
Switch(vlan)#vtp v2-mode
Switch(vlan)#vtp domain cisco
Switch(vlan)#vtp {client | server | transparent}
Routers and VLANs
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Need for routers
To pass traffic between subnets(VLANs) a router is required
Segment broadcast domains
Trunks may connect to router
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Routers Trunked Links
Not cost effective to have dedicated ethernet port per vlan
Can split physical interface into logical sub-interfaces
A logical interface for each subnet
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Sub-interfaces
A logical interface within a physical interface
Each sub-interface has own IP address and belongs to one VLAN
Denoted by FA0/1.1, FA0/1.2 etc
Router passes traffic between sub-interfaces
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Sub-interface setup
Router(config)#int fa port-number.subint-number
Router(config-if)#encapsulation dot1q vlan-no
Router(config-if)#ip address ip-address subnet-mask
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