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Chapter 8
VLAN & Trunking
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Objectives
Upon completion of this chapter, you will beable to perform the following tasks:
Configure a VLAN
Configure VLAN Trunking Protocol (VTP) Configure a switch for trunkingConfigure a router for trunkingRouting between Vlans
Verify VLAN connectivity
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VLAN introduction
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VLAN Overview
Segmentation
Flexibility
Security
A VLAN = A broadcast domain = Logical network (subnet)
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Benefits of VLANs
Easily move workstations on the LAN.Easily add workstations to the LAN.Easily change the LAN configuration.Easily control network traffic.Improve security.
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VLAN Operations
Each logical VLAN is like a separate physical bridge VLANs can span across multiple switches Trunks carries traffic for multiple VLANs Trunks use special encapsulation to distinguish
between different VLANs
Switch A
GreenVLAN
BlackVLAN
RedVLAN
GreenVLAN
BlackVLAN
RedVLAN
Switch B
Trunk
Fast Ethernet
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VLAN Membership Modes
VLAN5
Static VLAN Dynamic VLAN
MAC = 1111.1111.1111
Trunk
VMPS
1111.1111.1111 = vlan 10
VLAN10
Port e0/9Port e0/4
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VLAN CONFIGURATION
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Configuring Static VLANsCreate VLANs:
Switch# vlan databaseSwitch(vlan)# vlan < vlan_number> name Switch(vlan)# exit
Assign ports to VLANs:Switch(config)# interface fastethernet 0/9Switch(config-if)#switchport access vlan < vlan_number>
Deleting VLANs
Use the no form of the command .EX :
Create VLANs:Switch# vlan databaseSwitch(vlan)# vlan 2 name VlanBGD Switch(vlan)# exit
Assign ports to VLANs:Switch(config)# interface fastethernet 0/9Switch(config-if)#switchport access vlan 2
Deleting VLANs Use the no form of the command .
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Verifying and save VLANconfiguration
show vlan
copy running-config startup-configCopy running-config TFTP
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TRUNKING
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Trunking concept
VLAN1
VLAN2
VLAN1
VLAN2
VLAN1
VLAN2
VLAN1
VLAN2
VLAN1
VLAN2
VLAN1 and VLAN2
TRUNK
A trunk is a single communications line that carriesdata of multiple Vlans.
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VLANs and trunking
ISL802.1Q (dot1q)
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Frames Tagging
ISL802.1Q (dot1q)
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ISL Tagging
Not intrusive to clientstations, client does notsee the ISL headerEffective betweenswitches, routers andswitches, switches andservers with ISL networkinterface cards
ISL trunks enable VLANs across a backbone
VLAN Tag addedby incoming
port
VLAN Tagstripped by
forwarding port
Inter-SwitchLink carries
VLAN identifier
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ISL Encapsulation
ISL Header26 bytes Encapsulated Ethernet frame
CRC4 bytes
Frames encapsulated with ISL header and CRCSupport for many VLANs (1024)
VLAN field
BPDU bit
DA Type User SA LEN
VLAN
AAAA03
BPDU
HSA VLAN BPDUBPDU INDEX RES
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IEEE 802.1q
802.1q does not actually encapsulate the originalframe instead, it adds an extra 4-byte header to themiddle of the original Ethernet header.802.1q encapsulation forces a recalculation of theoriginal FCS field in the Ethernet trailer
Dest Src Len/Type Data FCS
Dest Src Type Tag Len/Type Data FCS
Priority VLAN ID
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ISL and 802.1q Compared
Both ISL and 802.1q provide trunking.
Both allow the use of a 12-bit-long VLAN ID field. So,either works fine and supports the same number ofVLANs.Both support a separate instance of spanning tree foreach VLAN.
The header used by each varies, and only ISL actuallyencapsulates the original frame.
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Trunking implementation
Switch# show port capabilities
Switch # configure terminalSwitch(config)# interface f0/1Switch(config-if)# Switchport mode trunk.Switch(config-if)# Switchport trunk encapsulation dot1q/islSwitch # Show trunk
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Inter-VLAN issues and solutions
Every Vlan is one subnetwork.When a host on one VLAN wants to communicate with ahost on another, a router must be involved. Networks with many VLANs must use VLAN trunking to
assign multiple VLANs to a single router interface.
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Dividing a Physical Interface intoSubinterfaces
Physical interfaces can be divided into multiplesubinterfaces.
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Physical and logical interfaces
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Configuring Router for Trunking
Router(config)#Interface fastethernet port-numberRouter(config-if)#no shutdownRouter(config-if)#exit
Router(config)# interface fastethernet port-number . Subinterface-number
Router(config-if)#encapsulation dot1q/isl vlan-numberRouter(config-if)#ip address
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R1
SW1SW2
Vlan1 Vlan1 Vlan2 Vlan3Vlan3Vlan2
F0/0.10:192.168.1.254F0/0.12:192.168.2.254F0/0.13:192.168.3.254
.1.20 .2.20 .3.20 .1. 10 .2.10 .3.10
EX : Configuring Router for Trunking
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VTP
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VLAN Trunking Protocol (VTP)
A messaging system that advertises VLANconfiguration informationMaintains VLAN configuration consistencythroughout a common administrative domainVTP sends advertisements on trunk ports onlySupport mixed media trunks (Fast Ethernet,
FDDI, ATM)
1.new vlan added
3.Sync to the latest vlan information
2
VTP Domain ICND
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VTP Modes
Transparent
Forwardsadvertisements
Synchronize Not saved in
NVRAM
Create vlans Modify vlans Delete vlans Sends/forwardsadvertisements
Synchronize Saved in NVRAM
Create vlans Modify vlans Delete vlans Forwardsadvertisements
Does notsynchronize
Saved in NVRAM
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VTP advertisements are sent as multicast frames
VTP servers and clients synchronized to latest revisionnumber
VTP advertisement are sent every five minutes or whenthere is a change
How VTP Works
1.Add new VLAN2.Rev 3 --> Rev 4
Server
Client Client
4.Rev 3 --> Rev 45.Sync new vlan info
3 3
4.Rev 3 --> Rev 45.Sync new vlan info
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VTP Pruning Increases available bandwidth by reducing unnecessaryflooded traffic
Example: Station A sends broadcast, broadcast is onlyflooded toward any switch with ports assigned to the redVLAN
Switch 4
Switch 2
Switch 6 Switch 3 Switch 1
Port 2
Floodedtraffic is
pruned
RedVLAN
Port 1
Switch 5
A
B
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VLAN Configuration Guidelines
Maximum number of VLANs is switch-dependent
Catalyst 1900 supports 64 VLANs with a separatespanning tree per VLANVLAN1 is One of the factory default VLANsCDP and VTP advertisements are sent on VLAN1
Must be in VTP server or transparent mode tocreate, add, or delete VLANs
VLAN Config ration Steps
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VLAN Configuration Steps
Enable VTP (optional)
Enable trunking
Create VLANs
Assign VLAN to ports
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VTP domain name
VTP mode (server/client/transparent)
VTP server mode is the default
VTP pruning
VTP password
VTP Configuration Guidelines
Use caution when adding a new switch into an existing domain. Anew switch should be added in client mode to prevent the newswitch from propagating incorrect VLANs information
Use the delete vtp command to reset the VTP revision number
wg_sw_2950#vlan database wg_sw_2950(vlan)#vtp [ server | client | transparent ] wg_sw_2950(vlan)#vtp domain domain-name wg_sw_2950(vlan)#vtp password password wg_sw_2950(vlan)#vtp pruning wg_sw_2950(vlan)#exit
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Verifying VTP Configurations
wg_sw_a#show vtpVTP version: 1Configuration revision: 4Maximum VLANs supported locally: 1005Number of existing VLANs: 6VTP domain name : switchlabVTP password :
VTP operating mode : TransparentVTP pruning mode : EnabledVTP traps generation : EnabledConfiguration last modified by: 10.1.1.40 at 00-00-0000 00:00:00
wg_sw_a#show vtp
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