Chapter 2: VLAN Trunks Part Two (Preview)

Inter-Switch-Link (ISL)

ISL is a Cisco proprietary protocol.

IEEE 802.1Q

Inserts a 4 byte 802.1Q header which consists of the 12 bit VLAN ID field. This is where the VLAN information is going to be included.

 

802-1q-frame-format

Figure 802.1Q Header Format

 

  • Theoretically supported maximum VLANs are 2^12 (4096)
  • In practice supports a maximum of 4094 VLANs
  • VLANs 0 and 4095 are reserved by both 802.1Q and ISL
  • VLANs 1 – 1005 are Normal Range VLANs.
  • VTP 1 and VTP 2 supports only Normal Range VLANs
  • VLAN IDs 1002 through 1005 are reserved for Token Ring and FDDI VLANs.
  • In VTP 1 and VTP 2 switch must be in VTP transparent mode when you create VLAN IDs from 1006 to 4094 (Extended Range).
  • VTP 3 supports VLAN IDs from 1 – 4094

VTP stands for VLAN Trunkin Protocol and we will discuss its operations in a later chapter.

 

Let’s try to create an Extended Range VLAN 1006 on a switch in VTP Server mode and running VTP Version 1.

WS-2960#show vtp status 
VTP Version capable             : 1 to 3
VTP version running             : 1
VTP Domain Name                 : 
VTP Pruning Mode                : Disabled
VTP Traps Generation            : Disabled
Device ID                       : 001c.5777.c980
Configuration last modified by 0.0.0.0 at 3-1-93 01:11:32
Local updater ID is 192.168.1.91 on interface Vl1 (lowest numbered VLAN interface found)

Feature VLAN:
--------------
VTP Operating Mode                : Server
Maximum VLANs supported locally   : 255
Number of existing VLANs          : 7
Configuration Revision            : 8
MD5 digest                        : 0x5E 0x35 0xA8 0x74 0xD9 0x6E 0xCB 0xF6 
                                    0x3A 0x12 0xAE 0xF4 0xCE 0xC6 0x25 0x16 
WS-2960#
WS-2960#
WS-2960#configure terminal  
Enter configuration commands, one per line.  End with CNTL/Z.
WS-2960(config)#vlan 1006
WS-2960(config-vlan)#name Research
WS-2960(config-vlan)#exit
% Failed to create VLANs 1006 Extended VLAN(s) not allowed in current VTP mode.
%Failed to commit extended VLAN(s) changes.
WS-2960(config)#
*Mar  1 02:34:32.850: %SW_VLAN-4-VLAN_CREATE_FAIL: Failed to create VLANs 1006: extended VLAN(s) not allowed in current VTP mode
WS-2960(config)#
WS-2960(config)#exit

Example Creating an Extended Range VLAN on a Cisco Catalyst 2960 switch using VTP V1

 

Let’s change the VTP version to 2 and see what happens when we try to create an Extended range VLAN 1006.

WS-2960# 
WS-2960#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
WS-2960(config)#vtp version 2
WS-2960(config)#exit
WS-2960#show vtp status
VTP Version capable             : 1 to 3
VTP version running             : 2
VTP Domain Name                 : 
VTP Pruning Mode                : Disabled
VTP Traps Generation            : Disabled
Device ID                       : 001c.5777.c980
Configuration last modified by 192.168.1.91 at 3-1-93 02:44:13
Local updater ID is 192.168.1.91 on interface Vl1 (lowest numbered VLAN interface found)

Feature VLAN:
--------------
VTP Operating Mode                : Server
Maximum VLANs supported locally   : 255
Number of existing VLANs          : 7
Configuration Revision            : 11
MD5 digest                        : 0x29 0x48 0xCA 0xED 0x2B 0x5F 0x56 0xCB 
                                    0xE7 0xE3 0xD9 0xF1 0x71 0xCB 0x63 0x41 
WS-2960#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
WS-2960(config)#vlan 1006
WS-2960(config-vlan)#name Research
WS-2960(config-vlan)#exit
% Failed to create VLANs 1006 Extended VLAN(s) not allowed in current VTP mode.
%Failed to commit extended VLAN(s) changes.
WS-2960(config)#
*Mar  1 02:44:59.119: %SW_VLAN-4-VLAN_CREATE_FAIL: Failed to create VLANs 1006: extended VLAN(s) not allowed in current VTP mode
WS-2960(config)#
WS-2960(config)#exit
WS-2960#

Example Creating an Extended Range VLAN on a Cisco Catalyst 2960 switch running VTP Version 2

 

If you carefully observe the error message, it is saying “extended VLAN(s) not allowed in current VTP mode”. Let’s change it to VTP Transparent mode and see whether we could create VLAN 1006.

WS-2960#
WS-2960#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
WS-2960(config)#vtp mode transparent
Setting device to VTP Transparent mode for VLANS.
WS-2960(config)#do show vtp status
VTP Version capable             : 1 to 3
VTP version running             : 2
VTP Domain Name                 : 
VTP Pruning Mode                : Disabled
VTP Traps Generation            : Disabled
Device ID                       : 001c.5777.c980
Configuration last modified by 192.168.1.91 at 3-1-93 02:44:13

Feature VLAN:
--------------
VTP Operating Mode                : Transparent
Maximum VLANs supported locally   : 255
Number of existing VLANs          : 7
Configuration Revision            : 0
MD5 digest                        : 0x29 0x48 0xCA 0xED 0x2B 0x5F 0x56 0xCB 
                                    0xE7 0xE3 0xD9 0xF1 0x71 0xCB 0x63 0x41 
WS-2960(config)#vlan 1006
WS-2960(config-vlan)#name Research
WS-2960(config-vlan)#exit
WS-2960(config)#exit
WS-2960#
*Mar  1 02:49:33.032: %SYS-5-CONFIG_I: Configured from console by console
WS-2960#show vlan brief 

VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------
1    default                          active    Fa0/1, Fa0/2, Fa0/3, Fa0/4
                                                Fa0/5, Fa0/6, Fa0/7, Fa0/8
                                                Fa0/9, Fa0/10, Fa0/11, Fa0/12
                                                Fa0/13, Fa0/14, Fa0/15, Fa0/16
                                                Fa0/17, Fa0/18, Fa0/19, Fa0/20
                                                Fa0/21, Fa0/22, Fa0/23, Fa0/24
                                                Gi0/1, Gi0/2
10   LAB                              active    
100  VLAN0100                         active    
1002 fddi-default                     act/unsup 
1003 trcrf-default                    act/unsup 
1004 fddinet-default                  act/unsup 
1005 trbrf-default                    act/unsup 
1006 Research                         active    
WS-2960#

Example Creating an Extended Range VLAN on a Cisco Catalyst 2960 switch running VTP Version 2 and in VTP Transparent mode

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