Max VLANs on ISL??

Answered Question
Aug 12th, 2009

802.1Q frame has a 12 Bits VLAN ID which translate to maximum possible 4096 vlans on a trunk.

ISL frame has a 15 Bits VLAN ID. based on this I would assumed ISL can support more than 4096 vlans. however, I'm being told only up to 1024 vlans are supported on a ISL trunk. is this true? if so, is there something in the ISL frame I can use to calculate the max vlan?

I have this problem too.
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Correct Answer by Giuseppe Larosa about 7 years 5 months ago

Hello Wayne,

original ISL specification supported only 1024 vlans but later a revision of the protocol allows for support of 4096 vlans like it happens for 802.1Q.

see

http://www.cisco.com/en/US/partner/products/hw/switches/ps700/products_white_paper09186a00801b49a4.shtml#pre4

the vlan-id field has been 15 bits wide since first version of ISL.

You need also to take in account that ISL has more overhead and that 802.1Q is standard based.

Your lab results look like correct.

to be noted that VTP is still limited to vlans 1-1024 in version 2 and vtp mode transparent is needed to use the extended range

Hope to help

Giuseppe

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iyde Wed, 08/12/2009 - 20:37

I believe that ISL has been designed to use only VLAN 1-1005. So it is not a matter of how many bits are available for VLAN id but more of a design decision.

HTH

situwayne Thu, 08/13/2009 - 15:58

That's what all the documents said. I've configured ISL on these Cat3560 switches. I don't have 1000+ vlans created, but the trunk is clearly allowing more than 1005.

SW1#sh run int gi0/1

Building configuration...

Current configuration : 145 bytes

!

interface GigabitEthernet0/1

switchport trunk encapsulation isl

switchport trunk allowed vlan 1-2048

switchport mode dynamic desirable

end

SW1#sh run int gi0/2

Building configuration...

Current configuration : 107 bytes

!

interface GigabitEthernet0/2

switchport trunk encapsulation isl

switchport mode dynamic desirable

end

SW1#sh int trunk

Port Mode Encapsulation Status Native vlan

Gi0/1 desirable isl trunking 1

Gi0/2 desirable isl trunking 1

Port Vlans allowed on trunk

Gi0/1 1-2048

Gi0/2 1-4094

Port Vlans allowed and active in management domain

Gi0/1 1,5-6

Gi0/2 1,5-6,4000

Port Vlans in spanning tree forwarding state and not pruned

Gi0/1 1,5-6

Gi0/2 1,5-6,4000

SW1#

SW1#sh int gi0/1 switchport

Name: Gi0/1

Switchport: Enabled

Administrative Mode: dynamic desirable

Operational Mode: trunk

Administrative Trunking Encapsulation: isl

Operational Trunking Encapsulation: isl

Negotiation of Trunking: On

Access Mode VLAN: 1 (default)

Trunking Native Mode VLAN: 1 (default)

Administrative Native VLAN tagging: enabled

Voice VLAN: none

Administrative private-vlan host-association: none

Administrative private-vlan mapping: none

Administrative private-vlan trunk native VLAN: none

Administrative private-vlan trunk Native VLAN tagging: enabled

Administrative private-vlan trunk encapsulation: dot1q

Administrative private-vlan trunk normal VLANs: none

Administrative private-vlan trunk private VLANs: none

Operational private-vlan: none

Trunking VLANs Enabled: 1-2048

Pruning VLANs Enabled: 2-1001

Capture Mode Disabled

Capture VLANs Allowed: ALL

Protected: false

Unknown unicast blocked: disabled

Unknown multicast blocked: disabled

Appliance trust: none

fjcardenas-1 Mon, 08/17/2009 - 05:12

I read that both ISL and 802.1Q support 4094 vlans. What you see as allowed VLAN is configured with a command: "switchport trunk allowed vlan [add|all|except|remove] vlan-list" ..... So one thing is the number of VLANs it supports and a different thing is the number of VLANs allowed in a trunk. Hope this helps.

Correct Answer
Giuseppe Larosa Mon, 08/17/2009 - 06:52

Hello Wayne,

original ISL specification supported only 1024 vlans but later a revision of the protocol allows for support of 4096 vlans like it happens for 802.1Q.

see

http://www.cisco.com/en/US/partner/products/hw/switches/ps700/products_white_paper09186a00801b49a4.shtml#pre4

the vlan-id field has been 15 bits wide since first version of ISL.

You need also to take in account that ISL has more overhead and that 802.1Q is standard based.

Your lab results look like correct.

to be noted that VTP is still limited to vlans 1-1024 in version 2 and vtp mode transparent is needed to use the extended range

Hope to help

Giuseppe

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