Solved: max number of vlans

zeeshanr202 · ‎12-30-2007

hi every body

i read in ccna book by wendom that isl and dot.1q have 12 bit vlan id and thus support same number of vlans.

But then i read somewhere isl supports max 1000 vlans while 802.1q can support max 4096 vlans.

which one is correct?

thanks alot

ankbhasi · ‎12-30-2007

Hi Zeeshan,

Both ISL and dot1q has feild for vlan. DOT1q has 12 bits for Vlan Field called VID and all 12 bits are used for vlan support which gives you support of 4096 vlans.

But ISL has 15 bits for vlan field but only lower 10 bits are used for vlans and thus only 1024 vlans are supported.

HTH

Ankur

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View solution in original post

shrikar.dange · ‎12-30-2007

hi,

The total number of vlans supported by Isl is 1005. (Though it can support extended lan range up to 4096. The valns after 1005 are not stored in tha Vlan.dat file so can not be used with the VTP) Vlan numbers 1,1002,1003,1004,1005 are reserved for special networks such as FFDI and token ring so the total number of vlans supported by ISL is 1000 (excluding extended valn range.

The total number of vlans supported by dot1q is 4096 which is correct.

Now cisco is going away from ISL and is encouraging engineers to use dot1q as all the main features of the ISL whih were not present in the dot1q have been added now.