Re: use of internal vlans

Internal VLANs are reserved for certain hardware functions.
For example, on the catalyst platforms, the requirements between the
following two options to implement an IP interface are the same:
1.
interface gig3/3
switchport
switch mode acceess
switch acc vlan 4000

int vlan 4000
ip add 1.2.3.4 255.255.255.0

 2.
interface gig3/3
no switchport
ip address 1.2.3.4 255.255.255.0

In the first case, you use manually defined VLAN to server packets. In the
second case, there is no VLANs, but the way hardware is built, it wtill
requires a VLAN to function. Therefore a system has to allocate an "internal
VLAN".

Many other features require internal VLANs. Like GOLD tests, that are
continuously running in the background. Certain QoS or Multicast features,
etc.

You can only change if the internal VLANs are allocated in ascending mode
(from bottom up) or Descending mode (from top to bottom). Only extended
VLANs can be used for Internal VLAN purposes, so "classic" VLANs are never
used for this.

Internal VLANs are always dynamically allocated and deallocated when the
feature is turned on or turned off.

You can test the importance of Internal VLANs by the following simple test:

conf t
vlan 1-4094 (this may vary, depending on the hardware)
interface gig 1/1
shut
no switchport
ip address 1.1.1.1 255.255.255.252
no shut <<< Here you will get an error message, saying that Internal VLAN
could not be allocated, and that feature will not be enabled (i.e. the
interface will not come up)

Internal VLANs function on first come, first serve basis. So if the system
is allocated VLAN 1050, and you do "vlan 1050" then the system says "Sorry,
That's mine". Likewise, when the system want to allocate 1050, and you have
already created 1050, then the system will use the next available VLAN.

Hope this helps.

On Thu, Oct 13, 2011 at 4:36 PM, imran ali <immrccie_at_gmail.com> wrote:

> hi all
>
> what is the purpose of internal vlan . what vlans are reserved for them . ?
> can we change them ? .
>
> *To view used internal VLANs, enter the show vlan internal usage privileged
> EXEC command*
>
>
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Pavel Bykov
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