Cisco switch

The fairy tale of the VLAN internal allocation policy

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Once upon a time there was a Cisco switch called CatOS, it had the power to transform its ports into a layer3 links, But it had a secret … vlan tale

…and yes the secret is this:

vlan internal allocation policy ascending

This conf command! You can find him in CatOS and on more recent IOS Cisco firmware.

When we configure a routed-port on a Cisco switch:

CatLab01(config)#int gi0/3
CatLab01(config-if)#no switchport
CatLab01(config-if)#ip address

We tell our L2 device to act as a router on that port. But the Switch is not a Router so it may have a dedicated L2 VLAN to segregate the broadcast domain.

We can decide only ascending (from 1006 to 4094) or descending (from 4094 to 1006) VLAN id of the policy but we will never see them in our vlan-database. If we accidentally overlap these reserved VLANs we receive this message:

%Failed to create VLANs 1006
VLAN(s) not available in Port Manager.
%Failed to commit extended VLAN(s) changes.

The only way to verify the use of internal vlan is the following command:

CatLab01#sh vlan internal usage 

VLAN Usage
---- --------------------
1006 GigabitEthernet0/3

REMEMBER when you do vlan planning!

In the datacenter and backhaul networks where many vlans are needed, it would be better to avoid using this feature. It is preferable to have a dedicated router to manage L3 links and let the switches do their L2 job.

more information con Cisco Community
device involved: Cisco 3800, Cisco 3700, Cisco 2400


… this were the VLAN tale.

.glitchlist crew

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