VLAN Internal Allocation

Answered Question
Jul 25th, 2007

I am facing issues with my 7613 router after creating VRF's.Configuration follows :

ip vrf A

rd 1:1

route-target both 1:1

int G5/1.1

encapsulation dot1q 100

ip vrf forwarding A

ip address x.x.x.x y.y.y.y

router bgp <as>

address-family ipv4 vrf A

redistribute connected.

Once I write memory after doing all this, the router allocated a VLAN between 1006-4094 for internal uses. Does anyone have a clear explanation as to why this internal allocation is done when I already allocated VLAN 100 for my sub-interface???

In case of SVIs, no allocation is done...

Correct Answer by jayjorda about 9 years 6 months ago

The internal allocation of VLANs for Layer 3 interfaces is in the VLAN configuration guide.

http://www.cisco.com/en/US/customer/products/hw/routers/ps368/products_configuration_guide_chapter09186a008069a3d4.html

It has to do with the architecture of the box. Pretty much everything in the 7600 is based off VLANs. Each Layer 3 interface (including SVIs internal allocation equals SVI#)is assigned a VLAN internally. The internal association of a Layer 3 interface is based off a VLAN value. You can't stop that. You can change where it starts allocating VLANs at in the extended range, either by starting at 1006 and going up (default) or starting at 4094 and going down.

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jayjorda Wed, 07/25/2007 - 13:59

Thats normal allocation on the 7600. Layer 3 LAN ports, WAN interfaces and subinterfaces, and some software features use internal VLANs in the extended range on the 7600.

You've configured it as a layer 3 interface and therefore assigned an internal vlan out of the extended range. You're by setting the encapsulation to vlan 100 you're just saying this layer 3 interface is part of vlan 100. Best thing to do if possible would be to configure the interface as a Layer 2 802.1Q trunk interface, then create an SVI for the VLAN and make the SVI the layter 3 interface. Then you would just have an allocation for the VLAN only.

dinesh.thathana... Wed, 07/25/2007 - 22:18

But any idea whyand for what this internal allocation is used. Any URls or documents where cisco has disclosed this?

My scenario is too complex with more than 900 sub-interfaces with vrf, additional 3000 VLANS (SVI).Due to internal allocation on extended VLANs, I cannot acheive my VLAN targets. Any way out...?

Correct Answer
jayjorda Wed, 07/25/2007 - 22:37

The internal allocation of VLANs for Layer 3 interfaces is in the VLAN configuration guide.

http://www.cisco.com/en/US/customer/products/hw/routers/ps368/products_configuration_guide_chapter09186a008069a3d4.html

It has to do with the architecture of the box. Pretty much everything in the 7600 is based off VLANs. Each Layer 3 interface (including SVIs internal allocation equals SVI#)is assigned a VLAN internally. The internal association of a Layer 3 interface is based off a VLAN value. You can't stop that. You can change where it starts allocating VLANs at in the extended range, either by starting at 1006 and going up (default) or starting at 4094 and going down.

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