Wednesday, June 3, 2009

VLAN resource vs VLAN significance

These are two terms that get used quite often when talking about hardware switching platforms (i.e. 6500/7600).

As you all know, VLAN is a virtual LAN defined initially for L2 bridging. One VLAN represents a broadcast domain and packet forwarding within the same VLAN is based on mac-address learning. Another name for this is multipoint bridging (you'll meet that term quite frequently in ES/ES+ setups).

VLANs are also used in the 802.1q ethernet encapsulation method. Services like L2 point-to-point are usually using this functionality. Such services, as EoMPLS xconnect or local-connect (more on that on another post about ES/ES+), do not require a VLAN for bridging, but they might require a vlan as a packet encapsulation method.

Actually you might differentiate the above 2 VLAN types like below:

system VLAN or global VLAN : used for L2 bridging
port VLAN or access VLAN : used for L2 point-to-point services (do not confuse it with the vlan-id used under switchports in access mode)

Global VLAN resource is per system and is limited to 4094 (0 & 4095 are reserved) on 6500s/7600s (you can use "sh platform hardware capacity vlan" to check this).
Access VLAN resource is per linecard and is limited to 16000 on the ES/ES+ cards (probably lower on some SIPs), which equals the number of service instances supported.

L3 services are a different story. Each physical/logical L3 interface requires a global VLAN resource (use "sh vlan internal usage" to find these). An exception on this are the PPP and ISG sessions (supported on SIP-400 cards).

Bridge-domains require also a global VLAN resource each. A bridge-domain, which is used in the ES/ES+ cards, is like a traditional L2 bridging instance, where many physical or logical ports connect to. What is interesting about this, is the ability to have L2 interworking between normal switchports, EVC service instances, ATM/FR PVCs/DLCIs, EoMPLS/VPLS VCs, etc.

Regarding the VLAN significance, there are also 2 VLAN types here:

per system or global significant : when a vlan-id must be unique across the whole system
per port or local significant : when a vlan-id can be the same across different ports

Although it might seem a little bit confusing, VLAN resource is somewhat independent of VLAN significance. i.e. in a case of a L3 subinterface, you always require a global VLAN resource, regardless of the linecard used. But whether you'll have local VLAN significance or not depends on the linecard itself.

SIP-400 and ES/ES+ cards support local VLAN significance, but keep in mind that the "older" ES cards do not support it when using single-tagged subinterfaces.

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