VLAN-Transparent Forwarding

By Vikas Bagwe -

VLANs The Ethernet frames that have to be forwarded may reside in different VLANs. This implies that all Ethernet frames contain a (802.1Q) VLAN ID. Communication between different VLANs is not allowed on the link layer. Only routers can make connections between different VLANs. VLANs create thus separated logical Ethernet segments within a single physical segment.

Ethernet QoS Ethernet frames optionally contain a (802.1p) user priority indication. If Ethernet QoS is taken into account during bridging, it can be based on two steps:

  1. Mapping the user priority of an incoming frame to an internal priority class. This classification can be based on:
    • The type of the interface on which the frame is entering the bridge.
    • The (802.1p) user priority value.
    • The IP Type of Service octet (TOS-byte) for IP packets, using the Precedence or DSCP notation.
  2. Sending out the frame while taking into account its internal priority class. This class can be used to:
    • Perform priority queuing on a single PVC.
    • Perform traffic multiplexing over a range of PVCs.
    Scenario Overview Introduction In this scenario, the Gateway is intended to forward all VLAN-tagged frames between the local Ethernet segment and a PVC transparently. This scenario configures the Thomson Gateway as bridge with one PVC. All incoming frames on all local interfaces have to be forwarded transparently to this PVC. The PVC to which all traffic has to be forwarded, is chosen at configuration time. The exact value of the VLAN ID (VID) or the user priority indication should not influence the result. Following illustration shows the VLAN-transparent forwarding scenario:
    Mechanism To set up this scenario, we use following key mechanism: we explicitly define that the Ethernet bridge is not VLAN aware. Ethernet frames coming in on the Thomson Gateway have a VLAN tag in their header. However, neither the VLAN ID nor the user priority indication should be taken into account. If the Ethernet bridge is VLAN aware, we have to define all VLANs that have to pass through the Thomson Gateway. Otherwise, unknown VLANs are dropped. Configuration overview Following configuration steps have to be performed to configure the Gateway for this scenario:
    1. Define which PVC must be used by configuring an ATM interface for it.
    2. Connect the ATM interface to the Ethernet Bridge.
    3. Define that the bridge is not VLAN aware.
    4. Save the configuration.

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