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Transporting 802.1Q Tagged Frames
When an 802.1Q Ethernet frame is encapsulated for transport over VC tunnel, the entire
frame is included, except for the preamble and FCS. The 4-byte VLAN tag field is
transmitted as is, but may be overwritten by the egress LER. The option to overwrite
the VLAN tag allows two (possibly independently administered) VLAN segments with
different VLAN IDs to be treated as a single VLAN.
Establishing LDP LSPs to TLS Tunnel Endpoints
The TLS tunnel endpoint is identified using an IP address configuration parameter, and
LDP must set up a tunnel LSP to the configured IP address before Layer-2 traffic can be
transported. To ensure that the tunnel LSP is established, both an OSPF route and a
MPLS label mapping must be advertised for the configured IP address.
When the peer LSR is also an Extreme switch, the following options are available for
ensuring that an OSPF route is advertised for the tunnel endpoint IP address:
• A route is advertised when OSPF is enabled on the VLAN to which the IP address is
assigned (using the config ospf add vlan command on the peer switch).
• A route is advertised when the peer switch is configured to distribute direct routes
into the OSPF domain (via the enable ospf export direct command). The
export option should be used when the tunnel LSP needs to cross OSPF area
boundaries or when the Extreme Standby Routing Protocol (ESRP) is enabled on the
VLAN to which the IP address is assigned.
In either case, LDP must be configured to advertise label mappings for direct routing
interfaces.
In some configurations, you may want to enable loopback mode on the VLAN to which
the tunnel endpoint IP address is assigned. One situation where loopback mode may be
useful is when multiple physical interfaces, associated with different VLANs, are
connected to the MPLS backbone. In this case, use of loopback-mode can provide
redundancy by enabling TLS traffic to continue even when the physical interfaces
associated with the tunnel endpoint IP address VLAN fail.
MPLS Module Installation and User Guide
Overview of MPLS Layer-2 VPNs
7-3
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