The subject of this topic is divided into two parts L2VPN & Interworking, Starting with the first part “L2VPN” we are not going deeply with the L2VPN architecture but in brief L2VPNs are tunneling the layer 2 packets over IP/MPLS backbones as these packets are carried transparently over the IP/MPLS backbone from one site to other with the impression that the two sites are directly connected and the mechanisms/protocols used to achieve L2VPNs are :
- AToM “Any Transport over MPLS“: this mechanism requires MPLS encapsulation over MPLS backbone to operate.
- L2TPv2/3 “Layer 2 Transport Protocol versions“: and this protocol does not need any MPLS encapsulation and operates in native IP backbones.
- VPLS “Virtual Private LAN Service“: This mechanism emulates LANs overseas.
- and more …
In L2VPNs methods/mechanisms most L2 protocols are supported like “Ethernet, PPP, HDLC, ATM & FR”
Basically Interworking is playing a really big role in L2VPNs as it enables the translation between two different L2 encapsulations end-points for example if one side is Ethernet and the the other is Frame-Relay they need Interworking to make connectivity.
Actually there are three modes of Interworking:
1- Ethernet (bridged) mode:
– Native Ethernet frames are accepted and forwarded to the L2 pseudowire and if there’s a 802.1Q “VLAN” header attached with the frame, this header will be discarded leaving the extracted Ethernet frame to be forwarded.
2- Ethernet (VLAN) mode:
-It is the same like Ethernet (bridged) mode but the 802.1Q header is included as a part of the Ethernet frame transported to the pseudowire.
3-IP (routed) mode:
– In this mode any type of L2 headers are discarded leaving the IP payload to be forwarded on the pseudowire to the other end and any non IP packets will be discarded.
– This mode can be used with any L2 encapsulation end-points as these mode depends on the IP payload and the L2 headers are being dropped at the ingress PE interface.
|Ethernet/VLAN||Ethernet/VLAN||bridged or routed|
|Ethernet/VLAN||Frame-Relay||bridged or routed|