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Translational Cross-Connect and Layer 2.5 VPNs Overview


Translational cross-connect (TCC) is a switching concept that allows you to establish interconnections between a variety of Layer 2 protocols or circuits. It is similar to its predecessor, circuit cross-connect (CCC). However, while CCC requires the same Layer 2 encapsulations on both sides of a Juniper Networks router (such as PPP-to-PPP or Frame Relay-to-Frame Relay), TCC lets a network administrator connect different types of Layer 2 protocols interchangeably. With TCC, combinations such as PPP-to-ATM and Ethernet-to-Frame Relay cross-connections are possible. Also, TCC can be used to create Layer 2.5 VPNs and Layer 2.5 circuits.

Junos OS makes interworking between unlike protocols possible. The software strips off the Layer 2 header when frames enter the router and adds a different Layer 2 header before the frames exit the router. TCC supports these Layer 2 protocols:

  • ATM

  • Cisco HDLC

  • Ethernet

  • Extended VLAN

  • Frame Relay

  • PPP

In Figure 1, the PPP header is stripped from frames arriving at Router B and an ATM header is added before the frames are sent to Router C. All Layer 2 negotiations are terminated at the interconnecting router (Router B). Examples include Link Control Protocol (LCP) and Network Control Protocol (NCP) for PPP, keepalives for Cisco HDLC, and Local Management Interface (LMI) for Frame Relay.

Figure 1: TCC Concept Example
TCC Concept Example


TCC functionality is different from standard Layer 2 switching. TCC only swaps Layer 2 headers. No other processing, such as header checksums, time-to-live (TTL) decrementing, or protocol handling, is performed. Currently, TCC is supported in IPv4, ISO, and MPLS.