TCC and Layer 2.5 Switching Overview
Translational cross-connect (TCC) allows you to forward traffic between a variety of Layer 2 protocols or circuits. It is similar to its predecessor, CCC. However, while CCC requires the same Layer 2 encapsulations on both sides of a router (such as Point-to-Point Protocol [PPP] or Frame Relay-to-Frame Relay), TCC lets you 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.
Consider a sample topology (Figure 1) in which you can configure a full-duplex Layer 2.5 translational cross-connect between Router A and Router C, using a Juniper Networks router, Router B, as the TCC interface. In this topology, Router B strips all PPP encapsulation data from frames arriving from Router A and adds ATM encapsulation data before the frames are sent to Router C. All Layer 2 negotiations are terminated at the interconnecting router (Router B).
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.
Ethernet TCC is supported on interfaces that carry IPv4 traffic only. For 8-port, 12-port, and 48-port Fast Ethernet PICs, TCC and extended VLAN CCC are not supported. For 4-port Gigabit Ethernet PICs, extended VLAN CCC and extended VLAN TCC are not supported.