The JUNOS software 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:
In Figure 83, 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 83: 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.
This guide shows you how to use the Layer 2 interworking nature of TCC for interface switching and for constructing Layer 2.5 virtual private networks (VPNs).
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