Cisco CCNA Service Provider Question of the Week

CCNASPofw01At what layer does a router de-encapsulate and re-encapsulate?

    1. Data link
    2. Network
    3. Transport
    4. Session
    5. Presentation

The correct answer is 1.

When traffic is passing through a router, the frames have to enter the router (with the router as the destination at layer 2, some remote host for layer 3).  Once the router determines layer 2 is it (and it is a valid frame and layer 3 protocol is enabled this interface), it removes the layer 2 header.  The router then looks at the destination layer 3 address, if that address is not one of its own addresses, then it has to determine if it has a route.  If so, it will determine the egress interface, layer 2 encapsulation type and addressing for layer 2.  It then does a re-write of the layer 2 and dispatches the frame out the egress interface.  This process happens (more or less) for every frame and every packet sent from host to host that passes through a router..

Related Courses:
SPNGN1 – Building Cisco Service Provider Next-Generation Networks, Part 1
SPNGN2 – Building Cisco Service Provider Next-Generation Networks, Part 2

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