relation between kernel FIB and hardware FIB

Discussion in 'Cisco' started by Mark, Sep 20, 2012.

  1. Mark

    Mark Guest


    This is more a genric question on the implementation of routers. Does a
    software in the control plane (let's consider RIP, OSPF or BGP protocols)
    have to install routing entries in both kernel and underlying hardware
    (given that the hardware is capable to maintain its FIB), or in such case
    the kernel doesn't need to have a routing table and the routing table is
    constructed and maintained entirely by the control plane routing prototols ?


    Mark, Sep 20, 2012
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  2. Usually they each maintain separate tables. Each routing protocol
    typically needs to maintain its own state, because they operate
    independently -- a RIP route shouldn't normally be advertised via BGP.
    The hardware FIB is created by merging all the information from the
    different protocols.

    Also, the memory for the hardware FIB is usually limited, so you just
    want the minimal forwarding information. But routing protocols need to
    maintain state, sometimes quite a bit of it (e.g. the AS path in BGP).
    Barry Margolin, Sep 20, 2012
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  3. Mark

    Mark Guest

    Thanks for the reply. To be more specific -- a routing protocol maintains
    its state and RIB table should reflect the RIB information down to the
    kernel or/and hardware; so my question is -- should it be done on both
    kernel and hardware, i.e. FIB in the hardware, or only FIB in hardware? (By
    the hardware let's consider L3 switch, or network processor). Obviously if
    we have a generic x86 machine with NICs and OS Linux, then the only FIB we
    update is the kernel's FIB, but what if we have Linux and user-level
    applications implementing routing protocols, and underlying network
    processor capable of maintaining its own FIB tables?
    Mark, Sep 20, 2012
  4. If you're implementing a router, you can do whatever you want. The main
    benefit of keeping a copy of the FIB in the kernel is that it doesn't
    have to interrupt the hardware if an application asks to view the FIB.
    Barry Margolin, Sep 21, 2012
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