XPost: comp.dcom.lans.ethernet, comp.dcom.sys.cisco   
   From: barmar@alum.mit.edu   
      
   In article ,   
    "Mark"  wrote:   
      
   > Hello   
   >   
   > 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 ?   
      
   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, barmar@alum.mit.edu   
   Arlington, MA   
   *** PLEASE post questions in newsgroups, not directly to me ***   
      
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