When RIP router sends update to routers, why it is limited to use only 512 bytes of UDP data. Why not more than that, even when it has a 1500 byte MTU. How is the maximum of 512 bytes determined?


You can find this on Page 19 of RFC 1058: RIP, the maximum UDP payload of a RIP update is 512 bytes:

   The maximum datagram size is 512 octets.  This includes only the
   portions of the datagram described above.  It does not count the IP
   or UDP headers.  The commands that involve network information allow
   information to be split across several datagrams.  No special
   provisions are needed for continuations, since correct results will
   occur if the datagrams are processed individually.

This means RIP can carry a maximum of 25 routes in a RIP update packet. When RIP was conceived, 512 bytes was a reasonable maximum

  • Thanks for your answer, from the above answer i understood for avoiding fragmentation they are using 512 bytes right ? Normally minimum MTU is 576 bytes, so in RIP update we can send upto 576 bytes, out of 576 bytes 512 bytes is RIP data and UDP header is 8 bytes and IP header is 20 bytes and MAC header is 14 bytes so totally (512+8+20+14) 554 bytes. And still 22 bytes remaining for in RIP packet, where is that gone? can you clarify on this
    – velpandian
    Apr 25 '14 at 13:50
  • The bytes are not gone; RFC 1058 chose a 512 byte UDP payload. There was no requirement for RIP updates to fill a specific frame size. Apr 25 '14 at 17:05
  • ok i came to understand, mean while i want verify the update packet capture with 25 routes.
    – velpandian
    Apr 25 '14 at 18:34

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