RFC 1879 (rfc1879) - Page 3 of 6

Class A Subnet Experiment Results and Recommendations

RFC 1879               Class A Subnet Experiment            January 1996


Routing

   A classic cisco configuration that has the following statements

                ip route 39.1.28.0 255.255.255.0
                router bgp 64000
                redistribute static

   will, by default, promote any classful subnet route to a full
   classful route (supernet routes will be left alone).  This behaviour
   can be changed in at least the following two ways:

        1:
                ip route 39.1.28.0 255.255.255.0
                router bgp 64000
                no auto-summary
                redistribute static

        2:
                ip route 39.1.28.0 255.255.255.0
                router bgp 64000
                network 39.1.28.0 mask 255.255.255.0
                redistribute static route-map static-bgp
                ....
                access-list 98 deny 39.1.28.0 0.255.255.255
                access-list 98 permit any
                ....
                route-map static-bgp
                match ip address 98

   Users of cisco gear currently need to code the following two
   statements:

                ip classless
                ip subnet-zero

   The implication of the first directive is that it eliminates the idea
   that if you know how to talk to a subnet of a network, you know how
   to talk to ALL of the network.

   The second is needed since it is no longer clear where the all-ones
   or all-zeros networks are [6].

   Other infrastructure gear exhibited similar or worse behaviour.
   Equipment that depends on use of a classful routing protocol, such a
   RIPv1 are prone to misconfiguration.  Tested examples are current
   Ascend and Livingston gear, which continue to use RIPv1 as the
   default/only routing protocol.  RIPv1 use will create an aggregate



Manning                      Informational