Classful Routing Notes


  1. Classful routing protocols do not send the network mask in routing updates
  2. Classful routing protocols do not differentiate between subnets within the same major network
  3. Doesn’t support VLSM
  4. For routing updates to be sent out …
    1. Major network and mask must match the outgoing network interface
    2. Otherwise, route update is sent out summarised, regardless of the auto-summary setting (see foot note)
  5. The network mask is applied by the receiving router:
    1. there is a major network match with the incoming interface => use the incoming interface mask
    2. otherwise, use the default major network classful mask

Let’s look at a few examples:


The ip-classless command affect routing decision when choosing the default route; we can distinguish two variations:

  1. When ip classless is used (default), the longest match lookup occur; should there be no match, the default route is used, if any.
  2. Otherwise, the default route is only used if the router is has no routes to the major network.

An example … Given the routing table below:

  • If we ping, the longest match would be route /16; the will be used as a gateway;
  • If we ping, no match occurs; the default gateway is used (coincidentally, this is the same gateway as above);
  • Lastly, if we ping, a major network is matched; but there is no match within the major network. In this case, the packet is dropped.

To be noted that the ip classless command only affects the ip forwarding algorithm in regards to default route selection. It has no bearing in the building of the routing table itself.


Thank you,

Rafael A. Couto Cabral • LinkedIn Profile
Cisco​ | F5 | VMware Certified • PRINCE2 Practitioner

Originally posted 2017-11-07 08:53:06.

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