- Classful routing protocols do not send the network mask in routing updates
- Classful routing protocols do not differentiate between subnets within the same major network
- Doesn’t support VLSM
- For routing updates to be sent out …
- Major network and mask must match the outgoing network interface
- Otherwise, route update is sent out summarised, regardless of the auto-summary setting (see foot note)
- The network mask is applied by the receiving router:
- there is a major network match with the incoming interface => use the incoming interface mask
- 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:
- When ip classless is used (default), the longest match lookup occur; should there be no match, the default route is used, if any.
- 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 172.16.1.1, the longest match would be route 172.16.0.0 /16; the 10.1.1.1 will be used as a gateway;
- If we ping 188.8.131.52, no match occurs; the default gateway 10.1.1.1 is used (coincidentally, this is the same gateway as above);
- Lastly, if we ping 10.0.2.3, a major network is matched; but there is no match within the major network. In this case, the packet is dropped.
Rafael A. Couto Cabral • LinkedIn Profile
Cisco | F5 | VMware Certified • PRINCE2 Practitioner
Originally posted 2017-11-07 08:53:06.