4.3. How the rules work

Rule #1 guarantees that the routing algorithm used is consistent across implementations and consistent with other routing protocols, such as OSPF. Multi-homed networks are always explicitly advertised by every service provider through which they are routed even if they are a specific subset of one service provider's aggregate (if they are not, they clearly must be explicitly advertised). It may seem as if the "primary" service provider could advertise the multi-homed site implicitly as part of its aggregate, but the assumption that longest-match routing is always done causes this not to work.

Rule #2 guarantees that no routing loops form due to aggregation. Consider a mid-level network which has been allocated the 2048 class C networks starting with 192.24.0.0 (see the example in section 5 for more on this). The mid-level advertises to a "backbone" 192.24.0.0/255.248.0.0. Assume that the "backbone", in turn, has been allocated the block of networks 192.0.0.0/255.0.0.0. The backbone will then advertise this aggregate route to the mid-level. Now, if the mid-level loses internal connectivity to the network 192.24.1.0/255.255.255.0 (which is part of its aggregate), traffic from the "backbone" to the mid-level to destination 192.24.1.1 will follow the mid-level's advertised route. When that traffic gets to the mid-level, however, the mid-level *must not* follow the route 192.0.0.0/255.0.0.0 it learned from the backbone, since that would result in a routing loop. Rule #2 says that the mid-level may not follow a less-specific route for a destination which matches one of its own aggregated routes. Note that handling of the "default" route (0.0.0.0/0.0.0.0) is a special case of this rule - a network must not follow the default to destinations which are part of one of it's aggregated advertisements.