blank.gif (43 bytes)

Church Of The
Swimming Elephant

4.3. How the rules work Connected: An Internet Encyclopedia
4.3. How the rules work

Up: Connected: An Internet Encyclopedia
Up: Requests For Comments
Up: RFC 1519
Up: 4. Changes to inter-domain routing protocols and practices
Prev: 4.2. Rules for route advertisement
Next: 4.4. Responsibility for and configuration of aggregation

4.3. How the rules work

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 (see the example in section 5 for more on this). The mid-level advertises to a "backbone" Assume that the "backbone", in turn, has been allocated the block of networks The backbone will then advertise this aggregate route to the mid-level. Now, if the mid-level loses internal connectivity to the network (which is part of its aggregate), traffic from the "backbone" to the mid-level to destination 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 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 ( 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.

Next: 4.4. Responsibility for and configuration of aggregation

Connected: An Internet Encyclopedia
4.3. How the rules work


Protect yourself from cyberstalkers, identity thieves, and those who would snoop on you.
Stop spam from invading your inbox without losing the mail you want. We give you more control over your e-mail than any other service.
Block popups, ads, and malicious scripts while you surf the net through our anonymous proxies.
Participate in Usenet, host your web files, easily send anonymous messages, and more, much more.
All private, all encrypted, all secure, all in an easy to use service, and all for only $5.95 a month!

Service Details

Have you gone to church today?
All pages ©1999, 2000, 2001, 2002, 2003 Church of the Swimming Elephant unless otherwise stated
Church of the Swimming Elephant©1999, 2000, 2001, 2002, 2003 is a wholly owned subsidiary of Packetderm, LLC.

Packetderm, LLC
210 Park Ave #308
Worcester, MA 01609