Frequently Asked Questions

Q— What is the difference between a domain and an autonomous system?

A— Both terms are used to indicate a collection of routers. The domain notation is usually used to indicate a collection of routers running the same routing protocol, such as a RIP domain or an OSPF domain. The AS represents one or more domains under a single administration that have a unified routing policy with other ASs.

Q— My company is connected to an ISP via RIP. Should I use BGP instead?

A— If you are thinking of connecting to multiple providers in the near future, you should start discussing the option of using BGP with your provider. If your traffic needs do not require multiple provider connectivity, you should be okay with what you have.

Q— I have a single IGP connection to a provider. I am thinking of connecting to the same provider in a different location. Can I connect via an IGP, or should I use BGP?

A— This depends on the provider. Some providers will let you connect via IGP in multiple locations; others prefer that you use BGP. Practically speaking, when you use BGP, you will be in better control of your traffic, as you will see in the following chapters.

Q— I thought that BGP is to be used between ASs. I am a bit confused about using BGP inside the AS.

A— Think of BGP inside the AS (IBGP) as a tunnel through which routing information flows. If your AS is a transit AS, IBGP will shield all your internal nontransit routers from the potentially overwhelming number of external routing updates. On the other hand, even if you are not a transit AS, you will realize as this book progresses that IBGP will give you better control in choosing exit and entrance points for your traffic.

Q— You talk about BGP-4, but is anybody still using BGP-1, -2, or -3? What about EGP?

A— BGP-4 is the de facto interdomain routing protocol used on the Internet. EGP and BGP-1, 2, and 3 are obsolete. BGP-4's support of CIDR, incremental updates, and better filtering and policy-setting capabilities have prompted everybody to shift gears into using this new protocol.

Q— I'm planning to install a second connection to my current Internet service provider. Should I get an AS number from my RIR?

A— Getting an AS number is indeed an option, although you might first see if your provider has provisions in place to support the use of private ASs for customers multihomed to a single provider. In addition, you should check with your RIR to ensure that it will allocate AS numbers to networks connected to only a single provider.