Frequently Asked Questions

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

A—Both notations 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 of your traffic.

Q—You talk about BGP4, but is anybody still using BGP1, 2, or 3? What about EGP?

A—BGP4 is the de facto interdomain routing protocol used on the Internet. EGP and BGP1, 2, and 3 are obsolete. BGP4's support of CIDR and better filtering and policy setting capabilities have prompted everybody to shift gears into using this new protocol.

Q—Does BGP send periodic updates like RIP?

A—No. BGP exchanges routing information once, when the BGP session is being established. After that, only network changes are exchanged between BGP peers.

Q—Does the BGP session become "established" after all the routing updates have been exchanged between BGP neighbors?

A—No. It is the other way around. No routing exchange can take effect until both BGP neighbors agree on all parameters and the session becomes established.

Q—Is the Network Layer Reachability Information (NLRI) the actual BGP routing update?

A—No. The NLRI is one of the elements that is carried in a BGP UPDATE message. Other elements are the attributes and the unreachable networks.

Q—You talk about authentication as an example of the BGP optional parameters. How important is authentication?

A—Authentication is a means to validate the BGP peer. This is to prevent hackers from assuming the identity of one of your peers and feeding you wrong routing information. With authentication, both peers will validate the connection via password mechanisms.

Q—Where does BGP carry information about AS numbers?

A—AS numbers are listed as part of the AS_path attribute carried in the UPDATE message.