What is BGP, anyway?
When you make a modem connection to your ISP and want to connect to, for
instance, www.bgpexpert.com, all the routers along the way have to know where
to send the packets you're sending to our Web server, and the packets from the
server have to find their way back to your computer.
For the first few hops, this isn't much of the problem. For instance,
your computer only knows the packets don't have a local destination,
so they should be sent over the modem connection. This can continue
for a while, but at some point the decision where to send the packet
next becomes more complex than just "local: keep it" / "not local: send it
to a smarter router".
The router making this decision will have to know where
to send the packet based on the destination IP address contained in it.
Since IP addresses are distributed fairly randomly
around the globe, there aren't any shortcuts or calculations that make it
possible for the router to decide this for itself.
The only way a router can know where to send a packet, is when another router
tells it "send those packets to me, I know how to deliver them". The Border
Gateway Protocol (BGP) is a protocol that is used between routers to convey
this information. Since the routers that talk BGP to each other aren't
owned by the same organization (that would kind of defeat the purpose of
creating global reachability) this is often called "inter-domain" routing.
BGP and Interdomain Routing Terms
- AS Number
Autonomous System Number. Each AS has a unique number that is used to
identify it in BGP processing.
- Autonomous System
An Autonomous System is a network that has its own routing policy. In most
cases, customers belong to their ISP's Autonomous System, but multihomed
customers obviously have their own routing policy that is different from
either ISP so they must be a separate AS.
Border Gateway Protocol.
Exterior Gateway Protocol: a routing protocol used between
organizations/networks. BGP is an EGP, but there is also an older
EGP called EGP.
Older term for router. Sometimes the word "gateway" is used to describe
a system that connects two dissimilar networks or protocols.
Interior Gateway Protocol: a routing protocol used within an
organization/network. Examples are RIP, OSPF, IS-IS and EIGRP.
The practice of connecting to two or more ISPs. Most multihomed networks
run BGP so the rest of the Internet knows where to send packets for the
multihomed network even if one of the connections fails.
1. Any system that will receive packets over one network connection and then
forward them to another by looking at the network address inside the packet.
2. A special-purpose system (like a computer, but usually without a screen,
keyboard and harddisks) that forwards packets.
- Routing Policy
A policy that defines how a network is connected to other networks and how
packets are allowed to flow.