The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  TCP/IP Routing Protocols (Gateway Protocols)                 9  TCP/IP Exterior Gateway/Routing Protocols (BGP and EGP)                      9  TCP/IP Border Gateway Protocol (BGP/BGP-4)                           9  BGP Fundamentals and General Operation

BGP Topology, Speakers, Border Routers and Neighbor Relationships (Internal and External Peers) 1 2 3 BGP Route Storage and Advertisement, and BGP Routing Information Bases (RIBs)

When we connect autonomous systems (ASes) together to form an internetwork, the paths between AS border routers form the conduit by which messages move from one AS to another. It is very important that the flow of messages between ASes be carefully controlled. Depending on circumstances, we may wish to limit or even prohibit certain types of messages from going to or from a certain AS. These decisions in turn have a direct impact on BGP route determination.

The flow of messages in an internetwork is sometimes collectively called traffic. This term presents a good analogy, because we can consider the matter of traffic flow control in a BGP internetwork in much the same way we do on the streets of a city. You have probably in the past seen signs on residential streets that say “No Through Traffic” or “Local Traffic Only”. These are attempt to control the flow of traffic over those streets. A more extreme example of this would be a street in the neighborhood where I used to live, where a barricade was intentionally erected in the middle to turn a busy through street into a pair of dead-ends. Again, the goal was traffic control.

These measures highlight a key distinction between local traffic and through traffic in a neighborhood. The very same categorization is important in BGP:

• Local Traffic: Traffic carried within an autonomous system that either originated in that same AS, or is intended to be delivered within that AS. This is like local traffic on a street.
  
  
• Transit Traffic: Traffic that was generated outside that AS and is intended to be delivered outside the AS. This is like what we commonly call through traffic on the streets.

In the previous topic we discussed the distinction between internal routers and border (or boundary) routers in an AS. If we look at the entire BGP internetwork, we can make a similar distinction between different types of ASes, based on how they are interconnected in the overall BGP topology. There are two main types of AS:

• Stub AS: This is an AS that is connected to only one other AS. It is comparable to a cul-de-sac (dead-end street) in our road analogy; usually, only vehicles coming from or going to houses on the street will be found on that street.
  
  
• Multihomed AS: This is an AS that is connected to two or more other ASes. It is comparable to a through street in our road analogy, because it is possible that cars may enter the street and pass through it without stopping at any of the street's houses.

In the example BGP internetwork in Figure 191, I have linked border routers in AS #2 to both AS #1 and AS #3. While traffic from AS #2 can flow both to and from AS #1 and AS #3, it is possible that traffic from AS #1 may also flow to AS #3 and vice-versa. AS #2 acts as the “through street” for these datagrams.

BGP Topology, Speakers, Border Routers and Neighbor Relationships (Internal and External Peers) 1 2 3 BGP Route Storage and Advertisement, and BGP Routing Information Bases (RIBs)