Exterior routing protocols are used to exchange routing information between autonomous systems. The routing information passed between autonomous systems is called reachability information. Reachability information is simply information about which networks can be reached through a specific autonomous system.
RFC 1771 defines Border Gateway Protocol (BGP), the leading exterior routing protocol, and provides the following description of the routing function of an autonomous system:
The classic definition of an Autonomous System is a set of routers under a single technical administration, using an interior gateway protocol and common metrics to route packets within the AS, and using an exterior gateway protocol to route packets to other ASs.... The administration of an AS appears to other ASs to have a single coherent interior routing plan and presents a consistent picture of what networks are reachable through it. From the standpoint of exterior routing, an AS can be viewed as monolithic...
Moving routing information into and out of these monoliths is the function of exterior routing protocols. Exterior routing protocols are also called exterior gateway protocols. Don't confuse an exterior gateway protocol with the Exterior Gateway Protocol (EGP). EGP is not a generic term; it is a particular exterior routing protocol, and an old one at that.
A gateway running EGP announces that it can reach networks that are part of its autonomous system. It does not announce that it can reach networks outside its autonomous system. For example, the exterior gateway for our imaginary autonomous system book-as can reach the entire Internet through its external connection, but only one network is contained in its autonomous system. Therefore, it would announce only one network (172.16.0.0) if it ran EGP.
Before sending routing information, the systems exchange EGP Hello and I-Heard-You (I-H-U) messages. These messages establish a dialogue between two EGP gateways. Computers communicating via EGP are called EGP neighbors, and the exchange of Hello and I-H-U messages is called acquiring a neighbor.
Once a neighbor is acquired, routing information is requested via a poll. The neighbor responds by sending a packet of reachability information called an update. The local system includes the routes from the update into its local routing table. If the neighbor fails to respond to three consecutive polls, the system assumes that the neighbor is down and removes the neighbor's routes from its table. If the system receives a poll from its EGP neighbor, it responds with its own update packet.
Unlike the interior protocols discussed above, EGP does not attempt to choose the "best" route. EGP updates contain distance-vector information, but EGP does not evaluate this information. The routing metrics from different autonomous systems are not directly comparable. Each AS may use different criteria for developing these values. Therefore, EGP leaves the choice of a "best" route to someone else.
When EGP was designed, the network relied upon a group of trusted core gateways to process and distribute the routes received from all of the autonomous systems. These core gateways were expected to have the information necessary to choose the best external routes. EGP reachability information was passed into the core gateways, where the information was combined and passed back out to the autonomous systems.
A routing structure that depends on a centrally controlled group of gateways does not scale well and is therefore inadequate for the rapidly growing Internet. As the number of autonomous systems and networks connected to the Internet grew, it became difficult for the core gateways to keep up with the expanding workload. This is one reason why the Internet moved to a more distributed architecture that places a share of the burden of processing routes on each autonomous system. Another reason is that no central authority controls the commercialized Internet. The Internet is composed of many equal networks. In a distributed architecture, the autonomous systems require routing protocols, both interior and exterior, that can make intelligent routing choices. Because of this, EGP is no longer popular.
Border Gateway Protocol (BGP) is the leading exterior routing protocol of the Internet. It is based on the OSI InterDomain Routing Protocol (IDRP). BGP supports policy-based routing, which uses non-technical reasons (for example, political, organizational, or security considerations) to make routing decisions. Thus BGP enhances an autonomous system's ability to choose between routes and to implement routing policies without relying on a central routing authority. This feature is important in the absence of core gateways to perform these tasks.
Routing policies are not part of the BGP protocol. Policies are provided externally as configuration information. As described in Chapter 2, the National Science Foundation provides Routing Arbiters (RAs) at the Network Access Points (NAPs) where large Internet Service Providers (ISPs) interconnect. The RAs can be queried for routing policy information. Most ISPs also develop private policies based on the bilateral agreements they have with other ISPs. BGP can be used to implement these policies by controlling the routes it announces to others and the routes it accepts from others. In the gated section later in this chapter, we discuss the import command and the export command, which control what routes are accepted (import) and what routes are announced (export). The network administrator enforces the routing policy through configuring the router.
BGP is implemented on top of TCP, which provides BGP with a reliable delivery service. BGP uses well-known TCP port 179. It acquires its neighbors through the standard TCP three-way handshake. BGP neighbors are called peers. Once connected, BGP peers exchange OPEN messages to negotiate session parameters, such as the version of BGP that is to be used.
The UPDATE message lists the destinations that can be reached through a specific path and the attributes of the path. BGP is a path-vector protocol. It is called a path-vector protocol because it provides the entire end-to-end path of a route in the form of a sequence of autonomous system numbers. Having the complete AS path eliminates the possibility of routing loops and count-to-infinity problems. A BGP UPDATE contains a single path vector and all of the destinations reachable through that path. Multiple UPDATE packets may be sent to build a routing table.
BGP peers send each other complete routing table updates when the connection is first established. After that, only changes are sent. If there are no changes, just a small (19-byte) KEEPALIVE message is sent to indicate that the peer and the link are still operational. BGP is very efficient in its use of network bandwidth and system resources.
By far the most important thing to remember about exterior protocols is that most systems never run them. Exterior protocols are required only when an AS must exchange routing information with another AS. Most routers within an AS run an interior protocol such as OSPF. Only those gateways that connect the AS to another AS need to run an exterior routing protocol. Your network is probably an independent part of an AS run by someone else. ISPs are good examples of autonomous systems made up of many independent networks. Unless you provide a similar level of service, you probably don't need to run an exterior routing protocol.
Although there are many routing protocols, choosing one is usually easy. Most of the interior routing protocols mentioned above were developed to handle the special routing problems of very large networks. Some of the protocols have been used only by large national and regional networks. For local area networks, RIP is still a common choice. For larger networks, OSPF is the choice.
If you must run an exterior routing protocol, the protocol that you use is often not a matter of choice. For two autonomous systems to exchange routing information, they must use the same exterior protocol. If the other AS is already in operation, its administrators have probably decided which protocol to use, and you will be expected to conform to their choice. Most often this choice is BGP.
The type of equipment affects the choice of protocols. Routers support a wide range of protocols, though individual vendors may have a preferred protocol. Hosts don't usually run routing protocols at all, and most Unix systems are delivered with only RIP. Allowing host systems to participate in dynamic routing could limit your choices. gated, however, gives you the option to run many different routing protocols on a Unix system. While the performance of hardware designed specifically to be a router is generally better, gated gives you the option of using a Unix system as a router.
In the following sections we discuss the Gateway Routing Daemon (gated) software that combines interior and exterior routing protocols into one software package. We look at examples of running RIP, RIPv2, OSPF, and BGP with gated.