IPv4 Autonomous System Topology

Sites with multiple routers and networks typically administer their network topology as a single routing domain, or autonomous system (AS) . The following figure shows a typical network topology that would be considered a small AS. This topology is referenced in the examples throughout this section.

Figure 5–3 Autonomous System With Multiple IPv4 Routers

The figure shows an AS that is divided into three local networks, 10.0.5.0, 172.20.1.0, and 192.168.5. Four routers share packet-forwarding and routing responsibilities. The AS includes the following types of systems:

• Border routers connect an AS to an external network, such as the Internet. Border routers interconnect with networks external to the IGP running on the local AS. A border router can run an EGP, such as Border Gateway Protocol (BGP), to exchange information with external routers, for example, the routers at the ISP. In Figure 5–3, the border router's interfaces connect to internal network 10.0.5.0 and to a high-speed router to a service provider.

  For information on configuring a border router, refer to the Open Source Quagga documentationfor BGP information.

  If you plan to use BGP to connect your AS to the Internet, you should obtain an autonomous system number (ASN) from the Internet Registry for your locale. Regional registries, such as the American Registry for Internet Numbers (ARIN), offer guidelines on how to obtain an ASN. For example, the ARIN Number Resource Policy Manual contains instructions for getting an ASN for autonomous systems in the United States and Canada. Alternatively, your ISP might be able to obtain an ASN for you.

• Default routers maintain routing information about all the systems on the local network. These routers typically run IGPs such as RIP. In Figure 5–3, Router 1s interfaces are connected to internal network 10.0.5.0 and internal network 192.168.5. Router 1 also serves as the default router for 192.168.5. Router 1 maintains routing information for all systems on 192.168.5 and routes to other routers, such as the border router. Router 2s interfaces connect to internal network 10.0.5.0 and internal network 172.20.1.

  For an example of configuring a default router, refer to Example 5–4.

• Packet-forwarding routers forward packets but do not run routing protocols. This type of router receives packets from one of its interfaces that is connected to a single network. These packets are then forwarded through another interface on the router to another local network. In Figure 5–3, Router 3 is a packet-forwarding router with connections to networks 172.20.1 and 192.168.5.

• Multihomed hosts have two or more interfaces that are connected to the same network segment. A multihomed host can forward packets, which is the default for all systems that run Oracle Solaris. Figure 5–3 shows a multihomed host with both interfaces connected to network 192.168.5. For an example of configuring a multihomed host, refer to Example 5–6.

• Single interface hosts rely on the local routers, not only for packet forwarding but also for receiving valuable configuration information. Figure 5–3 includes Host A on the 192.168.5 network, which implements dynamic routing, and Host B on the 172.20.1 network, which implements static routing. To configure a host to run dynamic routing, refer to How to Enable Dynamic Routing on a Single-Interface Host. To configure a host to run static routing, refer to How to Enable Static Routing on a Single-Interface Host.