Static and Dynamic Routes

Static path information can be manually programmed into the router and simply force the router to utilize a particular interface or next-hop IP address for forwarding packets with matching destination addresses. Static routes potentially could match a broad range of network addresses. Yet another way to obtain routing information is to use distributed applications enabled on routers that allow automatic collection and sharing of routing infor-mation. These routing applications frequently are referred to as dynamic routing protocols because they are not only automated route-gathering tools; they also work in almost real time, tracking the state of connectivity in the network to provide routing information that is as current and as valid as possible.

Contrast this behavior with static routes, which are manual route entries and require manual intervention to reprogram the network routers in case of any path changes. Obviously, dynamic routing protocols provide more convenience to the network operator than static routes in managing routing information. The price for this convenience, however, is configuration and troubleshooting complexity. Operation of dynamic routing protocols also can be resource-intensive, requiring large amounts of memory and processing resources. Hence, working with dynamic routing protocols frequently requires advanced knowledge and sophisticated expertise for handling related network design, router configuration, tuning, and troubleshooting chores.

Even though static routing is less demanding on system resources and requires a lower level of technical skill to configure and troubleshoot, the sheer effort of manually entering routes for a sizeable network makes it a less attractive option. Obviously, static routing is not a good candidate for today's large enterprise and Internet service provider (ISP) IP-based networks. Another drawback to static routing is that it is less flexible for implementation of complicated routing policies. When it comes to routing policy implementation, there is no better substitute for the intelligence and flexibility provided by dynamic routing protocols, such as BGP, OSPF, and IS-IS. The next section further discusses dynamic routing protocols.

Dynamic Routing

The last section discusses the essence of IP routing and indicates that dynamic automatic routing is very necessary for large network deployments. This section discusses the characteristics and classification of various IP routing protocols. Although all routing protocols have a common goal of gathering routing information to support packet-forwarding decisions, they can be classified into two broad categories, unicast and multicast, based on the type of data traffic they are designed to provide forwarding information for.

The previous section indicates that IP provides an addressing scheme for identifying various locations or subnets in the network. The destination IP address in an IP packet indicates the target address of the packet. The sender's address is stored in the source address field. An important concept to understand about IP addressing is IP subnetworks. IP subnetworks—or subnets, for short—are mentioned earlier in the section on IP address-ing concepts. Physically, an IP subnet is a collection of interconnected network devices whose IP interface addresses share the same network ID and have a common mask.

The earlier section "IPv4 Address Classes" discusses unicast and multicast addresses. The unicast address space is used for addressing network devices, whereas addresses from the multicast space are used for specifying groups or users tuned in to receive information from the same multicast application.

For any IP unicast subnet, the last address, such as in, is known as the broadcast address. This address can be used to target all nodes on the subnet at the same time in what is referred to as a directed broadcast.

A unicast routing protocol is optimized for processing unicast network information and provides routing intelligence for forwarding IP packets to unicast destination addresses. Multicast forwarding is conceptually different and requires special routing applications to support forwarding of multicast packets.

Unicast Versus Multicast IP Routing

Two devices in an IP network normally communicate by sending unicast traffic to each other's IP address. An IP node might have many active interfaces, each of which needs to be configured with an IP address from the unicast space. The address on an interface uniquely defines the device on the subnet directly connected to that interface.

Cisco routers also support the concept of secondary logical subnets, many of which can be configured on a router's interface in addition to the primary address on that interface. Additionally, you can enable tunnel and loopback interfaces on a Cisco router, both of which provide it with unicast IP reachability. Packets with unicast addresses in their destination field are forwarded based on information in the IP routing table. The IP routing table on a Cisco router is displayed with the show ip route command.

If the address in the destination field of a packet is from the multicast address space (Class D), the packet is directed to a multicast group with potentially many receivers. Multicast forwarding uses special mechanisms that enable efficient utilization of network resources. If an application is designed for multidestination delivery, using unicast routing to forward packets of the application's data stream would require unnecessary replication at the source, resulting in a waste of network resources. This can be avoided by using multicast propagation, which replicates multicast packets only when necessary at branches in the network toward the location of receivers.

Figure 1-7 illustrates a situation in which a packet is forwarded from SRC1 to two separate destinations, RCV1 and RCV2, by unicast forwarding.

Figure 1-7. Multidestination Unicast Forwarding

Multidestination Unicast Forwarding

In this case, SRC1 generates two identical streams of packets with destination addresses and, respectively. Packets belonging to each stream are handled indepen-dently and are delivered through RT1 and RT2 to their respective destinations, consuming network resources (bandwidth and processing time) along the paths that they traverse. Contrast this scenario with that shown in Figure 1-8, where IP Multicast forwarding mechanisms are employed.

Figure 1-8. Multicast Forwarding

Multicast Forwarding

Multicast forwarding provides a more efficient way to deliver information by replicating packets only at fork points of the network where paths to receivers follow divergent directions. Therefore, as shown in the Figure 1-8, SRC1 originates only a single stream, and packets in this stream are forwarded through RT1 to RT2. They are then replicated at RT2 and fanned out to RCV1 and RCV2.

Multicast routing protocols are functionally different from unicast routing protocols, in that they build multicast forwarding state in the multicast-enabled routers by using a concept known as reverse path forwarding (RPF). RPF is used to ensure that a multicast packet is received from the interface leading to the expected location of the multicast source, as dictated by the routing table in place.

RPF is discussed further in "Understanding Protocol Independent Multicast (PIM)," which covers IP Multicast routing.

Table 1-2 shows a table of popular multicast and unicast routing protocols.

Table 1-2. Unicast and Multicast Routing Protocols

Unicast and Multicast Routing Protocols

All the listed unicast routing protocols are supported in Cisco IOS Software; however, from the listed multicast routing protocols, only Protocol Independent Multicast (PIM) sparse mode/dense mode (SM/DM), Multicast Source Discovery Protocol (MSDP), and Multiprotocol BGP are supported.

Multicast routing environments also need an additional protocol called the Internet Gateway Multicast Protocol (IGMP). Multicast OSPR (MOSPF) is not supported at all, but IOS provides special capabilities for interoperability with the Distance Vector Multicast Routing Protocol (DVMRP).

As of this writing, multicast routing protocols are not widely deployed on the Internet. However, this situation obviously will change in the near future as more multicast-oriented applications, such as radio broadcasting, video streaming, remote training, videoconferencing, and gaming, become more popular on the Internet.