Routing Basics 679 ■ Routing protocol—Protocol that supports a routed protocol by providing mech- anisms for sharing routing information. Routing protocol messages move between the routers. A routing protocol allows the routers to communicate with other routers to update and maintain tables. TCP/IP examples of routing protocols are listed here: — Routing Information Protocol (RIP) — Interior Gateway Routing Protocol (IGRP) — Enhanced Interior Gateway Routing Protocol (EIGRP) — Open Shortest Path First (OSPF) Figure 15-4 Router and Routing Protocols Network Layer Protocol Operations Suppose that a host application needs to send a packet to a destination on a different network. The host addresses the data link frame to the router, using the address of one of the router’s interfaces. The router’s network layer process examines the incoming packet’s Layer 3 header to determine the destination network and then references the routing table, which associates networks to outgoing interfaces (see Figure 15-5). The packet is encapsulated again in the data link frame that is appropriate for the selected interface and is queued for delivery to the next hop in the path. Destination Network 1.0 2.0 3.0 Exit Port to Use 1.1 2.1 3.1 Routed Protocol (Example: IP) Routing Protocol (Examples: RIP, IGRP) Network Protocol Protocol Name 1102.book Page 679 Tuesday, May 20, 2003 2:53 PM 680 Chapter 15: Routing and Routing Protocols Figure 15-5 Router Services This process occurs each time that the packet is forwarded through another router. When the packet reaches the router that is connected to the destination host’s network, it is encapsulated in the destination LAN’s data link frame type and is delivered to the destination host. Multiprotocol Routing Routers are capable of supporting multiple independent routing protocols and main- taining routing tables for several routed protocols. This capability allows a router to deliver packets from several routed protocols over the same data links (see Figure 15-6). Figure 15-6 Router Traffic X Y A B XY ABC C Application Presentation Session Transport Network Data Link Physical Network Data Link Physical Application Presentation Session Transport Network Data Link Physical Network Data Link Physical Network Data Link Physical Token Ring Token Ring Novell Apple Routing Tables IPX 3a.0800.5678.12ab IP 15.16.50.3 AppleTalk 100.110 DECnet 10.1 IP 15.16.42.8 IP 15.17.132.6AppleTalk 200.167 DECnet 5.8 IPX 4b.0800.0121.ab13 Digital IP 1102.book Page 680 Tuesday, May 20, 2003 2:53 PM Static Routing 681 Static Routing Routing is nothing more than directions for getting from one network to another. These directions, also known as routes, can be given to the router dynamically by another router. An administrator also can statically assign these directions to the router. This section concentrates on routes that are assigned by an administrator. Static Versus Dynamic Routes Static route knowledge is administered manually by a network administrator who enters it into a router’s configuration. The administrator must update this static route entry manually whenever an internetwork topology change requires an update. Dynamic route knowledge works differently. After a network administrator enters configuration commands to start dynamic routing, the route knowledge automatically is updated by a routing process whenever new information is received from the inter- network. Changes in dynamic knowledge are exchanged between routers as part of the update process. The Purpose of a Static Route Static routing has several useful applications. Dynamic routing tends to reveal every- thing known about an internetwork; for security reasons, however, you might want to hide parts of an internetwork. Static routing enables you to specify the information that you want to reveal about restricted networks. When a network is accessible by only one path, a static route to the network can be sufficient. This type of network is called a stub network. A stub network is an OSPF area that carries a default route, intra-area routes, and interarea routes, but that does not carry external routes. Configuring static routing to a stub network avoids the over- head of dynamic routing, as shown in Figure 15-7. Static Route Operation Static route operations can be summarized into a three-part sequence: 1. The network administrator configures the route. 2. The router installs the route in the routing table. 3. Packets are routed using the static route. 1102.book Page 681 Tuesday, May 20, 2003 2:53 PM 682 Chapter 15: Routing and Routing Protocols Figure 15-7 Static Route Operation Because a static route is configured manually, the administrator must configure the static route on the router using the ip route command. The correct syntax for the ip route command is as follows: Router(config)# ip route prefix mask { address | interface }[ distance ] In Figure 15-8, the network administrator of the Hoboken router needs to configure a static route pointing to the 172.16.1.0/24 and 172.16.5.0/24 networks on the other routers. Figure 15-8 Static Routes B A Point-to-Point or Circuit-Switched Connection Only a Single Network Connection with no Need for Routing Updates Stub Network 1102.book Page 682 Tuesday, May 20, 2003 2:53 PM Static Routing 683 The administrator could enter one of two commands to accomplish this objective. The method in Example 15-1 specifies the outgoing interface (Serial 0). The method in Example 15-2 specifies the next-hop IP address of the adjacent router (172.16.2.2). Either of the commands installs a static route in the routing table of Hoboken. The only difference between the two commands is in the administrative distance that the router assigns to the route as it is placed in the routing table. The administrative distance is an optional parameter that provides a measure of the reliability of the route. A lower value for the administrative distance indicates a more reliable route. This means that a route with a lower administrative distance will be installed before an identical route with a higher administrative distance. The default administrative distance when using next-hop address is 1. The default administrative distance when using the outgoing interface is 0. Table 15-1 show Cisco’s administrative distance values for each supported protocol. Routes with lower administrative distances are trusted over identical routes with higher administrative distances. If an administra- tive distance other than the default is desired, a value between 0 and 255 is entered after the next-hop or outgoing interface, as follows: ip route 172.16.3.0 255.255.255.0 192.168.2.1 255 Example 15-1 IP Route Using Interface Sterling(config)# ip route 172.16.3.0 255.255.255.0 s0 Example 15-2 IP Route Using Next-Hop Router IP Sterling(config)# ip route 172.16.3.0 255.255.255.0 172.16.2.2 Table 15-1 Cisco Administrative Distances Route Source Default Distance Values Connected interface 0 Static route 1 Enhanced Interior Gateway Routing Protocol (EIGRP) summary route 5 External Border Gateway Protocol (BGP) 20 Internal EIGRP 90 IGRP 100 continues 1102.book Page 683 Tuesday, May 20, 2003 2:53 PM 684 Chapter 15: Routing and Routing Protocols If the router cannot reach the outgoing interface that is being used in the route, the route will not be installed in the routing table. This means that if the interface is down, the route will not be placed in the routing table. Sometimes static routes are used for backup purposes. A static route can be configured on a router that will be used only when the dynamically learned route has failed. To use a static route in this manner, set the administrative distance higher than that of the dynamic routing protocol being used. Configuring Static Routes This section lists the steps for configuring static routes and gives an example of a simple network for which static routes might be configured. Use the following steps to configure static routes: Step 1 Determine all desired destination networks, their subnet masks, and their gateways. A gateway can be either a local interface or a next-hop address that leads to the desired destination. Step 2 Enter global configuration mode. Step 3 Type the ip route command with a destination network address and subnet mask followed by their corresponding gateway from Step 1. Including an administrative distance is optional. Step 4 Repeat Step 3 for as many destination networks as were defined in Step 1. Step 5 Exit global configuration mode. Step 6 Save the active configuration to NVRAM by using the copy running- config startup-config or write memory commands. OSPF 110 Intermediate System-to-Intermediate System (IS-IS) 115 Routing Information Protocol (RIP) 120 Exterior Gateway Protocol (EGP) 140 External EIGRP 170 Internal BGP 200 Unknown 255 Table 15-1 Cisco Administrative Distances (Continued) Route Source Default Distance Values 1102.book Page 684 Tuesday, May 20, 2003 2:53 PM Static Routing 685 The example network in Figure 15-9 is a simple three-router configuration. Hoboken must be configured so that it can reach the 172.16.1.0 network and the 172.16.5.0 network. Both of these networks have a subnet mask of 255.255.255.0. Figure 15-9 Static Route Operation Packets that have a destination network of 172.16.1.0 need to be routed to Sterling. Pack- ets that have a destination address of 172.16.5.0 need to be routed to Waycross. Static routes can be configured to accomplish this using the outgoing router interfaces (S0 and S1), as shown in Example 15-3. Both static routes first are configured to use a local interface as the gateway to the destination networks, as in Figure 15-10. Because the administrative distance was not specified, it defaults to 0 when the route is installed in the routing table. Remember that an administrative distance of 0 is the same as a directly connected network. Example 15-3 IP Route with Outgoing Interfaces Hoboken(config)#ip route 172.16.1.0 255.255.255.0 s1 Hoboken(config)#ip route 172.16.5.0 255.255.255.0 s0 1102.book Page 685 Tuesday, May 20, 2003 2:53 PM 686 Chapter 15: Routing and Routing Protocols Figure 15-10 Static Routes Configured by Administrator The same two static routes also can be configured using a next-hop address as their gateway. The first route to the 172.16.1.0 network has a gateway of 172.16.2.1. The second route to the 172.16.5.0 network has a gateway of 172.16.4.2. Example 15-4 shows configuring static routes using the next-hop interface address and includes comments (preceded by !) that will show up in the configuration file. Because the administrative distance was not specified, it defaults to 1. Example 15-4 IP Route with Next Hop and Comment Hoboken(config)# ip route 172.16.1.0 255.255.255.0 172.16.2.1 !This command points to Sterling's LAN Hoboken(config)# ip route 172.16.1.0 255.255.255.0 172.16.4.2 !This command points to Waycross LAN Lab Activity Using Static Routes In this lab, you configure static routes between routers to allow data transfers between them without the use of dynamic routing protocols. 1102.book Page 686 Tuesday, May 20, 2003 2:53 PM Static Routing 687 How a Default Route Is Used Figure 15-11 shows a use for a default route, a routing table entry that directs packets to the next hop when that hop is not explicitly listed in the routing table. You can set default routes as part of the static configuration. In this example, the Company X routers possess specific knowledge of the topology of the Company X network, but not of other networks. Maintaining knowledge of every other network accessible by way of the Internet cloud is unnecessary and unreasonable, if not impossible. Instead of maintaining specific network knowledge, each router in Company X is informed of the default route that it can use to reach any unknown destination by directing the packet to the Internet. Figure 15-11 Static Default Route Using Next Hop Configuring Default Route Forwarding Default routes route packets with destinations that do not match any of the other routes in the routing table. Routers typically are configured with a default route for Internet-bound traffic because it is often impractical and unnecessary to maintain routes to all networks in the Internet. A default route is actually a special static route that uses the following format: ip route 0.0.0.0 0.0.0.0 [ next-hop-address | outgoing interface ] Use the following steps to configure default routes: Step 1 Enter global configuration mode. A B Company X Routing Table No entry for destination net. Try Router B default route. C Internet 10.0.0.0 192.34.56.0 NOTE The 0.0.0.0 mask, when logically ANDed to the destination IP address of the packet to be routed, always yields the network 0.0.0.0. If the packet does not match a more specific route in the routing table, it is routed to the 0.0.0.0 network. 1102.book Page 687 Tuesday, May 20, 2003 2:53 PM 688 Chapter 15: Routing and Routing Protocols Step 2 Type the ip route command with 0.0.0.0 for the destination network address and 0.0.0.0 for the subnet mask. The gateway for the default route can be either the local router interface that connects to the outside networks or the IP address of the next-hop router. In most cases, it is pre- ferred that the IP address of the next hop router is specified. Step 3 Exit global configuration mode. Step 4 Save the active configuration to NVRAM by using the copy running- config startup-config command. Earlier in this chapter, in Figure 15-8, static route configuration was demonstrated on router Hoboken to make networks 172.16.1.0 on Sterling and 172.16.5.0 on Waycross accessible. It should now be possible to route packets to both of these networks from Hoboken. However, as configured, neither Sterling nor Waycross will know how to return packets to any network that is not directly connected. A static route could be configured on Sterling and Waycross for each of the destination networks that are not directly connected, but on a larger network this would not be a scalable solution. Sterling connects to all non-directly connected networks via interface s0. Waycross has only one connection to all non-directly connected networks. This is through interface Serial 1. A default route on both Sterling and Waycross provides routing for all packets that are destined for networks that are not directly connected, as demonstrated in Figure 15-12. Examples 15-5 and 15-6 show the commands necessary to define a default static route on Waycross and Sterling, respectively. Figure 15-12 Static Route for Waycross Sterling 172.16.1.1/24 S0 Hoboken 172.16.3.1/24 S0 Waycross 172.16.5.1/24 S1 S1 172.16.2.1/24 172.16.2.2/24 172.16.4.1/24 172.16.4.2/24 My administrator has told me how to reach all networks not dirrectly connected to me. My administrator has told me how to reach all networks not dirrectly connected to me. 1102.book Page 688 Tuesday, May 20, 2003 2:53 PM . Waycross Sterling 17 2 .16 .1. 1 /24 S0 Hoboken 17 2 .16 .3 .1/ 24 S0 Waycross 17 2 .16 .5 .1/ 24 S1 S1 17 2 .16 .2 .1/ 24 17 2 .16 .2. 2 /24 17 2 .16 .4 .1/ 24 17 2 .16 .4 .2/ 24 My administrator has told me how to reach all networks. and Comment Hoboken(config)# ip route 17 2 .16 .1. 0 25 5 .25 5 .25 5.0 17 2 .16 .2 .1 !This command points to Sterling's LAN Hoboken(config)# ip route 17 2 .16 .1. 0 25 5 .25 5 .25 5.0 17 2 .16 .4 .2 !This command. 3a.0800.5678 . 12 ab IP 15 .16 .50.3 AppleTalk 10 0 .11 0 DECnet 10 .1 IP 15 .16 . 42. 8 IP 15 .17 .13 2. 6AppleTalk 20 0 .16 7 DECnet 5.8 IPX 4b.0800. 0 12 1.ab13 Digital IP 11 02. book Page 680 Tuesday, May 20 , 20 03 2: 53 PM Static