Semestre 1 CCNP, Módulo 3

Module 3: Routing Overview

Overview

Many of the scalable design features explored in the first two modules, such as load balancing and route summarization, work very differently depending on the routing protocol used. Routing protocols are the rules that govern the exchange of routing information between routers. The open architecture and global popularity of TCP/IP has encouraged the development of more than a half-dozen prominent IP routing protocols. Each protocol has a unique combination of strengths and weaknesses. Because routing protocols are key to network performance, there must be a clear understanding of the following attributes of each protocol:

* Convergence times
* Overhead
* Scalability features

This module explores various routing processes including default routing, floating static routes, convergence, and route calculation.

3.1.1 Routing fundamentals

One of the primary jobs of a router is to determine the best path to a given destination. A router learns paths, or routes, from the static configuration entered by an administrator or dynamically from other routers, through routing protocols. Routers keep a routing table in RAM. A routing table is a list of the best known available routes. Routers use this table to make decisions about how to forward a packet. Issue the show ip route command to view the TCP/IP routing table.

A routing table maps network prefixes to an outbound interface. When RTA receives a packet destined for 192.168.4.46, it looks for the prefix 192.168.4.0/24 in the routing table. RTA then forwards the packet out an interface, such as Ethernet0, based on the routing table entry. If RTA receives a packet destined for 10.3.21.5, it sends that packet out Serial0 (S0).

The first few lines in Figure list the possible codes that designate how the router learned the route. This table shows four routes for directly connected networks. They are labeled with a C in the routing table. RTA drops any packet destined for a network that is not listed in the routing table. To forward to other destinations, the routing table for RTA will need to include more routes. New routes can be added by using one of the following two methods:

* Static routing – An administrator manually defines routes to one or more destination networks.
* Dynamic routing – Routers follow rules defined by routing protocols to exchange routing information and independently select the best path.

Administratively defined routes are referred to as static routes because they do not change until a network administrator manually programs the changes. Routes learned from other routers are dynamic because they change automatically as neighboring routers update each other with new information. Each method has fundamental advantages and disadvantages.

The following sections describe how to configure both static and dynamic routing on a Cisco router.

3.1.2 Static routing

Static routing is useful in networks that do not have multiple paths to any destination network. Static routing reduces the memory and processing burdens on a router. Even on large internetworks, administrators often configure static routes on access routers that connect stub networks. Stub networks have only one way in and one way out. In Figure , RTZ is configured with a static route to 172.24.4.0 /24.

To configure static routing on a Cisco router, use the ip route command. This command uses the following syntax:

Router(config)#ip routedestination-prefix destination-prefix-mask {address | interface} [distance] [tagtag] [permanent]

Figure describes the parameters that are used with this command.

An entry can be manually added to a routing table by using one of the following two versions of the ip route command:

RTA(config)#ip route 10.6.0.0 255.255.0.0 s1

or

RTA(config)#ip route 10.7.0.0 255.255.0.0 10.4.0.2

Both of these global configuration commands will add a static route to the routing table. The first example maps a network prefix, 10.6.0.0/16, to a local physical interface, S1, on the router. A directly connected network is also mapped to an interface. The second example maps the network prefix, 10.7.0.0/16, to the next hop address, 10.4.0.2. Each of these commands will add valid static routes to the routing table.

In Figure , the route to 10.6.0.0 is identified as a static route by the S at the beginning of the line. However, it is formatted similarly to a directly connected route. This is because the router has been configured to forward packets for 10.6.0.0 out S1. The entry in the routing table for the static route to 10.7.0.0 is configured with a next hop address. This entry in the routing table is similar to dynamic routes. The next hop address is included in the routing table because the packets destined for 10.7.0.0 should be forwarded to an interface at 10.4.0.2 on another router. What is the difference between these two kinds of static routes?

When using a routing protocol such as RIP or IGRP, static routes that are shown as directly connected will be automatically advertised to other routers if the appropriate network command has been issued. The next hop static route will not be advertised without additional configuration. These static routes can be included in updates if they are injected, or redistributed into the dynamic routing protocol.

When an interface goes down, all static routes mapped to that interface are removed from the IP routing table. If the router can no longer find a valid next hop for the address specified in a static route, the static route is removed from the table. An alternative method is to map a static IP address to a loopback interface.

NOTE:

As a rule, the next hop address should always be used when defining a static route on a multi-access network such as Ethernet. A router interface on a multi-access network could have several link partners, so the next hop address must be used to specify which neighbor should receive traffic for a given network.

Static routing is not suitable for large, complex networks that include redundant links, multiple protocols, and meshed topologies. Routers in complex networks must adapt to topology changes quickly and select the best route from multiple candidates. Therefore, dynamic routing is the better choice.

3.1.3 Configuring dynamic routing

Dynamic routing of TCP/IP can be implemented using one or more protocols. These protocols are often grouped according to where they are used. Routing protocols designed to work inside an autonomous system are categorized as interior gateway protocols (IGPs), and protocols that work between autonomous systems are classified as exterior gateway protocols (EGPs). Figure lists widely supported EGPs and IGPs for TCP/IP routing.

This module focuses on IGPs. A comprehensive discussion of EGPs, particularly BGP4, will be presented in Module 8. These protocols can be further categorized as either distance vector or link-state routing protocols, depending on their method of operation.

Routing protocols for IPX and AppleTalk
Despite the dominance of IP, a significant number of organizations continue to support legacy protocols, such as Novell IPX and AppleTalk. A legacy technology is one that is supported because of a significant past investment or deployment. Many organizations continue to support IPX and AppleTalk because of a past investment in protocol-specific printers, software, and servers. Although Cisco EIGRP offers comprehensive support for both IPX and AppleTalk, it is important to be familiar with three proprietary routing protocols. These three protocols are IPX or Novell RIP, NetWare Link Services Protocol (NLSP), and AppleTalk Routing Table Maintenance Protocol (RTMP). Implementation of AppleTalk and Novell proprietary routing protocols is beyond the scope of this curriculum.

IP routing protocols and the routing table
Cisco IOS commands enable dynamic routing based on the routing protocol used. Figure displays the routing table of a router configured to use four IP routing protocols, RIP, IGRP, EIGRP, and OSPF. Notice that most organizations would not normally use more than one or two routing protocols.

Figure examines the specific table entry for 192.168.1.0/24. Routes in the routing table that are not directly connected include two numbers offset by brackets [administrative distance/metric]. For example, [120/3] means that the administrative distance is 120 and the metric is 3. Routers base their evaluations of routes on these two numbers. Since this is a RIP route, the metric represents hop count.

Routers use metrics to evaluate, or measure, routes. When multiple routes to the same network exist and the routes are from the same routing protocol, the route with the lowest metric is considered the best. Hop count is the only factor that IP RIP uses to determine the metric. In the sample entry shown in Figure , the number 3 indicates that the destination network is three hops away.

Each routing protocol calculates its metrics differently. EIGRP uses a complex combination of factors that include bandwidth and reliability to calculate its metric. Using the default settings, the EIGRP metric for the same route to 192.168.1.0 is 3,219,456. If RTA receives a RIP update and an EIGRP update for this same network, the router will use the administrative distance metric to compare the two. This process is similar to comparing three apples to over three million oranges.

When a router receives updates from different routing protocols about the same network, it cannot use dissimilar metrics to evaluate a route. The router will use administrative distance as the deciding metric for route validation. Cisco IOS assigns a default administrative distance to every routing protocol. A lower value signifies a more trustworthy routing protocol. A complete list of administrative distances can be found in Module 8, Route Optimization.

3.1.4 Distance vector routing protocols

Routing protocols may be classified as either distance vector or link-state routing protocols. These classifications describe the algorithm, or formula, that routers use to calculate and exchange routing information. Distance vector routing protocols are based on the Bellman-Ford algorithm.

Routers configured to use a distance vector routing protocol typically send their complete routing table at regular intervals to neighbor routers. Simple distance vector protocols, such as RIP and IGRP, broadcast their routing tables on all configured interfaces. This broadcast is also referred to as multicasting. Routers that use these protocols do not actually identify their neighbors for direct communication.

A neighbor router receiving the broadcast update examines it and compares the information to its current routing table. Routes to new networks, or routes to known networks with better metrics, are inserted in the table. The neighbor then broadcasts its routing table, which includes any updated routes.

Distance vector routing protocols are concerned with the distance and vector, or direction, of destination networks. Before sending an update, each router adds its own distance value to the route metric. When a router receives an update, it maps the learned network to the receiving interface. The router then uses that interface to reach those destinations.

Simple distance vector routing protocols offer two primary advantages over link-state protocols. They are relatively easy to configure and they generally use less memory and processing power. RIP v1 has the added advantage of almost universal support among all routing software and is often used as a common denominator in mixed vendor or legacy routing environments.

Simple distance vector routing protocols do not scale as well as their link-state counterparts. RIP v1 and IGRP are classful routing protocols, which means they do not send subnet information in updates. They are unable to support scalability features such as Variable Length Subnet Masking (VLSM) or supernetting. In general, simple distance vector routing protocols converge more slowly than link-state protocols. Most complex and scalable internetworks require routing protocols that achieve convergence quickly when all routers agree on the state of the network topology. Therefore, distance vector protocols are usually not appropriate. RIP restricts networks from growing beyond 15 hops between any two destinations, which is too limiting for large networks. IGRP overcomes this limitation by supporting a 255-hop maximum. IGRP is a proprietary protocol developed by Cisco and does not support a multi-vendor routing environment.

Because of the limitations of simple distance vector routing protocols, network administrators often use link-state routing in complex internetworks.

3.1.5 Link-state routing protocols

Link-state routing protocols offer greater scalability and faster convergence than distance vector protocols such as RIP and IGRP. Link-state routing protocols require more memory and processing power from the router and more knowledge and expertise from the administrator than distance vector routing protocols.

Link-state protocols are based on the Dijkstra algorithm, sometimes referred to as the shortest path first (SPF) algorithm. The most common link-state routing protocol, Open Shortest Path First (OSPF), is examined in Module 6.

Routers running a link-state protocol are concerned with the state of link interfaces on other routers in the network. A link-state router builds a complete database of all the link states of every router in its area. In other words, a link-state router gathers enough information to create its own map of the network. Each router then individually runs the SPF algorithm on its own map, or link-state database, to identify the best paths to be installed in the routing table. These paths to other networks form a tree with the local router as its root.

Instead of learning routes and then broadcasting the routes with incremented distances to neighbors, link-state routers advertise the states of their own links to all other routers in the area so that each router can build a complete link-state database. These advertisements are called link-state advertisements (LSAs). Unlike distance vector routers, link-state routers can form special relationships with their neighbors and other link-state routers, to ensure that the LSA information is properly and efficiently exchanged.

There is an initial flood of LSAs to provide routers with the information that they need to build a link-state database. Routing updates occur only when a link-state changes or if no changes have occurred, after a specific interval. If a link state changes, a partial update is sent immediately. The partial update contains only link states that have changed, not a complete routing table. An administrator concerned about WAN link utilization will find these partial and infrequent updates an efficient alternative to distance vector routing, which sends out a complete routing table at each update interval. When a change occurs link-state routers are all notified immediately by the partial update. Distance vector routers have to wait for neighbors to note the change, increment the change, and then pass it on to the next neighbor down the line.

The benefits of link-state routing include faster convergence and improved bandwidth utilization over distance vector protocols. Link-state protocols support classless interdomain routing (CIDR), variable-length subnet mask (VLSM), and supernetting. This makes them a good choice for complex, scalable networks. In fact, link-state protocols generally outperform distance vector protocols on any size network. Link-state protocols do have two major disadvantages:

* Link-state routing may overtax low-end hardware. Link-state routers require more memory and processing power than distance vector routers, potentially making link-state routing cost-prohibitive for organizations with tight budgets and legacy hardware.
* Link-state routing protocols require complex administration. Configuring link-state routing can be a daunting task, and many administrators prefer to avoid its complexity and implement distance vector routing. Even capable administrators may choose a distance vector protocol on a small network.

3.1.6 Hybrid routing protocol: EIGRP

The proprietary EIGRP offered by Cisco is an advanced distance vector protocol that employs the best features of link-state routing. For the most part, EIGRP configuration is similar to configuring a simple distance vector protocol such as IGRP. However, like their link-state counterparts, EIGRP routers use partial updates, special neighbor relationships, and topological databases to provide optimal convergence. EIGRP, which is sometimes referred to as a hybrid protocol, is discussed in Module 5.

3.2.1 Default routing overview

It is not feasible, or even desirable, for every router to maintain routes to every possible destination. Instead, routers keep a default route or a gateway of last resort. Default routes are used when the router cannot match a destination network with a specific entry in the routing table. The router must use the default route, or the gateway of last resort, to send the packet to another router. The next router will have a route to that destination or its own default route to a third router. If it is a default route to a third router, that router must have the route to the destination or another default route, and so on. Eventually, the packet should be routed to a router that has a route to the destination.

Using default routes keep routing tables small is a key scalability feature. They make it possible for routers to forward packets destined to any Internet host without having to maintain a table entry for every destination network. Default routes can be statically entered by an administrator or dynamically learned through a routing protocol.

Before routers can dynamically exchange default information, an administrator must configure at least one router with a default route. An administrator can use two different commands to statically configure default routes, ip route 0.0.0.0 0.0.0.0 and ip default-network.

The following sections explore these two methods in detail.

3.2.2 Configuring static default routes

Creating an ip route to 0.0.0.0/0 is the simplest way to configure a default route. This is done using the following syntax:

Router(config)#ip route 0.0.0.0 0.0.0.0 [next-hop-ip-address exit-interface]

To the Cisco IOS, network 0.0.0.0 /0 has special meaning as the gateway of last resort. All destination addresses match this route because a mask of all 0s requires none of the 32 bits in an address to be an exact match.

A route to 0.0.0.0 /0 is often called a quad-zero route. Manually configuring 0.0.0.0/0 routes on every router might suffice in a simple network. Routers may need to dynamically exchange default routes in more complex situations. The exchange of default information works differently depending on the routing protocol being used and can create severe problems when improperly configured. Default routes typically point outside the network. Therefore, default routes are noticeable when they fail.

Static routes to 0.0.0.0/0 are automatically propagated to other routers in RIP updates. The only way to stop this automatic update is to use a route filter. A route filter configuration option is discussed in Module 8, Route Optimization.

In IOS releases 12.1 and later, RIP does not propagate a static default route automatically. When using RIP and IOS 12.1, the RIP process must be manually configured to advertise the static default route by issuing the network 0.0.0.0 command.

Alternately, the default-information originate or the redistribute static command may be used to configure static default route propagation. Regardless of the IOS version, the OSPF requires the default-information originate command to propagate the static default routes. The following example illustrates this configuration for RIP. Figure .

RTY(config)#ip route 0.0.0.0 0.0.0.0 172.16.1.2
RTY(config)#router rip
RTY(config-router)#default-information originate

Using the default-information originate command, an administrator can statically configure a single RIP router with a 0.0.0.0/0 route, and that default route will be propagated to other routers. The default-information originate command can also be used with OSPF to achieve the same effect.

3.2.3 Default routing with IGRP

IGRP treats 0.0.0.0/0 routes differently. In fact, IGRP does not recognize the network 0.0.0.0/0 and will not include it in updates. To configure a dynamic exchange of default information in an IGRP network, the ip default-network command must be used. The ip default-network command can be used to flag a route to any IP network, not just 0.0.0.0/0, as a candidate default route. Use the following command syntax:

Router(config)#ip default-network ip-network-address

An example of this command is shown in the following internetwork figure. An asterisk in the routing table denotes a candidate default route. The candidate default route will be considered, along with any other candidates for the role as gateway of last resort.

As boundary router, RTB must be manually configured to send default traffic to its link partner, the ISP router. A 0.0.0.0/0 route can be configured on RTB, but this route will not be propagated by IGRP to the other routers. To avoid manually configuring 0.0.0.0/0 routes on all routers, configure RTB to flag its route to 207.21.20.0/24 as a candidate default route, as shown in the following:

RTB(config)#ip default-network 207.21.20.0

The network 207.21.20.0/24 now has special properties as an exterior network. The exterior network is the outside network that serves as a gateway of last resort. RTB will send this information in IGRP routing updates to RTA and RTC. These routers can now dynamically learn that network 207.21.20.0/24 is an exterior network, making RTB the gateway of last resort for both of these routers. Both RTA and RTC will propagate this route, flagged as a candidate default, to other IGRP neighbors, if present.

Unlike a static 0.0.0.0/0 route configuration, the ip default-network command provides an administrator with a great deal of flexibility. In complex topologies, several networks can be flagged as candidate defaults. Routers can then choose from among the available candidates to pick the lowest cost route.

Running IGRP, ip default-network command must be used to enable the exchange of default information. Using RIP a 0.0.0.0/0 route will usually suffice. However, use the ip default-network command on a RIP router, but RIP routers propagate IP default networks as 0.0.0.0/0 routes. In other words, a RIP router configured with the ip default-network 192.168.1.0 command will send neighbors a route to 0.0.0.0/0, not a flagged route to 192.168.1.0.

NOTE:

The ip default-network command is classful, which means that if the router has a route to the subnet indicated by this command. It installs a static route to the major net and then flags that route as a candidate default route.

3.2.4 Default route caveats

A router does not use a gateway of last resort for addresses that are part of its local domain. A local domain is a major network to which the router is connected. In the figure, RTX has two interfaces configured with IP addresses that belong to the major network, 172.16.0.0.

If all three routers are running IGRP, RTX will not learn about the subnet 172.16.1.1/30 because a variable-length subnet mask is used. The IGRP does not support VLSM. After statically configuring a 0.0.0.0/0 route, RTX routing table is shown in the example.

A ping issued to 172.16.1.1 from RTX would be expected to use its default route to send the ping to RTZ. RTX has interfaces connected to the major net 172.16.0.0, and so RTX considers 172.16.0.0 a local domain and will not use a default route to reach 172.16.1.0 or any other local domain address. Without additional configuration, the ping will fail.

This problem may be solved in a number of ways. One approach is to configure the router with the ip classless global configuration command. With ip classless enabled, the default in Cisco IOS versions 11.3 and greater, the router uses the best prefix match available, including a supernet route, such as 172.0.0.0/8 or 0.0.0.0/0. By enabling ip classless RTX will use the 0.0.0.0/0 route to reach unknown subnets within its local domain, 172.16.0.0.

A second approach is to configure RTX with an explicit route for the major network 172.16.0.0.

RTX(config)#ip route 172.16.0.0 255.255.0.0 172.16.3.1

Without a more specific route available for 172.16.1.1, RTX uses the static route to the major network number 172.16.0.0/16 and successfully routes packets destined for 172.16.1.1.

3.3.1 Configuring floating static routes

One of the disadvantages of static routing is that it cannot adapt to topology changes. However, static routing can be configured to have limited adaptability by creating floating static routes.

Floating static routes are static routes configured with an administrative distance value that is greater than that of the primary route or routes. Floating static routes are fallback routes, or backup routes, that do not appear in the routing table until another route to the same destination fails. Assume that RTB is connected to network 10.0.0.0/8 through two different links. Additionally, that the RTB preferred route to network 10.0.0.0/8 is through RTC because that link has a higher bandwidth. This route is learned by RIP. RTB should use the slower link to 10.0.0.0/8 through RTA, only if the primary route fails. The route to RTA is statically configured.

A floating static route must be used to statically configure RTB, so that it will use the slower link to reach 10.0.0.0/8 after the RIP route fails.

RTB(config)#ip route 10.0.0.0 255.0.0.0 1.1.1.1 130

This ip route command includes an administrative distance of 130. Recall that static routes have a default administrative distance of 1. To create a floating static route, the administrative distance value must be manually configured. This value must be greater than the primary route administrative distance value. In this example, the primary route is learned by RIP and has an administrative distance of 120. By configuring the static route with an administrative distance of 130, the static route will be less desirable than the primary route. The RIP route through RTC is preferred. However, if the RIP route is lost, the floating static route takes its place in the routing table.

Floating static routes can be used in conjunction with other static routes to create a semi-adaptable static routing scheme.

RTZ(config)#ip route 0.0.0.0 0.0.0.0 s0
RTZ(config)#ip route 0.0.0.0 0.0.0.0 s1 5
RTZ(config)#ip route 4.0.0.0 255.0.0.0 s2
RTZ(config)#ip route 4.0.0.0 255.0.0.0 s3 5
RTZ(config)#ip route 4.0.0.0 255.0.0.0 s4 10

If RTZ is configured with these commands, it installs one route to 0.0.0.0/0 using S0, and one route to 4.0.0.0/8, using S2. If S0 becomes unavailable, RTZ will install the floating static route to 0.0.0.0/0, using S1, into its routing table. If S2 fails, RTZ will go back to using S3 to reach 4.0.0.0/8. Finally, if both S2 and S3 go down, RTZ will use the least desirable static route to 4.0.0.0/8, with an administrative distance 10.

3.4.1 ODR overview

The configuration of static routes is a fairly simple process in a hub-and-spoke Frame Relay topology, such as the network shown in Figure . However, many network administrators still view static routes as administratively undesirable. An administrator must add static routes as new stub networks are brought on line and remove routes that are taken off line. Beginning with the release of IOS 11.2, Cisco offers a proprietary alternative for hub routers called On Demand Routing (ODR).

With ODR, a hub router can automatically discover stub networks while the stub routers still use a default route to the hub. ODR utilizes CDP to provide address prefixes (the network portion of the IP address) for the routing table entries. The network portion does not have to be strictly classful. VLSM is supported. Further, because only minimal route information is traversing the link between the stub and hub routers, bandwidth use is minimal.

It is important to note that ODR is not a true routing protocol. It discovers information about stub networks, but does not provide any routing information to the stub routers. The link information is conveyed by a data link protocol and, therefore, does not go further than from the stub router to the hub router. However, ODR discovered routes can be redistributed into dynamic routing protocols.

Figure shows a routing table containing ODR entries. The table shows that the administrative distance is 160 and the metric of the routes is 1. Because ODR routes are always from a hub router to a stub router, the metric (hop count) will never be more than 1. The routes also show that VLSM is supported.

The transport mechanism for ODR routes is the Cisco Discovery Protocol (CDP), a proprietary data link protocol that gathers information about neighboring Cisco network devices. CDP runs on any media that supports the Subnetwork Access Protocol (SNAP), which means that ODR also depends on SNAP support. Although CDP is enabled by default on all interfaces of all Cisco devices running IOS 10.3 and later, ODR support does not begin until Release 11.2. ODR is configured on the hub router only. However, the stub routers must run IOS 11.2 or later for the hub router to discover their attached networks.

Although static routes can be used on both the spoke and hub routers, ODR presents a more scalable solution.

3.4.2 Configuring ODR

ODR is enabled with a single global configuration command, router odr. Turn off any dynamic routing protocols in the spoke routers. Spoke routers will automatically start advertising their subnets using CDP. However, ODR and a dynamic routing protocol can be run on a hub router. CDP is enabled by default. It would need to be enabled only if it has been turned off for some reason. The command to enable the CDP process on a router is cdp run. To enable CDP on a specific interface, the command is cdp enable.

The figure shows a typical hub-and-spoke topology. To configure ODR, use the router odr command on the hub router. As long as all routers are running IOS 11.2 or later and the connecting medium supports SNAP (such as the Frame Relay shown), ODR is operational and the hub will learn the addresses of the stub networks. The only configuration necessary at the stub routers is a static default route to the hub router. Cisco IOS versions 12.0.5.T and higher include a new feature that sends an automatic default route to the spokes from the hub router.

ODR routes can also be redistributed into routing protocols. If RTA in the graphic needs to advertise the ODR discovered routes into OSPF, RTA configuration could include the commands shown:

RTA(config)#router odr
RTA(config)#router ospf 1
RTA(config-router)#redistribute odr metric 100 subnets
RTA(config-router)#network 10.2.0.0 0.0.255.255 area 1

3.5.1 Convergence issues

This process is both collaborative and independent. The routers share information with each other, but must individually recalculate their own routing tables. For individual routing tables to be accurate, all routers must have a common view of the network topology. When all routers in a network agree on the topology they are considered to have converged. Rapid convergence means rapid recovery from link failure or other network changes. Routing protocols and network designs are ultimately judged by how rapidly they converge.

When routers are in the process of convergence, the network is susceptible to routing problems. Some routers learn that a link is down while others incorrectly believe that the link is still up. If this happens the individual tables will be contradictory and may lead to dropped packets or devastating routing loops.

It is virtually impossible for all routers in a network to simultaneously detect a topology change. Dependent on the routing protocol in use, a significant amount of time may pass before all the routers in a network converge. Factors affecting the convergence time include the following:

* The routing protocol used
* The distance of the router, or the number of hops from the point of change
* The number of routers in the network that use dynamic routing protocols
* The bandwidth and traffic load on communications links
* The load on the router
* The traffic patterns in relation to the topology change

The effects of some of these factors can be minimized through careful network design. A network could be designed to minimize the load on any given router or communications link. Other factors, such as the number of routers in the network, must be accepted as risks inherent to the design of a network. Large internetworks can reduce the number of routers that must converge by using static default routes for stub networks.

Although proper network design can significantly reduce convergence time, the capability of the routing protocol to update and calculate routes efficiently may also improve convergence.

3.6.1 Route calculation fundamentals

The capability of a routing protocol to update and calculate routes efficiently is based on several factors:

* Whether the protocol calculates and stores multiple routes to each destination
* The manner in which routing updates are initiated
* The metrics used to calculate distances or costs

The following sections discuss these three factors in detail.

Multiple routes to a single destination

Some routing protocols allow the router to install only a single route to a destination network in its routing table. Other routing protocols permit the router to store multiple routes to each destination, at the cost of additional overhead. One advantage of multiple routes is that equal-cost load balancing or unequal-cost load balancing may be used. Another advantage is that maintaining multiple routes to a single destination reduces the network vulnerability to routing loops and dropped packets when a link fails. If a router maintains two different routes to 10.0.0.0 and one route fails, the router can continue to route to 10.0.0.0 using the second route, without waiting for an alternate route to propagate. Maintaining multiple routes does not reduce convergence time, but it can insulate a router from instabilities during the convergence process.

Routing protocols can instruct a router to update neighbors after a specific amount of time has passed, after a certain event has occurred, or both. Time-driven routing protocols wait for the update timer to expire and then send an update. RIP will send a complete update every 30 seconds by default even if its routing table is unchanged since the last update. By contrast, protocols that are event-driven do not require the router to update neighbors until the router detects a change in the network topology. Link-state protocols and EIGRP send a partial update that includes only the changed information. Other protocols may send their entire table when triggered by an event.

Routing protocols that are exclusively time-driven react poorly to topology changes. If a router detects a change but has to wait 30 seconds before alerting neighbors, routing in that network could break down and it could take several minutes before convergence. In the meantime, routers unaware of the change may send packets the wrong way, leading to routing loops or loss of connectivity.

Routing protocols that are exclusively event-driven could go for extended periods of time without sending updates. If there is no other mechanism to ensure that routers regularly communicate, such as a Hello protocol, routers could base their routing decisions on outdated information.

For these reasons, most routing protocols use a combination of time-driven and event-driven updates. RIP is time-driven, but the Cisco implementation of RIP sends triggered updates whenever a change is detected. Topology changes also trigger immediate updates in IGRP routers, regardless of the update timer. Without triggered updates, RIP and IGRP will not perform.

Protocols that are primarily event-driven typically use timers as well. OSPF routers assign a MaxAge to the routing information. Once the information has reached its MaxAge it can no longer be used in the routing table and a new update must be requested.

3.6.3 Routing metrics

Routing metric is a value that measures desirability. Some routing protocols use only one factor to calculate a metric. IP RIP uses hop count as the only factor to determine the metric of a route. Other protocols base their metric on two or more factors, including hop count, bandwidth, delay, load, reliability, and maximum transmission unit (MTU).

Bandwidth and delay are static. They remain the same for each interface until the router is reconfigured or the network is redesigned. Load and reliability are dynamic and calculated for each interface in real time by the router.

The more factors that make up a metric, the greater the ability is to adapt network operation to meet specific needs. IGRP, by default, uses two static factors to calculate the metric, bandwidth and delay. These can be configured manually, allowing for precise control over what routes a router chooses. IGRP can also be configured to include two dynamic factors in the metric calculation, load, and reliability. By using dynamic factors, IGRP routers can make decisions based on changing conditions. Therefore, if a link becomes heavily loaded or unreliable, IGRP will increase the metric of routes using that link and alternate routes presenting a lower metric would be used.

3.7.1 Migrating from RIP to EIGRP

3.7.2 Configuring IGRP

3.7.3 Configuring default routing with RIP and IGRP

3.7.4 Configuring floating static routes

Summary

One of the most important decisions in a network design is the selection of an appropriate routing protocol or protocols. Such selection should be done carefully and with an appreciation for the long-term implications of the decision. This module has provided an overview of the various ways that routing can be performed, as well as the benefits and limitations of each.

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