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# Finding an Alternative Path to a Remote Network

14 Mar

Finding an Alternative Path to a Remote Network
When the path to a network is lost, EIGRP goes to a lot of trouble to ﬁnd an alternative path. This process is one of the major beneﬁts of EIGRP. The method that EIGRP uses to ﬁnd alternative paths is very reliable and very fast. Figure 13-4 illustrates the steps in this section. In the ﬁgure, the routers that are participating in the selection process are circled.

Figure 13-4 Campus Topology Map Showing Alternative Path Selection

NOTE The metric shown in Figure 13-4 has been simpliﬁed for the purposes of this example.

The following list describes the process after Router G goes ofﬂine:

1. Router D marks the routes that were reached by sending the trafﬁc to Router G.
2. Router D looks in the topology table, which has every network and path of the network, to determine whether there is an alternative route. It is looking for an FS.
3. An FS is determined by a clearly deﬁned equation. The topology table has an AD and an FD listed for every route or successor. This comprises the metric by which the route was selected.
4. Router D adds the alternative route to Router X via Router H, found in the topology table, without moving into active mode because the AD is still less than the original FD. The AD is 5; the original FD was 15. It needs to send updates to its neighbors because the distance has changed.

5. If the router did not have an FS, it would have placed the route into an active state while it actively queried other routers for an alternative path.
6. After interrogating the topology table, if a feasible route is found, the neighbor replies with the alternative path. This alternative path is then added to the topology table.
7. Next, in the last steps of DUAL, the routing table is updated.
8. The network is placed back into a passive state as the router returns to the normal forwarding and maintenance of EIGRP tables until the next change in the network.
9. If a neighbor that has been queried has no alternative path or FS, it places the network into active mode and queries its neighbors.
10. If no answer is heard, the messages are propagated until they hit a network or autonomous system boundary.

When the router sends a query packet, it is recorded in the topology table. This is to ensure a timely reply. If the router does not hear a reply, the neighbor is removed from the neighbor table; all the networks held in the topology table for that neighbor are seen as suspect, and the networks are queried. Occasionally, because of slow links and burdened routers in a large network, problems can occur. In particular, a router might not receive a reply from all the queries that it sent out. This leads to the route being declared SIA; the neighbor that failed to reply is removed from the neighbor table, and DUAL assumes a reply was received, giving an inﬁnite metric.

NOTE SIA can take minutes to resolve. First, it is important to discover if neighbors are consistently failing to answer the query and why. This failure is due to a resource problem either on the router or on the link to the router. It is always better to redesign the network. Summarization and distribution (route ﬁlters) both reduce the scope of the query range. This subject is well documented in the Cisco White Paper “EIGRP—Enhanced Interior Gateway Routing Protocol” available at the Cisco Web site, Cisco.com.

Creating the Routing Table
The routing table is built from the topology table after DUAL has been run. The topology table is the foundation of EIGRP: This is where all the routes are stored, even after DUAL has been run. It is in the routing table that the best paths are stored and accessed by the routing process.

Once the tables have been built, the router can make routing decisions.

EIGRP Metrics
The metrics used in EIGRP are very similar to those of IGRP. The main difference is that the result of the calculation is held in a 32-bit ﬁeld. This means that the decision can be much ﬁner or more detailed. The DUAL algorithm uses this metric to select the best path or paths to a destination. The computation is performed on paths held in the topology table to identify the best path to place into the routing table. Up to six paths can be held for one destination, and there can be three different types of paths. These three path types are described in Table 13-3.

Table 13-3 EIGRP Routing Types

The metric is the same composite metric used by IGRP, with the default calculated from bandwidth and delay. Although it is possible to change the metric, this must be done only with great care and consideration to the network design. Any changes made must be effected on every router in the EIGRP autonomous system.

The equation for the default metric used is this:
metric = [(10000000 | smallest bandwidth kbps) + sum of delays] * 256
Table 13-4 explains the metric values.
Table 13-4 EIGRP Metric Values

The default for the K constants are
K1 = 1, K2 = 0, K3 = 1, K4 = 0, K5 = 0
If K5 = 0, the composite metric is calculated using the following formula:
metric = [K1 * bandwidth(K2 * bandwidth)(256-load) + K3 * delay]
If K5 is not 0, the added formula is used:
metric = metric * [K5(reliability + K4)]
Given the overall understanding of how EIGRP works, the next section considers the topology table and its components, which will help explain the details of EIGR operation.

The Topology Table and the DUAL Finite-State Machine
DUAL is responsible for maintenance of the topology table and the creation of the routing table. The topology table records the metric as received from the advertising router, or the next hop. It then adds the cost of getting to that neighbor, the one that is advertising the route. The cost to the destination network from the advertising router, plus the cost to that router, equals the metric to the destination network from the router.

The metric or cost from the neighbor advertising the route is known as the advertised distance (AD). The metric or cost from the router that is determining the metric or the local router is referred to as the feasible distance (FD). If the AD is less than the FD, the next-hop router is downstream and there is no loop. Put simply: The downstream neighbor or next hop must be closer to the destination. This is fundamental to EIGRP.

Figures 13-4 and 13-5, shown earlier in this chapter, illustrate these distances. Note that the metric shown in these ﬁgures has been simpliﬁed for the purposes of this example.

Updating the Routing Table in Passive Mode with DUAL
DUAL determines whether there is an acceptable route in the topology table to replace the current path in the routing table. In EIGRP terms, this is replacing a successor in the routing table with a feasible successor from the topology table.

Use the network in Figure 13-5 as an example.

Figure 13-5 The Use of Feasible and Advertised Distance—Passive Mode

The following list explains the ﬁgure with the metrics and actions that EIGRP takes in determining the path:

■ The FD from Router A to Router G is 10 (A to D to G).
■ The AD from Router A to Router G is 5 (advertised from Neighbor D).
■ Because 10 > 5, then FD > AD. This means that the FD is a feasible condition (FC), allowing it to become an FS. If you follow the diagram, it is very straightforward and less algebraic.
■ If the link between Router D and Router G were down, Router A would look in its topology table.
■ The alternative routes through Router A to D to H to E to G have an AD of 19 (7 + 5 + 7).
■ Because 19 is greater than the original FD of 10, it does not qualify as an FS.
■ The path through Router D to H to F to G has an AD of 20 and cannot be an FS.

■ The path through Router A to E to G has an AD of 7, however, which is less than the original 10. Therefore, this is an FS and can be replaced as a route without Router A changing from passive to active mode.
■ The original topology table would show that the primary route (successor) is Router D, while the backup route (FS) is Router E. After the link between D and G dies, the routing table would be updated from the topology table while the route remains passive.

The following section illustrates what happens when the topology table is interrogated and no feasible route is found.

Updating the Routing Table in Active Mode with DUAL
When no alternative route is found in the routing table, the following actions are taken (using the network in Figure 13-6 as an example). The following list describes the ﬁgure and explains the actions taken on the information provided:

■ The topology table of Router A has a path (successor) of A to D to G to X.
■ The FD is 20, and the AD from Router D is 15.
■ When Router D dies, Router A must ﬁnd an alternative path to X.
■ Neighbors B, C, E, and F have ADs of 27, 27, 20, and 21, respectively.
■ Because all the neighbors have an AD that is the same or greater than the successor FD, none of these are acceptable as FSs.
■ Router A must go into active mode and send queries to the neighbors.
■ Both Routers E and F reply with an FS because both have an AD from Router G of 5. Remember the equation FD > AD; the Routers E and F have an FD of 21, and 21 > 5.
■ Because the FD is acceptable, the topology and routing tables will be updated, DUAL will be calculated, and the network will be returned to passive mode.
■ From this information received from Routers E and F, the router selects the path through E as the best route because it has the lower cost.
■ The result is placed in the routing table as the valid neighboring router. EIGRP refers to this neighboring router as a successor.
■ Router F will be stored as an FS in the topology table.

NOTE Figure 13-6 is simpliﬁed to explain the concepts. In reality, the split horizon rule dictates that Routers B and C would not readvertise routes it learned through an interface out of that same interface. Because all routes to X are learned through one interface, no routes to X would be readvertised out of this interface.

Figure 13-6 The Use of Feasible and Advertised Distance—Active Mode

The details on how EIGRP computes successors are complex, but the concept is simple, as described in the next section.

Choosing a Successor
To determine whether a path to a remote network is feasible, EIGRP considers the feasible condition (FC) of the route. Essentially, each router holds a routing table that is a list of the available networks and the best or most efﬁcient path to each of them. The term used to describe this is the feasible distance of the successor, otherwise known as the metric for the route. The router also holds the routing table of its neighbors, referred to as the AD. If the AD is within scope, this route may be identiﬁed as an alternative route, or an FS.

A neighbor can become an FS for a route only if its AD is less than the FD. This is DUAL’s fundamental key to remaining loop-free; if a route contains a loop, the AD will be greater than the FD and therefore will fail the FC. By holding the routing tables of the neighbors, the amount of network overhead and computation is reduced. When a path to a remote network is lost, the router might be capable of ﬁnding an alternative route with minimal fuss, computation, or network trafﬁc. This gives the much-advertised beneﬁt of very fast convergence.

As you can see in the explanation for ﬁnding an FS in the previous section, “Updating the Routing Table in Active Mode with DUAL,” queries can be sent throughout the organization’s network. This is the design key to ensuring that EIGRP scales.

EIGRP Network Design
EIGRP is designed to work in very large networks. However, EIGRP, as with OSPF, is designsensitive. Scaling a network—or, in other words, improving its capability to grow in size and complexity—is a major concern in today’s organizations. New demands are constantly driving the networks to use applications that require more bandwidth and other resources from the network. For example, simply consider the need for every desktop and every user to be able to attach to centralized resources as well as to the Internet.

The factors that can affect the scaling of EIGRP are as follows:
■ The amount of information sent between neighbors
■ The number of routers that are sent updates
■ How far away the routers are that have to send updates
■ The number of alternative paths to remote networks
Poorly scaled EIGRP networks can result in the following:
■ A route being SIA
■ Network congestion

— Delays
— Routing information being lost
— Flapping routes
— Retransmission
■ Router memory running low
■ Router CPU overutilized
■ Unreliable circuit or unidirectional link

Some of these symptoms are caused by other factors, such as poor design, with resources overwhelmed by the tasks assigned. Often, many of these symptoms are characterized by a route being ﬂagged as SIA, as the router waits for a reply from a neighbor across a network that cannot handle the demands made upon it.

Careful design and placement of network devices can remedy many of the problems seen in a network.

Solutions to EIGRP Scaling Issues
The design of the network is very important to the ability to scale any network. The following solutions revolve around a carefully thought-out network:

■ Allocation of addresses should be contiguous to allow summarization.
■ A hierarchical tiered network design should be used to allow summarization.
■ Sufﬁcient network resources (both hardware and software) on network devices.
■ Sufﬁcient bandwidth should be used on WAN links.
■ Appropriate EIGRP conﬁguration should be used on WAN links. By default, EIGRP only uses 50 percent of the bandwidth of the link for its trafﬁc. This default may be tuned manually.
■ Filters should be used.
■ Network monitoring should be used.

EIGRP Design Issues
The major concern in scaling an organizational network is controlling the network overhead that is sent over slow WAN links in particular. The less information about the network, its services, and networks that needs to be sent, the greater the capacity available for the data between clients and servers. Although sending less routing information relieves the network, it gives the routers less information with which to make decisions. Every designer of routing protocols and every network administrator must deal continually with this trade-off. As seen with summarization, static and default routes can lead to poor routing decisions and loss of connectivity.

EIGRP automatically summarizes at the autonomous system boundary and at the classful network boundary. To conﬁgure manual conﬁguration, it is ﬁrst necessary to disable automatic summarization. Summarization is conﬁgured at the interface level. This obviously requires careful consideration of the network design in reference to the ﬂow of data and the network topology. Although still a distance vector protocol and proprietary, EIGRP addresses many of the problems related to scaling the network.

Remember that queries must be limited to ensure that EIGRP can properly scale. If queries are allowed to traverse the entire organization, the problems and symptoms described will ravage your network.

Many believe that dividing the organization’s network into different EIGRP autonomous systems is a good way of limiting the query range. This is true, because EIGRP does not share updates with another autonomous systems. However, many organizations that created autonomous systems to replicate OSPF areas naturally redistribute between them so that the entire organization can share routing information. At this point, the query is propagated into the new autonomous system, and the problem continues. Summarization is the best way to limit the query range of EIGRP networks. If a subnet is hidden by summarization, the query will stop at the ﬁrst router that has no knowledge of it.

Certain topologies, although valid in most instances, pose problems for the EIGRP network. This is true in particular for the hub-and-spoke design often seen implemented between remote sites and regional ofﬁces. The popular dual-homed conﬁguration, although providing redundancy, also allows the potential for routers to reﬂect queries back to one another. Summarization and ﬁlters make this network design work well while also allowing queries to be managed effectively.

Foundation Summary

The “Foundation Summary” section of each chapter lists the most important facts from the chapter. Although this section does not list every fact from the chapter that will be on your exam, a wellprepared candidate should, at a minimum, know all the details in each “Foundation Summary” before going to take the exam.

The main concepts of EIGRP are as follows:
■ DUAL
■ Loop-free networks
■ Loop-free routing tables
■ Support for different topologies
■ Rapid convergence
■ Reduced bandwidth use
■ Protocol independence at Layer 3
■ Compatibility with IGRP
■ Easy conﬁguration
■ Use of a composite metric
Cisco identiﬁes four main components of EIGRP:
■ Protocol-dependent modules
■ RTP
■ Neighbor discovery and recovery
■ DUAL

Table 13-5 summarizes the EIGRP packet types sent between neighbors.
Table 13-5 Summary of Packet Types

Figure 13-7 shows the actions taken when a router receives a query from another router asking for an alternative route to a destination. Note that if the queried router has no route to offer, it is still obliged to respond to the querying router.

Figure 13-7 EIGRP—Maintaining the Topology Table, Router D

Figure 13-8 illustrates the logic ﬂow in a router that realizes a link has been lost, which may occur because a directly connected interface has lost a carrier signal or because the router has received an update or query.

Figure 13-8 EIGRP—Maintaining the Topology Table, Choosing a Feasible Successor

Q&A
As mentioned in the introduction, “All About the CCNP, CCDP, and CCIP Certiﬁcations,” you have two choices for review questions. The questions that follow next give you a bigger challenge than the exam itself by using an open-ended question format. By reviewing now with this more difﬁcult question format, you can exercise your memory better and prove your conceptual and factual knowledge of this chapter. The answers to these questions are found in Appendix A.

For more practice with examlike question formats, including questions using a router simulator and multichoice questions, use the exam engine on the CD-ROM.

1. If a router does not have a feasible successor, what action will it take?
2. When does EIGRP need to be manually redistributed into another EIGRP process?
3. Which timers are tracked in the neighbor table?
4. What is the difference between an update and a query?
5. When does EIGRP recalculate the topology table?
6. EIGRP has a default limit set on the amount of bandwidth that it can use for EIGRP packets.
What is the default percentage limit?
7. State two rules for designing a scalable EIGRP network.
8. EIGRP can be used to send information about which three routed protocols?
9. Which EIGRP packets are sent reliably?
10. In what instances will EIGRP automatically redistribute?
11. How long is the holdtime, by default?
12. What is an EIGRP topology table, and what does it contain?
13. What is the advertised distance in EIGRP, and how is it distinguished from the feasible distance?
14. What EIGRP algorithm is run to create entries for the routing table?
15. When does EIGRP place a network in active mode?
16. By default, EIGRP summarizes at which boundary?
17. What is Stuck in Active?
18. State two factors that inﬂuence EIGRP scalability.
19. What are reply packets in EIGRP?
20. What conditions must be met for a router to become a neighbor?