EIGRP

Chapter 03 (EIGRP)

Overview
EIGRP is a Cisco-proprietary routing protocol that is based on IGRP.
EIGRP supports CIDR and VLSM which allows network designers to maximize address space. When compared to IGRP which is a classful routing protocol, EIGRP boasts faster convergence times, improved scalability, and superior management of routing loops.
Furthermore, EIGRP can replace Novell RIP and AppleTalk Routing Table Maintenance Protocol (RTMP). EIGRP serves both IPX and AppleTalk networks with powerful efficiency.
EIGRP is often described as a hybrid routing protocol that offers the best of distance vector and link-state algorithms.
EIGRP is an advanced routing protocol that relies on features commonly associated with link-state protocols. Some of the best features of OSPF, such as partial updates and neighbor discovery, are similarly put to use by EIGRP. However, EIGRP is easier to configure than OSPF.
EIGRP is an ideal choice for large, multi-protocol networks built primarily on Cisco routers.
This module covers common EIGRP configuration tasks. The emphasis is on ways in which EIGRP establishes relationships with adjacent routers, calculates primary and backup routes, and responds to failures in known routes to a particular destination.
A network is made up of many devices, protocols, and media that allow data communication to occur. When a network component does not work correctly, it can affect the entire network. In any case, network administrators must quickly identify and troubleshoot problems when they arise. The following are some reasons why network problems occur:
• Commands are entered incorrectly
• Access lists are constructed or placed incorrectly
• Routers, switches, or other network devices are misconfigured
• Physical connections are bad


A network administrator should troubleshoot in a methodical manner with the use a general problem-solving model. It is often useful to check for physical layer problems first and then move up the layers in an organized manner. Although this module focuses on how to troubleshoot Layer 3 protocols, it is important to troubleshoot and eliminate any problems that may exist at the lower layers.
This module covers some of the objectives for the CCNA 640-801 and ICND 640-811 exams.







Students who complete this module should be able to perform the following tasks:
• Describe the differences between EIGRP and IGRP
• Describe the key concepts, technologies, and data structures of EIGRP
• Understand EIGRP convergence and the basic operation of the Diffusing Update Algorithm (DUAL)
• Perform basic EIGRP configuration
• Configure EIGRP route summarization
• Describe the processes used by EIGRP to build and maintain routing tables
• Verify EIGRP operations
• Describe the eight-step process for general troubleshooting
• Apply a logical process to troubleshoot routing
• Use the show and debug commands to troubleshoot RIP
• Use the show and debug commands to troubleshoot IGRP
• Use the show and debug commands to troubleshoot EIGRP
• Use the show and debug commands to troubleshoot OSPF
3.1 EIGRP Concepts
3.1.1 Comparing EIGRP and IGRP
Cisco released EIGRP in 1994 as a scalable and improved version of its proprietary distance vector routing protocol, IGRP. This page will explain how EIGRP and IGRP compare to each other. The distance vector technology and distance information found in IGRP is also used in EIGRP.
EIGRP has improved convergence properties and operates more efficiently over IGRP. This allows a network to have improved architecture as well as retain the current investment in IGRP.
The comparisons between EIGRP and IGRP fall into the following major categories:
• Compatibility mode
• Metric calculation
• Hop count
• Automatic protocol redistribution
• Route tagging
IGRP and EIGRP are compatible with each other. This compatibility provides seamless interoperability with IGRP routers. This is important as users can take advantage of the benefits of both protocols. EIGRP offers multiprotocol support, but IGRP does not.
EIGRP and IGRP use different metric calculations. EIGRP scales the metric of IGRP by a factor of 256. That is because EIGRP uses a metric that is 32 bits long, and IGRP uses a 24-bit metric. EIGRP can multiply or divide by 256 to easily exchange information with IGRP.


IGRP has a maximum hop count of 255. EIGRP has a maximum hop count limit of 224. This is more than adequate to support large, properly designed internetworks.
To enable dissimilar routing protocols such as OSPF and RIP to share information requires advanced configuration. Redistribution, or route sharing, is automatic between IGRP and EIGRP as long as both processes use the same AS number. In Figure , RTB automatically redistributes routes learned from EIGRP to the IGRP AS, and vice versa.



EIGRP tags routes learned from IGRP or any outside source as external because they did not originate from EIGRP routers. IGRP cannot differentiate between internal and external routes.


Notice that in the show ip route command output for the routers in Figure , EIGRP routes are flagged with D, and external routes are denoted by EX. RTA identifies the difference between the 172.16.0.0 network, which was learned through EIGRP, and the 192.168.1.0 network that was redistributed from IGRP. In the RTC table, the IGRP protocol makes no such distinction. RTC, which uses IGRP only, just sees IGRP routes, despite the fact that both 10.1.1.0 and 172.16.0.0 were redistributed from EIGRP.
The Interactive Media Activity will help students recognize the characteristics of IGRP and EIGRP.
The next page will explain EIGRP in greater detail.
3.1.2 EIGRP concepts and terminology
This page will discuss the three tables that EIGRP uses to store network information.
EIGRP routers keep route and topology information readily available in RAM so they can react quickly to changes. Like OSPF, EIGRP saves this information in several tables and databases.
EIGRP saves routes that are learned, in specific ways. Routes are given a particular status and can be tagged to provide additional useful information.
The following three tables are maintained by EIGRP:
• Neighbor table
• Topology table
• Routing table
The neighbor table is the most important table in EIGRP. Each EIGRP router maintains a neighbor table that lists adjacent routers. This table is comparable to the adjacency database used by OSPF. There is a neighbor table for each protocol that EIGRP supports.


When newly discovered neighbors are learned, the address and interface of the neighbor is recorded. This information is stored in the neighbor data structure. When a neighbor sends a hello packet, it advertises a hold time. The hold time is the amount of time a router treats a neighbor as reachable and operational. If a hello packet is not received within the hold time, then the hold time expires. When the hold time expires, the Diffusing Update Algorithm (DUAL), which is the EIGRP distance vector algorithm, is informed of the topology change and must recalculate the new topology.


The topology table is made up of all the EIGRP routing tables in the autonomous system. DUAL takes the information supplied in the neighbor table and the topology table and calculates the lowest cost routes to each destination. EIGRP tracks this information so that EIGRP routers can identify and switch to alternate routes quickly. The information that the router learns from the DUAL is used to determine the successor route, which is the term used to identify the primary or best route. This information is also entered into the topology table.
EIGRP routers maintain a topology table for each configured network protocol. All learned routes to a destination are maintained in the topology table.
The following are fields in the topology table:
• Feasible distance (FD) - This is the lowest calculated metric to each destination. For example, the feasible distance to 32.0.0.0 is 2195456.
• Route source - The identification number of the router that originally advertised that route. This field is populated only for routes learned externally from the EIGRP network. Route tagging can be particularly useful with policy-based routing. For example, the route source to 32.0.0.0 is 200.10.10.10 through 200.10.10.10.
• Reported distance (RD) - The distance reported by an adjacent neighbor to a specific destination. For example, the reported distance to 32.0.0.0 is 2195456 as indicated by (90/2195456).
• Interface information - The interface through which the destination can be reached.
• Route status - The status of a route. Routes are identified as being either passive, which means that the route is stable and ready for use, or active, which means that the route is in the the process of being recomputed by DUAL.
The EIGRP routing table holds the best routes to a destination. This information is retrieved from the topology table. EIGRP routers maintain a routing table for each network protocol.



A successor is a route selected as the primary route to reach a destination. DUAL identifies this route from the information contained in the neighbor and topology tables and places it in the routing table. There can be up to four successor routes for any particular destination. These can be of equal or unequal cost and are identified as the best loop-free paths to a given destination. A copy of the successor routes is also placed in the topology table.



A feasible successor (FS) is a backup route. These routes are identified at the same time as the successors, but these routes are only kept in the topology table. Multiple feasible successors for a destination can be retained in the topology table although it is not mandatory.
A router views the feasible successors as neighbors downstream, or closer to the destination than it is. Feasible successor cost is computed by the advertised cost of the neighbor router to the destination. If a successor route goes down, the router will look for an identified feasible successor. This route will be promoted to successor status. A feasible successor must have a lower advertised cost than the current successor cost to the destination. If a feasible successor is not identified from the current information, the router places an Active status on a route and sends out query packets to all neighbors in order to recompute the current topology. The router can identify any new successor or feasible successor routes from the new data that is received from the reply packets that answer the query requests. The router will then place a Passive status on the route.



The topology table can record additional information about each route. EIGRP classifies routes as either internal or external. EIGRP adds a route tag to each route to identify this classification. Internal routes originate from within the EIGRP AS.


External routes originate outside the EIGRP AS. Routes learned or redistributed from other routing protocols, such as RIP, OSPF, and IGRP, are external. Static routes that originate outside the EIGRP AS are external. The tag can be configured to a number between 0-255 to customize the tag.




The next page will list some advantages of EIGRP.
3.1.3 EIGRP design features
This page will describe some key design features of EIGRP.
EIGRP operates quite differently from IGRP. EIGRP is an advance distance vector routing protocol, but also acts as a link-state protocol in the way that it updates neighbors and maintains routing information. The following are advantages of EIGRP over simple distance vector protocols:


• Rapid convergence
• Efficient use of bandwidth
• Support for VLSM and CIDR.
• Multiple network layer support
• Independence from routed protocols.
Independence from routed protocols means that protocol-dependent modules (PDMs) protect EIGRP from lengthy revision. As routed protocols evolve, they may need new protocol modules, but changes to EIGRP will not be necessary.
EIGRP routers converge quickly because they rely on DUAL. DUAL guarantees loop-free operation throughout a route computation which allows all routers involved in a topology change to synchronize at the same time.
EIGRP sends partial, bounded updates and makes efficient use of bandwidth. EIGRP uses minimal bandwidth when the network is stable. EIGRP routers do not send the complete tables, but instead, send partial, incremental updates. This is similar to OSPF operation, except that EIGRP routers send these partial updates only to the routers that need the information, not to all routers in an area. For this reason, they are called bounded updates. Instead of timed routing updates, EIGRP routers use small hello packets to keep in touch with each other. Though exchanged regularly, hello packets do not use up a significant amount of bandwidth.
EIGRP supports IP, IPX, and AppleTalk through PDMs. EIGRP can redistribute IPX, RIP, and SAP information to improve overall performance. In effect, EIGRP can take over for these two protocols. EIGRP routers receive routing and service updates, and update other routers only when changes in the SAP or routing tables occur. In EIGRP networks, routing updates occur in partial updates.
EIGRP can also take over for the AppleTalk RTMP. As a distance vector routing protocol, RTMP relies on periodic and complete exchanges of routing information. To reduce overhead, EIGRP uses event-driven updates to redistributes AppleTalk routing information. EIGRP also uses a configurable composite metric to determine the best route to an AppleTalk network. RTMP uses hop count, which can result in suboptimal routing. AppleTalk clients expect RTMP information from local routers, so EIGRP for AppleTalk should be run only on a clientless network, such as a WAN link.
The next page will discuss some EIGRP technologies.
3.1.4 EIGRP technologies
This page will discuss some of the new technologies that EIGRP includes. Each new technology represents an improvement in EIGRP operation efficiency, speed of convergence, or functionality relative to IGRP and other routing protocols. These technologies fall into one of the following four categories:
• Neighbor discovery and recovery
• Reliable Transport Protocol
• DUAL finite-state machine algorithm
• Protocol-dependent modules
Simple distance vector routers do not establish any relationship with their neighbors. RIP and IGRP routers merely broadcast or multicast updates on configured interfaces. In contrast, EIGRP routers actively establish relationships with their neighbors, much the same way that OSPF routers do.
EIGRP routers establish adjacencies as described in Figure . EIGRP routers use small hello packets to accomplish this. Hellos are sent by default every five seconds. An EIGRP router assumes that as long as it receives hello packets from known neighbors, those neighbors and their routes remain viable or passive. The following are possible when EIGRP routers form adjacencies:
• Dynamically learn of new routes that join the network
• Identify routers that become either unreachable or inoperable
• Rediscover routers that had previously been unreachable


Reliable Transport Protocol (RTP) is a transport layer protocol that guarantees ordered delivery of EIGRP packets to all neighbors. On an IP network, hosts use TCP to sequence packets and ensure their timely delivery. However, EIGRP is protocol-independent. This means it does not rely on TCP/IP to exchange routing information the way that RIP, IGRP, and OSPF do. To stay independent of IP, EIGRP uses RTP as its own proprietary transport layer protocol to guarantee delivery of routing information.
EIGRP can call on RTP to provide reliable or unreliable service as the situation warrants. For example, hello packets do not require the overhead of reliable delivery because they are frequent and should be kept small. The reliable delivery of other routing information can actually speed convergence because then EIGRP routers do not wait for a timer to expire before they retransmit.
With RTP, EIGRP can multicast and unicast to different peers simultaneously. This allows for maximum efficiency.
The centerpiece of EIGRP is the DUAL, which is the EIGRP route-calculation engine. The full name of this technology is DUAL finite-state machine (FSM). An FSM is an algorithm machine, not a mechanical device with parts that move. FSMs define a set of possible states that something can go through, the events that cause those states, and the events that result from those states. Designers use FSMs to describe how a device, computer program, or routing algorithm will react to a set of input events. The DUAL FSM contains all the logic used to calculate and compare routes in an EIGRP network.
DUAL tracks all the routes advertised by neighbors. Composite metrics of each route are used to compare them. DUAL also guarantees that each path is loop free. DUAL inserts lowest cost paths into the routing table. These primary routes are known as successor routes. A copy of the successor routes is also placed in the topology table.





EIGRP keeps important route and topology information readily available in a neighbor table and a topology table. These tables supply DUAL with comprehensive route information in case of network disruption. DUAL uses the information in these tables to select alternate routes quickly. If a link goes down, DUAL looks for an alternative route path, or feasible successor, in the topology table.


One of the best features of EIGRP is its modular design. Modular, or layered designs, prove to be the most scalable and adaptable. Support for routed protocols, such as IP, IPX, and AppleTalk, is included in EIGRP through PDMs. In theory, EIGRP can add PDMs to easily adapt to new or revised routed protocols such as IPv6.
Each PDM is responsible for all functions related to its specific routed protocol. The IP-EIGRP module is responsible for the following functions:
• Send and receive EIGRP packets that bear IP data
• Notify DUAL of new IP routing information that is received
• Maintain the results of DUAL routing decisions in the IP routing table
• Redistribute routing information that was learned by other IP-capable routing protocols
The next page will discuss the EIGRP packet types.
3.1.5 EIGRP data structure
Like OSPF, EIGRP relies on different types of packets to maintain its tables and establish relationships with neighbor routers. This page will describe these packet types.


The following are the five types of EIGRP packets:
• Hello
• Acknowledgment
• Update
• Query
• Reply
EIGRP relies on hello packets to discover, verify, and rediscover neighbor routers. Rediscovery occurs if EIGRP routers do not receive hellos from each other for a hold time interval but then re-establish communication.
EIGRP routers send hellos at a fixed, but configurable interval called the hello interval. The default hello interval depends on the bandwidth of the interface. On IP networks, EIGRP routers send hellos to the multicast IP address 224.0.0.10.
EIGRP routers store information about neighbors in the neighbor table. The neighbor table includes the Sequence Number (Seq No) field to record the number of the last received EIGRP packet that each neighbor sent. The neighbor table also includes a Hold Time field which records the time the last packet was received. Packets should be received within the Hold Time interval period to maintain a Passive state. The Passive state is a reachable and operational status.






If EIGRP does not receive a packet from a neighbor within the hold time, EIGRP considers that neighbor down. DUAL then steps in to re-evaluate the routing table. By default, the hold time is three times the hello interval, but an administrator can configure both timers as desired.
OSPF requires neighbor routers to have the same hello and dead intervals to communicate. EIGRP has no such restriction. Neighbor routers learn about each of the other respective timers through the exchange of hello packets. They then use that information to forge a stable relationship regardless of unlike timers.
Hello packets are always sent unreliably. This means that no acknowledgment is transmitted.
EIGRP routers use acknowledgment packets to indicate receipt of any EIGRP packet during a reliable exchange. RTP provides reliable communication between EIGRP hosts. A message that is received must be acknowledged by the recipient to be reliable. Acknowledgment packets, which are hello packets without data, are used for this purpose. Unlike multicast hellos, acknowledgment packets are unicast. Acknowledgments can be attached to other kinds of EIGRP packets, such as reply packets.
Update packets are used when a router discovers a new neighbor. EIGRP routers send unicast update packets to that new neighbor so that it can add to its topology table. More than one update packet may be needed to convey all the topology information to the newly discovered neighbor.
Update packets are also used when a router detects a topology change. In this case, the EIGRP router sends a multicast update packet to all neighbors, which alerts them to the change. All update packets are sent reliably.
An EIGRP router uses query packets whenever it needs specific information from one or all of its neighbors. A reply packet is used to respond to a query.
If an EIGRP router loses its successor and cannot find a feasible successor for a route, DUAL places the route in the Active state. A query is then multicasted to all neighbors in an attempt to locate a successor to the destination network. Neighbors must send replies that either provide information on successors or indicate that no information is available. Queries can be multicast or unicast, while replies are always unicast. Both packet types are sent reliably.
The next page will describe the EIGRP algorithm.
3.1.6 EIGRP algorithm
This page will describe the DUAL algorithm, which results in the exceptionally fast convergence of EIGRP.
The sophisticated DUAL algorithm results in the exceptionally fast convergence of EIGRP. To better understand convergence with DUAL, consider the example in Figure . Each router has constructed a topology table that contains information about how to route to destination Network A.


Each topology table identifies the following information:
• The routing protocol or EIGRP
• The lowest cost of the route, which is called feasible distance (FD)
• The cost of the route as advertised by the neighboring router, which is called reported distance (RD)
The Topology column identifies the primary route called the successor route (successor), and, where identified, the backup route called the feasible successor (FS). Note that it is not necessary to have an identified feasible successor.
The EIGRP network follows a sequence of actions to allow convergence between the routers, which currently have the following topology information:
• Router C has one successor route by way of Router B.
• Router C has one feasible successor route by way of Router D.
• Router D has one successor route by way of Router B.
• Router D has no feasible successor route.
• Router E has one successor route by way of Router D.
• Router E has no feasible successor.


The feasible successor route selection rules are specified in Figure .









The following example demonstrates how each router in the topology will carry out the feasible successor selection rules when the route from Router D to Router B goes down:
In Router D:
• Route by way of Router B is removed from the topology table.
• This is the successor route. Router D has no feasible successor identified.
• Router D must complete a new route computation.
In Router C:
• Route to Network A by way of Router D is down.
• Route by way of Router D is removed from the table.
• This is the feasible successor route for Router C.


In Router D:
• Router D has no feasible successor. It cannot switch to an identified alternative backup route.
• Router D must recompute the topology of the network. The path to destination Network A is set to Active.
• Router D sends a query packet to all connected neighbors to request topology information.
• Router C does have a previous entry for Router D.
• Router D does not have a previous entry for Router E.
In Router E:
• Route to Network A through Router D is down.
• The route by way of Router D is removed from the table.
• This is the successor route for Router E.
• Router E does not have a feasible route identified.
• Note that the RD cost of routing by way of Router C is 3. That is the same cost as the successor route by way of Router D.


In Router C:
• Router E sends a query packet to Router C.
• Router C removes Router E from the table.
• Router C replies to Router D with a new route to Network A.
In Router D:
• Route status to destination Network A is still marked as Active. The computation has not been completed yet.
• Router C has replied to Router D to confirm that a route to destination Network A is available with a cost of 5.
• Router D still waits for a reply from Router E.
In Router E:
• Router E has no feasible successor to reach destination Network A.
• Router E, therefore, tags the status of the route to destination network as Active.
• Router E has to recompute the network topology.
• Router E removes the route by way of Router D from the table.
• Router E sends a query to Router C, to request topology information.
• Router E already has an entry by way of Router C. It is at a cost of 3, the same as the successor route.



In Router E:
• Router C replies with an RD of 3.
• Router E can now set the route by way of Router C as the new successor with an FD of 4 and an RD of 3.
• Router E replaces the Active status of the route to destination Network A with a Passive status. Note that a route will have a Passive status by default as long as hello packets are received. In this example, only Active status routes are flagged.


In Router E:
• Router E sends a reply to Router D to inform it of the Router E topology information.
In Router D:
• Router D receives the reply packed from Router E.
• Router D enters this data for the route to destination Network A by way of Router E.
• This route becomes an additional successor route as the cost is the same as routing by way of Router C and the RD is less than the FD cost of 5.
Convergence occurs among all EIGRP routers that use the DUAL algorithm.
This page concludes this lesson. The next lesson will discuss the configuration of EIGRP. The first page will explain how EIGRP is configured.
3.2 EIGRP Configuration
3.2.1 Configuring EIGRP
Despite the complexity of DUAL, configuring EIGRP can be relatively simple. EIGRP configuration commands vary depending on the protocol that is to be routed. Some examples of these protocols are IP, IPX, and AppleTalk. This page describes EIGRP configuration for the IP protocol.


Perform the following steps to configure EIGRP for IP:
1. Use the following to enable EIGRP and define the autonomous system:
router(config)#router eigrp autonomous-system-number
The autonomous system number is used to identify all routers that belong within the internetwork. This value must match all routers within the internetwork.
2. Indicate which networks belong to the EIGRP autonomous system on the local router by using the following command:
router(config-router)#networknetwork-number
The network-number is the network number that determines which interfaces of the router are participating in EIGRP and which networks are advertised by the router.
The network command configures only connected networks. For example, network 3.1.0.0, which is on the far left of the main Figure, is not directly connected to Router A. Consequently, that network is not part of the configuration of Router A.
3. When configuring serial links using EIGRP, it is important to configure the bandwidth setting on the interface. If the bandwidth for these interfaces is not changed, EIGRP assumes the default bandwidth on the link instead of the true bandwidth. If the link is slower, the router may not be able to converge, routing updates might become lost, or suboptimal path selection may result. To set the interface bandwidth, use the following syntax:
router(config-if)#bandwidthkilobits
The bandwidth command is only used by the routing process and should be set to match the line speed of the interface.
4. Cisco also recommends adding the following command to all EIGRP configurations:
router(config-router)#eigrp log-neighbor-changes
This command enables the logging of neighbor adjacency changes to monitor the stability of the routing system and to help detect problems.
In the Lab Activities, students will set up an IP address scheme and configure EIGRP.
The next page will discuss EIGRP summarization.
3.2.2 Configuring EIGRP summarization
This page will teach students how to manually configure summary addresses.
EIGRP automatically summarizes routes at the classful boundary. This is the boundary where the network address ends, as defined by class-based addressing. This means that even though RTC is connected only to the subnet 2.1.1.0, it will advertise that it is connected to the entire Class A network, 2.0.0.0. In most cases auto summarization is beneficial because it keeps routing tables as compact as possible.
However, automatic summarization may not be the preferred option in certain instances. For example, if there are discontiguous subnetworks auto-summarization must be disabled for routing to work properly. To turn off auto-summarization, use the following command:
router(config-router)#no auto-summary





With EIGRP, a summary address can be manually configured by configuring a prefix network. Manual summary routes are configured on a per-interface basis, so the interface that will propagate the route summary must be selected first. Then the summary address can be defined with the ip summary-address eigrp command:
router(config-if)#ip summary-address eigrpautonomous-system-number ip-address mask administrative-distance



EIGRP summary routes have an administrative distance of 5 by default. Optionally, they can be configured for a value between 1 and 255.
In Figure , RTC can be configured using the commands shown:
RTC(config)#router eigrp 2446
RTC(config-router)#no auto-summary
RTC(config-router)#exit
RTC(config)#interface serial 0/0
RTC(config-if)#ip summary-address eigrp 2446 2.1.0.0 255.255.0.0
Therefore, RTC will add a route to its table as follows:
D 2.1.0.0/16 is a summary, 00:00:22, Null0
Notice that the summary route is sourced from Null0 and not from an actual interface. This is because this route is used for advertisement purposes and does not represent a path that RTC can take to reach that network. On RTC, this route has an administrative distance of 5.
RTD is not aware of the summarization but accepts the route. The route is assigned the administrative distance of a normal EIGRP route, which is 90 by default.
In the configuration for RTC, auto-summarization is turned off with the no auto-summary command. If auto-summarization was not turned off, RTD would receive two routes, the manual summary address, which is 2.1.0.0 /16, and the automatic, classful summary address, which is 2.0.0.0 /8.
In most cases when manually summarizing, the no auto-summary command should be issued.
The next page will show students how to verify EIGRP.
3.2.3 Verifying basic EIGRP
This page will explain how show commands can be used to verify EIGRP configurations. Figure lists the key EIGRP show commands and briefly discusses their functions.
The Cisco IOS debug feature also provides useful EIGRP monitoring commands.
The Lab Activities will require students to set up an IP address scheme and verify EIGRP configurations.
The next page will discuss EIGRP neighbor tables.











3.2.4 Building neighbor tables
This page will explain how EIGRP builds neighbor tables. Students will also learn about the information that is stored in a neighbor table and how it is used.
Simple distance vector routers do not establish any relationship with their neighbors. RIP and IGRP routers merely broadcast or multicast updates on configured interfaces. In contrast, EIGRP routers actively establish relationships with their neighbors as do OSPF routers.


The neighbor table is the most important table in EIGRP. Each EIGRP router maintains a neighbor table that lists adjacent routers. This table is comparable to the adjacency database used by OSPF. There is a neighbor table for each protocol that EIGRP supports.


EIGRP routers establish adjacencies with neighbor routers by using small hello packets. Hellos are sent by default every five seconds. An EIGRP router assumes that, as long as it is receiving hello packets from known neighbors, those neighbors and their routes remain viable or passive. By forming adjacencies, EIGRP routers do the following:
• Dynamically learn of new routes that join their network
• Identify routers that become either unreachable or inoperable
• Rediscover routers that had previously been unreachable
The following fields are found in a neighbor table:
• Neighbor address - This is the network layer address of the neighbor router.
• Hold time - This is the interval to wait without receiving anything from a neighbor before considering the link unavailable. Originally, the expected packet was a hello packet, but in current Cisco IOS software releases, any EIGRP packets received after the first hello will reset the timer.
• Smooth Round-Trip Timer (SRTT) - This is the average time that it takes to send and receive packets from a neighbor. This timer is used to determine the retransmit interval (RTO).
• Queue count (Q Cnt) - This is the number of packets waiting in a queue to be sent. If this value is constantly higher than zero, there may be a congestion problem at the router. A zero means that there are no EIGRP packets in the queue.
• Sequence Number (Seq No) - This is the number of the last packet received from that neighbor. EIGRP uses this field to acknowledge a transmission of a neighbor and to identify packets that are out of sequence. The neighbor table is used to support reliable, sequenced delivery of packets and can be regarded as analogous to the TCP protocol used in the reliable delivery of IP packets.
The next page will describe how route and topology information is used to route data.
3.2.5 Discover routes
This page will explain how EIGRP stores route and topology information. Students will also learn how DUAL uses this information to route data.


EIGRP routers keep route and topology information available in RAM, so changes can be reacted to quickly. Like OSPF, EIGRP keeps this information in several tables or databases.
The EIGRP distance vector algorithm, DUAL, uses the information gathered in the neighbor and topology tables and calculates the lowest cost route to the destination. The primary route is called the successor route. When calculated, DUAL places the successor route in the routing table and a copy in the topology table.
DUAL also attempts to calculate a backup route in case the successor route fails. This is called the feasible successor route. When calculated, DUAL places the feasible route in the topology table. This route can be called upon if the successor route to a destination becomes unreachable or unreliable.
The Interactive Media Activity will help students understand some important EIGRP terms and concepts.
The next page will provide more information about how DUAL selects a route.
3.2.6 Select routes
This page will explain how DUAL selects an alternative route in the topology table when a link goes down. - If a feasible successor is not found, the route is flagged as Active, or unusable at present. Query packets are sent to neighboring routers requesting topology information. DUAL uses this information to recalculate successor and feasible successor routes to the destination.







Once DUAL has completed these calculations, the successor route is placed in the routing table. Then both the successor route and feasible successor route are placed in the topology table. The route to the final destination will now pass from an Active status to a Passive status. This means that the route is now operational and reliable.


The sophisticated algorithm of DUAL results in EIGRP having exceptionally fast convergence. To better understand convergence using DUAL, consider the example in Figure . All routers have built a topology table that contains information about how to route to destination network Z.
Each table identifies the following:
• The routing protocol or EIGRP
• The lowest cost of the route or Feasible Distance (FD)
• The cost of the route as advertised by the neighboring router or Reported Distance (RD)


The Topology heading identifies the preferred primary route, which is called the successor route (Successor). If it is identified, the Topology heading will also identify the backup route, which is called the feasible successor (FS). Note that it is not necessary to have an identified feasible successor.
The next page will explain how DUAL maintains routing tables.
3.2.7 Maintaining routing tables
This page will explain how DUAL maintains and updates routing tables.
DUAL tracks all routes advertised by neighbors using the composite metric of each route to compare them. DUAL also guarantees that each path is loop-free.
Lowest-cost paths are then inserted by the DUAL algorithm into the routing table. These primary routes are known as successor routes. A copy of the successor paths is placed in the topology table.
EIGRP keeps important route and topology information readily available in a neighbor table and a topology table. These tables supply DUAL with comprehensive route information in case of network disruption. DUAL selects alternate routes quickly by using the information in these tables.


If a link goes down, DUAL looks for an alternative route path, or feasible successor, in the topology table. If a feasible successor is not found, the route is flagged as active, or unusable at present. Query packets are sent to neighboring routers requesting topology information. DUAL uses this information to recalculate successor and feasible successor routes to the destination.
Once DUAL has completed these calculations, the successor route is placed in the routing table. Then both the successor route and feasible successor route are placed in the topology table. The route to the final destination will now pass from an active status to a passive status. This means that the route is now operational and reliable.
EIGRP routers establish and maintain adjacencies with neighbor routers by using small hello packets. Hellos are sent by default every five seconds. An EIGRP router assumes that, as long as it is receiving hello packets from known neighbors, those neighbors and their routes remain viable, or passive.


When newly discovered neighbors are learned, the address and interface of the neighbor is recorded. This information is stored in the neighbor data structure. When a neighbor sends a hello packet, it advertises a hold time. The hold time is the amount of time a router treats a neighbor as reachable and operational. In other words, if a hello packet is not heard from within the hold time, the hold time expires. When the hold time expires, DUAL is informed of the topology change, and must recalculate the new topology.










In the example in Figures - , DUAL must reconstruct the topology following the discovery of a broken link between router D and router B.
The new successor routes will be placed in the updated routing table.
This page concludes this lesson. The next lesson will discuss routing protocols. The first page will show students how to troubleshoot routing protocols.
3.3 Troubleshooting Routing Protocols
3.3.1 Routing protocol troubleshooting process
This page will explain the logical sequence of steps that should be used to troubleshoot all routing protocols.
All routing protocol troubleshooting should begin with a logical sequence, or process flow. This process flow is not a rigid outline for troubleshooting an internetwork. However, it is a foundation from which a network administrator can build a problem-solving process to suit a particular environment.




1. When analyzing a network failure, make a clear problem statement.
2. Gather the facts needed to help isolate possible causes.
3. Consider possible problems based on the facts that have been gathered.
4. Create an action plan based on the remaining potential problems.
5. Implement the action plan, performing each step carefully while testing to see whether the symptom disappears.
6. Analyze the results to determine whether the problem has been resolved. If it has, then the process is complete.
7. If the problem has not been resolved, create an action plan based on the next most likely problem in the list. Return to Step 4, change one variable at a time, and repeat the process until the problem is solved.
8. Once the actual cause of the problem is identified, try to solve it.


Cisco routers provide numerous integrated commands to assist in monitoring and troubleshooting an internetwork:
• show commands help monitor installation behavior and normal network behavior, as well as isolate problem areas
• debug commands assist in the isolation of protocol and configuration problems
• TCP/IP network tools such as ping, traceroute, and telnet
Cisco IOS show commands are among the most important tools for understanding the status of a router, detecting neighboring routers, monitoring the network in general, and isolating problems in the network.


EXEC debug commands can provide a wealth of information about interface traffic, internal error messages, protocol-specific diagnostic packets, and other useful troubleshooting data. Use debug commands to isolate problems, not to monitor normal network operation. Only use debug commands to look for specific types of traffic or problems. Before using the debug command, narrow the problems to a likely subset of causes. Use the show debugging command to view which debugging features are enabled.
The next page will describe how to troubleshoot RIP.
3.3.2 Troubleshooting RIP configuration
This page will discuss VLSM as the most common problem that occurs in RIP networks. VLSM prevents the advertisement of RIP routes.
The most common problem found in Routing Information Protocol (RIP) that prevents RIP routes from being advertised is the variable-length subnet mask (VLSM). This is because RIP Version 1 does not support VLSM. If the RIP routes are not being advertised, check the following:
• Layer 1 or Layer 2 connectivity issues exist.
• VLSM subnetting is configured. VLSM subnetting cannot be used with RIP v1.
• Mismatched RIP v1 and RIP v2 routing configurations exist.
• Network statements are missing or incorrectly assigned.
• The outgoing interface is down.
• The advertised network interface is down.
The show ip protocols command provides information about the parameters and current state of the active routing protocol process. RIP sends updates to the interfaces in the specified networks. If interface FastEthernet 0/1 was configured but the network was not added to RIP routing, no updates would be sent out or received from the interface.
Use the debug ip rip EXEC command to display information on RIP routing transactions. The no debug ip rip, no debug all, or undebug all commands will turn off all debugging.
Figure shows that the router being debugged has received an update from another router at source address 192.168.3.1. That router sent information about two destinations in the routing table update. The router being debugged also sent updates. Both routers broadcasted address 255.255.255.255 as the destination. The number in parentheses is the source address encapsulated into the IP header.






An entry most likely caused by a malformed packet from the transmitter is shown in the following output:
RIP: bad version 128 from 160.89.80.43.
The next page will discuss IGRP.



3.3.3 Troubleshooting IGRP configuration
This page will teach students how to troubleshoot IGRP. IGRP is an advanced distance vector routing protocol that was developed by Cisco in the 1980s. IGRP has several features that differentiate it from other distance vector routing protocols such as RIP.


Use the router igrpautonomous-system command to enable the IGRP routing process:
R1(config)#router igrp 100
Use the router configuration networknetwork-number command to enable interfaces to participate in the IGRP update process:
R1(config-router)#network 172.30.0.0
R1(config-router)#network 192.168.3.0








Verify IGRP configuration with the show running-configuration and show ip protocols commands:
R1#show ip protocols
Verify IGRP operation with the show ip route command:
R1#show ip route


If IGRP does not appear to be working correctly, check the following:
• Layer 1 or Layer 2 connectivity issues exist.
• Autonomous system numbers on IGRP routers are mismatched.
• Network statements are missing or incorrectly assigned.
• The outgoing interface is down.
• The advertised network interface is down.
To view IGRP debugging information, use the following commands:
• debug ip igrp transactions [host ip address] to view IGRP transaction information
• debug ip igrp events [host ip address] to view routing update information
To turn off debugging, use the no debug ip igrp command.
If a network becomes inaccessible, routers running IGRP send triggered updates to neighbors to inform them. A neighbor router will then respond with poison reverse updates and keep the suspect network in a holddown state for 280 seconds.
The next page will teach students how to troubleshoot EIGRP.
3.3.4 Troubleshooting EIGRP configuration
This page will provide some commands that are used to troubleshoot EIGRP.
Normal EIGRP operation is stable, efficient in bandwidth utilization, and relatively simple to monitor and troubleshoot.
Use the router eigrpautonomous-system command to enable the EIGRP routing process:
R1(config)#router eigrp 100
To exchange routing updates, each router in the EIGRP network must be configured with the same autonomous system number.
Use the router configuration networknetwork-number command to enable interfaces to participate in the EIGRP update process:
R1(config-router)#network 172.30.0.0
R1(config-router)#network 192.168.3.0
Verify EIGRP configuration with the show running-configuration and show ip protocols commands:
R1#show ip protocols


Some possible reasons why EIGRP may not be working correctly are:
• Layer 1 or Layer 2 connectivity issues exist.
• Autonomous system numbers on EIGRP routers are mismatched.
• The link may be congested or down.
• The outgoing interface is down.
• The advertised network interface is down.
• Auto-summarization is enabled on routers with discontiguous subnets.
• Use no auto-summary to disable automatic network summarization.
One of the most common reasons for a missing neighbor is a failure on the actual link. Another possible cause of missing neighbors is an expired holddown timer. Since hellos are sent every 5 seconds on most networks, the hold-time value in a show ip eigrp neighbors command output should normally be a value between 10 and 15.


To effectively monitor and troubleshoot an EIGRP network, use the commands described in Figures - .











The next page will discuss OSPF.
3.3.5 Troubleshooting OSPF configuration
This page will show students how to troubleshoot OSPF. OSPF is a link-state protocol.
Open Shortest Path First (OSPF) is a link-state protocol. A link is an interface on a router. The state of the link is a description of that interface and of its relationship to its neighboring routers. For example, a description of the interface would include the IP address, the mask, the type of network to which it is connected, the routers connected to that network, and so on. This information forms a link-state database.
The majority of problems encountered with OSPF relate to the formation of adjacencies and the synchronization of the link-state databases. The show ip ospf neighbor command is useful for troubleshooting adjacency formation. OSPF configuration commands are shown in Figure .


Use the debug ip ospf events Privileged EXEC command to display the following information about OSPF-related events:
• Adjacencies
• Flooding information
• Designated router selection
• Shortest path first (SPF) calculation
If a router configured for OSPF routing is not seeing an OSPF neighbor on an attached network, perform the following tasks:
• Verify that both routers have been configured with the same IP mask, OSPF hello interval, and OSPF dead interval.
• Verify that both neighbors are part of the same area.
To display information about each Open Shortest Path First (OSPF) packet received, use the debug ip ospf packet Privileged EXEC command. The no form of this command disables debugging output.
The debug ip ospf packet command produces one set of information for each packet received. The output varies slightly, depending on which authentication is used.
This page concludes this lesson. The next page will summarize the main points from this module.
Summary
This page summarizes the topics discussed in this module.


Although IGRP and EIGRP are compatible with each other, there are some differences. EIGRP offers multiprotocol support, but IGRP does not. EIGRP and IGRP use different metric calculations. IGRP has a maximum hop count of 255. EIGRP has a maximum hop count limit of 224.
EIGRP routers keep route and topology information readily available in RAM. Like OSPF, EIGRP saves this information in three tables. The neighbor table lists adjacent routers, the topology table which is made up of all the EIGRP routing tables in the autonomous system, and the routing table which holds the best routes to a destination. DUAL (the EIGRP distance vector algorithm) takes the information supplied in the neighbor table and the topology table and calculates the lowest cost routes to each destination. The preferred primary route is called the successor route and the backup route is called the feasible successor (FS).
EIGRP is an advanced distance vector routing protocol and acts as a link-state protocol when updating neighbors and maintaining routing information. Advantages include rapid convergence, efficient use of bandwidth, support for VLSM and CIDR, support for multiple network layers, and independence from routed protocols.
The DUAL algorithm results in the fast convergence of EIGRP. Each router has constructed a topology table that contains information about how to route to specific destinations. Each topology table identifies the routing protocol or EIGRP, the lowest cost of the route, which is called Feasible Distance (FD), and the cost of the route as advertised by the neighboring router called Reported Distance (RD).
EIGRP configuration commands vary depending on which protocol is used. Some examples of these protocols are IP, IPX, and AppleTalk. The network command configures only connected networks. EIGRP automatically summarizes routes at the classful boundary. If there are discontiguous subnetworks, auto-summarization must be disabled for routing to work properly. Verifying EIGRP operation is performed by the use of various show commands.
The most important table in EIGRP is the neighbor table that lists adjacent routers. Hello packets are used to establish adjacencies with neighboring routers. By default, hellos are sent every five seconds. Neighbor tables contain fields for the neighbor address, hold time, smooth round-trip timer (SRTT), queue count (Q Cnt), and a sequence number (Seq NO).
If a link goes down, DUAL looks for an alternative route path, or feasible successor, in the topology table. If a feasible successor is not found, the route is flagged as active, or unusable at present. Query packets are sent to neighboring routers requesting topology information. DUAL uses this information to recalculate successor and feasible successor routes to the destination.
The eight steps of the troubleshooting process should be followed when determining the cause of routing protocol problems. Variable-length subnet mask (VLSM) is the most common problem found in Routing Information Protocol (RIP) that prevents RIP routes from being advertised. The show ip protocols command provides information about the parameters and current state of the active routing protocol process. For IGRP, use the router igrp autonomous-system command to enable the IGRP routing process for troubleshooting. For EIGRP, use the router eigrp autonomous-system command to enable the EIGRP routing process. The show ip ospf neighbor command is useful for troubleshooting adjacency formation for OSPF since the majority of problems relate to the formation of adjacencies and the synchronization of the link-state database.