In this sample chapter from CCNP Enterprise Advanced Routing ENARSI 300-410 Official Cert Guide, 2nd Edition, you will learn the underlying mechanics of the EIGRP routing protocol and the path metric calculations, and how to configure EIGRP on a router for IPv4 and IPv6. This book covers topics from the Cisco Enterprise Advanced Routing v1.1 (ENARSI 300-410) exam.
This chapter covers the following topics:
EIGRP Fundamentals: This section explains how EIGRP establishes a neighborship with other routers and how routes are exchanged with other routers.
EIGRP Configuration Modes: This section defines the two methods of configuring EIGRP with a baseline configuration.
Path Metric Calculation: This section explains how EIGRP calculates the path metric to identify the best and alternate loop-free paths.
Enhanced Interior Gateway Routing Protocol (EIGRP) is an enhanced distance vector routing protocol commonly found in enterprise networks. EIGRP is a derivative of Interior Gateway Routing Protocol (IGRP) but includes support for variable-length subnet masking (VLSM) and metrics capable of supporting higher-speed interfaces. Initially, EIGRP was a Cisco proprietary protocol, but it was released to the Internet Engineering Task Force (IETF) through RFC 7868, which was ratified in May 2016.
This chapter explains the underlying mechanics of the EIGRP routing protocol and the path metric calculations, and it demonstrates how to configure EIGRP on a router. This is the first of several chapters in the book that discuss EIGRP:
Chapter 2, “EIGRP”: This chapter describes the fundamental concepts of EIGRP.
Chapter 3, “Advanced EIGRP”: This chapter describes EIGRP’s failure detection mechanisms and techniques to optimize the operations of the routing protocol. It also includes topics such as route filtering and traffic manipulation.
Chapter 4, “Troubleshooting EIGRP for IPv4”: This chapter reviews common problems with the routing protocols and the methodology to troubleshoot EIGRP from an IPv4 perspective.
Chapter 5, “EIGRPv6”: This chapter demonstrates how IPv4 EIGRP concepts carry over to IPv6 and the methods used to troubleshoot common problems.
“Do I Know This Already?” Quiz
The “Do I Know This Already?” quiz allows you to assess whether you should read this entire chapter thoroughly or jump to the “Exam Preparation Tasks” section. If you are in doubt about your answers to these questions or your own assessment of your knowledge of the topics, read the entire chapter. Table 2-1 lists the major headings in this chapter and their corresponding “Do I Know This Already?” quiz questions. You can find the answers in Appendix A, “Answers to the ‘Do I Know This Already?’ Quiz Questions.”
Table 2-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping
Foundation Topics Section |
Questions |
|---|---|
EIGRP Fundamentals |
1–6 |
EIGRP Configuration Modes |
7–9 |
Path Metric Calculation |
10 |
EIGRP uses protocol number ____ for inter-router communication.
87
88
89
90
How many packet types does EIGRP use for inter-router communication?
Three
Four
Five
Six
Seven
Which of the following are not required to match in order to form an EIGRP adjacency?
Metric K values
Primary subnet
Hello and hold timers
Authentication parameters
What is an EIGRP successor?
The next-hop router for the path with the lowest path metric for a destination prefix
The path with the lowest metric for a destination prefix
The router selected to maintain the EIGRP adjacencies for a broadcast network
A route that satisfies the feasibility condition where the reported distance is less than the feasible distance
What attributes does the EIGRP topology table contain? (Choose all that apply.)
Destination network prefix
Hop count
Total path delay
Maximum path bandwidth
List of EIGRP neighbors
What destination addresses does EIGRP use when feasible? (Choose two.)
IP address 224.0.0.9
IP address 224.0.0.10
IP address 224.0.0.8
MAC address 01:00:5E:00:00:0A
MAC address 0C:15:C0:00:00:01
Which of the following techniques can be used to initialize the EIGRP process? (Choose two.)
Use the interface command ip eigrp as-number ipv4 unicast.
Use the global configuration command router eigrp as-number.
Use the global configuration command router eigrp process-name.
Use the interface command router eigrp as-number.
True or false: The EIGRP router ID (RID) must be configured for EIGRP to be able to establish neighborship.
True
False
True or false: When using MD5 authentication between EIGRP routers, the keychain sequence numbers used on the routers can be different, as long as the password is the same.
True
False
Which value can be modified on a router to manipulate the path taken by EIGRP but does not have an impact on other routing protocols, like OSPF?
Interface bandwidth
Interface MTU
Interface delay
Interface priority
EIGRP Fundamentals
EIGRP overcomes the deficiencies of other distance vector routing protocols, such as Routing Information Protocol (RIP), with features such as unequal-cost load balancing, support for networks 255 hops away, and rapid convergence features. EIGRP uses a diffusing update algorithm (DUAL) to identify network paths and provides for fast convergence using precalculated loop-free backup paths. Most distance vector routing protocols use hop count as the metric for routing decisions. However, a route-selection algorithm that uses only hop count for path selection does not take into account link speed and total delay. EIGRP adds logic to the route-selection algorithm to use factors other than hop count alone.
Autonomous Systems
A router can run multiple EIGRP processes. Each process operates under the context of an autonomous system, which represents a common routing domain. Routers within the same domain use the same metric calculation formula and exchange routes only with members of the same autonomous system (AS). Do not confuse an EIGRP autonomous system with a Border Gateway Protocol (BGP) autonomous system.
In Figure 2-1, EIGRP AS 100 consists of R1, R2, R3, and R4, and EIGRP AS 200 consists of R3, R5, and R6. Each EIGRP process correlates to a specific autonomous system and maintains an independent EIGRP topology table. R1 does not have knowledge of routes from AS 200 because it is different from its own autonomous system, AS 100. R3 is able to participate in both autonomous systems and, by default, does not transfer routes learned from one autonomous system into a different autonomous system.
Figure 2-1 EIGRP Autonomous Systems
EIGRP uses protocol-dependent modules (PDMs) to support multiple network protocols, such as IPv4, IPv6, AppleTalk, and IPX. EIGRP is written so that the PDM is responsible for the functions to handle the route selection criteria for each communication protocol. In theory, new PDMs can be written as new communication protocols are created. Current implementations of EIGRP support only IPv4 and IPv6.
EIGRP Terminology
This section explains some of the core concepts of EIGRP, along with the path selection process. Figure 2-2 is a reference topology for this section, showing R1 calculating the best path and alternative loop-free paths to the 10.4.4.0/24 network. A value in parentheses represents the link’s calculated metric for a segment based on bandwidth and delay.
Figure 2-2 EIGRP Reference Topology
Table 2-2 defines important terms related to EIGRP and correlates them to Figure 2-2.
Table 2-2 EIGRP Terminology
Term |
Definition |
|---|---|
Successor route |
The route with the lowest path metric to reach a destination. The successor route for R1 to reach 10.4.4.0/24 on R4 is R1→R3→R4. |
Successor |
The first next-hop router for the successor route. R1’s successor for 10.4.4.0/24 is R3. |
Feasible distance (FD) |
The metric value for the lowest path metric to reach a destination. The feasible distance is calculated locally using the formula shown in the “Path Metric Calculation” section, later in this chapter. The FD calculated by R1 for the 10.4.4.0/24 destination network is 3328 (that is, 256 + 256 + 2816). |
Reported distance (RD) |
Distance reported by a router to reach a destination. The reported distance value is the feasible distance for the advertising router. R3 advertises the 10.4.4.0/24 destination network to R1 and R2 with an RD of 3072. R4 advertises the 10.4.4.0/24 destination network to R1, R2, and R3 with an RD of 2816. |
Feasibility condition |
For a route to be considered a backup route, the RD received for that route must be less than the FD calculated locally. This logic guarantees a loop-free path. |
Feasible successor |
A route that satisfies the feasibility condition is maintained as a backup route. The feasibility condition ensures that the backup route is loop free. The route R1→R4 is the feasible successor because the RD of 2816 is lower than the FD of 3328 for the R1→R3→R4 path. |
Topology Table
EIGRP contains a topology table, which makes it different from a true distance vector routing protocol. EIGRP’s topology table is a vital component of DUAL and contains information to identify loop-free backup routes. The topology table contains all the network prefixes advertised within an EIGRP autonomous system. Each entry in the table contains the following:
Network prefix
EIGRP neighbors that have advertised that prefix
Metrics from each neighbor (reported distance and hop count)
Values used for calculating the metric (load, reliability, total delay, and minimum bandwidth)
The command show ip eigrp topology [all-links] provides the topology table. By default, only the successor and feasible successor routes are displayed, but the optional all-links keyword shows the paths that did not pass the feasibility condition.
Figure 2-3 shows the topology table for R1 from Figure 2-2. This section focuses on the 10.4.4.0/24 network when explaining the topology table.
Figure 2-3 EIGRP Topology Output
Examine the 10.4.4.0/24 prefix and notice that R1 calculates an FD of 3328 for the successor route. The successor (upstream router) advertises the successor route with an RD of 3072. The second path entry has a metric of 5376 and has an RD of 2816. Because 2816 is less than 3328, the second entry passes the feasibility condition, which means the second entry is classified as the feasible successor for the 10.4.4.0/24 prefix.
The 10.4.4.0/24 route is passive (P), which means the topology is stable. During a topology change, routes go into an active (A) state when computing a new path.
EIGRP Neighbors
Unlike a number of routing protocols—such as Routing Information Protocol (RIP), Open Shortest Path First (OSPF), and Intermediate System-to-Intermediate System (IS-IS)—EIGRP does not rely on periodic advertisement of all the network prefixes in an autonomous system. EIGRP neighbors exchange the entire routing table when forming an adjacency, and they advertise incremental updates only as topology changes occur within a network. The neighbor adjacency table is vital for tracking neighbor status and the updates sent to each neighbor.
Inter-Router Communication
EIGRP uses five different packet types to communicate with other routers, as shown in Table 2-3. EIGRP uses IP protocol number (88) and uses multicast packets where possible; it uses unicast packets when necessary. Communication between routers is done with multicast using the group address 224.0.0.10 or the MAC address 01:00:5e:00:00:0a when possible.
Table 2-3 EIGRP Packet Types
Opcode Value |
Packet Type |
Function |
|---|---|---|
1 |
Update |
Used to transmit routing and reachability information with other EIGRP neighbors |
2 |
Request |
Used to get specific information from one or more neighbors |
3 |
Query |
Sent out to search for another path during convergence |
4 |
Reply |
Sent in response to a query packet |
5 |
Hello |
Used for discovery of EIGRP neighbors and for detecting when a neighbor is no longer available |
EIGRP uses Reliable Transport Protocol (RTP) to ensure that packets are delivered in order and to ensure that routers receive specific packets. A sequence number is included in each EIGRP packet. The sequence value zero does not require a response from the receiving EIGRP router; all other values require an ACK packet that includes the original sequence number.
Ensuring that packets are received makes the transport method reliable. All update, query, and reply packets are deemed reliable, and hello and ACK packets do not require acknowledgment and could be unreliable.
If the originating router does not receive an ACK packet from the neighbor before the retransmit timeout expires, it notifies the non-acknowledging router to stop processing its multicast packets. The originating router sends all traffic by unicast until the neighbor is fully synchronized. Upon complete synchronization, the originating router notifies the destination router to start processing multicast packets again. All unicast packets require acknowledgment. EIGRP retries up to 16 times for each packet that requires confirmation, and it resets the neighbor relationship when the neighbor reaches the retry limit of 16.
Forming EIGRP Neighbors
Unlike other distance vector routing protocols, EIGRP requires a neighbor relationship to form before routes are processed and added to the Routing Information Base (RIB). Upon hearing an EIGRP hello packet, a router attempts to become the neighbor of the other router. The following parameters must match for the two routers to become neighbors:
Metric formula K values
Primary subnet matches
Autonomous system number (ASN) matches
Authentication parameters
Figure 2-4 shows the process EIGRP uses for forming neighbor adjacencies.
Figure 2-4 EIGRP Neighbor Adjacency Process from R1’s Perspective
EIGRP Configuration Modes
This section describes the two methods of EIGRP configuration: classic mode and named mode.
Classic Configuration Mode
With classic EIGRP configuration mode, most of the configuration takes place in the EIGRP process, but some settings are configured under the interface configuration submode. This can add complexity for deployment and troubleshooting as users must scroll back and forth between the EIGRP process and individual network interfaces. Some of the settings that are set individually are hello advertisement interval, split-horizon, authentication, and summary route advertisements.
Classic configuration requires the initialization of the routing process with the global configuration command router eigrp as-number to identify the ASN and initialize the EIGRP process. The second step is to identify the network interfaces with the command network ip-address [wildcard-mask]. The network statement is explained in the following sections.
EIGRP Named Mode
EIGRP named mode configuration was released to overcome some of the difficulties network engineers have with classic EIGRP autonomous system configuration, including scattered configurations and unclear scope of commands.
EIGRP named configuration provides the following benefits:
All the EIGRP configuration occurs in one location.
It supports current EIGRP features and future developments.
It supports multiple address families (including virtual routing and forwarding [VRF] instances). EIGRP named configuration is also known as multi-address family configuration mode.
Commands are clear in terms of the scope of their configuration.
EIGRP named mode provides a hierarchical configuration and stores settings in three subsections:
Address Family: This submode contains settings that are relevant to the global EIGRP AS operations, such as selection of network interfaces, EIGRP K values, logging settings, and stub settings.
Interface: This submode contains settings that are relevant to the interface, such as hello advertisement interval, split-horizon, authentication, and summary route advertisements. In actuality, there are two methods of the EIGRP interface section’s configuration. Commands can be assigned to a specific interface or to a default interface, in which case those settings are placed on all EIGRP-enabled interfaces. If there is a conflict between the default interface and a specific interface, the specific interface takes priority over the default interface.
Topology: This submode contains settings regarding the EIGRP topology database and how routes are presented to the router’s RIB. This section also contains route redistribution and administrative distance settings.
EIGRP named configuration makes it possible to run multiple instances under the same EIGRP process. The process for enabling EIGRP interfaces on a specific instance is as follows:
Step 1. Initialize the EIGRP process by using the command router eigrp process-name. (If a number is used for process-name, the number does not correlate to the autonomous system number.)
Step 2. Initialize the EIGRP instance for the appropriate address family with the command address-family {IPv4 | IPv6} {unicast | vrf vrf-name} autonomous-system as-number.
Step 3. Enable EIGRP on interfaces by using the command network network wildcard-mask.
EIGRP Network Statement
Both configuration modes use a network statement to identify the interfaces that EIGRP will use. The network statement uses a wildcard mask, which allows the configuration to be as specific or ambiguous as necessary.
The syntax for the network statement, which exists under the EIGRP process, is network ip-address [wildcard-mask]. The optional wildcard-mask can be omitted to enable interfaces that fall within the classful boundaries for that network statement.
A common misconception is that the network statement adds prefixes to the EIGRP topology table. In reality, the network statement identifies the interface to enable EIGRP on, and it adds the interface’s connected network to the EIGRP topology table. EIGRP then advertises the topology table to other routers in the EIGRP autonomous system.
EIGRP does not add an interface’s secondary connected network to the topology table. For secondary connected networks to be installed in the EIGRP routing table, they must be redistributed into the EIGRP process. Chapter 16, “Route Redistribution,” provides additional coverage of route redistribution.
To help illustrate the concept of the wildcard mask, Table 2-4 provides a set of IP addresses and interfaces for a router. The following examples provide configurations to match specific scenarios.
Table 2-4 Table of Sample Interface and IP Addresses
Router Interface |
IP Address |
|---|---|
Gigabit Ethernet 0/0 |
10.0.0.10/24 |
Gigabit Ethernet 0/1 |
10.0.10.10/24 |
Gigabit Ethernet 0/2 |
192.0.0.10/24 |
Gigabit Ethernet 0/3 |
192.10.0.10/24 |
The configuration in Example 2-1 enables EIGRP only on interfaces that explicitly match the IP addresses in Table 2-4.
Example 2-1 EIGRP Configuration with Explicit IP Addresses
Example 2-2 shows the EIGRP configuration using network statements that match the subnets used in Table 2-4. Setting the last octet of the IP address to 0 and changing the wildcard mask to 255 cause the network statements to match all IP addresses within the /24 network range.
Example 2-2 EIGRP Configuration with an Explicit Subnet
The following snippet shows the EIGRP configuration using network statements for interfaces that are within the 10.0.0.0/8 or 192.0.0.0/8 network ranges:
router eigrp 1
network 10.0.0.0 0.255.255.255
network 192.0.0.0 0.255.255.255
The following snippet shows the configuration to enable all interfaces with EIGRP:
router eigrp 1
network 0.0.0.0 255.255.255.255
Sample Topology and Configuration
Figure 2-5 shows a sample topology for demonstrating EIGRP configuration in classic mode for R1 and named mode for R2.
Figure 2-5 EIGRP Sample Topology
R1 and R2 enable EIGRP on all of their interfaces. R1 configures EIGRP using multiple specific network interface addresses, and R2 enables EIGRP on all network interfaces with one command. Example 2-3 provides the configuration that is applied to R1 and R2.
Example 2-3 Sample EIGRP Configuration
As mentioned earlier, EIGRP named mode has three configuration submodes. The configuration in Example 2-3 uses only the EIGRP address-family submode section, which uses the network statement. The EIGRP topology base submode is created automatically with the command topology base and exited with the command exit-af-topology. Settings for the topology submode are listed between those two commands.
Example 2-4 demonstrates the slight difference in how the configuration is stored on the router between EIGRP classic and named mode configurations.
Example 2-4 Comparison of EIGRP Configuration Mode Structures
Confirming Interfaces
Upon configuring EIGRP, it is a good practice to verify that only the intended interfaces are running EIGRP. The command show ip eigrp interfaces [{interface-id [detail] | detail}] shows active EIGRP interfaces. Appending the optional detail keyword provides additional information, such as authentication, EIGRP timers, split horizon, and various packet counts.
Example 2-5 demonstrates R1’s non-detailed EIGRP interface and R2’s detailed information for the Gi0/1 interface.
Example 2-5 Verifying EIGRP Interfaces
Table 2-5 provides a brief explanation to the key fields shown with the EIGRP interfaces.
Table 2-5 EIGRP Interface Fields
Field |
Description |
|---|---|
Interface |
Interfaces running EIGRP. |
Peers |
Number of peers detected on the interface. |
Xmt Queue Un/Reliable |
Number of unreliable/reliable packets remaining in the transmit queue. The value zero is an indication of a stable network. |
Mean SRTT |
Average time for a packet to be sent to a neighbor and a reply from that neighbor to be received, in milliseconds. |
Multicast Flow Timer |
Maximum time (seconds) that the router sent multicast packets. |
Pending Routes |
Number of routes in the transmit queue that need to be sent. |
Verifying EIGRP Neighbor Adjacencies
Each EIGRP process maintains a table of neighbors to ensure that they are alive and processing updates properly. If EIGRP didn’t keep track of neighbor states, an autonomous system could contain incorrect data and could potentially route traffic improperly. EIGRP must form a neighbor relationship before a router advertises update packets containing network prefixes.
The command show ip eigrp neighbors [interface-id] displays the EIGRP neighbors for a router. Example 2-6 shows the EIGRP neighbor information obtained using this command.
Example 2-6 EIGRP Neighbor Confirmation
Table 2-6 provides a brief explanation of the key fields shown in Example 2-6.
Table 2-6 EIGRP Neighbor Columns
Field |
Description |
|---|---|
Address |
IP address of the EIGRP neighbor |
Interface |
Interface the neighbor was detected on |
Holdtime |
Time left to receive a packet from this neighbor to ensure that it is still alive |
SRTT |
Time for a packet to be sent to a neighbor and a reply to be received from that neighbor, in milliseconds |
RTO |
Timeout for retransmission (waiting for ACK) |
Q Cnt |
Number of packets (update/query/reply) in queue for sending |
Seq Num |
Sequence number that was last received from this router |
Displaying Installed EIGRP Routes
You can see EIGRP routes that are installed into the RIB by using the command show ip route eigrp. EIGRP routes that originate within the autonomous system have an administrative distance (AD) of 90 and are indicated in the routing table with a D. Routes that originate from outside the autonomous system are external EIGRP routes. External EIGRP routes have an AD of 170 and are indicated in the routing table with D EX. Placing external EIGRP routes into the RIB with a higher AD acts as a loop-prevention mechanism.
Example 2-7 displays the EIGRP routes from the sample topology in Figure 2-5. The metric for the selected route is the second number in brackets.
Example 2-7 EIGRP Routes for R1 and R2
Router ID
The router ID (RID) is a 32-bit number that uniquely identifies an EIGRP router and is used as a loop-prevention mechanism. The RID can be set dynamically, which is the default, or manually.
The algorithm for dynamically choosing the EIGRP RID uses the highest IPv4 address of any up loopback interfaces. If there are not any up loopback interfaces, the highest IPv4 address of any active up physical interfaces becomes the RID when the EIGRP process initializes.
IPv4 addresses are commonly used for the RID because they are 32 bits and are maintained in dotted-decimal format. You use the command eigrp router-id router-id to set the RID, as demonstrated in Example 2-8, for both classic and named mode configurations.
Example 2-8 Static Configuration of EIGRP Router ID
Passive Interfaces
Some network topologies must advertise a network segment into EIGRP but need to prevent neighbors from forming adjacencies with other routers on that segment. This might be the case, for example, when advertising access layer networks in a campus topology. In such a scenario, you need to put the EIGRP interface in a passive state. Passive EIGRP interfaces do not send out or process EIGRP hellos, which prevents EIGRP from forming adjacencies on those interfaces.
To configure an EIGRP interface as passive, you use the command passive-interface interface-id under the EIGRP process for classic configuration. Another option is to configure all interfaces as passive by default with the command passive-interface default and then use the command no passive-interface interface-id to allow an interface to process EIGRP packets, preempting the global passive interface default configuration.
Example 2-9 demonstrates making R1’s Gi0/2 interface passive and also the alternative option of making all interfaces passive but setting Gi0/1 as non-passive.
Example 2-9 Passive EIGRP Interfaces for Classic Configuration
For a named mode configuration, you place the passive-interface state on af-interface default for all EIGRP interfaces or on a specific interface with the af-interface interface-id section. Example 2-10 shows how to set the Gi0/2 interface as passive while allowing the Gi0/1 interface to be active, using both configuration strategies.
Example 2-10 Passive EIGRP Interfaces for Named Mode Configuration
Example 2-11 shows what the named mode configuration looks like with some settings (that is, passive-interface and no passive-interface) placed under the af-interface default and af-interface interface-id settings.
Example 2-11 Viewing the EIGRP Interface Settings with Named Mode
A passive interface does not appear in the output of the command show ip eigrp interfaces even though it was enabled. Connected networks for passive interfaces are still added to the EIGRP topology table so that they are advertised to neighbors.
Example 2-12 shows that the Gi0/2 interface on R1 no longer appears; compare this to Example 2-5, where it does exist.
Example 2-12 show ip eigrp interfaces Output
To accelerate troubleshooting of passive interfaces, as well as other settings, use the command show ip protocols, which provides a lot of valuable information about all the routing protocols. With EIGRP, it displays the EIGRP process identifier, the ASN, K values that are used for path calculation, RID, neighbors, AD settings, and all the passive interfaces.
Example 2-13 provides sample output for both classic and named mode instances on R1 and R2.
Example 2-13 show ip protocols Output
Authentication
Authentication is a mechanism for ensuring that only authorized routers are eligible to become EIGRP neighbors. It is possible for someone to add a router to a network and introduce invalid routes accidentally or maliciously. Authentication prevents such scenarios from happening. A precomputed password hash is included with all EIGRP packets, and the receiving router decrypts the hash. If the passwords do not match for a packet, the router discards the packet.
EIGRP encrypts the password by using Message Digest 5 (MD5) authentication and the keychain function. The hash consists of the key number and a password. EIGRP authentication encrypts just the password rather than the entire EIGRP packet.
To configure EIGRP authentication, you need to create a keychain and then enable EIGRP authentication on the interface. The following sections explain the steps.
Keychain Configuration
Keychain creation is accomplished with the following steps:
Step 1. Create the keychain by using the command key chain key-chain-name.
Step 2. Identify the key sequence by using the command key key-number, where key-number can be anything from 0 to 2147483647.
Step 3. Specify the preshared password by using the command key-string password.
Enabling Authentication on the Interface
When using classic configuration, authentication must be enabled on the interface under the interface configuration submode. The following commands are used in the interface configuration submode:
ip authentication key-chain eigrp as-number key-chain-name ip authentication mode eigrp as-number md5
The named mode configuration places the configurations under the EIGRP interface submode, under af-interface default or af-interface interface-id. Named mode configuration supports MD5 or Hashed Message Authentication Code-Secure Hash Algorithm-256 (HMAC-SHA-256) authentication. MD5 authentication involves the following commands:
authentication key-chain eigrp key-chain-name authentication mode md5
HMAC-SHA-256 authentication involves the command authentication mode hmac-sha-256 password.
Example 2-14 demonstrates MD5 configuration on R1 with classic EIGRP configuration and on R2 with named mode configuration. Remember that the hash is computed using the key sequence number and key string, which must match on the two nodes.
Example 2-14 Configuring EIGRP Authentication
The command show key chain provides verification of the keychain. Example 2-15 shows that each key sequence provides the lifetime and password.
Example 2-15 Verifying Keychain Settings
The EIGRP interface detail view provides verification of EIGRP authentication on a specific interface. Example 2-16 shows detailed EIGRP interface output.
Example 2-16 Verifying EIGRP Authentication
Path Metric Calculation
Metric calculation is a critical component for any routing protocol. EIGRP uses multiple factors to calculate the metric for a path. Metric calculation uses bandwidth and delay by default but can include interface load and reliability, too. Figure 2-6 shows the EIGRP classic metric formula.
Figure 2-6 EIGRP Metric Formula
EIGRP uses K values to define which factors the formula uses and the impact associated with a factor when calculating the metric. A common misconception is that the K values directly apply to bandwidth, load, delay, or reliability; this is not accurate. For example, K1 and K2 both reference bandwidth (BW).
BW represents the slowest link in the path, scaled to a 10 Gbps link (107). Link speed correlates to the configured interface bandwidth on an interface and is measured in kilobits per second (Kbps). Delay is the total measure of delay in the path, measured in tens of microseconds (μs).
Taking these definitions into consideration, look at the formula for classic EIGRP metrics in Figure 2-7.
Figure 2-7 EIGRP Classic Metric Formula with Definitions
By default, K1 and K3 each has a value of 1, and K2, K4, and K5 are all set to 0. Figure 2-8 places default K values into the formula and shows a streamlined version of the formula.
The EIGRP update packet includes path attributes associated with each prefix. The EIGRP path attributes can include hop count, cumulative delay, minimum bandwidth link speed, and RD. The attributes are updated each hop along the way, allowing each router to independently identify the shortest path.
Figure 2-8 EIGRP Classic Metric Formula with Default K Values
Figure 2-9 shows the information in the EIGRP update packets for the 10.1.1.0/24 network propagating through the autonomous system. Notice that the hop count increments, minimum bandwidth decreases, total delay increases, and the RD changes with each EIGRP update.
Figure 2-9 EIGRP Attribute Propagation
Table 2-7 shows for some common network types the link speed, delay, and EIGRP metric, based on the streamlined formula in Figure 2-8.
Table 2-7 Default EIGRP Interface Metrics for Classic Metrics
Interface Type |
Link Speed (Kbps) |
Delay |
Metric |
|---|---|---|---|
Serial |
64 |
20,000 μs |
40,512,000 |
T1 |
1544 |
20,000 μs |
2,170,031 |
Ethernet |
10,000 |
1000 μs |
281,600 |
FastEthernet |
100,000 |
100 μs |
28,160 |
GigabitEthernet |
1,000,000 |
10 μs |
2816 |
TenGigabitEthernet |
10,000,000 |
10 μs |
512 |
Using the topology from Figure 2-2, the metrics from R1 and R2 for the 10.4.4.0/24 network are calculated using the formula in Figure 2-10. The link speed for both routers is 1 Gbps, and the total delay is 30 μs (10 μs for the 10.4.4.0/24 link, 10 μs for the 10.34.1.0/24 link, and 10 μs for the 10.13.1.0/24 link).
Figure 2-10 Calculating EIGRP Metrics with Default K Values
If you are unsure of the EIGRP metrics, you can query the parameters for the formula directly from EIGRP’s topology table by using the command show ip eigrp topology network/prefix-length.
Example 2-17 shows R1’s topology table output for the 10.4.4.0/24 network. Notice that the output includes the successor route, any feasible successor paths, and the EIGRP state for the prefix. Each path contains the EIGRP attributes minimum bandwidth, total delay, interface reliability, load, and hop count.
Example 2-17 EIGRP Topology for a Specific Prefix
Wide Metrics
The original EIGRP specifications measured delay in 10-microsecond (μs) units and bandwidth in kilobits per second, which did not scale well with higher-speed interfaces. In Table 2-7, notice that the delay is the same for the GigabitEthernet and TenGigabitEthernet interfaces.
Example 2-18 provides some metric calculations for common LAN interface speeds. Notice that there is not a differentiation between an 11 Gbps interface and a 20 Gbps interface. The composite metric stays at 256, despite the different bandwidth rates.
Example 2-18 Metric Calculation for Common LAN Interface Speeds
EIGRP includes support for a second set of metrics, known as wide metrics, that addresses the issue of scalability with higher-capacity interfaces. Just as EIGRP scaled by 256 to accommodate IGRP, EIGRP wide metrics scale by 65,536 to accommodate higher-speed links. This provides support for interface speeds up to 655 Tbps (65,536 × 107) without any scalability issues.
Figure 2-11 shows the explicit EIGRP wide metrics formula. Notice that an additional K value (K6) is included that adds an extended attribute to measure jitter, energy, or other future attributes.
Figure 2-11 EIGRP Wide Metrics Formula
Latency is the total interface delay measured in picoseconds (1012) instead of in microseconds (106). Figure 2-12 shows an updated formula that takes into account the conversions in latency and scalability.
Figure 2-12 EIGRP Wide Metrics Formula with Definitions
The interface delay varies from router to router, depending on the following logic:
If the interface’s delay was specifically set, the value is converted to picoseconds. Interface delay is always configured in tens of microseconds and is multiplied by 107 for picosecond conversion.
If the interface’s bandwidth was specifically set, the interface delay is configured using the classic default delay, converted to picoseconds. The configured bandwidth is not considered when determining the interface delay. If delay was configured, this step is ignored.
If the interface supports speeds of 1 Gbps or less and does not contain bandwidth or delay configuration, the delay is the classic default delay, converted to picoseconds.
If the interface supports speeds over 1 Gbps and does not contain bandwidth or delay configuration, the interface delay is calculated by 1013/interface bandwidth.
The EIGRP classic metrics exist only with EIGRP classic configuration, and EIGRP wide metrics exist only in EIGRP named mode. The metric style used by a router is identified with the command show ip protocols. If a K6 metric is present, the router is using wide-style metrics.
Example 2-19 shows the commands to verify the operational mode of EIGRP on R1 and R2. It shows that R1 does not have a K6 metric and is using EIGRP classic metrics. R2 has a K6 metric and is using EIGRP wide metrics.
Example 2-19 Verifying EIGRP Metric Style
Metric Backward Compatibility
EIGRP wide metrics were designed with backward compatibility in mind. EIGRP wide metrics set K1 and K3 to a value of 1 and set K2, K4, K5, and K6 to 0, which allows backward compatibility because the K value metrics match with classic metrics. As long as K1 through K5 are the same and K6 is not set, the two metric styles allow adjacency between routers.
EIGRP is able to detect when peering with a router is using classic metrics, and it unscales the metric by using the formula in Figure 2-13.
Figure 2-13 Formula for Calculating Unscaled EIGRP Metrics
This conversion results in loss of clarity if routes pass through a mixture of classic metric and wide metric devices. An end result of this intended behavior is that paths learned from wide metric peers always look better than paths learned from classic peers. Using a mixture of classic metric and wide metric devices could lead to suboptimal routing, so it is best to keep all devices operating with the same metric style.
Interface Delay Settings
If you do not remember the delay values from Table 2-7, you can query the values dynamically by using the command show interface interface-id. The output displays the EIGRP interface delay, in microseconds, after the DLY field. Example 2-20 provides sample output of the command on R1 and R2. The output shows that both interfaces have a delay of 10 μs.
Example 2-20 Verifying EIGRP Interface Delay
EIGRP delay is set on an interface-by-interface basis, allowing for manipulation of traffic patterns flowing through a specific interface on a router. Delay is configured with the interface parameter command delay tens-of-microseconds under the interface.
Example 2-21 demonstrates the modification of the delay on R1 to 100, increasing the delay to 1000 μs on the link between R1 and R2. To ensure consistent routing, modify the delay on R2’s Gi0/1 interface as well. Afterward, you can verify the change.
Example 2-21 Configuring Interface Delay
Custom K Values
If the default metric calculations are insufficient, you can change them to modify the path metric formula. K values for the path metric formula are set with the command metric weights TOS K1 K2 K3 K4 K5 [K6] under the EIGRP process. TOS always has a value of 0, and K6 is used for named mode configurations.
To ensure consistent routing logic in an EIGRP autonomous system, the K values must match between EIGRP neighbors to form an adjacency and exchange routes. The K values are included as part of the EIGRP hello packet. The K values are displayed with the show ip protocols command, as demonstrated with the sample topology in Example 2-13. Notice that both routers are using the default K values, with R1 using classic metrics and R2 using wide metrics.
Load Balancing
EIGRP allows multiple successor routes (with the same metric) to be installed into the RIB. Installing multiple paths into the RIB for the same prefix is called equal-cost multipathing (ECMP). At the time of this writing, the default maximum ECMP setting is four routes. You change the default ECMP setting with the command maximum-paths maximum-paths under the EIGRP process in classic mode and under the topology base submode in named mode.
Example 2-22 shows the configuration for changing the maximum paths on R1 and R2 so that classic and named mode configurations are visible.
Example 2-22 Changing the EIGRP Maximum Paths
EIGRP supports unequal-cost load balancing, which allows installation of both successor routes and feasible successors into the EIGRP RIB. To use unequal-cost load balancing with EIGRP, change EIGRP’s variance multiplier. The EIGRP variance value is the feasible distance (FD) for a route multiplied by the EIGRP variance multiplier. Any feasible successor’s FD with a metric below the EIGRP variance value is installed into the RIB. EIGRP installs multiple routes where the FD for the routes is less than the EIGRP variance value up to the maximum number of ECMP routes, as discussed earlier.
Dividing the feasible successor metric by the successor route metric provides the variance multiplier. The variance multiplier is a whole number, and any remainders should always round up.
Using the topology shown in Figure 2-2 and output from the EIGRP topology table in Figure 2-3, the minimum EIGRP variance multiplier can be calculated so that the direct path from R1 to R4 can be installed into the RIB. The FD for the successor route is 3328, and the FD for the feasible successor is 5376. The formula provides a value of about 1.6 and is always rounded up to the nearest whole number to provide an EIGRP variance multiplier of 2. Figure 2-14 shows the calculation.
Figure 2-14 EIGRP Variance Multiplier Formula
The command variance multiplier configures the variance multiplier under the EIGRP process for classic configuration and under the topology base submode in named mode. Example 2-23 provides a sample configuration for each configuration mode.
Example 2-23 Configuring EIGRP Variance
Example 2-24 shows how to verify that both paths were installed into the RIB. Notice that the metrics for the paths are different. One path metric is 3328, and the other path metric is 5376. To see the traffic load-balancing ratios, you use the command show ip route network, as demonstrated in the second output. The load-balancing traffic share is highlighted.
Example 2-24 Verifying Unequal-Cost Load Balancing
References in This Chapter
Edgeworth, Brad, Foss, Aaron, and Garza Rios, Ramiro, IP Routing on Cisco IOS, IOS XE, and IOS XR, Cisco Press, 2014.
RFC 7868, Cisco’s Enhanced Interior Gateway Routing Protocol (EIGRP), D. Savage, J. Ng, S. Moore, D. Slice, P. Paluch, and R. White. http://tools.ietf.org/html/rfc7868, May 2016.
Cisco, Cisco IOS Software Configuration Guides, http://www.cisco.com.
Exam Preparation Tasks
As mentioned in the section “How to Use This Book” in the Introduction, you have a couple choices for exam preparation: the exercises here, Chapter 24, “Final Preparation,” and the exam simulation questions in the Pearson Test Prep software.
Review All Key Topics
Review the most important topics in this chapter, noted with the Key Topic icon in the outer margin of the page. Table 2-8 lists these key topics and the page number on which each is found.
Table 2-8 Key Topics
Key Topic Element |
Description |
Page Number |
|---|---|---|
Paragraph |
EIGRP terminology |
76 |
Paragraph |
Topology table |
76 |
Table 2-3 |
EIGRP packet types |
78 |
Paragraph |
Forming EIGRP neighbors |
79 |
Paragraph |
Classic configuration mode |
80 |
Paragraph |
EIGRP named mode |
80 |
Paragraph |
Passive interfaces |
88 |
Paragraph |
Authentication |
91 |
Paragraph |
Path metric calculation |
94 |
Paragraph |
EIGRP attribute propagation |
94 |
Figure 2-11 |
EIGRP wide metrics formula |
97 |
Paragraph |
Custom K values |
100 |
Paragraph |
Unequal-cost load balancing |
100 |
Define Key Terms
Define the following key terms from this chapter and check your answers in the glossary:
autonomous system (AS)
successor route
successor
feasible distance
reported distance
feasibility condition
feasible successor
topology table
classic EIGRP configuration mode
EIGRP named mode configuration
passive interface
K values
wide metrics
variance value
Use the Command Reference to Check Your Memory
The ENARSI 300-410 exam focuses on the practical, hands-on skills that networking professionals use. Therefore, you should be able to identify the commands needed to configure, verify, and troubleshoot the topics covered in this chapter.
This section includes the most important configuration and verification commands covered in this chapter. It might not be necessary to memorize the complete syntax of every command, but you should be able to remember the basic keywords that are needed.
To test your memory of the commands in Table 2-9, go to the companion website and download Appendix B, “Command Reference Exercises.” Fill in the missing commands in the tables based on each command description. You can check your work by downloading Appendix C, “Command Reference Exercise Answer Key,” from the companion website.
Table 2-9 Command Reference
Task |
Command Syntax |
|---|---|
Initialize EIGRP in a classic configuration. |
router eigrp as-number network network wildcard-mask |
Initialize EIGRP in a named mode configuration. |
router eigrp process-name address-family {ipv4 | ipv6} {unicast | vrf vrf-name} autonomous-system as-number network network wildcard-mask |
Define the EIGRP router ID. |
eigrp router-id router-id |
Configure an EIGRP-enabled interface to prevent neighbor adjacencies. |
Classic: (EIGRP process) passive-interface interface-id Named mode: af-interface {default | interface-id} passive-interface |
Configure a keychain for EIGRP MD5 authentication. |
key chain key-chain-name key key-number key-string password |
Configure MD5 authentication for an EIGRP interface. |
Classic: (EIGRP process) ip authentication key-chain eigrp as-number key-chain-name ip authentication mode eigrp as-number md5 Named mode: af-interface {default | interface-id} authentication key-chain eigrp key-chain-name authentication mode md5 |
Configure SHA authentication for EIGRP named mode interfaces. |
Named mode: af-interface {default | interface-id} authentication mode hmac-sha-256 password |
Modify the interface delay for an interface. |
delay tens-of-microseconds |
Modify the EIGRP K values. |
metric weights TOS K1 K2 K3 K4 K5 [K6] |
Modify the default number of EIGRP maximum paths that can be installed into the RIB. |
maximum-paths maximum-paths |
Modify the EIGRP variance multiplier for unequal-cost load balancing. |
variance multiplier |
Display the EIGRP-enabled interfaces. |
show ip eigrp interface [{interface-id [detail] | detail}] |
Display the EIGRP topology table. |
show ip eigrp topology [all-links] |
Display the configured EIGRP keychains and passwords. |
show key chain |
Display the IP routing protocol information configured on the router. |
show ip protocols |