The Enhanced Interior
Gateway Routing Protocol (EIGRP) is an enhanced version of IGRP developed by
Cisco. This module describes the concepts and tasks you need to implement basic
EIGRP configuration using Cisco IOS XR software. EIGRP uses distance vector
routing technology, which specifies that a router need not know all the router
and link relationships for the entire network. Each router advertises
destinations with a corresponding distance and upon receiving routes, adjusts
the distance and propagates the information to neighboring routes.
For more information
about EIGRP on the Cisco IOS XR software and complete descriptions of the EIGRP
commands listed in this module, see the
Commands chapter in the
Routing Command Reference for
Cisco NCS 6000 Series Routers. To
locate documentation for other commands that might appear while executing a
configuration task, search online in the
Cisco IOS XR software
master command index.
for Implementing EIGRP
feature was introduced.
Prerequisites for Implementing EIGRP
You must be in a user group associated with a task group that includes the proper task IDs. The command reference guides include
the task IDs required for each command. If you suspect user group assignment is preventing you from using a command, contact
your AAA administrator for assistance.
restrictions are employed when running EIGRP on this version of
Cisco IOS XR software:
allowed for EIGRP process name are
@ . # : - _
Management Protocol (SNMP) MIB is not supported.
routes are not automatically redistributed into EIGRP, because there are no
configuration (either through the
default-metric command or a route policy) is required
for redistribution of connected and static routes.
Auto summary is
disabled by default.
Stub leak maps are
Information About Implementing EIGRP
To implement EIGRP, you need to understand the following concepts:
EIGRP Functional Overview
Enhanced Interior Gateway Routing Protocol (EIGRP) is an interior gateway protocol suited for many different topologies and
media. EIGRP scales well and provides extremely quick convergence times with minimal network traffic.
EIGRP has very low usage of network resources during normal operation. Only hello packets are transmitted on a stable network.
When a change in topology occurs, only the routing table changes are propagated and not the entire routing table. Propagation
reduces the amount of load the routing protocol itself places on the network. EIGRP also provides rapid convergence times
for changes in the network topology.
The distance information in EIGRP is represented as a composite of available bandwidth, delay, load utilization, and link
reliability with improved convergence properties and operating efficiency. The fine-tuning of link characteristics achieves
The convergence technology that EIGRP uses is based on research conducted at SRI International and employs an algorithm referred
to as the Diffusing Update Algorithm (DUAL). This algorithm guarantees loop-free operation at every instant throughout a route
computation and allows all devices involved in a topology change to synchronize at the same time. Routers that are not affected
by topology changes are not involved in recomputations. The convergence time with DUAL rivals that of any other existing routing
EIGRP offers the
convergence—The DUAL algorithm allows routing information to converge as
quickly as any currently available routing protocol.
updates—EIGRP sends incremental updates when the state of a destination
changes, instead of sending the entire contents of the routing table. This
feature minimizes the bandwidth required for EIGRP packets.
mechanism—This is a simple hello mechanism used to learn about neighboring
routers. It is protocol independent.
subnet masks (VLSMs).
scales to large networks.
Support for IPv4
and IPv6 address families.
(PE)-Customer Edge (CE) protocol support with Site of Origin (SoO) and Border
Gateway Protocol (BGP) cost community support.
EIGRP has the
following four basic components:
or neighbor recovery
DUAL finite state
Neighbor discovery or
neighbor recovery is the process that routers use to dynamically learn of other
routers on their directly attached networks. Routers must also discover when
their neighbors become unreachable or inoperative. Neighbor discovery or
neighbor recovery is achieved with low overhead by periodically sending small
hello packets. As long as hello packets are received, the
Cisco IOS XR software can determine that a neighbor is alive
and functioning. After this status is determined, the neighboring routers can
exchange routing information.
The reliable transport
protocol is responsible for guaranteed, ordered delivery of EIGRP packets to
Some EIGRP packets must be sent reliably and others need not be. For
efficiency, reliability is provided only when necessary.
The DUAL finite state
machine embodies the decision process for all route computations. It tracks all
routes advertised by all neighbors. DUAL uses the distance information (known
as a metric) to select efficient, loop-free paths. DUAL selects routes to be
inserted into a routing table based on a calculation of the feasibility
condition. A successor is a neighboring router used for packet forwarding that
has a least-cost path to a destination that is guaranteed not to be part of a
routing loop. When there are no feasible successors but there are neighbors
advertising the destination, a recomputation must occur. This is the process
whereby a new successor is determined. The amount of time required to recompute
the route affects the convergence time. Recomputation is processor intensive;
it is advantageous to avoid unneeded recomputation. When a topology change
occurs, DUAL tests for feasible successors. If there are feasible successors,
it uses any it finds to avoid unnecessary recomputation.
modules are responsible for network layer protocol-specific tasks. An example
is the EIGRP module, which is responsible for sending and receiving EIGRP
packets that are encapsulated in IP. It is also responsible for parsing EIGRP
packets and informing DUAL of the new information received. EIGRP asks DUAL to
make routing decisions, but the results are stored in the IP routing table.
EIGRP is also responsible for redistributing routes learned by other IP routing
EIGRP Configuration Grouping
Cisco IOS XR software groups all EIGRP configuration under router EIGRP configuration mode, including interface configuration portions associated
with EIGRP. To display EIGRP configuration in its entirety, use the show running-config router eigrp command. The command output displays the running configuration for the configured EIGRP instance, including the interface
assignments and interface attributes.
The following examples
show how to enter each of the configuration modes. From a mode, you can enter
command to display the commands available in that mode.
example shows how to enter router configuration mode:
Family Configuration Mode
example shows how to enter IPv4 address family configuration mode:
EIGRP interfaces can be configured as either of the following types:
Active—Advertises connected prefixes and forms adjacencies. This is the default type for interfaces.
Passive—Advertises connected prefixes but does not form adjacencies. The passive command is used to configure interfaces as passive. Passive interfaces should be used sparingly for important prefixes, such
as loopback addresses, that need to be injected into the EIGRP domain. If many connected prefixes need to be advertised, then
the redistribution of connected routes with the appropriate policy should be used instead.
an EIGRP Process
Routes from other
protocols can be redistributed into EIGRP. A route policy can be configured
along with the
redistribute command. A metric is required, configured
either through the
default-metric command or under the route policy
configured with the
redistribute command to import routes into EIGRP.
A route policy allows
the filtering of routes based on attributes such as the destination,
origination protocol, route type, route tag, and so on.
Metric Weights for EIGRP Routing
EIGRP uses the minimum bandwidth on the path to a destination network and the total delay to compute routing metrics. You
can use the metric weights command to adjust the default behavior of EIGRP routing and metric computations. For example, this adjustment allows you
to tune system behavior to allow for satellite transmission. EIGRP metric defaults have been carefully selected to provide
optimal performance in most networks.
By default, the EIGRP composite metric is a 32-bit quantity that is a sum of the segment delays and lowest segment bandwidth
(scaled and inverted) for a given route. For a network of homogeneous media, this metric reduces to a hop count. For a network
of mixed media (FDDI, Ethernet, and serial lines running from 9600 bits per second to T1 rates), the route with the lowest
metric reflects the most desirable path to a destination.
Mismatched K Values
Mismatched K values (EIGRP metrics) can prevent neighbor relationships from being established and can negatively impact network
convergence. The following example explains this behavior between two EIGRP peers (ROUTER-A and ROUTER-B).
The following error message is displayed in the console of ROUTER-B because the K values are mismatched:
RP/0//CPU0:Mar 13 08:19:55:eigrp:%ROUTING-EIGRP-5-NBRCHANGE:IP-EIGRP(0) 1:Neighbor 184.108.40.206 (GigabitEthernet0/6/0/0) is down: K-value mismatch
Two scenarios occur in which this error message can be displayed:
The two routers are connected on the same link and configured to establish a neighbor relationship. However, each router is
configured with different K values.
The following configuration is applied to ROUTER-A. The K values are changed with the metric weights command. A value of 2 is entered for the k1 argument to adjust the bandwidth calculation. The value of 1 is entered for the k3 argument to adjust the delay calculation.
The bandwidth calculation is set to 2 on ROUTER-A and set to 1 (by default) on ROUTER-B. This configuration prevents these
peers from forming a neighbor relationship.
The K-value mismatch error message can also be displayed if one of the two peers has transmitted a “goodbye” message and the
receiving router does not support this message. In this case, the receiving router interprets this message as a K-value mismatch.
The goodbye message is a feature designed to improve EIGRP network convergence. The goodbye message is broadcast when an EIGRP
routing process is shut down to inform adjacent peers about the impending topology change. This feature allows supporting
EIGRP peers to synchronize and recalculate neighbor relationships more efficiently than would occur if the peers discovered
the topology change after the hold timer expired.
The following message is displayed by routers that run a supported release when a goodbye message is received:
RP/0//CPU0:Mar 13 09:13:17:eigrp:%ROUTING-EIGRP-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 10.0.0.20 (GigabitEthernet0/6/0/0) is down: Interface Goodbye received
A Cisco router that runs a software release that does not support the goodbye message can misinterpret the message as a K-value
mismatch and display the following message:
The receipt of a goodbye message by a nonsupporting peer does not disrupt normal network operation. The nonsupporting peer
terminates the session when the hold timer expires. The sending and receiving routers reconverge normally after the sender
Percentage of Link Bandwidth Used for EIGRP Packets
By default, EIGRP packets consume a maximum of 50 percent of the link bandwidth, as configured with the bandwidth interface configuration command. You might want to change that value if a different level of link utilization is required
or if the configured bandwidth does not match the actual link bandwidth (it may have been configured to influence route metric
Floating Summary Routes for an EIGRP Process
You can also use a floating summary route when configuring the summary-address command. The floating summary route is created by applying a default route and administrative distance at the interface level.
The following scenario illustrates the behavior of this enhancement.
shows a network with three routers, Router-A, Router-B, and Router-C. Router-A learns a default route from elsewhere in the
network and then advertises this route to Router-B. Router-B is configured so that only a default summary route is advertised
to Router-C. The default summary route is applied to interface 0/1 on Router-B with the following configuration:
The configuration of the default summary route on Router-B sends a 0.0.0.0/0 summary route to Router-C and blocks all other
routes, including the 10.1.1.0/24 route, from being advertised to Router-C. However, this configuration also generates a local
discard route on Router-B, a route for 0.0.0.0/0 to the null 0 interface with an administrative distance of 5. When this route
is created, it overrides the EIGRP learned default route. Router-B is no longer able to reach destinations that it would normally
reach through the 0.0.0.0.0/0 route.
This problem is resolved by applying a floating summary route to the interface on Router-B that connects to Router-C. The
floating summary route is applied by relating an administrative distance to the default summary route on the interface of
Router-B with the following statement:
The administrative distance of 250, applied in the above statement, is now assigned to the discard route generated on Router-B.
The 0.0.0.0/0, from Router-A, is learned through EIGRP and installed in the local routing table. Routing to Router-C is restored.
If Router-A loses the connection to Router-B, Router-B continues to advertise a default route to Router-C, which allows traffic
to continue to reach destinations attached to Router-B. However, traffic destined for networks to Router-A or behind Router-A
is dropped when the traffic reaches Router-B.
shows a network with two connections from the core: Router-A and Router-D. Both routers have floating summary routes configured
on the interfaces connected to Router-C. If the connection between Router-E and Router-C fails, the network continues to operate
normally. All traffic flows from Router-C through Router-B to the hosts attached to Router-A and Router-D.
However, if the link between Router-D and Router-E fails, the network may dump traffic into a black hole because Router-E
continues to advertise the default route (0.0.0.0/0) to Router-C, as long as at least one link (other than the link to Router-C)
to Router-E is still active. In this scenario, Router-C still forwards traffic to Router-E, but Router-E drops the traffic
creating the black hole. To avoid this problem, you should configure the summary address with an administrative distance on
only single-homed remote routers or areas in which only one exit point exists between the segments of the network. If two
or more exit points exist (from one segment of the network to another), configuring the floating default route can cause a
black hole to form.
Split Horizon for an EIGRP Process
Split horizon controls the sending of EIGRP update and query packets. When split horizon is enabled on an interface, update
and query packets are not sent for destinations for which this interface is the next hop. Controlling update and query packets
in this manner reduces the possibility of routing loops.
By default, split horizon is enabled on all interfaces.
Split horizon blocks route information from being advertised by a router on any interface from which that information originated.
This behavior usually optimizes communications among multiple routing devices, particularly when links are broken. However,
with nonbroadcast networks (such as Frame Relay and SMDS), situations can arise for which this behavior is less than ideal.
For these situations, including networks in which you have EIGRP configured, you may want to disable split horizon.
Adjustment of Hello Interval and Hold Time for an EIGRP Process
You can adjust the interval between hello packets and the hold time.
Routing devices periodically send hello packets to each other to dynamically learn of other routers on their directly attached
networks. This information is used to discover neighbors and learn when neighbors become unreachable or inoperative. By default,
hello packets are sent every 5 seconds.
You can configure the hold time on a specified interface for a particular EIGRP routing process designated by the autonomous
system number. The hold time is advertised in hello packets and indicates to neighbors the length of time they should consider
the sender valid. The default hold time is three times the hello interval, or 15 seconds.
Stub Routing for an EIGRP Process
The EIGRP Stub Routing feature improves network stability, reduces resource usage, and simplifies stub router configuration.
Stub routing is commonly used in a hub-and-spoke network topology. In a hub-and-spoke network, one or more end (stub) networks
are connected to a remote router (the spoke) that is connected to one or more distribution routers (the hub). The remote router
is adjacent only to one or more distribution routers. The only route for IP traffic to follow into the remote router is through
a distribution router. This type of configuration is commonly used in WAN topologies in which the distribution router is directly
connected to a WAN. The distribution router can be connected to many more remote routers. Often, the distribution router is
connected to 100 or more remote routers. In a hub-and-spoke topology, the remote router must forward all nonlocal traffic
to a distribution router, so it becomes unnecessary for the remote router to hold a complete routing table. Generally, the
distribution router need not send anything more than a default route to the remote router.
When using the EIGRP Stub Routing feature, you need to configure the distribution and remote routers to use EIGRP and configure
only the remote router as a stub. Only specified routes are propagated from the remote (stub) router. The stub router responds
to all queries for summaries, connected routes, redistributed static routes, external routes, and internal routes with the
message “inaccessible.” A router that is configured as a stub sends a special peer information packet to all neighboring routers
to report its status as a stub router.
Any neighbor that receives a packet informing it of the stub status does not query the stub router for any routes, and a router
that has a stub peer does not query that peer. The stub router depends on the distribution router to send the proper updates
to all peers.
The stub routing feature by itself does not prevent routes from being advertised to the remote router. In the example in Figure 1 , the remote router can access the corporate network and the Internet through the distribution router only. Having a full
route table on the remote router, in this example, would serve no functional purpose because the path to the corporate network
and the Internet would always be through the distribution router. The larger route table would only reduce the amount of memory
required by the remote router. Bandwidth and memory can be conserved by summarizing and filtering routes in the distribution
router. The remote router need not receive routes that have been learned from other networks because the remote router must
send all nonlocal traffic, regardless of destination, to the distribution router. If a true stub network is desired, the distribution
router should be configured to send only a default route to the remote router. The EIGRP Stub Routing feature does not automatically
enable summarization on the distribution router. In most cases, the network administrator needs to configure summarization
on the distribution routers.
Without the stub feature, even after the routes that are sent from the distribution router to the remote router have been
filtered or summarized, a problem might occur. If a route is lost somewhere in the corporate network, EIGRP could send a query
to the distribution router, which in turn sends a query to the remote router even if routes are being summarized. If there
is a problem communicating over the WAN link between the distribution router and the remote router, an EIGRP stuck in active
(SIA) condition could occur and cause instability elsewhere in the network. The EIGRP Stub Routing feature allows a network
administrator to prevent queries from being sent to the remote router.
Route Policy Options for an EIGRP Process
Route policies comprise series of statements and expressions that are bracketed with the route-policy and end-policy keywords. Rather than a collection of individual commands (one for each line), the statements within a route policy have
context relative to each other. Thus, instead of each line being an individual command, each policy or set is an independent
configuration object that can be used, entered, and manipulated as a unit.
Each line of a policy configuration is a logical subunit. At least one new line must follow the then , else , and end-policy keywords. A new line must also follow the closing parenthesis of a parameter list and the name string in a reference to an
AS path set, community set, extended community set, or prefix set (in the EIGRP context). At least one new line must precede
the definition of a route policy or prefix set. A new line must appear at the end of a logical unit of policy expression and
may not appear anywhere else.
This is the command to set the EIGRP metric in a route policy:
RP/0/RP0/CPU0:router(config-rpl)# set eigrp-metric bandwidth delay reliability loading mtu
This is the command to provide EIGRP offset list functionality in a route policy:
RP/0/RP0/CPU0:router(config-rpl)# add eigrp-metric bandwidth delay reliability loading mtu
A route policy can be used in EIGRP only if all the statements are applicable to the particular EIGRP attach point. The following
commands accept a route policy:
default-information allowed—Match statements are allowed for destination. No set statements are allowed.
route-policy—Match statements are allowed for destination, next hop, and tag. Set statements are allowed for eigrp-metric and tag.
redistribute—Match statements are allowed for destination, next hop, source-protocol, tag and route-type. Set statements are allowed for
eigrp-metric and tag.
The range for setting a tag is 0 to 255 for internal routes and 0 to 4294967295 for external routes.
EIGRP v4/v6 Authentication Using Keychain
EIGRP authentication using keychain introduces the capability to authenticate EIGRP protocol packets on a per-interface basis.
The EIGRP routing authentication provides a mechanism to authenticate all EIGRP protocol traffic on one or more interfaces,
based on Message Digest 5 (MD5) authentication.
The EIGRP routing authentication uses the Cisco IOS XR software security keychain infrastructure to store and retrieve secret keys and to authenticate incoming and outgoing traffic on a
How to Implement EIGRP
This section contains instructions for the following tasks:
To save configuration changes, you must commit changes when the system prompts you.
This task enables
EIGRP routing and establishes an EIGRP routing process.
Before you begin
Although you can
configure EIGRP before you configure an IP address, no EIGRP routing occurs
until at least one IP address is configured.
Configures a router-id for an EIGRP process.
It is good
practice to use the
router-id command to explicitly specify a unique
32-bit numeric value for the router ID. This action ensures that EIGRP can
function regardless of the interface address configuration.
Configures the percentage of bandwidth that may be used by EIGRP on an
Summarization for an EIGRP Process
This task configures
route summarization for an EIGRP process.
You can configure a
summary aggregate address for a specified interface. If any more specific
routes are in the routing table, EIGRP advertises the summary address from the
interface with a metric equal to the minimum of all more specific routes.
Before you begin
You should not use
summarization command to generate the default route (0.0.0.0)
from an interface. This command creates an EIGRP summary default route to the
null 0 interface with an administrative distance of 5. The low administrative
distance of this default route can cause this route to displace default routes
learned from other neighbors from the routing table. If the default route
learned from the neighbors is displaced by the summary default route or the
summary route is the only default route present, all traffic destined for the
default route does not leave the router; instead, this traffic is sent to the
null 0 interface, where it is dropped.
way to send only the default route from a given interface is to use a
RP/0/RP0/CPU0:router(config-eigrp-af)# route-policy IN-IPv4 in
routing policy to updates advertised to or received from an EIGRP neighbor.
Routing for an EIGRP Process
This task configures
the distribution and remote routers to use an EIGRP process for stub routing.
Before you begin
EIGRP stub routing
should be used only on remote routers. A stub router is defined as a router
connected to the network core or distribution layer through which core transit
traffic should not flow. A stub router should not have any EIGRP neighbors
other than distribution routers. Ignoring this restriction causes undesirable
RP/0/RP0/CPU0:router# show eigrp topology 10.0.0.1 253.254.255.255 summary
entries in the EIGRP topology table.
RP/0/RP0/CPU0:router# show eigrp traffic
number of EIGRP packets sent and received.
Configuration Examples for Implementing EIGRP
This section provides the following configuration examples:
Configuring a Basic EIGRP Configuration: Example
The following example shows how to configure EIGRP with a policy that filters incoming routes. This is a typical configuration
for a router that has just one neighbor, but advertises other connected subnets.
No new or
modified RFCs are supported by this feature, and support for existing standards
has not been modified by this feature.
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