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Cisco IOS Software Releases 12.0 S

EIGRP Stub Routing

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EIGRP Stub Routing

Table Of Contents

EIGRP Stub Routing

Feature Overview

Dual-Homed Remote

Benefits

Restrictions

Related Features and Technologies

Related Documents

Supported Platforms

Supported Standards, MIBs, and RFCs

Configuration Tasks

Configuring EIGRP Stub Routing

Verifying EIGRP Stub Routing

Monitoring and Maintaining EIGRP Stub Routing

Configuration Examples

Command Reference

eigrp stub


EIGRP Stub Routing


Feature History

Release
Modification

12.0(7)T

This feature was introduced.

12.0(15)S

This feature was integrated into Cisco IOS Release 12.0(15)S.


This feature module describes the EIGRP Stub Routing feature and includes the following sections:

Feature Overview, page 1

Benefits, page 5

Restrictions, page 5

Related Documents, page 6

Supported Platforms, page 6

Supported Standards, MIBs, and RFCs, page 6

Configuration Tasks, page 6

Monitoring and Maintaining EIGRP Stub Routing, page 7

Configuration Examples, page 8

Command Reference, page 10

Feature Overview

The Enhanced Interior Gateway Routing Protocol (EIGRP) Stub Routing feature improves network stability, reduces resource utilization, 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 where the distribution router is directly connected to a WAN. The distribution router can be connected to many more remote routers. Often, the distribution router will be 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 to configure only the remote router as a stub. Only specified routes are propagated from the remote (stub) router. The router responds to 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 will send 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 will not query the stub router for any routes, and a router that has a stub peer will not query that peer. The stub router will depend on the distribution router to send the proper updates to all peers.

Figure 1 shows a simple spoke and hub configuration.

Figure 1 Simple Spoke and Hub Network

The stub feature by itself does not prevent routes from being advertised to he 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 have 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 increase the amount of memory required by the remote router.

Bandwidth and memory can further be conserved by summarizing and filtering routes on 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 will need to configure summarization on the distribution routers.


Note When configuring the distribution router to send only a default route to the remote router, you must use the ip classless command on the remote router. By default, the ip classless command is enabled in all Cisco IOS images that support the EIGRP Stub Routing feature.


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 will send 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.

Dual-Homed Remote

In addition to a simple hub and spoke network where a remote router is connected to a single distribution router, the remote router can be dual-homed to two or more distribution routers. This configuration adds redundancy and introduces unique issues, and the stub feature helps to address some of these issues.

A dual-homed remote will have two or more distribution (hub) routers. However, the principles of stub routing are the same as they are with a hub and spoke topology. Figure 2 shows a common dual-homed remote topology with one remote router, but 100 or more routers could be connected on the same interfaces on distribution router 1 and distribution router 2. The remote router will use the best route to reach its destination. If distribution router 1 experiences a failure, the remote router can still use distribution router 2 to reach the corporate network.

Figure 2 Simple Dual-Homed Remote Topology

Figure 2 shows a simple dual-homed remote with one remote router and two distribution routers. Both distribution routers maintain routes to the corporate network and stub network (10.1.1.0/24).

Dual-homed routing can introduce instability into an EIGRP network. In Figure 3, distribution router 1 is directly connected to network 10.3.1.0/24. If summarization or filtering is applied on distribution router 1, the router will advertise network 10.3.1.0/24 to all of its directly connected EIGRP neighbors (distribution router 2 and the remote router).

Figure 3

Dual-Homed Remote Topology Where Distribution Router 1 Is Connected to 2 Networks

Figure 3 shows a simple dual-homed remote topology where distribution router 1 is connected to both network 10.3.1.0/24 and network 10.2.1.0/24.

If the 10.2.1.0/24 link between distribution router 1 and distribution router 2 has failed, the lowest cost path to network 10.3.1.0/24 from distribution router 2 is through the remote router (see Figure 4). This route is not desirable because the traffic that was previously traveling across the corporate network 10.2.1.0/24 would now be sent across a much lower bandwidth connection. The overutilization of the lower bandwidth WAN connection can cause a number of problems that might affect the entire corporate network. The use of the lower bandwidth route that passes through the remote router might cause WAN EIGRP distribution routers to be dropped. Serial lines on distribution and remote routers could also be dropped, and EIGRP SIA errors on the distribution and core routers could occur.

Figure 4

Dual-Homed Remote Topology with a Failed Route to a Distribution Router

It is not desirable for traffic from distribution router 2 to travel through any remote router in order to reach network 10.3.1.0/24. If the links are sized to handle the load, it would be acceptable to use one of the backup routes. However, most networks of this type have remote routers located at remote offices with relatively slow links. This problem can be prevented if proper summarization is configured on the distribution router and remote router.

It is typically undesirable for traffic from a distribution router to use a remote router as a transit path. A typical connection from a distribution router to a remote router would have much less bandwidth than a connection at the network core. Attempting to use a remote router with a limited bandwidth connection as a transit path would generally produce excessive congestion to the remote router. The EIGRP Stub Routing feature can prevent this problem by preventing the remote router from advertising core routes back to distribution routers. Routes learned by the remote router from distribution router 1 will not be advertised to distribution router 2. Since the remote router will not advertise core routes to distribution router 2, the distribution router will not use the remote router as a transit for traffic destined for the network core.

Benefits

Greater Network Stability

In the event of network instability, the EIGRP Stub Routing feature prevents EIGRP queries from being sent over limited bandwidth links to nontransit routers. Instead, distribution routers to which the stub router is connected answers the query on behalf of the stub router. This feature greatly reduces the chance of further network instability due to congested or problematic WAN links.

Simplified Stub Router Configuration

The EIGRP Stub Routing feature simplifies the configuration and maintenance of hub and spoke networks. When stub routing is enabled in dual-homed remote configurations, it is no longer necessary to configure filtering on remote routers to prevent those remote routers from appearing as transit paths to the hub routers.

Restrictions

Supports Only Stub Routers

This feature should only be used on stub 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 will cause undesirable behavior.

Multi-Access Interfaces

Multi-access interfaces, such as ATM, Ethernet, Frame Relay, ISDN PRI, and X.25, are supported by the EIGRP Stub Routing feature only when all routers on that interface, except the hub, are configured as stub routers.

Related Features and Technologies

The EIGRP Stub Routing feature is an extension of the Enhanced Interior Gateway Routing Protocol (EIGRP). For more information about configuring EIGRP and configuring route summarization and filtering, refer to the "Configuring EIGRP" chapter of the Cisco IOS Release 12.0 Network Protocols Configuration Guide, Part 1 and Cisco IOS Release 12.0 Network Protocols Command Reference, Part 1.

Related Documents

Cisco IOS Release 12.0 Network Protocols Configuration Guide, Part 1

Cisco IOS Release 12.0 Network Protocols Command Reference, Part 1

Supported Platforms

This feature is supported on all platforms that support EIGRP in Cisco IOS Release 12.0(15)S, including the following platforms:

Cisco 7200 series

Cisco 7500 series

Cisco 12000 series

Determining Platform Support Through Cisco Feature Navigator

Cisco IOS software is packaged in feature sets that are supported by specific platforms. To get updated information regarding platform support for this feature, access Cisco Feature Navigator. Cisco Feature Navigator dynamically updates the list of supported platforms as new platform support is added for the feature.

Cisco Feature Navigator is a web-based tool that enables you to determine which Cisco IOS software images support a specific set of features and which features are supported in a specific Cisco IOS image. You can search by feature or release. Under the release section, you can compare releases side by side to display both the features unique to each software release and the features in common.

To access Cisco Feature Navigator, you must have an account on Cisco.com. If you have forgotten or lost your account information, send a blank e-mail to cco-locksmith@cisco.com. An automatic check will verify that your e-mail address is registered with Cisco.com. If the check is successful, account details with a new random password will be e-mailed to you. Qualified users can establish an account on Cisco.com by following the directions at http://www.cisco.com/register.

Cisco Feature Navigator is updated regularly when major Cisco IOS software releases and technology releases occur. For the most current information, go to the Cisco Feature Navigator home page at the following URL:

http://www.cisco.com/go/fn

Availability of Cisco IOS Software Images

Platform support for particular Cisco IOS software releases is dependent on the availability of the software images for those platforms. Software images for some platforms may be deferred, delayed, or changed without prior notice. For updated information about platform support and availability of software images for each Cisco IOS software release, refer to the online release notes or, if supported, Cisco Feature Navigator.

Supported Standards, MIBs, and RFCs

Standards

No new or modified standards are supported by this feature.

MIBs

No new or modified MIBs are supported by this feature.

To obtain lists of supported MIBs by platform and Cisco IOS release, and to download MIB modules, go to the Cisco MIB website on Cisco.com at the following URL:

http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml

RFCs

No new or modified MIBs are supported by this feature.

Configuration Tasks

See the following sections for configuration tasks for the EIGRP Stub Routing feature. Each task in the list is identified as either optional or required:

Configuring EIGRP Stub Routing(required)

Verifying EIGRP Stub Routing(optional)

Configuring EIGRP Stub Routing

To configure a remote or spoke router for EIGRP stub routing, use the following commands beginning in router configuration mode:

 
Command
Purpose

Step 1 

router(config)# router eigrp as-number

Configures a remote or distribution router to run an EIGRP process.

Step 2 

router(config-router)# network network-number

Specifies the network address of the EIGRP distribution router.

Step 3 

router(config-router)# eigrp stub [receive-only | connected | static | summary]

Configures a remote router as an EIGRP stub router.

Verifying EIGRP Stub Routing

To verify that a remote router has been configured as a stub router with the EIGRP Stub Routing feature, use the show ip eigrp neighbor detail command from the distribution router in privileged EXEC mode. The last line of the output will show the stub status of the remote or spoke router. The following example output is from the show ip eigrp neighbor detail command:

router# show ip eigrp neighbor detail

IP-EIGRP neighbors for process 1

H Address Interface Hold Uptime SRTT RTO Q Seq Type

(sec) (ms) Cnt Num

0 10.1.1.2 Se3/1 11 00:00:59 1 4500 0 7

Version 12.1/1.2, Retrans: 2, Retries: 0

Stub Peer Advertising ( CONNECTED SUMMARY ) Routes

Monitoring and Maintaining EIGRP Stub Routing

To enable EIGRP stub packet debugging, use the following command in privileged EXEC mode:

Command
Purpose

Router# debug eigrp packet stub

Displays debug information about the stub status of peer routers.


Configuration Examples

A router that is configured as a stub with the eigrp stub command shares connected and summary routing information with all neighbor routers by default. Four optional keywords can be used with the eigrp stub command to modify this behavior:

receive-only

connected

static

summary

This section provides configuration examples for all forms of the eigrp stub command. The eigrp stub command can be modified with several options, and these options can be used in any combination except for the receive-only keyword. The receive-only keyword will restrict the router from sharing any of its routes with any other router in that EIGRP autonomous system, and the receive-only keyword will not permit any other option to be specified because it prevents any type of route from being sent. The three other optional keywords (connected, static, and summary) can be used in any combination but cannot be used with the receive-only keyword. If any of these three keywords is used individually with the eigrp stub command, connected and summary routes will not be sent automatically.

The connected keyword will permit the EIGRP Stub Routing feature to send connected routes. If the connected routes are not covered by a network statement, it may be necessary to redistribute connected routes with the redistribute connected command under the EIGRP process. This option is enabled by default.

The static keyword will permit the EIGRP Stub Routing feature to send static routes. Without the configuration of this option, EIGRP will not send any static routes, including internal static routes that normally would be automatically redistributed. It will still be necessary to redistribute static routes with the redistribute static command.

The summary keyword will permit the EIGRP Stub Routing feature to send summary routes. Summary routes can be created manually with the summary address command or automatically at a major network border router with the auto-summary command enabled. This option is enabled by default.

In the following example, the eigrp stub command is used to configure the router as a stub that advertises connected and summary routes:

router eigrp 1
network 10.0.0.0
eigrp stub

In the following example, the eigrp stub command is issued with the connected and static keywords to configure the router as a stub that advertises connected and static routes (sending summary routes will not be permitted):

router eigrp 1
network 10.0.0.0
eigrp stub connected static

In the following example, the eigrp stub command is issued with the receive-only keyword to configure the router as a receive-only neighbor (Connected, summary, and static routes will not be sent):

router eigrp 1
network 10.0.0.0 eigrp 
eigrp stub receive-only

Command Reference

This section documents the eigrp stub command. All other commands used with this feature are documented in the Cisco IOS Release 12.0 command reference publications.

eigrp stub

To configure a router as a stub using Enhanced Interior Gateway Routing Protocol (EIGRP), use the eigrp stub command in router configuration mode. To disable the EIGRP Stub Routing feature, use the no form of this command.

eigrp stub [receive-only | connected | static | summary]

no eigrp stub [receive-only | connected | static | summary]

Syntax Description

receive-only

(optional) Sets the router as a receive-only neighbor.

connected

(optional) Advertises connected routes.

static

(optional) Advertises static routes.

summary

(optional) Advertises summary routes.


Defaults

Stub routing is not enabled by default.

Command Modes

Router configuration

Command History

Release
Modification

12.0(7)T

This command was introduced.

12.0(15)S

This command was integrated into Cisco IOS Release 12.0(15)S.


Usage Guidelines

Use the eigrp stub command to configure a router as a stub where the router directs all IP traffic to a distribution router.

The eigrp stub command can be modified with several options, and these options can be used in any combination except for the receive-only keyword. The receive-only keyword will restrict the router from sharing any of its routes with any other router in that EIGRP autonomous system, and the receive-only keyword will not permit any other option to be specified because it prevents any type of route from being sent. The three other optional keywords (connected, static, and summary) can be used in any combination but cannot be used with the receive-only keyword. If any of these three keywords is used individually with the eigrp stub command, connected and summary routes will not be sent automatically.

The connected keyword will permit the EIGRP Stub Routing feature to send connected routes. If the connected routes are not covered by a network statement, it may be necessary to redistribute connected routes with the redistribute connected command under the EIGRP process. This option is enabled by default.

The static keyword will permit the EIGRP Stub Routing feature to send static routes. Without the configuration of this option, EIGRP will not send any static routes, including internal static routes that normally would be automatically redistributed. It will still be necessary to redistribute static routes with the redistribute static command.

The summary keyword will permit the EIGRP Stub Routing feature to send summary routes. Summary routes can be created manually with the summary address command or automatically at a major network border router with the auto-summary command enabled. This option is enabled by default.


Note Multi-access interfaces, such as ATM, Ethernet, Frame Relay, ISDN PRI, and X.25, are supported by the EIGRP Stub Routing feature only when all routers on that interface, except the hub, are configured as stub routers.


Examples

In the following example, the eigrp stub command is used to configure the router as a stub that advertises connected and summary routes:

router eigrp 1
network 10.0.0.0
eigrp stub

In the following example, the eigrp stub command is issued with the connected and static keywords to configure the router as a stub that advertises connected and static routes (sending summary routes will not be permitted):

router eigrp 1
network 10.0.0.0
eigrp stub connected static

In the following example, the eigrp stub command is issued with the receive-only keyword to configure the router as a receive-only neighbor (Connected, summary, and static routes will not be sent):

router eigrp 1
network 10.0.0.0 eigrp 

eigrp stub receive-only