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The EIGRP stub routing feature improves network stability, reduces resource utilization, and simplifies the stub device configuration.
Stub routing is commonly used in hub-and-spoke network topologies. In a hub-and-spoke network, one or more end (stub) networks
are connected to a remote device (the spoke) that is connected to one or more distribution devices (the hub). The remote device
is adjacent to one or more distribution devices. The only route for IP traffic to reach the remote device is through a distribution
device.
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Information About EIGRP Stub Routing
EIGRP Stub Routing
The EIGRP stub routing feature improves network stability, reduces resource utilization,
and simplifies the stub device configuration.
Stub routing is commonly used in hub-and-spoke network topologies. In a hub-and-spoke
network, one or more end (stub) networks are connected to a remote device (the spoke)
that is connected to one or more distribution devices (the hub). The remote device is
adjacent to one or more distribution devices. The only route for IP traffic to reach the
remote device is through a distribution device. This type of configuration is commonly
used in WAN topologies, where the distribution device is directly connected to a WAN.
The distribution device can be connected to many remote devices, which is often the
case. In a hub-and-spoke topology, the remote device must forward all nonlocal traffic
to a distribution device, so it becomes unnecessary for the remote device to have a
complete routing table. Generally, the distribution device need not send anything more
than a default route to the remote device.
When using the EIGRP stub routing feature, you need to configure the distribution and
remote devices to use EIGRP and configure only the remote device as a stub. Only
specified routes are propagated from the remote (stub) device. The stub device responds
to all queries for summaries, connected routes, redistributed static routes, external
routes, and internal routes with the message “inaccessible.” A device that is configured
as a stub will send a special peer information packet to all neighboring devices to
report its status as a stub device.
Any neighbor that receives a packet informing it of the stub status will not query the
stub device for any routes, and a device that has a stub peer will not query that peer.
The stub device will depend on the distribution device to send proper updates to all
peers.
The figure below shows a simple hub-and-spoke network.
Figure 1. Simple Hub-and-Spoke Network
The stub routing feature by itself does not prevent routes from being advertised to the
remote device. In the above example, the remote device can access the corporate network
and the Internet only through the distribution device. Having a complete route table on
the remote device would serve no functional purpose because the path to the corporate
network and the Internet would always be through the distribution device. The large
route table would only reduce the amount of memory required by the remote device.
Bandwidth and memory can be conserved by summarizing and filtering routes in the
distribution device. The remote device need not receive routes that have been learned
from other networks because the remote device must send all nonlocal traffic, regardless
of the destination, to the distribution device. If a true stub network is desired, the
distribution device should be configured to send only a default route to the remote
device. The EIGRP stub routing feature does not automatically enable summarization on
distribution devices. In most cases, the network administrator will need to configure
summarization on distribution devices.
Note
When configuring the distribution device to send only a default route to the remote
device, you must use the ipclassless command on the remote device. By default, the
ipclassless command is enabled in all Cisco images that support
the EIGRP stub routing feature.
Without the EIGRP stub routing feature, even after routes that are sent from the
distribution device to the remote device 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 device, which in turn would send a query to the remote device,
even if routes are being summarized. If there is a communication problem (over the WAN
link) between the distribution device and the remote device, 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 device.
Dual-Homed Remote
Topology
In addition to a simple hub-and-spoke network, where a remote device is connected to a
single distribution device, the remote device can be dual-homed to two or more
distribution devices. This configuration adds redundancy and introduces unique issues,
and the stub feature helps to address some of these issues.
A dual-homed remote device will have two or more distribution (hub) devices. However,
the principles of stub routing are the same as they are with a hub-and-spoke topology.
The figure below shows a common dual-homed remote topology with one remote device:
however, 100 or more devices could be connected on the same interfaces on distribution
Device 1 and distribution Device 2. The remote device will use the best route to reach
its destination. If distribution Device 1 experiences a failure, the remote device can
still use distribution Device 2 to reach the corporate network.
Figure 2. Simple Dual-Homed Remote Topology
The figure above shows a simple dual-homed remote topology with one remote device and
two distribution devices. Both distribution devices 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 the figure below,
distribution Device 1 is directly connected to network 10.3.1.0/24. If summarization or
filtering is applied on distribution Device 1, the device will advertise network
10.3.1.0/24 to all of its directly connected EIGRP neighbors (distribution Device 2 and
the remote device).
Figure 3. Dual-Homed Remote Topology with Distribution Device 1 Connected to Two Networks
The figure above shows a simple dual-homed remote topology, where distribution Device 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 Device 1 and distribution Device 2 fails,
the lowest cost path to network 10.3.1.0/24 from distribution Device 2 will be through
the remote device (see the figure below). 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 many problems that might affect the entire corporate
network. The use of the lower bandwidth route that passes through the remote device may
cause WAN EIGRP distribution devices to be dropped. Serial lines on distribution and
remote devices may also be dropped, and EIGRP SIA errors on the distribution and core
devices can occur.
Figure 4. Dual-Homed Remote Topology with a Failed Route to a Distribution Device
It is not desirable for traffic from distribution Device 2 to travel through any remote
device to reach network 10.3.1.0/24. Backup routes can be used if links are sized to
manage the load. However, most networks, of the type shown in the figure above, have
remote devices located at remote offices with relatively slow links. To ensure that
traffic from distribution devices are not routed through a remote device, you can
configure route summarization on the distribution device and the remote device.
It is typically undesirable for traffic from a distribution device to use a remote
device as a transit path. A typical connection from a distribution device to a remote
device would have much less bandwidth than a connection at the network core. Attempting
to use a remote device with a limited bandwidth connection as a transit path would
generally produce excessive congestion at the remote device. The EIGRP stub routing
feature can prevent this problem by preventing the remote device from advertising core
routes back to the distribution devices. In the above example, routes learned by the
remote device from distribution Device 1 will not be advertised to distribution Device
2. Therefore, distribution Device 2 will not use the remote device as a transit for
traffic destined to the network core.
The EIGRP stub routing feature provides network stability. If the network is not stable,
this feature prevents EIGRP queries from being sent over limited bandwidth links to
nontransit devices. Instead, distribution devices to which the stub device is connected
answer queries on behalf of the stub device. This feature greatly reduces the chance of
further network instability due to congested or problematic WAN links. The EIGRP stub
routing feature also 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 devices to prevent those devices from
appearing as transit paths to hub devices.
Caution
The EIGRP stub routing feature should be used only on stub devices. A stub device is
defined as a device connected to the network core or distribution layer through
which core transit traffic should not flow. A stub device should not have any EIGRP
neighbors other than distribution devices. Ignoring this restriction will cause
undesirable behavior.
Note
Multiaccess interfaces such as ATM, Gigabit Ethernet, Frame Relay, ISDN PRI, and
X.25 are supported by the EIGRP stub routing feature only when all devices on that
interface, except the hub, are configured as stub devices.
How to Configure EIGRP Stub Routing
Configuring the EIGRP Stub Routing Autonomous System Configuration
Device# show eigrp address-family ipv4 neighbors detail
(Optional)
Displays neighbors discovered by EIGRP.
Configuration Examples for EIGRP Stub Routing
Example: EIGRP Stub Routing—Autonomous System Configuration
A device that is configured as a stub with the
eigrpstub command shares connected and summary routing information with all neighbor devices by default. The following six keywords
can be used with the
eigrpstub command to modify this behavior:
connected
leak-map
receive-only
redistributed
static
summary
This section provides configuration examples for all forms of the
eigrpstub command for an EIGRP autonomous system configuration.
Example: eigrp stub Command
In the following example, the
eigrpstub command is used to configure the device as a stub that advertises connected and summary routes:
In the following example, the
eigrpstub command is used with the
connected and
static keywords to configure the device as a stub that advertises connected and static routes (sending summary routes will not be
permitted):
In the following example, the
eigrpstub command is issued with the
leak-mapname keyword-argument pair to configure the device to reference a leak map that identifies routes that would have been suppressed:
In the following example, the
eigrpstub command is issued with the
receive-only keyword to configure the device as a receive-only neighbor (connected, summary, and static routes will not be sent):
In the following example, the
eigrpstub command is issued with the
redistributed keyword to configure the device to advertise other protocols and autonomous systems:
A device that is configured as a stub with the
eigrpstub command shares connected and summary routing information with all neighbor devices by default. The following six keywords
can be used with the
eigrpstub command to modify this behavior:
connected
leak-map
receive-only
redistributed
static
summary
This section provides configuration examples for all forms of the
eigrpstub command for an EIGRP named configuration.
Example: eigrp stub Command
In the following example, the
eigrpstub command is used to configure the device as a stub that advertises connected and summary routes:
In the following named configuration example, the
eigrpstub command is issued with the
connected and
static keywords to configure the device as a stub that advertises connected and static routes (sending summary routes will not be
permitted):
In the following named configuration example, the
eigrpstub command is issued with the
leak-mapname keyword-argument pair to configure the device to reference a leak map that identifies routes that would normally have been
suppressed:
In the following named configuration example, the
eigrpstub command is issued with the
receive-only keyword to configure the device as a receive-only neighbor (connected, summary, and static routes will not be sent):
In the following named configuration example, the
eigrpstub command is issued with the
redistributed keyword to configure the device to advertise other protocols and autonomous systems:
Feature Information for Overview of Cisco TrustSec
The following table provides release information about the feature or features described in this module. This table lists
only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise,
subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco
Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Table 1. Feature Information for Overview of Cisco TrustSec
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