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
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.
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.
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
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.