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 router 1 and distribution router 2. The remote device will use the best route to reach its destination. If distribution router 1 experiences a failure, the remote device can still use distribution router 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 router 1 is directly connected to network 10.3.1.0/24. If summarization or filtering is applied on distribution router 1, the device will advertise network 10.3.1.0/24 to all of its directly connected EIGRP neighbors (distribution router 2 and the remote device).
Figure 3. Dual-Homed Remote Topology with Distribution Router 1 Connected to Two Networks
The figure above 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 fails, the lowest cost path to network 10.3.1.0/24 from distribution router 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 router 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 router 1 will not be advertised to distribution router 2. Therefore, distribution router 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 undesirable behavior.
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.