Cisco Catalyst 4500 series Administration Manual page 857

Chapter 34 Configuring Layer 3 Interfaces
network. The use of the lower bandwidth route that passes using 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 34-7
Corporate
network
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 is 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 has 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 generally produces 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 are not advertised to
distribution router 2. Because the remote router does not advertise core routes to distribution router 2,
the distribution router does not use the remote router as a transit for traffic destined for the network core.
The EIGRP stub routing feature can help to provide greater network stability. In the event of network
instability, this feature prevents EIGRP queries from being sent over limited bandwidth links to
nontransit routers. Instead, distribution routers to which the stub router is connected answer 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. 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 routers to prevent those remote
routers from appearing as transit paths to the hub routers.
EIGRP stub routing should only be used on stub routers. A stub router is defined as a router connected
Caution
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 behavior.
OL_28731-01
Dual-Homed Remote Topology with a Failed Route to a Distribution Router
10.3.1.0/24
X
Distribution
router 1
(hub)
Distribution
router 2
(hub)
Software Configuration Guide—Release IOS XE 3.6.0E and IOS 15.2(2)E
Configuring EIGRP Stub Routing
Remote
router
(spoke)
34-17