Dual-Homed Remote Topology - Cisco Catalyst 4500 series Administration Manual

Chapter 34 Configuring Layer 3 Interfaces
Configuring EIGRP Stub Routing
Figure 34-4 Simple Hub-and-Spoke Network
Internet
Corporate
network
Distribution Remote
router router
(hub) (spoke)
The stub routing feature does not prevent routes from being advertised to the remote router. In the
example in Figure 34-4, the remote router can access the corporate network and the Internet using a
distribution router only. In this example, having a full route table on the remote router serves no purpose
because the path to the corporate network and the Internet always uses a distribution router. The larger
route table only reduces the amount of memory required by the remote router. Bandwidth and memory
can be conserved by summarizing and filtering routes in 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 needs 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 sends 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 Topology

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 router has 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 34-5 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 uses 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.
Software Configuration Guide—Release IOS XE 3.6.0E and IOS 15.2(2)E
34-15
OL_28731-01