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Example Configurations
Example 7: Using Local Preference to Promote a BGP Route as the Primary
Internet Connection over a Backup Static Route
The following example illustrates how to configure an AOS device to prefer a default route learned from
an ISP using eBGP as superior to a manually configured static route for the on-site backup Internet
connection. In addition, the AOS device will advertise the default route to a local iBGP neighbor.
AOS only supports the no default-originate command, which prevents a unit from sending
the default route to a BGP neighbor. However, transmission of default routes to BGP
neighbors can be accomplished by issuing the command network 0.0.0.0 mask 0.0.0.0.
An inbound prefix list (DEFAULT) is used in conjunction with a route map to ensure only the default route
is learned from the ISP, and an outbound prefix list (FILTER) ensures no routes are advertised to the ISP
from the AOS device. The DEFAULT prefix list is also used outbound to only allow the default route to be
advertised to the iBGP neighbor.
In this example, a default route is advertised from the ISP to the AOS device using BGP. A static default
backup route with an administrative distance of 30 is also specified in the AOS device (using the command
ip route 0.0.0.0 0.0.0.0 192.168.1.254 30) for the on-site backup Internet connection. In addition, the
network 0.0.0.0 mask 0.0.0.0 command is issued to ensure that the AOS device transmits the default route
learned using BGP to its iBGP neighbor. Anytime a route is specified with the above network command or
redistributed into BGP, it is subject to the BGP rules for determining the best path. Since the prefix of the
static default backup route is specifically advertised by the network statement under BGP, the backup route
is injected into the BGP process. This means that the default route advertised from the ISP and the static
default backup route will be compared by BGP to determine the best path.
Both the advertised default route and the injected static route will be assigned the default LOCAL_PREF
value of 100 in BGP. The learned default route advertised from the ISP is the preferred primary route.
However, since both routes have the same value for LOCAL_PREF, the static default backup route will be
selected as the best path by the BGP algorithm. The reason for this selection is because the static default
backup route is a locally originated route, which is preferable to the BGP selection process than routes
learned from a BGP neighbor. The eBGP learned route can be promoted as the preferred route by
increasing its LOCAL_PREF value to 110. This is accomplished by applying an inbound route map
(DEFAULT-ROUTE-IN) to the eBGP neighbor (ISP).
The route map match criteria specifies a prefix list (DEFAULT) that defines the default route advertised by
the eBGP neighbor to the AOS device. When the match is made, the LOCAL_PREF value for that route is
set to 110. Thus, the advertised default route is now more desirable with a LOCAL_PREF value of 110
than the static backup route with the default LOCAL_PREF value of 100. As a result, the eBGP learned
default route is exported to the route table of the AOS device anytime the route is available. When the
eBGP learned default route is not available, the AOS device will send traffic to the internal backup Internet
router (192.168.1.254) from the floating static default route.
61200860L1-29.4E Copyright © 2012 ADTRAN, Inc. 49