Troubleshooting guide
Example Configurations
61200860L1-29.4E Copyright © 2012 ADTRAN, Inc. 39
Example 3: Filtering Routes with Prefix Lists
BGP routes advertised and received on an interface can be filtered using prefix lists. The following
example illustrates the use of prefix lists to discard incoming routing information and to limit the routes
advertised to certain peers. The AOS device in Figure 5 is expecting a specific route from
its eBGP
neighbor (208.61.209.253). All other advertised routes from this neighbor are to be discarded. A prefix list
(EXPECTE
D-ROUTE) is used to define the specific subnet (208.61.209.0 /29) the AOS device is
expecting from the eBGP neighbor. The implicit deny at the end of a prefix list denies all other routes. This
prefix list is applied inbound from the eBGP neighbor.
All BGP routes learned from one neighbor are advertised
to all other BGP neighbors by default. The
customer wants to advertise a specific route from the AOS device to the eBGP neighbor (208.61.209.253)
and at the same time prevent the eBGP neighbor from learning about other BGP routes advertised to the
AOS device from its iBGP neighbor (65.162.109.202). A prefix list (ADVERTISE) is used to define the
specific route (65.162.109.201 /29) that is to be advertised from the AOS device to the eBGP neighbor.
The implicit deny at the end of the prefix list will prevent any other BGP routes from being advertised on
the link. The prefix list is applied outbound toward the eBGP neighbor.
Lastly, it is desired that the AOS device in Figure 5 learn routes from its iBGP neighbor (65.162.109.202),
but not
advertise any routes to this neighbor. A prefix list (FILTER) is used to create a deny all statement.
The prefix list is applied outbound toward the iBGP neighbor.
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eBGP Neighbor
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eth 0/1
65.162.109.201 /29
65.162.109.201 /29
iBGP Neighbor
65.162.109.202
208.61.209.254 /30
PPP 1
T1
208.61.209.253 /30
208.61.209.0 /29
Figure 5. Using Prefix Lists to Filter Routes Sent to and Received from BGP Neighbors
The following configuration applies to Example 3:
!
interface eth 0/1
ip address 65.162.109.201 255.255.255.248
no shutdown
!
interface t1 1/1
clock source line
tdm-group 1 timeslots 1-24 speed 64
no shutdown
!