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NetVanta 1234<br />
STAT<br />
1 5 9 13 17 21 G1<br />
2 6 10 14 18 22 G2<br />
3 7 11 15 19 23 G3<br />
4 8 12 16 20 24 G4<br />
1 3 5 7 9 11 13 15 17 19 21 23<br />
2 4 6 8 10 12 14 16 18 20 22 24<br />
G1<br />
G2<br />
G3<br />
G4<br />
Example Configurations<br />
Example 3: Filtering Routes with Prefix Lists<br />
BGP routes advertised and received on an interface can be filtered using prefix lists. The following<br />
example illustrates the use of prefix lists to discard incoming routing information and to limit the routes<br />
advertised to certain peers. The AOS device in Figure 5 is expecting a specific route from its eBGP<br />
neighbor (208.61.209.253). All other advertised routes from this neighbor are to be discarded. A prefix list<br />
(EXPECTED-ROUTE) is used to define the specific subnet (208.61.209.0 /29) the AOS device is<br />
expecting from the eBGP neighbor. The implicit deny at the end of a prefix list denies all other routes. This<br />
prefix list is applied inbound from the eBGP neighbor.<br />
All BGP routes learned from one neighbor are advertised to all other BGP neighbors by default. The<br />
customer wants to advertise a specific route from the AOS device to the eBGP neighbor (208.61.209.253)<br />
and at the same time prevent the eBGP neighbor from learning about other BGP routes advertised to the<br />
AOS device from its iBGP neighbor (65.162.109.202). A prefix list (ADVERTISE) is used to define the<br />
specific route (65.162.109.201 /29) that is to be advertised from the AOS device to the eBGP neighbor.<br />
The implicit deny at the end of the prefix list will prevent any other BGP routes from being advertised on<br />
the link. The prefix list is applied outbound toward the eBGP neighbor.<br />
Lastly, it is desired that the AOS device in Figure 5 learn routes from its iBGP neighbor (65.162.109.202),<br />
but not advertise any routes to this neighbor. A prefix list (FILTER) is used to create a deny all statement.<br />
The prefix list is applied outbound toward the iBGP neighbor.<br />
AS 65001<br />
AS 65000<br />
65.162.109.201 /29<br />
eBGP Neighbor<br />
eth 0/1<br />
65.162.109.201 /29<br />
PPP 1<br />
208.61.209.254 /30<br />
T1<br />
208.61.209.253 /30<br />
208.61.209.0 /29<br />
iBGP Neighbor<br />
65.162.109.202<br />
Figure 5. Using Prefix Lists to Filter Routes Sent to and Received from BGP Neighbors<br />
The following configuration applies to Example 3:<br />
!<br />
interface eth 0/1<br />
ip address 65.162.109.201 255.255.255.248<br />
no shutdown<br />
!<br />
interface t1 1/1<br />
clock source line<br />
tdm-group 1 timeslots 1-24 speed 64<br />
no shutdown<br />
!<br />
61200860L1-29.4E Copyright © 2012 <strong>ADTRAN</strong>, Inc. 39