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Another restriction for multipath BGP is that both neighboring routers must be in<br />

the same AS. This is necessary because BGP can advertise only a single path, and it<br />

makes assumptions regarding the forwarding path matching the routing information.<br />

This recipe creates an EBGP group external-group on Router1, in AS 65500, with two<br />

neighbors in AS 65505 (configured with the set peer-as 65505 command) (see<br />

Figure 13-5). The set multipath command configures BGP multipath. However, for<br />

BGP multipath to work, you must also set up a load-balancing routing policy that is<br />

applied to the router’s forwarding table.<br />

Looking in the routing table, you see the routes learned from both routers:<br />

aviva@Router1> show route 172.16.14.0/24<br />

inet.0: 176580 destinations, 1421698 routes (176272 active, 1 holddown, 79688 hidden)<br />

+ = Active Route, - = Last Active, * = Both<br />

172.16.14.0/24 *[BGP/170] 3w4d 05:01:06, MED 0, localpref 100<br />

AS path: (65505) 65510 65520 ?<br />

> to 192.168.1.1 via ge-0/1/0.0<br />

to 192.168.1.3 via ge-1/1/0.0<br />

[BGP/170] 3w4d 05:01:15, MED 0, localpref 100<br />

AS path: (65505) 65510 65520 ?<br />

> to 192.168.1.3 via ge-1/1/0.0<br />

The routing table shows two paths to this destination, the first goes through<br />

Router3 (192.168.1.1) and the ge-0/1/0 interface, and the second goes through<br />

Router4 (192.168.1.3) and the ge-1/1/0 interface. The first path is active (marked<br />

with an asterisk) because it was learned from the router with the lower router ID<br />

(192.168.1.1 is lower than 192.168.1.3). The second path is the alternate next hop.<br />

The first path lists two next hops:<br />

> to 192.168.1.1 via ge-0/1/0.0<br />

to 192.168.1.3 via ge-1/1/0.0<br />

The first hop is learned from 192.168.1.1, and the second is learned from 192.168.1.3.<br />

The second next hop has been copied from the second path up to the first one.<br />

The detail version of the show route command gives more information about the two<br />

paths:<br />

aviva@Router1> show route 172.16.14.0/24 detail<br />

inet.0: 176576 destinations, 1426247 routes (176265 active, 4 holddown, 79690 hidden)<br />

172.16.14.0/24 (12 entries, 1 announced)<br />

*BGP Preference: 170/-101<br />

Next-hop reference count: 14184<br />

Source: 192.168.1.1<br />

Next hop: 192.168.1.1 via ge-0/1/0.0, selected<br />

Next hop: 192.168.1.3 via ge-1/1/0.0<br />

Protocol next hop: 192.168.1.1<br />

Indirect next hop: ac0a200 524714<br />

Protocol next hop: 192.168.1.3<br />

Indirect next hop: 24479700 524586<br />

State: <br />

Local AS: 65000 Peer AS: 65505<br />

Age: 3w4d 5:08:45 Metric: 0 Metric2: 0<br />

This is the Title of the Book, eMatter Edition<br />

Copyright © 2008 O’Reilly & Associates, Inc. All rights reserved.<br />

Load-Balancing BGP Traffic | 473

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