Troubleshooting guide
Example Configurations
61200860L1-29.4E Copyright © 2012 ADTRAN, Inc. 41
Example 4: Multihoming and Influencing Traffic over a Preferred Path
Multihoming is when a router has more than one connection to the Internet. The following example
illustrates a customer multihoming to two different Internet service providers (ISPs). The customer owns a
public block 208.61.209.0 /29 that will be advertised to both ISPs. The preferred path for incoming traffic
is the high speed Metro-Ethernet connection to ISP 1. The secondary path is the T1 connection to ISP 2.
AS path prepend is used to influence ISP 2 to direct inbound traffic destined to the public block over the
Metro-Ethernet connection versus the T1 connection, except when the Metro-Ethernet connection is
unavailable. This is accomplished with the route map (NOT-PREFERRED). This route map also
automatically filters the routes that are advertised by matching only prefixes defined in the prefix list
(PUBLIC-SUBNET) and dropping the rest due to the implicit discard all at the end of the route map.
Since the wide area network (WAN) connections are not of equa
l bandwidth, the customer also prefers to
send outbound traffic over the Metro-Ethernet connection. A route map (PREFERRED) is used to create a
preference for the default route learned from the neighbor across the Metro-Ethernet connection rather than
the neighbor across the T1 connection. The route map is assigned to the Metro-Ethernet eBGP neighbor
(65.162.109.202) and matches a prefix list (DEFAULT) specifying the default route. The route map also
applies a LOCAL_PREF value of 110 to the specified default route; making it more desirable than the
default route learned from the T1 connection to ISP 2, which is assigned a default LOCAL_PREF value of
100.
When multihoming to two different ISPs, it is good practice to advertise
only intended networks to prevent
becoming a transit AS. The customer’s network in Figure 6 could become a transit AS, if ISP 1 sent traffic
destined for ISP 2 through the customer’s AS (AS 500) or vice versa. This example uses an outbound
prefix list (PUBLIC-SUBNET) to advertise only the customer public block to both ISPs. This prefix list
will prevent any routes learned by the AOS device using BGP from one ISP from being advertised to the
other ISP. The prefix list is applied explicitly to the Metro-Ethernet neighbor with the prefix-list
PUBLIC-SUBNET out command, and implicitly to the T1 neighbor through the NOT-PREFERRED
route map applied outbound. If only default routes are learned from the ISPs, the potential of becoming a
transit AS is not an issue. However, it is good practice to use outbound prefixes as a preventative measure
for multihoming setups.
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Internet
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ISP 1
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eth 0/1
eth 0/2
Metro-Ethernet
PPP 1
208.61.209.254 /30
208.61.209.1 /29 (Public Subnet)
65.162.109.201 /30
65.162.109.202 /30
208.61.209.253 /30
Figure 6. Multihoming to Two Different ISPs