Troubleshooting guide

Example Configurations

Example 5: Load Sharing When Multihomed to Multiple ISPs

AOS allows multiple equal cost routes to be used for the purposes of load sharing outbound traffic.

The need for load sharing is typically found in BGP applications where an AOS device is multihoming

with multiple connections to different ISPs. The BGP protocol does not provide support for load sharing.

Therefore, BGP will always export the single best path for a given prefix to the IP route table. However,

there are methods that can be implemented that will allow multiple BGP-derived routes to be imported into

the IPv4 route table. Aside from the BGP-specific configuration, load sharing must be globally enabled on

the AOS device to allow the presence of multiple equal cost routes in the IP route table.

The following example illustrates load sharing across multiple links where the customer’s router is

multihomed to two different ISPs. Each ISP is advertising a default route to the AOS device. The default

routes contain equal BGP attributes, therefore one route is no more desirable than the other according to

the BGP selection process. The objective is to ensure that outbound traffic from the customer’s network is

load balanced (load shared) between the two Internet connections.

Several configuration steps are needed to allow BGP load sharing to take place. The ip load-sharing

per-destination command must be enabled in Global Configuration mode. This command allows

duplicate routes to exist in the routing table. The command maximum-paths 2 is issued in BGP

Configuration mode to allow up to two equal cost routes from BGP to be exported to the routing table. In

order for multiple BGP routes to the same destination to be candidates for load sharing, they must be equal

cost and share the same AS number in the AS path attribute. Since each ISP in this example owns their

own unique AS number that is added on to the BGP path attribute, an arbitrary AS number must be

prepended to each eBGP neighbor’s BGP advertisement. This AS number enables the BGP routes to

become candidates for load sharing since the routes now appear to originate from the same AS. This is

accomplished with the route map (LOAD-SHARE) applied inbound to each eBGP neighbor. It matches the

prefix list (DEFAULT) that only allows the default route, which automatically filters any other advertised

routes. The route map then prepends the same arbitrary AS path (65009) to the default route prefix learned

by each neighbor. The prefix list (PUBLIC-BLOCK) is used to advertise only the customer public block

Consult with the ISP to determine which BGP attributes they will honor when making a

decision on routing traffic back to your advertised AS.

The command clock source line is enabled by default. Therefore, this command will not

appear in the output when the show running-config command is issued.

The maximum number of equal cost routes supported in AOS is 6.

At the global level, load sharing has two different implementation options: per packet and

per destination. Refer to Configuring IP Load Sharing in AOS (ADTRAN’s Knowledge Base

article 1994) for more information on these load sharing options.