Scenario 4: Customers of the Same Provider with a Backup Link

In some cases, customers with common interests agree to provide each other with internal connectivity and backup connectivity to the Internet. The customers are connected to the same provider and at the same time have an alternate private link to each other. Two scenarios might typically arise:

•  The private link can be used as a secondary (backup) link when an Internet link fails.

•  The private link can be used as a primary link for internal traffic between the two companies and as a backup link in the event of an Internet link failure. If a backup strategy is to work, customers must advertise each other's networks to the provider. One customer must be able to act as a transit AS for the other customer when the other customer's Internet link fails.

Private Link Used as a Pure Backup

Figure 6-18 illustrates the scenario where AS2 and AS3 are connected to the same provider—AS1. AS2 and AS3 have a private link that will be used only for backup. AS2 and AS3 will have the same policies.

Figure 6-18  Private link used as backup.

For the backup environment illustrated in figure 6-18, AS2's outbound traffic considerations are of particular interest. Whether AS2 is getting full routing or a combination of default, full, and partial routing from the provider and AS3, AS2 will have to set its local preference for routes coming from AS1 to be higher (200) than the ones coming from AS3 (100). As a result, provider AS1 is always preferred, and the private link acts as a backup only. In case of partial routing being accepted by AS2, AS2 can set defaults to both the provider (AS1) and AS3. Setting a higher local preference ensures that all the traffic will be sent toward the provider. If AS2 is getting full routing from the provider and partial routing from AS3, AS2 can keep a default route to AS3 to be used if the provider link fails.

Private Link Used as Primary Between AS2 and AS3

Figure 6-19 illustrates a case in which the link between AS2 and AS3 is used as a primary link for all traffic to AS3's local networks or to AS3's customers. For all other traffic, the link to the provider AS1 should be used. The two links (provider and private) should back up one another.

Figure 6-19  Private links used as primary AS interconnection.

---

Troubleshooting:  
Ch. 11, pp. 392-394. Customers of the Same Provider with a Backup Link

---

Assume that the environment illustrated in figure 6-19 features default, full, and partial routing; the discussions that follow focus on outbound and inbound traffic implications from AS2's perspective. This scenario is usually handled by the default BGP behavior. Since the shortest path is always preferred AS2 and AS3 will always use the private link to reach each others networks. For the sake of experimenting with BGP policies we will attempt to address this scenario by manipulating the local preference attribute.

AS2's Outbound Traffic

Whether AS2 is accepting default, full, or partial routing from the provider and AS3, AS2 should set the local preference of all updates that do not contain AS3 to be higher than all other updates. In the case illustrated in figure 6-19, these routes are given a local preference of 300. Updates coming on the private link are given a local preference of 200. Updates that contain AS3 and that are passed on to AS2 via the provider are kept at a default local preference of 100 to make sure that the private link between AS2 and AS3 is taken (200 > 100).

For all other traffic, AS2 will accept defaults or set its own defaults to the provider and AS3, with the provider being preferred.

It is also possible for AS2 to accept local routing from AS3 and not to accept routing from the provider. AS2 then defaults to both, with the provider being preferred. This way, any traffic containing AS3 will take the private link; any other traffic will take the link to the provider (better default). In case of a failure of the provider's link, the default to the private link will kick in.

AS2's Inbound Traffic

All the cases discussed so far in scenario 4 have the same inbound traffic behavior. Because of the shorter path length, incoming traffic from the Internet will always take the provider to AS2 link. For all traffic originating from AS3 or its customers, the private link will be taken also because of the shorter path. This is the desired behavior.