Multihoming to a Single Autonomous System

Figure 2-12 shows an improved topology, with redundant links to the same provider. How the incoming and outgoing traffic is manipulated across these links depends on how the two links are used. For example, a typical setup when multihoming to a single provider is for one of the links to be a primary, dedicated Internet access link—say, a T1—and for the other link to be used only for backup. In such a scenario, the backup link is likely to be some lower-speed connection.

Figure 2-12 Multihoming to a Single Autonomous System

Figure 2-12 Multihoming to a Single Autonomous System

When the redundant link is used only for backup, there is again no call for BGP. The routes can be advertised just as they were in the single-homed scenario, except that the routes associated with the backup link have the distances set high so that they are used only if the primary link fails.

Example 2-9 shows what the configurations of the routers carrying the primary and secondary links might look like.

Example 2-9 Primary and Secondary Link Configurations for Multihoming to a Single Autonomous System

Primary Router pouter ospf 100 network area 0 default-information originate metric 10

Backup Router router ospf 100 network area 0 default-information originate metric 100

In this configuration, the backup router has a default route whose administrative distance is set to 150 so that it is in the routing table only if the default route from the primary router is unavailable. Also, the backup default is advertised with a higher metric than the primary default route to ensure that the other routers in the OSPF domain prefer the primary default route. The OSPF metric type of both routes is E2, so the advertised metrics remain the same throughout the OSPF domain. This consistency ensures that the metric of the primary default route remains lower than the metric of the backup default route in every router, regardless of the internal cost to each border router. Example 2-10 shows the default routes in a router internal to the OSPF domain.

Example 2-10 The First Display Shows the Primary External Route; the Second Display Shows the Backup Route Being Used After the Primary Route Has Failed

Phoenix#show ip route

Routing entry for, supernet fSHHHBKSfiSBBS 1", distance 110, metric 10, candidate default path fag 1, type extern 2, forward metric 64 Redistributing via ospf 1

Last update from on Serial®, 00:01:24 ago Routing Descriptor Blocks:

*, from, 00:01:24 ago, via Serial©

Route metric is 10, traffic share count is 1

Phoenix#show ip route

Routing entry for, supernet

Known via "ospf 1M, distance 110, metric 100, candidate default path Tag 1, type extern 2, forward metric 64 Redistributing via ospf 1

Last update from on Seriall, 00:00:15 ago Routing Descriptor Blocks:

*, from, 00:00:15 ago, via Seriall

Route metric is 100, traffic share count is 1

Although a primary/backup design satisfies the need for redundancy, it does not efficiently use the available bandwidth. A better design is to use both paths, with each providing backup for the other in the event of a link or router failure. In this case, the configuration used in both routers is as indicated in Example 2-11.

Example 2-11 Configuration for Load Sharing When Multihomed to the Same AS

router ospf 100 network area 0 default-information originate metric 10 metric-type 1

The static routes in both routers have equal administrative distances, and the default routes are advertised with equal metrics (10). Notice that the default routes are now advertised with an OSPF metric type of El. With this metric type, each of the routers in the OSPF domain takes into account the internal cost of the route to the border routers in addition to the cost of the default routes themselves. As a result, every router chooses the closest exit point when choosing a default route (see Figure 2-13).

Figure 2-13 Border Routers Advertising a Default Route with a Metric of 10 and an OSPF Metric Type of El

Figure 2-13 Border Routers Advertising a Default Route with a Metric of 10 and an OSPF Metric Type of El

In most cases, advertising default routes into the AS from multiple exit points, and summarizing address space out of the AS at the same exit points, is sufficient for good internetwork performance. The one consideration is whether asymmetric traffic patterns will become a concern. If the geographical separation between the two (or more) exit points is large enough for delay variations to become significant, you might have a need for better control of the routing. You might now consider BGP.

Suppose, for example, that the two exit routers depicted in Figure 2-12 are located in Los Angeles and London. You might want all your exit traffic destined for the Eastern Hemisphere to use the London router and all your exit traffic for the Western Hemisphere to use the Los Angeles router. Remember that the incoming route advertisements influence your outgoing traffic. If the provider advertises routes into your AS via BGP, your internal routers have more-accurate information about external destinations. BGP also provides the tools for setting routing policies for the external destinations.

Similarly, outgoing route advertisements influence your incoming traffic. If internal routes are advertised to the provider via BGP, you have influence over which routes are advertised at which exit point, and also tools for influencing (to some degree) the choices the provider makes when sending traffic into your AS.

When considering whether to use BGP, carefully weigh the benefits gained against the cost of added routing complexity. You should use BGP only when you can realize an advantage in traffic control. Consider the incoming and outgoing traffic separately. If it is only important to control your incoming traffic, use BGP to advertise routes to your provider while still advertising only a default route into your AS.

On the other hand, if it is only important to control your outgoing traffic, use BGP only to receive routes from your provider. Consider carefully the ramifications of accepting routes from your provider. "Taking full BGP routes" means that your provider advertises to you the entire Internet routing table. As of this writing, that is approximately 88,000 route entries, as shown in Example 2-12. To store and process a table of this size, you need a reasonably powerful router and at least 64 MB of memory (although 128 MB is recommended). On the other hand, you can easily implement a simple default routing scheme with a low-end router and a moderate amount of memory.

Example 2-12 This Full Internet Routing Table Summary Shows 57,624 BGP Entries

route-server>show ip route summary

Route Source

Networks Subnets


Memory (bytes)


0 1




2 1



bgp 65000

76302 11967



External; 88269 Internal: 0 Local: 8






77083 11969




NOTE The routing table summary in Example 2-12 is taken from a publicly accessible route server at Another server to which you can Telnet is The number of BGP entries varies somewhat in each, but all indicate a similar size.

"Taking partial BGP routes" is a compromise between taking full routes and accepting no routes at all. As the name implies, partial routes are some subset of the full Internet routing table. For example, a provider might advertise only routes to its other subscribers, plus a default route to reach the rest of the Internet. The following section presents a scenario in which taking partial routes proves useful.

Another consideration is that when running BGP, a subscriber's routing domain must be identified with an autonomous system number. Like IP addresses, autonomous system numbers are limited and are assigned only by the regional address registries when there is a justifiable need. And like IP addresses, a range of autonomous system numbers is reserved for private use: the AS numbers 64512 to 65535. With few exceptions, subscribers that are connected to a single service provider (either single or multihomed) use an autonomous system number out of the reserved range. The service provider filters the private AS number out of the advertised BGP path.

Although the topology in Figure 2-12 is an improvement over the topology in Figure 2-10 because redundant routers and data links have been added, it still entails a single point of failure: the ISP itself. If the ISP loses connectivity to the rest of the Internet, so does the subscriber. And if the ISP suffers a major internal outage, the single-homed subscriber also suffers.

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