RSS
  Multihoming Your Internet Service With BGP But controlling which of the two links the Internet uses for inbound traffic is not really feasible. By one method or another, both ISPs have to advertise your Internet network number to the rest of the Internet (in our example, this is 158.6.0.0). This can be done by sending the provider's BGP advertisements, or having each set up a static route to your network number, which is redistributed through the Internet. It should be understood that you have no way of controlling which link remote hosts will ultimately use to send traffic to your site. Therefore, you have no dir ect control over the inbound traffic utilization on the Internet links. If your Internet traffic is predominately e-mail and Web access, in most instances you will not gain much by implementing BGP, since the majority of the traffic is inbound, and you have no control over that whether you implement BGP or not. If you do implement BGP, you also run the risk of becoming a transit network symbol--you could accidentally advertise network numbers other than your own back to the Internet, and general Internet traffic could be routed over your links and through your router. This could cause a "black hole" on the Internet with traffic being incorrectly routed to your site and discarded by your router. Generally, ISPs are reluctant to exchange route information with non-ISP organizations via BGP updates. The reason for this is that the stability of the ISPs routing process and routing on the Internet as a whole can be adversely affected by a BGP speaker introducing bad information to the ISP. Typically, a su rcharge is levied by the ISP to implement BGP. Implementing BGP is a very complex and expensive process in terms of router software and memory. This is only an introduction to the subject. For more information, see "Using the BGP for Interdomain Routing" at www.cisco.com/univercd/data/doc/ cintrnet/ics/icsbgp4.htm and "Issues Related to Having Multiple Connections to Multiple Providers," avail-able from MCI at its Web site. Additionally, look at the multihoming FAQ at www.netaxs.com/~freedman/multi.html. Chris Lewis is vice president of internat-ional operations at ILX Systems. He can be reached at chrisl@ilx.com. Thanh Nguyen is a senior WAN engineer with the American Red Cross. He can be reached at nguyent@usa.redcross.org.
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BGP Router Configuration
In our test network, we used a Cisco Systems router to define the primary link as Internet service provider (ISP) B, with a link to ISP A acting as backup. Border Gateway Protocol (BGP) is used to accept route advertisements from ISP A for the specific network numbers used by the other members of the joint venture. The result is the Cisco 7x00 router depicted in "Example of BGP Configuration" on page 121 will have routing table entries for the joint ventu re members listed as reachable via ISP A, and will have a default route listed to ISP B for all other Internet traffic. There is also a default route to ISP A, which is only used as the default for outbound traffic should the link to ISP B fail. To have ISP A take over in the event of a link failure to ISP B, assign a lower administrative distance to the default route that points to ISP B than the router that points to ISP A. The administrative distance value is assigned to all routes automatically in a Cisco router. If Open Shortest Path First (OSPF), Enhanced Interior Gateway Routing Protocol (EIGRP) and a static route present a route path for the same dest ination network, the router needs a way of comparing them to select the best one to put in the routing table. Since the metrics used in routing protocols differ among protocols, the metric value cannot be compared, so the administrative distance value is used. |
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by Chris Lewis Updated July 10, 1997 |
