So just a note before we conclude the multihoming examples: if we're distributing the default route within the AS whether we hear the default route by BGP getting the full routing table or simply in the partial routing example we should distribute the default route using our chosen IGP whether that is OSPF or IS-IS. The example on the slide shows how to originate the default route on the two border routers with different metrics. Remember in OSPF the lowest metric will win. So we had traffic would default out to router D unless we had more specific paths out through router C. So on router C we originated the default route in OSPF with a high metric 30 in the example. On router D we originated the default route with a low metric 10 in the example. Which means the primary default route is via router D with backup via router C. We'll have more specific paths to AS130 in the neighbors learned through iBGP on router C anyway. Partial routes from upstream is not expensive. It only carries the routes necessary for load sharing. You need to know how to filter on AS paths as in examples we've shown you and the previous example really is only an example. It shows you the steps and the process needed. Real life will need improved fine tuning. The previous example doesn't consider inbound traffic but we've covered those before. And just a note: when we're distributing internal default by iBGP or OSPF or IS-IS make sure that routers connected to private peers or to internet exchange points do not carry the default route. If it carries a default route or have the full BGP table, it means your peers could point a default route to you and unintentionally transit your backbone. The simple fix for private peer or exchange point based routers is to put a static default route to the null interface. An example shows how to do it for IPv4 and for IPv6.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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