Basic BGP and the Customer Side of Exterior Routing in .NET framework

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well-managed external connectivity Indeed, the department s link, had more bandwidth than the corporate link This division wanted to access the rest of the corporation, so totally separating it was not an alternative The solution here had distinct similarities to the problem discussed earlier in Non-BGP Multihoming, except it also required BGP In this case, you still have two autonomous system border routers One connects to a BGP speaker in the corporate backbone, while the other connects to your existing ISP You do import default from both, as type 2 external routes, and set a higher cost on the router to area 0000 The difference from the previous example is that you accept full corporate routes from the backbone As long as the nonbackbone router is up, however, you will prefer its default route for other than the more-specific corporate routes In the event the nonbackbone ASBR fails, all external traffic will default to the backbone ASBR If the backbone router fails, under the policies stated, all traffic will try to reach its destination via the nonbackbone router You may know that certain corporate destinations are not reachable from the Internet, so you might implement a policy to prevent such traffic from going outside This can be done either with destination address filters on the nonbackbone router or with blackhole routes on that router These blackhole routes must not be imported into OSPF
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In practice, blackhole routes are very commonly used in ISP operations The static route is announced to the general Internet, and attracts traffic to any address in your CIDR block Once the traffic reaches the router, there are more specific routes in the routing table of that router If any of those internal routes go up or down, information about them is not propagated into the general Internet, increasing the stability of the global routing system Here is an example An ISP is assigned 10100/16 Create a static route (using Cisco notation):
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ip route 10100 25525500 null0
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and import it into BGP Using directly connected and IGP-derived routes, make the router on which BGP runs aware of active, more specific routes to other destinations in the block, but do not redistribute them into BGP BGP assigns four customer blocks, 10100/24, 10110/24, 10120/24, and 10130/24, and has more specific routes to these four customers The customers are assumed to be single-homed, so the /24 never need appear on the Internet as long as the /16 is visible The ISP only advertises the /16 to the Internet If any of the sub-blocks go down, the failure is not advertised, thus avoiding forcing the other routers on the Internet to recompute their routing tables The other routers still send traffic to, for example, 10110/24 to the ISP boundary router If that router has a specific route to 10110/24, it sends the traffic there Assume, however, that the path to 10110/24 happens to be down On receiving a packet to this route, the router will find its route to 10100/16 to be the best match, better than the default route It will forward the packet to the next hop for the black hole, which will silently discard the packet In other words, whenever a sub-block of the ISP s allocation becomes unreachable, the ISP will handle the issue gracefully (not air its dirty laundry to the world) and simply discard the traffic because it knows the problem exists If 10110/24 becomes unavailable, the rest of the Internet still sends to the border router, which silently discards traffic to the sub-block No Internet Control Message Protocol (ICMP) or BGP withdrawals are generated Eventually, either the sending application will realize the specific destination is down, or routing will come up and connectivity will automatically be restored Perhaps even more important than avoiding ICMPs, having the blackhole route allows your router a useful default route If you did not have the blackhole route, the packet would match the default route to your upstream and be sent to the upstream in a loop that would end only when IP time to live was exceeded Incidentally, if you traceroute to the destination, the traceroute will fail at the border router
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