[vc_row][vc_column][vc_column_text]In the Figure 4 situation, all four routers are in the position to determine if the connection is up or down, as long the connections reliably provide this feedback. For instance, this is the case with a direct Ethernet UTP or fiber link. In that situation, it’s possible for the ISP to statically route the address blocks of the customer towards the interface that connects to the customer, and the customer sets a default route towards the interface that connects to the ISP. The routers on both sides then redistribute those static routes into their internal routing protocol, but those static routes will disappear if the interface in question goes down so traffic is rerouted over the other connection.
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Figure 4: Two connections terminating on two routers.
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[/vc_column_text][/vc_column][/vc_row][vc_row][vc_column][vc_column_text]However, in most cases a routing protocol will be used between the ISP and the customer. If there is link up/down feedback available, using a routing protocol provides an extra level of protection against failures, and in many cases link up/down feedback isn’t available because there are one or more switches in the path. Then, a routing protocol is necessary to detect when a connection goes down.
Because routing information is only exchanged between the ISP and the customer and doesn’t propagate to the rest of the internet, any routing protocol may be used, such as RIP or OSPF. RIPv2 doesn’t detect outages very quickly, so OSPF is a better choice. But in general, it’s best to use BGP in this situation, as BGP is designed to be used between networks belonging to different organizations and most ISPs routinely exchange BGP routing information with some of their customers already.
However, the BGP configuration is usually slightly different from one that’s used when a network connects to two or more ISPs. Often, the customer will use IP addresses from an address block that belongs to the ISP. For instance, the customer uses 10.0.16.0/22 and 10.0.20.0/24 out of the ISP’s 10.0.0.0/8 block. Because the ISP already announces the 10.0.0.0/8 block, there is no need to propagate the prefixes 10.0.16.0/22 and 10.0.20.0/24 towards the rest of the world. A packet for 10.0.20.100 will flow towards the ISP because of the 10.0.0.0/8 route that the ISP advertises to the rest of the world, and then further on to the customer because of the 10.0.20.0/24 route that the customer advertises to the ISP.
Because the advertisements of the customer’s prefixes aren’t seen by the rest of the world, the customer can simply use a private autonomous system number rather than request a “real” AS number from ARIN, LACNIC, APNIC, AFRINIC or the RIPE NCC. Private AS numbers are the ones from 64512 to 65534. A customer should coordinate with the ISP when choosing a private AS number to avoid the situation where multiple customers use the same private AS number. Of course if a public AS number is available, that can also be used.
On the customer side, the BGP configuration is the same as one that’s used towards multiple ISPs; see later in this document for examples. On the ISP side, the configuration is slightly different: the ISP has to accept the advertisements from the customer, but shouldn’t let them propagate towards the rest of the world. Usually, existing prefix lists and/or AS path filter lists will take care of that.
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