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I was wondering if global IPv4 Routing can be technically achieved only by importing from the ISP Provider (providing us with a IPv4 BGP full table) a subset from the BGPv4 routes (i.e. all prefixes up to /22), and without configuring any additional static default route towards the Provider.

Reason behind my question: we have an equipment which is only capable to import (due to licensing) a maximum amount of BGP routes and we would like to know if, from the routing point of view, this concept might be feasible.

Thanks.

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    Why don't you want to add a default (or better: receive it via BGP from your upstream) to cover for the missing routes? – Teun Vink Aug 14 at 19:00
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Prefixes as long as /24 are assigned for global routing, and the full BGP routing table contains many of those that are not part of an advertised supernet.

You really have three choices with your ISPs:

  • Default route
  • ISP routes and default route
  • Full global BGP table

Also, unless you have multiple ISP connections, having the BGP table doesn't really do much for you.

  • Thanks for the reply. If I understood correctly, by importing all prefixes up to /24 length I should be able to achieve full IPv4 coverage ? Is this correct ? – Ottootto Aug 15 at 5:15
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    The maximum length that the ISPs support is /24, so getting every prefix <= /24 will get you every BGP prefix being advertised. That is probably about 3/4 million prefixes. – Ron Maupin Aug 15 at 12:49
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No, because all routes are not summarized. The simpler way is to ask your ISP to advertise a default route, and only accept that one.

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