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Re: Emergency backup for a small net

  • From: Hank Nussbacher
  • Date: Sun May 18 09:59:19 1997

At 09:04 AM 5/18/97 -0400, Bradley Dunn wrote:

This brings up a problem I have been trying to solve for a while that I have
not found an answer to.  A company with an old style Class B (/16) wants to
be multihomed and split their incoming load between 2 ISPs (outgoing they
can live with going via 1 and if the primary fails they fallback to the
secondary conenction).  In order to split the load on incoming, one ISP has
to announce a /17 and the other ISP has to announce a /17 (or even the
entire /16).  But since this net is from the 147.x.x.x range, various ISPs
around the globe will filter out the /17.  This means that having a /16 from
the 192.x.x.x range is better (these days), unless someone can show me a
solution.

Thanks,
Hank 

>Hi,
>
>We have a small ISP customer that wants to run a circuit to another local
>ISP and the ISPs would use that pipe only in the case of primary link
>failure. The two ISPs would split the cost, etc.
>
>The obvious solution would be for both ISPs to set up BGP peering with
>their upstreams and not announce anything in normal operation. The
>upstreams would continue to statically route the smaller ISPs' blocks and
>the smaller ISPs would default to their upstreams. The smaller ISPs would
>also put in a default pointing at each other with a higher cost. Then in
>the case of primary link failure the ISP who still has a path to the net
>would begin announcing the other ISP's block(s) to their upstream. The
>upstream would in turn see this as a valid announcement and propagate it
>to the world. Therefore specificity should draw all the traffic to the
>correct place.
>
>The problem is both ISPs are small and have /24s from their providers. The
>/24s would be filtered by many, leading to only partial connectivity in
>the case of failure. (Partial connectivity is better than no connectivity,
>I guess...)
>
>Another possible solution I thought of is to use NAT. The small ISPs would
>use RFC1918 internally and use a block from their provider to translate
>into. When the primary link fails they switch over to using a block from
>the other ISP's provider. They would also have to use very low TTLs for
>their DNS zones and be prepared to switch the DNS zones to point to the
>other block. Does the NIC consider this efficient utilization
>to have a block lying around that only gets used when a link fails?
>
>An important thing to remember here is that the backup link will not be
>used in normal operation. This is not multihoming. They do not want load
>balancing.
>
>I would be interested to hear others' thoughts on this. If you reply
>privately I will summarize any interesting replies to the list.
>
>Thanks!
>
>pbd
>--
>You can make it illegal, but you can't make it unpopular.
>
>

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