Redundancy & Summarization

Brian Dickson Brian.Dickson at
Fri Aug 21 14:47:14 CDT 2009

> My institution has a single /16 spread across 2 sites: the lower /17 is
> used at site A, the upper /17 at site B.  Sites A & B are connected
> internally.  Currently both sites have their own ISPs and only advertise
> their own /17's.  For redundancy we proposed that each site advertise
> both their own /17 and the whole /16, so that an ISP failure at either
> site would trigger traffic from both /17s to reconverge towards the
> unaffected location.

There are two different ways to achieve almost-identical results.

However, one is a 50%  more "green" than the other, i.e. friendly to other network operators.

These two choices are functionally equivalent, and possible, only because things currently work for both your /17's.

Here are the two ways to do this:

One is:
- announce /17 "A" and /16 from uplink ISP-A
- announce /17 "B" and /16 from uplink ISP-B
- This results in 3 prefixes globally: A, B, and /16.

The other is:
- announce /17 "A" and /17 "B", with different policies (i.e. prepend your AS once or twice), at *both* ISPs.
- This results in 2 prefixes globally: A and B.

In all cases, as long as one ISP link is up, there is a path to both A and B.
In most cases, the best path to A or B, is *mostly*, but not completely, under your influence.

So, the main difference to everyone else is, the presence or absence of a routing slot (/16), and/or extra copies of A and/or B.

The routing slot occupies a slot in data-forwarding-plane hardware that is very limited.

The extra copies of A and B (and extra copies of your AS in the AS-path) only eat cheap control-plane memory.

If everyone did things nicely, we would not have as much of a crisis on the hardware side as we (collectively) do.

Please consider being part of the solution (announcing only /17's, but in both places) rather than part of the problem (adding a new redundant /16 to everyone's routers, including in the hardware slots.)


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