/27 the new /24

Leo Bicknell bicknell at ufp.org
Fri Oct 2 17:28:41 UTC 2015

In a message written on Fri, Oct 02, 2015 at 11:47:31AM -0500, Jason Baugher wrote:
> Are you suggesting that the Tier 1 and 2's that I connect to are not
> filtering out anything shorter than /24? My expectation is that they are
> dropping shorter than /24, just like I am.

Not exactly, but it's not what the other poster is implying either.

Many providers let a customer multi-home to the provider.  That is
they provide two circuits from two different POPs to the customer.
Allocate the customer a /27-/29 from the provider's supernet.  The
customer announces these small blocks back to the provider to get
high availability.  The provider does not announce externally, because
it is part of the supernet.

In Cisco speak:

ip prefix-list my-supernets-small-subnets permit ge 24
ip prefix-list my-supernets-small-subnets permit ge 24
...some route-map customer-in stuff...
route-map customer-in permit 100
  match ip prefix-list my-supernets-small-subnets
  set community 1234:1234 1234:5678 no-export
...some route-map customer-in stuff...

Yes, many tier 1's will allow longer than /24 _from their customers_
and _out of their supernets_, and will not reannounce them.

Leo Bicknell - bicknell at ufp.org
PGP keys at http://www.ufp.org/~bicknell/
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