[c-nsp] Typical BGP operational policies

Tom Simes simestd at netexpress.com
Fri Oct 17 10:55:57 EDT 2008

On Fri, 17 Oct 2008 08:53:52 -0500
"Church, Charles" <cchurc05 at harris.com> wrote:

> Hey all,
>     I support a small network, with own ASN.  They use address space
> given by provider A, and are dual homed to providers A and B.  We take
> full routes from each, and announce that address space (a /23) to
> both. In looking at a variety of looking glass sites out there, I see
> most only see that network via provider A's AS.  One I found did see
> it via provider B only.  Is filtering being done by provider B
> outbound to it's peers the only explanation for this (or the most
> likely one)?  One particular looking glass didn't have a path to us
> via provider B, yet does see our serial interface address (last hop
> that's still part of provider B AS) as reachable via provider B.  For
> what it's worth, address space is, ASN is 26296. 
> Provider A is 11456, B is 6389.  Just wondering if there is a real
> issue here, or if this partial reachability depending on where you are
> is normal...

Hi Chuck,

>From my little corner of the net (AS7782) this is what your address
space looks like.

! show me what's making it upstream via AS11456 (provider A)
gsr1.sea#sh ip bgp | include 11456 26296
  4323 11456 26296, (received-only)
  3356 11456 26296, (received & used)
  2914 3356 11456 26296, (received & used)

! show me what's making it upstream via AS6389 (provider B)
gsr1.sea#sh ip bgp | include 6389 26296 
  174 6389 26296, (received & used)

Ok, so both providers are advertising the space to at least one of their
upstreams - so let's compare your advertisement with how providers A &
B's own space is advertised:

! provider A
gsr1.sea#sh ip bgp | include 11456
  4323 11456, (received-only)
  174 4323 11456, (received & used)
  3356 1239 4323 11456, (received & used)
  2914 4323 11456, (received & used)

Ok, so it looks like provider A only has AS4323 as a transit peer
(immediate upstream).

! provider B
gsr1.sea#sh ip bgp | include 6389    
  4323 7018 6389, (received-only)
  174 6389, (received & used)
  3356 7018 6389, (received & used)
  2914 7018 6389, (received & used)

Provider B has both AS7018 and AS174 as transit peers, but I'm not
seeing your advertisement via the 7018 6389 path from provider B.

>From this quick look, I would guess that your advertisement via provider
A is more widespread simply because your /23 block is a small part of
their larger aggregate address space.  While it's possible provider B is
only showing your /23 block to AS174, it's much more likely that AS7018
is filtering out your advertisement based on prefix length because the
/23 block is so small.

   "Z-80 system stack overflow.  Shut 'er down Scotty, the system's
         sucking mud" - Error message on TRS 80 Model-16B

Tom Simes                                       simestd at netexpress.com 

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