[AusNOG] AARNet AS path prepending on Equinix IX

[Mark Prior]

On 28/04/2015 11:11, Ben Thompson wrote:
> Hi Nick,
> 
>  
> 
> Sorry, I got that output from our border router connected to the IX, to
> get a better picture I should have shown it from one of our core routers:
> 
>  
> 
> sh ip bgp 182.255.99.162/19 longer-prefixes
> 
>      Network          Next Hop            Metric LocPrf Weight Path
> 
> * i 182.255.96.0/19  103.29.52.2             64    100      0 7575 7575 i
> 
> *>i                  103.29.52.1              0    100      0 4826 7575 i
> 
> *>i 182.255.103.0/24 103.29.52.1              0    100      0 4826 7575 i
> 
>  
> 
> Because the AS Path is the same length from both transit and the IX, I
> think it is router Id being selected and thus our transit path
> (103.29.52.1) is being selected.
> 
>  
> 
> I did some digging around looking glasses and found that
> route-views.sydney.routeviews.org have a BGP session with AARnet and the
> AS Path includes 24115 which is the IX route servers ASN. I am guessing
> this makes the situation the same at AARNet with two routes with equal
> AS Path length (4826 then 56284 or 24115 then 56284), and the traffic
> from AARNet prefers Vocus over the IX.

Using the AARNet looking glass to look at your route I see

BGP routing table entry for 103.29.52.0/22
Paths: (3 available, best #1, table Default-IP-Routing-Table)
  Not advertised to any peer
  4826 56284
    202.158.215.59 (metric 4062) from 202.158.192.48 (202.158.215.59)
      Origin IGP, metric 0, localpref 85, valid, internal, best
      Community: 7575:1002 7575:2459 7575:3004 7575:6001 7575:8001
      Originator: 202.158.215.59, Cluster list: 202.158.192.48
202.158.215.1
      Last update: Mon Apr 20 19:31:20 2015

  4826 56284
    202.158.215.59 (metric 4062) from 202.158.192.62 (202.158.215.59)
      Origin IGP, metric 0, localpref 85, valid, internal
      Community: 7575:1002 7575:2459 7575:3004 7575:6001 7575:8001
      Originator: 202.158.215.59, Cluster list: 202.158.192.62
202.158.192.9 202.158.215.1
      Last update: Mon Apr 20 19:31:24 2015

  4826 56284
    202.158.215.59 (metric 4062) from 202.158.192.238 (202.158.215.59)
      Origin IGP, metric 0, localpref 85, valid, internal
      Community: 7575:1002 7575:2459 7575:3004 7575:6001 7575:8001
      Originator: 202.158.215.59, Cluster list: 202.158.192.238
202.158.192.9 202.158.215.1
      Last update: Mon Apr 20 19:31:25 2015

The community (7575:1002) indicates that they think the Vocus route is a
peering route and so they wouldn't necessarily consider it any different
to one from a peering session at Equinix IX. Possibly of concern is
7575:3004 which indicates that they are learning it in Brisbane, which a
traceroute from the looking glass confirms.

You could try stuffing your announcement to Vocus, if they are providing
you with transit, or announce more specifics to the IX.

Mark.