distinguishing eBGP from show ip BGP

Reza Motamedi motamedi at cs.uoregon.edu
Wed Mar 11 19:22:46 UTC 2015


Thanks Mark for the reply. Let me try to check what I understood is
correct. Does the 'i' on the left (status code) only shows whether the
prefix belongs to this AS?

What I want to figure out is if this two ASes (the owner of the router and
and the first one on the AS-PATH) connect at the location of the router, or
if packets need to stay for some hops in the local AS.

On Wed, Mar 11, 2015, 2:51 PM Mark Tinka <mark.tinka at seacom.mu> wrote:

>
>
> On 11/Mar/15 20:32, Reza Motamedi wrote:
> > Hi Nanog,
> >
> > For a research I want to distinguish the external AS peering from "show
> ip
> > BGP". In other words I want to see which entry show a path that
> immediately
> > sends packets to another AS. My understanding is that *status code* shows
> > if the route is internal, right? Does this mean if the *'i' *is not
> > present, the route is goes out of the AS in the next hop. On the same
> note,
> > can I use "Next Hop" to identify such entries?
> >
> > I just included a sample report from a public looking glass in XO.
> >
> >
> >   show ip bgp  207.108.0.0/15 longer-prefixes
> >   BGP table version is 529230540, local router ID is 65.106.7.145
> >   * * *Status codes: s suppressed, d damped, h history, * valid, > best,
> i -
> > internal,
> >                 r RIB-failure, S Stale, m multipath, b backup-path, x
> > best-external, f RT-Filter, a additional-path
> >   Origin codes: i - IGP, e - EGP, ? - incomplete
> >
> >      Network          Next Hop            Metric LocPrf Weight Path
> >   *  207.108.0.0/15   216.156.2.164            3             0 2828 209
> i
> >   *                   65.106.7.101             2             0 2828 209 i
> >   *                   65.106.7.246             3             0 2828 209 i
> >   *                   65.106.7.55              3             0 2828 209 i
> >   *>                  216.156.2.162            2             0 2828 209 i
> >   *                   65.106.7.54              3             0 2828 209 i
> >   *                   65.106.7.252             2             0 2828 209 i
> >   *                   216.156.2.160            2             0 2828 209 i
> >   *                   65.106.7.56              3             0 2828 209 i
> >   *                   216.156.2.165            2             0 2828 209 i
> >   *                   65.106.7.144             2             0 2828 209 i
>
> There are two uses of the "i" code in IOS:
>
>      1. "i" for Status codes refers to the route being learned via iBGP.
>      2. "i" for Origin codes refers to the route being learned via a
> locally-generated route at the origin (or more historically, the IGP).
>
> In IOS "show ip bgp" output, the "i" for Status code (iBGP) is to the
> left of the prefix. On the other hand, the "i" for Origin code
> (IGP-originated route) is to the right of the originating AS in the
> AS_PATH.
>
> So you need to be more interested in the "i" to the left of the prefix.
> In your output above, no such "i" exists; ergo, these are eBGP-learned
> routes from this router's point of view.
>
> Use of the NEXT_HOP attribute to identify whether a route is
> eBGP-learned is not reliable, especially if you do not own the network
> you're getting your data from.
>
> Mark.
>
>


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