[c-nsp] ebgp vs ibgp admin distance

[Tony Li]

The original implementation was done in an environment where IGP
synchronization was the expected configuration.  In such an environment,
you want EBGP to have a better distance than your IGP, which then needs
to have a better distance than IBGP.  Consider a case where your AS
learns a prefix via multiple entrances and redistributes into your IGP.
You want EBGP to be 'best', so that you learn exits and you don't get
into oscillation between EBGP and the IGP.  You want the IGP to be
better than IBGP so that synchronization works.

Of course, more modern BGP configuration has made this wholly irrelevant
and the snippet below seems like a good starting point.

Tony


> -----Original Message-----
> From: cisco-nsp-bounces at puck.nether.net 
> [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Michael Nicks
> Sent: Thursday, August 10, 2006 3:03 PM
> To: sthaug at nethelp.no
> Cc: cisco-nsp at puck.nether.net
> Subject: Re: [c-nsp] ebgp vs ibgp admin distance
> 
> If one didn't want standard hot-potato type routing, I could 
> see where 
> it would be applicable, but still a route w/ same info 
> received via ibgp 
> and ebgp is going to prefer ebgp. Did they give any 
> technical/operational reasons as to the benefit of such 
> implementation?
> 
> Maybe my mind is in neutral.
> 
> Thanks.
> -Michael
> 
> -- 
> Michael Nicks
> Network Engineer
> KanREN
> e: mtnicks at kanren.net
> o: +1-785-856-9800 x221
> m: +1-913-378-6516
> 
> 
> 
> sthaug at nethelp.no wrote:
> >> Routes are matched against BGP best path selection. If you 
> have same 
> >> metrics/variables of a prefix received via eBGP and iBGP, 
> eBGP will win.
> >>
> >> Then onto admin distance, 20 for ebgp, 200 for ibgp.
> > 
> > I have seen several places where Cisco recommends setting 
> admin distance
> > the same (200) for both EBGP and IBGP. A Cisco BGP workshop held in
> > Stockholm, Sweden in February 2003 (by Alvaro Retana and 
> Daniel Walton)
> > recommended the following BGP template:
> > 
> > 	 router bgp XXXX
> > 	  bgp deterministic-med
> > 	  no synchronization
> > 	  no auto-summary
> > 	  distance 200 200 200
> > 
> > Steinar Haug, Nethelp consulting, sthaug at nethelp.no
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