Re: Removing MED (New Issue 1.0)

[Curtis Villamizar <[e-mail]>]

In message <[e-mail]>, Enke Chen writes:
> Steve,
> 
> > From: "Stephen Gill" <[e-mail]>
> > To: "'Enke Chen'" <[e-mail]>,
> > 	"'Natale, Jonathan'" <[e-mail]>
> > Cc: <[e-mail]>
> > Subject: RE: Removing MED (New Issue 1.0) 
> > Date: Tue, 1 Oct 2002 13:01:04 -0500
> > 
> > Humm. I thought with a missing MED it was prefereable to be treated as
> > worst not as 0.
> 
> It was changed a long time ago. Please see the following text in
> Sect. 9.1.2.2 of the draft:
> 
>       In the pseudo-code above, MED(n) is a function which returns the
>       value of route n's MULTI_EXIT_DISC attribute. If route n has no
>       MULTI_EXIT_DISC attribute, the function returns the lowest
>       possible MULTI_EXIT_DISC value, i.e. 0.
> 
> -- Enke
> 
> >  Cisco treats a missing med as 0, and I think Juniper
> > treats it as worst.  Cisco has a knob to change this: bgp bestpath
> > missing-as-worst.  
> > 
> > -- steve


If Juniper treats missing MULTI_EXIT_DISC as worst and Cisco has a
knob to treat missing MULTI_EXIT_DISC as worst, then IMHO we should
change the spec to say that MED(n) returns the largest value possible
is MULTI_EXIT_DISC is missing since this has better loop suppression
bahaviour if the placement of MULTI_EXIT_DISC removal is moved in its
position in the flow from Adj-In-RIB to Loc-RIB to Adj-Rib-Out.  In
other words, no matter where the removal takes place, we are loop free
without special rules about comparing EBGP before MED removal and then
IBGP after MED removal.  The only argument for MED(n) going to zero
for missing MULTI_EXIT_DISC was that Cisco routers did that (and
change would pose an operational issue if there wasn't a knob to
facilitate the change).

Note that when explicitly jamming a MULTI_EXIT_DISC value, such as
zero, the issue of where in the decision process the MULTI_EXIT_DISC
learned from the EBGP peers could still be used becomes a concern
again.  Unfortunately these implementation hints are necessary to
remain loop free and so its hard to declare them out of scope, unless
we forbid changing MULTI_EXIT_DISC and just allow it to be removed
(which does not reflect current practice).

Curtis