AS 701 local-pref answer.

Daniel Golding dgolding at sockeye.com
Tue Dec 18 21:05:11 UTC 2001


Heh. Of course for AS's lacking usptreams, a more sensible sort of Local
Preference hierarchy might be...

Customer Prefered
Customer
Customer Backup
Private Peers
Congested Private Peers (perish the thought!)
Good Public Peers (usually gigE exchange points)
Bad Public Peers (Used to be FDDI, now ATM, i guess :)

This is the usual ranking system used, with each category having both a
local pref (and occasionally a range of LPs), and a destinctive community
value.

Although 701 has mechanisms for handling this (which work), the best
approach for most folks who have both peers and customers, is to pref your
customers, to ensure that their routes are always chosen in case of
prepending. There are several reasons for this...

1 - Customers generally WANT traffic from directly connected networks. They
also want to be able to prepend in order to balance traffic.
2 - Selecting a route through a peer, instead of a customer could adversely
effect both your peering traffic balance, and your burstable billing model
:)

One way of accomplishing this sort of thing, if one were completely adverse
to Local Preference, would be to use additive MEDs, and adding a large MED
cost for peers, and a smaller one for customer routes, at point of ingress.

- Daniel Golding


> -----Original Message-----
> From: owner-nanog at merit.edu [mailto:owner-nanog at merit.edu]On Behalf Of
> David Barak
> Sent: Monday, December 17, 2001 10:55 AM
> To: Mike Leber
> Cc: nanog at merit.edu; smentzer at mentzer.org; rekoil at semihuman.com;
> gmartine at nic.gip.net
> Subject: Re: AS 701 local-pref answer.
>
>
>
> Hi,
>
> I don't think AS 701 (or any of its peers) are
> particularly worried about different best-paths in
> different regions.  This is the old
> hot-potato/cold-potato discussion, which I don't see a
> need to re-hash.
>
> Let's pretend that Bob's bait & tackle shop (AS
> 30,000) is multihomed to AS 701 and AS 1.  Bob would
> probably want AS701 origin traffic to prefer his AS701
> link, and his AS1 Origin traffic to prefer his Genuity
> link.  No problem there - they both see a route 1
> AS-hop away.  The question only comes when Bob wants
> to have all other traffic prefer one link or the
> other.
>
> If he chose to prepend his AS to AS701, then he would
> run the risk of Genuity being the preferred path from
> AS701, and AS701 would not advertise a path.  This
> would be a useful situation if, for instance, the
> Genuity link was a DS3, and the 701 link was a T1.  If
> they were equal bandwidth links, and Bob was trying to
> do traffic-sharing, then that would not be a good
> solution.
>
> AS 701 does have mechanisa for customers to do this,
> and their support people are more than happy to assist
> customers with their routing needs.
>
> By the way, the gentleman who referred to "customers,
> peers, and upstreams" as useful loc-pref settings
> should remember that AS701 doesn't have upstreams.  :)
>
> David Barak
> I speak for myself only.
> "Quis custodes ipsos custodiet?" - Juvenal
>
> --- Mike Leber <mleber at he.net> wrote:
> >
> > Not to be like Columbo... However, there's just one
> > last question
> > bothering me.  Well ok, more than one :)
> >
> > If it's like mentzer at mentzer.org said and 701
> > doesn't deterministicly
> > prefer customer routes (customers and peer routes at
> > the same local pref)
> > wouldn't this mean that they wouldn't have
> > consistent route announcments
> > in various parts of their network?
> >
> > If a customer doesn't set the community to boost the
> > local pref, and 701
> > truly by default sets customers and peers to 100,
> > then 701 would be
> > announcing varying numbers of routes to the same
> > peer at different
> > locations.
> >
> > Do they expect consistent route annoucements from
> > their peers?
> >
> > Many networks out there insist upon this as a
> > requirement when peering.
> >
> > Mike.
> >
> > On Sun, 16 Dec 2001, Mike Leber wrote:
> >
> > >
> > > Thank you for pointing that out.  I was being
> > dense and reading way too
> > > much into the statements:
> > >
> > > smentzer at mentzer.org wrote:
> > > > All the responses I have gotten indicate that
> > UUnet does indeed set
> > > > local-pref on both customers and peers to 100
> > (or leave default in this
> > > > case).  Thanks for all the responses...
> > >
> > > Especially when the 701 communities were already
> > provided by German
> > > Martinez.  *DOH*
> > >
> > > In other words, 701 transit customers that
> > actually want to ensure their
> > > downstream customer routes are announced by 701
> > had better set the
> > > appropriate community so that local pref gets set
> > above 100.  By default
> > > this is not done.
> > >
> > > Pardon me while I get some much needed rest.
> > >
> > > Mike.
> > >
> > > On Sun, 16 Dec 2001, David Barak wrote:
> > >
> > > > Mike Leber wrote:
> > > >
> > > > >If they set local pref for both peers and
> > customers
> > > > >to 100 how do they
> > > > >ensure that the customer transit routes are
> > > > >announced to peers?
> > > >
> > > > >The reason I ask this is because if a customer
> > > > >announces a customer of
> > > > >theirs to you that a peer also has as a
> > customer >you
> > > > will have equal
> > > > >length routes for the same destination AS.
> > While
> > > > >there are many ways to
> > > > >deterministicly prefer customer routes, local
> > pref
> > > > >is the most common.
> > > >
> > > > AS 701 always announces the best route, as their
> > > > routers know it.  Their average AS-path length
> > is
> > > > under 2, so it doesn't seem to be a problem.  If
> > a
> > > > customer of AS 701 wants to insure that his/her
> > route
> > > > is advertised in all cases, s/he could send a
> > > > community which AS701 edge devices could use to
> > > > manipulate local-preference upward.  [this was
> > covered
> > > > in a previous posting on this topic]  I leave it
> > to
> > > > your imagination whether peers would be
> > permitted to
> > > > do this.
> > > >
> > > > -David Barak
> > > > I only speak for myself.
> > > > "Quis custodes ipsos custodiet?" - Juvenal
> > > >
> > > >
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