From: Nick Shah (nshah@connect.com.au)
Date: Thu Oct 03 2002 - 01:20:06 GMT-3
John/Adam
There is no single method for route loop prevention. I have made the
following observations.
- Broadly, there are 3 cases
- Single redistribution point
- Multiple redistribution points
- Many to many redistribution (I call this nightmare)
- For single redistribution point, permit "only that is needed" both ways,
so for eg. with IGRP - OSPF, use distribute lists (or route maps with access
lists), for EIGRP/OSPF use tags
- For multiple redistribution points, there are 2 possibilities (once again,
broadly speaking)
- One of the routing protocol is being redistributed into another at 2
points (like R1 ---- R2 ----- R3) where BB1 is pumping routes into R2 (both
running eigrp), R1 & R3 are running OSPF & EIGRP, and are connected to other
routers & area 0. R2 is pumping routes to both R1 & R3, and the need is to
provide REDUNDANCY, wherein a loss of single router (R1 or R3) doesnt matter
(eigrp routes still reach the ospf core).
In cases like this, you should use tags at redistribution point, increase
the distance of EXTERNAL EIGRP routes to be higher than OSPF routes, so that
the routes redistributed from OSPF into EIGRP dont over throw the OSPF
routes.
If REDUNDANCY is not expected, just filter what you are learning out from
EIGRP (just permit EIGRP internal and block everything else)
- Remember to use masks in EIGRP
- Since masks are not supported in IGRP & RIP, permit explicit routes only
at redistribution points.
- For many to many redistribution, you have to permit ONLY THE ROUTES THAT
ARE "NATIVE" to the particular routing protocol. By NATIVE I mean, if you
are redistributing ISIS & EIGRP & RIP & OSPF all into each other, just
permit OSPF routes to be redistributed from OSPF to ISIS (dont allow LSA
5's). Remember when using RIP/IGRP, since masks are not supported, they
tend to include all the prefixes belonging to the classful network (so be
careful with these).
Check the Routopia lab 1 for an example of this kind.
- Finally, make a list of prefixes & associate it with their NATIVE ROUTING
PROTOCOLS, what I mean here is that if OSPF is running on a link having ip
address 150.50.1.0/24 then in the list put 150.50.1.0/24 --- OSPF, so when
you redistribute you know which prefixes to be allowed from whcih protocols.
This will also enable you to know at remote router X that the following
protocol should send you the following route, if you are learning from
somewhere else, there is a loop happening.
This is not very easy to do at the lab, given the time constraints, but
something I do when I am studying, so that I fall into a mental habit.
- Last but not least, practise practice and more practise. (do the Fatkid
redistribution scenario, or Routopia 1 about 5 times, each time do the
redistribution differently, so you know the knobs)
rgds
Nick
----- Original Message -----
From: "Paglia, John (USPC.PCT.Hopewell)" <JPaglia@NA2.US.ML.com>
To: "'Adam Crisp'" <adam.crisp@totalise.co.uk>; "Paglia, John
(USPC.PCT.Hopewell)" <JPaglia@NA2.US.ML.com>; <ccielab@groupstudy.com>
Sent: Thursday, October 03, 2002 8:31 AM
Subject: RE: Loop Busters
> Yeah, I like tags as well, although I do alter btwn that and redist routes
> for no apparent reason.
>
> How about costs, distances and the like? What's the best method to
> configure? Any advice?
>
> John
>
> > -----Original Message-----
> > From: Adam Crisp [SMTP:adam.crisp@totalise.co.uk]
> > Sent: Wednesday, October 02, 2002 4:59 PM
> > To: Paglia, John (USPC.PCT.Hopewell); ccielab@groupstudy.com
> > Subject: RE: Loop Busters
> >
> > I'm a great fan of route tagging
> >
> > You can tag routes when redistributing from say ospf to eigrp, and vica
> > verca.
> > you can say as part of your route map, not do redistrbute routes that
are
> > tagged with a certain number, and avoid loops.
> > This is so easy and it is much cleaner than lengthy access-lists.
> >
> > Adam
> >
> > -----Original Message-----
> > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
> > Paglia, John (USPC.PCT.Hopewell)
> > Sent: 02 October 2002 21:14
> > To: 'ccielab@groupstudy.com'
> > Subject: Loop Busters
> >
> >
> > Group:
> >
> > Might I inquire what are everyone's preferred of breaking routing
loops???
> > I
> > have been having alot of issues with routing loops lately and I have
been
> > considerably unsuccessful at breaking them on a consistent basis, even
> > when
> > employing route-maps.
> >
> > Thanks in advance,
> > John
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