[c-nsp] BGP Route selection

[Howard Leadmon]

I use two different tweaks here to make sure stuff like this works as you
desire.   One I use 'bgp bestpath compare-routerid' so I can pretty much tell
which way things are going, as if not it will stay as you say in the oldest
pathway even when things come back.  By also adding this comparison in, when
announcements come and go it looks at it all again.

You can also use route-map's to decide your favorite paths by default, if you
normally want to go path A for your traffic flow, set something like 'set
local-preference 125' in a route-map for it.  That will make you prefer that
route, and I also always set path B to a value less than that.

Not sure if the above is the Cisco recommended way, but it's sure worked OK
for me..


---
Howard 

> -----Original Message-----
> From: cisco-nsp-bounces at puck.nether.net [mailto:cisco-nsp-
> bounces at puck.nether.net] On Behalf Of Gary Roberton
> Sent: Friday, May 23, 2008 10:09 AM
> To: Pete Templin
> Cc: cisco-nsp at puck.nether.net
> Subject: Re: [c-nsp] BGP Route selection
> 
> All
> 
> The network in question is actually 90.0.0.0.  All routers are in their own
> separate AS.  The route in question is a connected network not
> redistributed.
> 
> To make it clearer;
> Router X has network 90.0.0.0 connected
> Router X advertises to both Router1 and Router2.
> Router 1 sends it on to Router A
> Router 2 has a route map that does 'set metric 50' and then passes it onto
> RouterA.
> We want RouterA to go via Router1 whenever Router1 is up
> 
> Router A BGP table entry is shown here;
> 
> *  90.0.0.0         10.40.1.6               50             0 64604 1000 i
> 
> *>                  10.40.1.2                              0 64603 1000 i
> 
> Router A puts 10.40.1.2 route into global routing table
> Router1 goes down
> Router A puts 10.40.1.6 route into global routing table
> Router1 comes up
> RouterA puts entry back in BGP table but leaves route in global table alone.
> 
> Any help appreciated.
> 
> 
> 
> 
> 
> 
> On Fri, May 23, 2008 at 1:20 PM, Pete Templin <petelists at templin.org> wrote:
> 
> > Gary Roberton wrote:
> >
> >  I have router A receiving network 80.0.0.0 from router 1 and router 2.
> >> Router 2 weights its metric so that it is less favourable.
> >>
> >
> > Are routers 1 and 2 in your AS, or in another AS?  Also, please clarify
> > 'weights its metric' - do you mean it adjusts weight, it adjusts metric,
> it
> > adjusts origin, etc.?
> >
> >  In router A's BGP table I can see both routes and the route from Router 1
> >> is
> >> placed in the global routing table.  Fine.
> >>
> >
> > Are you seeing the various BGP knobs showing the settings you'd expect
> from
> > above?
> >
> >  When you turn off Router1, Router A removes the route from the routing
> >> table
> >> and installs the less favoured route from Router2.  What you would
> expect.
> >>
> >> When I turn on Router1, Router A does not put the better route back into
> >> the
> >> routing table, even though it sees both in its BGP table.
> >>
> >
> > Are you seeing the various BGP knobs showing the settings you'd expect
> from
> > above?
> >
> > pt
> >
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