Re: BGP advertise non best routes?

There is a feature in BGP called as BGP Best External Path. So a BGP Router
will advertise both the best route and also next best External route to the
neighbor

The logic here is that,w henever a prefix is learnt via an iBGP neighbor
and and eBGP neighbor and the best path is via the iBGP neighbor ( tweaking
the Local Preference ) , so now the particular router can still Update the
neighbors with both the iBGP and eBGP Route.

This is normally implemented in Route Reflectors configuration.

And there is also a feature called as BGP Diverse Path where for every iBGP
route , u can advertise more than one best path to the neighbor.

Multipath comes into picture where u have two IGP paths for the next-hop
for the prefix learned via BGP. So then u can load-balance according to
the next-hop

On Mon, May 14, 2012 at 3:22 AM, Yuri Bank <yuribank_at_gmail.com> wrote:

> I think everyone is misunderstanding what he is actually trying to
> accomplish. Changing the BGP best-path selection process, lowering the
> AD to induce rib-failure, or using multipath to install multiple
> routes into the rib, will not affect the behavior he is trying to
> circumvent. BGP will always select a "best path" no matter what, even
> if you're doing equal-cost load-balancing or bgp dmzlink. Whichever
> single route is selected as best, will be advertised to its neighbors.
>
> BGP will not advertise two of the same prefixes with different NLRI,
> unless these are vpnv4 routes which will have route distinguishers
> associated with them. One way you may be able to accomplish this would
> be with VRFs, tunnels, and multiple BGP sessions. ( So that your
> downstream peers could get both distinct copies of the BGP table).
>
> -Yuri
>
> On Sun, May 13, 2012 at 2:03 PM, Jay McMickle <jay.mcmickle_at_yahoo.com>
> wrote:
> > Hey Marc-
> > What you may be looking for is multipath, using the maxium-path
> > statement.
> >
> >
> http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431
> .
> > shtml
> >
> >
> > Regards,
> > Jay McMickle- CCIE #35355
> >
> >
> > From: marc abel
> > <marcabel_at_gmail.com>
> > To: john matijevic <john.matijevic_at_gmail.com>
> > Cc: Group
> > Study <ccielab_at_groupstudy.com>
> > Sent: Sunday, May 13, 2012 2:07 PM
> > Subject:
> > Re: BGP advertise non best routes?
> >
> > Thank you for your response john. I may
> > not have explained what I am trying
> > to do well enough.
> >
> >
> > what I am talking
> > about is if you have two paths in the bgp table such as
> > this:
> > R3(config-if)#do show ip bgp
> > BGP table version is 21, local router ID is
> > 1.1.3.3
> > Status codes: s suppressed, d damped, h history, * valid, > best, i -
> > internal,
> > r RIB-failure, S Stale
> > Origin codes: i - IGP, e - EGP,
> > ? - incomplete
> >
> > Network Next Hop Metric LocPrf Weight
> > Path
> > * i100.100.1.0 10.1.2.254 0 100 0 500 i
> > *>i
> > 10.1.1.254 0 100 0 500 200 i
> >
> > Your
> > router will only advertise out the route it considers best to its BGP
> > peer. So
> > in this case it would only advertise the route through 10.1.1.254.
> > What I want
> > is to be able to send both options to the next router in the
> > line and then let
> > it choose based on weight or as path or something.
> >
> > *note I have applied "bgp
> > bestpath as-path ignore" that is why this one
> > isn't preferring the shorter AS
> > path.
> >
> >
> >
> >
> > On Sun, May 13, 2012 at 1:49 PM, john matijevic
> > <john.matijevic_at_gmail.com>wrote:
> >
> >> Good Afternoon Marc,
> >>
> >> "Is there any way
> > to force BGP to advertise all of it's routes, even ones
> >> it
> >> doesn't
> > consider best? "
> >>
> >> You can use the network statement under the BGP process
> > to advertise
> >> the connected networks. For routes learned via another
> > protocol, you
> >> can use the redistribute command.
> >>
> >> Once you have all the
> > routes that you want advertised you can use
> >> different filter techniques to
> > advertise or bgp attributes command to
> >> prefer one path over another.
> >>
> >>
> > Hope this helps.
> >>
> >> Regards,
> >> John
> >>
> >>
> >> On 5/13/12, marc abel
> > <marcabel_at_gmail.com> wrote:
> >> > Is there any way to force BGP to advertise all
> > of it's routes, even ones
> >> it
> >> > doesn't consider best? Particularly looking
> > for a way to make an iBGP
> >> peer
> >> > prefer a different route than it's route
> > reflector.
> >> >
> >> >
> >> > Blogs and organic groups at http://www.ccie.net
> >> >
> >> >
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