Re: BGP advertise non best routes?

Hello Marc,

R3 will only advertise the best path to R1, assuming that R1 is only
peering with R3.

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800945ff.shtml

"Since only the best paths are announced to BGP peers, network
130.130.130.0/24 is not announced to R102. "
From above document.

Regards,
John

On 5/13/12, marc abel <marcabel_at_gmail.com> wrote:
> Suppose you have this topology
>
>
> R1------R3--------R2
> --------R4
>
>
> R3 is learning the same route from both R2 and R4. R3 will go through the
> BGP selection process and install the best route into the RIB. R3 will only
> advertise this preferred route to R1. What I am looking for is a way for R3
> to prefer one path, but R1 to prefer a different path without peering
> between R1 and any other router than R3, and without using PBR.
>
>
> On Sun, May 13, 2012 at 2:48 PM, john matijevic
> <john.matijevic_at_gmail.com>wrote:
>
>> 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."
>>
>> Actually 10.1.1.254 is the next hop, the network is 100.100.1.0, in
>> this case it is learning the same network from two different peers.
>> The networks that BGP advertise is different than what the best route
>> network that BGP learns from the peers.
>>
>>
>>
>> "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."
>>
>> Im not sure what you mean by being able so send both options. Can you
>> rephrase or be more specfic?
>>
>>
>>
>> Regards,
>> John
>>
>>
>>
>>
>> On 5/13/12, Tony Singh <mothafungla_at_gmail.com> wrote:
>> > Would as path ignore not cause loops and black holes?
>> >
>> > I understand what you want but I'm sure bgp will only offer the best
>> route
>> > to that prefix to the routing table, it might be worth experimenting
>> > with
>> > the as's and see if primary link goes down does your ping get through on
>> the
>> > other path?
>> >
>> >
>> > BR
>> >
>> > Tiny
>> >
>> > Sent from my iPhone on 3
>> >
>> > On 13 May 2012, at 20:07, marc abel <marcabel_at_gmail.com> wrote:
>> >
>> >> 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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>> >>
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>> >>
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Received on Sun May 13 2012 - 16:11:04 ART