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Beginner

BGP mutual advertisement for the same prefix

if tow routers advertise the same prefix to each other, which router is selected to advertise first ? 

 

Router1 and Router2 advertise 192.168.1.0/24 to each other,  which one will advertise it?  Can I control it ?

 

thanks !!

BGP.PNG

Sorry I did not describe the issue clearly.  This is the diagram for my question. R1 and R2 both receive default route from the ISP, then they advertise to each other.  When R1 receives default from R2, it does not advertise default route back R2, when R2 receive default route, it does not advertise it to R1.

what I need to achieve is either server-1 or server-2 can go to internet when either ISP connectivity is lost.

My question is how I can control R1 always advertise default route first.  right now both of them randomly take turns.

 

thanks !!

 

Anyone can help me on this ?  appreciate it !!

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1 ACCEPTED SOLUTION

Accepted Solutions
VIP Mentor

Re: BGP mutual advertisement for the same prefix

Hello
Now you have posted the diagram and elaborated it has now cleared some query’s so by default and if both rtrs are peering to each isp as ebgp peers them each rtr will receive and prefer their own directly connected default route but you can manipulate this by applying a default local preference to the rtr you want to be the preferred default for the whole site

Example:

R1-R2 have an ibgp peering between each other and an ebgp peering to their respective isp

R1 < facing ISP1
router bgp xx
bgp default local-preference 20000

 



kind regards
Paul

Please rate and mark posts accordingly if you have found any of the information provided useful.
It will hopefully assist others with similar issues in the future

View solution in original post

9 REPLIES 9
VIP Advisor

Re: BGP mutual advertisement for the same prefix

you need to look which one takes the best path to go out (if this in a different topic) is important here, that is your decision to send the traffic out with your Traffic engineering, BGP have many attributes to use - whichever works for you.

 

here is one good example to use it :

 

https://community.cisco.com/t5/networking-documents/bgp-conditional-advertisement-feature/ta-p/3343752

BB
*** Rate All Helpful Responses ***
VIP Mentor

Re: BGP mutual advertisement for the same prefix

Hello,

 

what are you trying to accomplish ? If both BGP neighbors advertise the exact same network to each other, these networks will never make it into each other routing tables anyway, since they are directly connected already. The only way to override a connected network is by having a more specific route in the routing table...

VIP Mentor

Re: BGP mutual advertisement for the same prefix

Hello
Depends on the origin of that subnet in the first place, if they’re directly connected on of the rtr themselves then they’ll prefer their own connected routes, however if they are being re-advertised to each from another source then the bgp selection process will take precedence



kind regards
Paul

Please rate and mark posts accordingly if you have found any of the information provided useful.
It will hopefully assist others with similar issues in the future
Rising star

Re: BGP mutual advertisement for the same prefix

Is it eBGP between R1 and R2?

Thanks.

VIP Mentor

Re: BGP mutual advertisement for the same prefix

Hello

I would have assumed an ibgp peering between those two rtrs ? Or is this two totally independent sites or just and lab senario?



kind regards
Paul

Please rate and mark posts accordingly if you have found any of the information provided useful.
It will hopefully assist others with similar issues in the future
Beginner

Re: BGP mutual advertisement for the same prefix

They are iBGP. Now it is in our lab, but eventually it will be for production.
Beginner

Re: BGP mutual advertisement for the same prefix

They are iBGP. thanks !!
VIP Mentor

Re: BGP mutual advertisement for the same prefix

Hello
Now you have posted the diagram and elaborated it has now cleared some query’s so by default and if both rtrs are peering to each isp as ebgp peers them each rtr will receive and prefer their own directly connected default route but you can manipulate this by applying a default local preference to the rtr you want to be the preferred default for the whole site

Example:

R1-R2 have an ibgp peering between each other and an ebgp peering to their respective isp

R1 < facing ISP1
router bgp xx
bgp default local-preference 20000

 



kind regards
Paul

Please rate and mark posts accordingly if you have found any of the information provided useful.
It will hopefully assist others with similar issues in the future

View solution in original post

Beginner

Re: BGP mutual advertisement for the same prefix

thanks so much !!
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