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BGP question

hi, I have been trying to find the right spot that indicates some light to my question, but if you think Im not understanding correctly, please do let me know

 

My understanding for local preference is that is the second best path in the equation on how BGP select the best path, 

and that it does not get exchanged to eBGP peers, only it gets propagated to all iBGP peers in the local AS, 

 

Now my question comes with the following image,

I understand why is in blank from the show ip bgp output and it is not displayed the default 100 value of local prefence, it is only showing for the iBGP peers respectively, 

 

BGP.PNG

 

 

but why did the instructor mention that despite the fact the prefix 119.0.0.0 was learned from the 155.1.13.3 (as shown in the image) it is still applying an implicit 100 value ( He even put a red mark there)

 

So my brain got a short circuit when everyone says that local preference does not get exchanged between iBGP peers, but why or how the invisible value of 100 is there?

 

Please keep in mind that Im not asking why has been chosen the best path in the image, Im just wondering how from the perspective of the local router in the output, how Im gonna determine if it has another value other than 100 like the instructor mentioned. 

 

so is there any cisco documentation that explain very well the blank value far beyond that only saying its because it comes from a eBGP peer, 

 

Or was just the instructor assuming things?

 

Simply put: I guess why if if does not get exchanged, why everyone says its default value is 100  for both eBGP (blank field) 

 

Regards, 

 

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VIP Advisor

Re: BGP question

Hello

Bgp default LP is 100 as you have stated and it seems what you are seeing is that prefix was advertised by other ibgp peers as such the LP was added from these peers

The best path neighbour is an ebgp peer hence you don’t see the default LP from that peer

Ebgp = no LP value advertised in updates

IBGP = LP value is advertised in updates 

 



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

2 REPLIES 2
VIP Advisor

Re: BGP question

Hello

Bgp default LP is 100 as you have stated and it seems what you are seeing is that prefix was advertised by other ibgp peers as such the LP was added from these peers

The best path neighbour is an ebgp peer hence you don’t see the default LP from that peer

Ebgp = no LP value advertised in updates

IBGP = LP value is advertised in updates 

 



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

Re: BGP question

Thank you
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