Interworking IBGP EBGP

Unanswered Question
Aug 20th, 2007
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Hi All,


I am just trying to clarify in my mind what happens in BGP routing.

---------D--------E

|

C

|

---------B--------A


Lets say in my example B,C,D run IBGP and A and B are different EBGP neighbors.


A advertises route to B.

NO SYNCHRONISATION

B propogates the route to C and D, and D will advertise the route to E. So the route will be in A,B,C,D,E (I guess B also advertises it back to A)

SYNCHRONISATION

B propogates the route to C but because of the synchronisation rule C won't pass it to D and D can't pass it to E?

If I then assume B,C,D are fully meshed, then I guess B,C,D get the route propogated, but D still can't advertise the route to E?


Hopefully Im on the right track. It seems that when I write it out like that it makes sense.


Ross.


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mohammedmahmoud Mon, 08/20/2007 - 22:24
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Hi Ross,


To make it simple we must separate between synchronization and the need of full mesh iBGP, as they are 2 separate issues.


iBGP rules state that, a BGP router won't advertise routes received from an iBGP peer to another iBGP peer, and thus you'll need to full mesh your iBGP peers or use Route Reflectors or even play with Confederations.


But for the synchronization, a BGP router with synchronization enabled will not advertise iBGP-learned routes to other eBGP peers if it is not able to validate those routes in its IGP (thus you'll have to redistribute BGP into IGP to have it working).


HTH,

Mohammed Mahmoud.

rossmillan Mon, 08/20/2007 - 22:38
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You have simplified this for me well.


So in both cases because it is not full meshed in my example B can only pass update to c. Because C receives the route from and IBGP peer, it CANNOT advertise the route.


Secondly assuming a full mesh, D could only pass on to another EBGP peer if synchronisation is off.


Ive got it, thanks!!!!!!!!!!!!!!!!

mohammedmahmoud Mon, 08/20/2007 - 23:07
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Ross,


You've got it right :), i am glade that you are comfortable with it now.


HTH,

Mohammed Mahmoud.

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