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BGP Synchronisation Doubt

What is the Correct Answer

Scenario /Daigram attached

Synchronisation.JPG

Conditions

1. synchronization is OFF
2. NO IGP is running in AS 65500


Which router in AS 65500 is going to end up advertising a route to network
172.16.0.0?
A. B only
B. E only
C. B and E only
D. B, C and D only
E. B, C, D, and E

2 ACCEPTED SOLUTIONS

Accepted Solutions
Hall of Fame Super Silver

Re: BGP Synchronisation Doubt

Hello Sohail,

RB advertises the net to all iBGP neighbors

because sync is OFF RE is going to advertise to its eBGP neighbor RF (it  wouldn't do it with sync on)

so answer C

Hope to help

Giuseppe

Hall of Fame Super Silver

Re: BGP Synchronisation Doubt

Hello Sohail,

>> and what about RB at that point will it also have to wait before adverstsing

RB learns the route from an eBGP neighbor that is directly connected and can advertise it to its iBGP neighbors actually RB should be the device that reditribute the route into IGP also.

The objective of synchronization is to avoid to have a routing black hole inside an AS so it is RE the device that can be affected by sync settings.

I may be wrong but it should be in this way

Hope to help

Giuseppe

4 REPLIES
Hall of Fame Super Silver

Re: BGP Synchronisation Doubt

Hello Sohail,

RB advertises the net to all iBGP neighbors

because sync is OFF RE is going to advertise to its eBGP neighbor RF (it  wouldn't do it with sync on)

so answer C

Hope to help

Giuseppe

New Member

Re: BGP Synchronisation Doubt

Hi Giuseppe

You mean IF SYNC is turned ON , then RE will advertise only if all IBGP have learned about the NET

and what about RB at that point will it also have to wait before adverstsing

Synchronization says

" When an AS provides transit service to other ASs and if there are non-BGP routers in the AS, transit
traffic might be dropped if the intermediate non-BGP routers have not learned routes for that traffic
via an IGP. The BGP synchronization rule states that if an AS provides transit service to another AS,
BGP should not advertise a route until all of the routers within the AS have learned about the route
via an IGP."

Hall of Fame Super Silver

Re: BGP Synchronisation Doubt

Hello Sohail,

>> and what about RB at that point will it also have to wait before adverstsing

RB learns the route from an eBGP neighbor that is directly connected and can advertise it to its iBGP neighbors actually RB should be the device that reditribute the route into IGP also.

The objective of synchronization is to avoid to have a routing black hole inside an AS so it is RE the device that can be affected by sync settings.

I may be wrong but it should be in this way

Hope to help

Giuseppe

Re: BGP Synchronisation Doubt

Sohail,

With the current scenario, Assuming no router reflectors are configured, and without any type of filteration in router B, Then AS 65000 can Only recieve routes from (Router B).  Router D Can also advertise  Networks to Router B and then to AS 65000 IF and Only IF the networks are originated from Router D itself.

The Synchronization rule here is not applicable.

HTH

Mohamed

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