BGP synchronization

Can you help me understand the case? Router A, B ,and C are in the same AS. Topology is A – B – C-loopback( and A and C are BGP neighbor. A,B,C are full BGP connection. Publish network Router A can see in routing table only after turning off synchronization. Why ? thank you.
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davidy2001Connect With a Mentor Author Commented:
AkinsdNetwork AdministratorCommented:
According to the synchronization rule - You would turn synchronization of when 2 conditions exist
1 - When 1 AS does not pass route information from one AS to another especially when the route has not been interjected via IGP

2 - All the transit routers run BGP.

You are using a loopback address and technically, BGP sees that as a next hop address. The traffic has to 1st pass through the interface before it gets retransmitted to the loopback address.
Sandeep GuptaConsultantCommented:
Here is the link where BGP sync is explained

since you have all routers in same AS thus no need to turn on bgp sync..

rule 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.
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davidy2001Author Commented:
thanks for your reply. i read that link article and understand it, which tell similar result with mine. but i still cannot see the relation between synchronization rule and why in the case Router A cannot see in its routing table after enabling synchronization.

All three routers are configured with BGP and OSPF. so, the synchronization rule should not make difference in the situation ?
AkinsdNetwork AdministratorCommented:
I answered this earlier
You are using a loopback address, (because you have a redundant connection).

You need synchronization turned off for routes to show when you use loopback addresses the way you did.

You wouldn't need it if you formed neighbors on the interfaces.
BGP treats loopbacks as additional hop.

There are ways to further tune BGP to work around this.

You can read a little more on BGP and loopback addresses here
AkinsdNetwork AdministratorCommented:
Answer these questions and I'll provide you with more info on your configuration

- Router A - Loopback IP? Physical Interfaces IP?
- Router B - Loopback IP? Physical Interfaces IP?
- Router C - Loopback IP? Physical Interfaces IP?

Did you advertise the loopback addresses into OSPF
AkinsdNetwork AdministratorCommented:
BGP Neighbors on loopback interfaces

R1 lo 1, fa0/1, fa0/2
R2 lo 2, fa0/1, fa0/2

AS 500
Advertise the All addresses (at least lo addresses) into OSPF

R1<config>#Router bgp 500
R1<config-router>#Neighbor remote-as 500

R2<config-router>#neighbor remote-as 500

Neighbor relationship will not form
Router 2 will attempt to form a neighbor relationship with R1 using a source address of one of the fastinternet interfaces through OSPF.
Problem is R1 has no neighbor entry for the fa interfaces in its BGP table. It only knows to form neighbor with

R2<config-router>#neighbor update-source loopback 2

Do the same on R1
R1<config-router>#neighbor update-source loopback 1

All the best
davidy2001Author Commented:
Thank you for your explanation. I am sorry I was not clear about the question. the loopback you mentioned are totally configured correctly in the case.

I read several papers. Their results are the same as mine. Now I am just repeating the experiment. but these papers did not give detail explanation.

Here is question: turning off synchronization at RouterA can let route into bgp table with marker * and >. When turning on the synchronization, route can go into bgp table only with marker *.

As synchronization rule states, RouterA do not send to EBGP neighbor(if is in BGP table with marker * and >, it should be able to reach the EBGP neighbor) until also reach RouterA via IGP.

Now already reach RouterA via IGP, why is still only with marker * without > after turning on synchronization? Only with *, RouterA cannot send to its EBGP neighbor.
AkinsdNetwork AdministratorCommented:
Note the difference between EBGP and IBGP neighbors.
They are configured the same way. The main difference is IBGP neighbors have the same AS while EBGP neighbors have different AS. Internal versus External

With that said, your reference to EBGP even though you mentioned that they all have the same AS is a bit confusing
davidy2001Author Commented:
Hi, i should have had a picture about it. please see attachment.

As synchronization rule states, RouterA do not send to EBGP neighbor(Router D) until also reach RouterA via IGP.

Now route already exits in RouterA via IGP, why is route1.1.1.1 marked only with marker "*" without ">" at RouterA bgp table after turning on synchronization? That is also why  route would not go to RouterD. Is this not consistent with synchronization rule ?
davidy2001Author Commented:
it's going to be solved soon
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