BGP No sync

Unanswered Question
Aug 31st, 2007

Guys can someone please tell me what does blackhole means in BGP as i was beenn asked this question in interview......secondly why we use no sync command in BGP......i cant get my head around this conecpyt of syn and no syn....can someone please explain it in easy words.......thanks a million

I have this problem too.
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mohammedmahmoud Fri, 08/31/2007 - 01:56


No sync (no synchronization) is used to disable synchronization, and synchronization was introduced to prevent blackhole (blackhole in the essence of BGP means that a router says that it can route traffic to destinations that it can't really route traffic to, and thus the traffic comes to it but it will eventually drop it), the problem is that enabling synchronization introduces a lot of problems and thus the best practice is to disable synchronization but have all routers in the network running BGP, and accordingly full meshed (or use a RR or Confederation) accordingly routing loop and back holes are prevented.


Mohammed Mahmoud.

Joseph W. Doherty Fri, 08/31/2007 - 04:19

(Another explanation.)


Q. What is synchronization, and how does it influence BGP routes installed in the IP routing table?

A. If your AS passes traffic from another AS to a third AS, BGP should not advertise a route before all routers in your AS learn about the route via IGP. BGP waits until IGP propagates the route within the AS and then advertises it to external peers. A BGP router with synchronization enabled does not install iBGP learned routes into its routing table if it is not able to validate those routes in its IGP. Issue the no synchronization command under router bgp in order to disable synchronization. This prevents BGP from validating iBGP routes in IGP. Refer to BGP Case Studies: Synchronization for a more detailed explanation.

What this means is that BGP often relies on another routing protocol (e.g. OSPF, EIGRP, etc.) under it to actually transfer the traffic. If BGP indicates is has a path from A to B, but the underlying protocol doesn't, BGP will be unable to actually forward the packets, it become a "black hole". The synchronization setting informs BGP whether to insure it and the underlying routing protocol are in agreement or not.

gaurav_thapar79 Tue, 09/04/2007 - 12:36


The thumb rule in bgp is to configure no sync. In large meshed network (like ISPs or Telcos), ibgp or igp (is-is/mbgp or ospf) are being used for intra-as routing. Running ebgp session with sync would result in validation of routes learned via iBGP or IGP as part of BGP scanner process.It may not install the prefix in eBGP for external advertisement.

Giving no-sync would help in better convergence of BGP.

2. Blackhole - The blackhole routing in BGP actually install a floating route in router toward NULL interface. like ip route 172.x.x.x NULL0. It was introduced as mitigation tool against DOS attacks. DDOS attacks actually consumes entire WAN bandwidth.


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