BGP synchronization

[ncnaveen_arasu]

HI team,

PLease expain in brief when do we use bgp synchronization and when do we disable this. what the synchronization does.

Thanks in advance.

Regards,

Naveen

[Muhammad Thanveer]

Hi Naveen,

Please have a look at this two links

http://www.cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a00800949e8.shtml#nineteen

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800c95bb.shtml#synch

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.

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Regards
Thanveer
"Everybody is genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is a stupid."

[Giuseppe Larosa]

Hello Naveen,

the BGP synchronization was an old routing loop prevention mechanism that worked in the following way:

a BGP border router would not propagate a BGP advertisement to an eBGP peer until the router does see the same IP prefix learned in IGP ( OSPF, ISIS or other internal protocol).

This would ensure that there are no black holes inside the ISP AS caused by routers not talking BGP. Intermediate routers only taking part in IGP would know how to route traffic with this destination.

In old times not all routers were BGP speakers so the rule was added by Cisco and enabled by default.

The rule is now by default disabled.

The rule can be disabled when:

- all routers in the AS take part in iBGP ( using BGP RR or confederations this is easy to achieve)

- in modern MPLS based networks internal routers ( P nodes) do not perform IP routing but MPLS switching so they can be unaware of IP prefixes and can be not part of the iBGP mesh.

So it is something that you should not worry about, it is good to know what is, but it is disabled by default.

Hope to help

Giuseppe

[bern81]

Hi Giuseppe,

Many thanks for the explanation.

I am running into a weird situation that could might be related to this issue.

I have 3 borders each peering with a different upstream provider via ebgp.

all the 3 router have full-mesh ibgp configured between them.

We are receiving the full-bgp table from our upstreams.

However when i perform : sh ip bgp sum

i don't see the same PfxRcd number from my 2 Ibgp peers (there is a big difference ).
There is no filtering between the ibgp peers.

if i type:  sh run | sec bgp ,    i don't see the word no synchronization in the output.


could it be the issue or this is normal behavior? (as far as i know if we don't have full-mesh and synchronization is on, we can see the routes learned from our IBGP peers in the BGP table but all marked with * , so none of them will be added into the routing table).

is that correct or normally they also don't make into the BGP table?

RQ: our ibgp peers are loopback interfaces advertised via ospf, next-hop-self is configured between them.

Please clarify and thank you in advance.

router version: Cisco IOS XE Software, Version 16.03.05
Cisco IOS Software [Denali], ASR1000 Software