hi jon,

when to enable synchronizationa and when to desable it?

and will you please explain this

"If synchronisation is on then before a route that is learned from an IBGP neighbour can be put into the IGP routing or before it can be advertised to another BGP peer the router must know about the route from an IGP." i m not clear with this.

it something like LOCAL_AS in migration of AS, so what is that exactly?

Dave

I'll try and explain with an example. You may need to draw this out :-)

R1 is in AS 150.

R2 is in AS 150.

R3 & R4 are also in AS 150

R1 & R2 run IBGP between each other.

R1 connects to R3. R3 connects to R4. R4 connects to R2. There is NO direct connection between R1 & R2.

All 4 routers are running EIGRP.

R1 is peering with REXT1 which is in AS 200

R2 is peering with REXT2 which is in AS 300.

Both these peerings are using EBGP.

So, with synchronisation on

1) R1 learns a route from REXT1 for 192.168.5.0/24

2) R1 advertises this route to R2 via IBGP. R1 advertises the next hop for this route as itself.

3) R2 now advertise this out to REXT2.

NOTE - 3 won't happen but i'm using this as an example. See below for what actually happens.

Now REXT2 wants to send traffic to the 192.168.5.0 subnet and sends it to R2.

R2 looks up the route for 192.168.5.0 and sees it is R1.

R2 then looks up how to get to R1 and sees from it's routing table that it needs to go to R4. This route was entered into the routing table by EIGRP.

R4 now receives the packet. It does a lookup for 192.168.5.0 but it has no entry in it's routing table for this. Remember the route for 192.168.5.0 was advertised from R1 to R2 via IBGP.

So R4 drops the packets as it has no route to that network.

So in the real world step 3 would not happen with synchronisation turned on. When R2 receives the 192.168.5.0 route from R1 it checks it's IGP routing table. If it isn't in there it cannot advertise that route out to REXT2.

The way to solve this is to redistribute BGP into EIGRP on R1. Then when R2 receives the advertisement from R1 for 192.168.5.0 it looks into it's routing table and finds an entry. So it can now advertise out that route to REXT2.

If you turn off synchronisation you must make sure you have a full mesh of IBGP connections between all your interior BGP speaking routers.

Generally speaking you don't want to be redistributing all your BGP learned routes into your IGP so you would want to turn synchronisation off.

Hope this makes sense.

Jon

you mean to say we can advertise some dummy number or we can establish the session with external peer with the different AS number than the actaul AS in which router is in.

then what is the "dual_as" key word in it?

will you please tell me about the synchronization: when should i enable it or disable it?

Friend,

Synchronization requires your IGP to synchronize with the BGP.

Say for eg you have 3 routers R1,R2 & R3 connected in a triangle.

If you plan to have a full mesh BGP between all the routers, then in this case you can disable synchronization.

The Idea is to have the all the routers having routing information so that packets are not blackholed

HTH, rate if it does

Narayan

and what are thos community, and what is mean by notation XX:YY in community configuration? i am not able to understand that configuration work.

Communities are a way to define policies to a certian set of routes.

The notation XX:YY is new BGP community format.

check this link

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00801475b2.shtml

HTH, rate if it does

Narayan