IBGP

Unanswered Question
May 29th, 2007

why do we need IBGP with IGP?

will any body explain me synchronization in details?

I have this problem too.
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smothuku Tue, 05/29/2007 - 21:25

Hi Dangal ,

Remember that the next-hop information from EBGP is carried into IBGP. If IBGP does not have a route to reach the next hop, then the route will be discarded. Typically an IGP needs to be used to exchange routes to the next hop, but this can be achieved by using static routes on all the routers running IBGP.

Synchronisation :

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.

checkout the below link.

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

Thanks,

Satish

mohammedmahmoud Tue, 05/29/2007 - 23:01

Hi,

Synchronization -> For a transit AS no BGP routes should be advertised unless matching in the IGP -> It was used in old network designs which depends on redistributing BGP inside IGP, which is not applicable any more due to the huge internet BGP table which no IGP protocol can stand it -> Modern network designs don't need it -> Synchronization should be disabled for proper iBGP operation.

Accordingly, synchronization should be disabled in all BGP implementations today (as we don't use redistribution between IGP and BGP these days) - Only enabled when using redistribution between BGP and IGP (which is not used now) - Makes problems inside a transit AS

If BGP synchronization is enabled, there must be a match for the prefix in the IP routing table in order for an internal BGP (iBGP) path to be considered a valid path. BGP synchronization is enabled by default in old Cisco IOS Software. With a special case for OSPF, if the matching route is learned from an Open Shortest Path First (OSPF) neighbor, its OSPF router ID must match the BGP router ID of the iBGP neighbor.

Note: Synchronization is disabled by default in Cisco IOS Software Release 12.2(8)T and later.

HTH, please do rate all helpful replies,

Mohammed Mahmoud.

guruprasadr Wed, 05/30/2007 - 02:38

HI Dangal, [PLS RATE / VOTE if Helps]

There are two alternative for injecting local routes into the bgp table. Using nework command / redistribute them.

If there are lot of networks to be advertised, and bgp is used primarily to acheive scalability, not routing security, it could be easier to let the local IGP find the routes & then redistribut them in BGP.

This introduce some risk that the IGP may find some networks that were not supposed to be advertised.

Private networks such as eg:10.0.0.0/8 are used within the AS but must never be advertised out to Internet. Careful filtering must be done to prevent unintentional advertising.

When the router injects a route listed with the network cmd into its BGP table, the origin code is set to "IGP".

If the origin code is injected into the table using redistribution means code is set to "unknown/incomplete".

Simple IGP-to-BGP redistribution:

1.configure redistn... in bgp process

2.configure route-filter using distribution list

Eg Config steps:

#router bgp

#redistribute

#distribute-list out

#exit

#access-list permit

Note: Receiving two routes, one with the IGP origin (inserted with the network cmd) and other one with Incomplete orign, the first route would alwasy be selected, no matter what value the MED attribute is set to (according to BGP route selection rules).

PLS RATE / VOTE if Helps

Best Regards,

Guru Prasad R

dangal.43 Wed, 05/30/2007 - 04:52

but why do we need IBGP, if i will have IGP in my internal network?

mohammedmahmoud Wed, 05/30/2007 - 05:02

Hi,

The IGP carries the interior routes between the interior routers, while iBGP carries only the routes learned from BGP sessions.

For example for a Service Provider, the IGP is responsible for routing between the SP interior routers, while the iBGP carries the routes learned from eBGP sessions on the PE routers between the SP PE routers.

HTH, please do rate all helpful replies,

Mohammed Mahmoud.

dangal.43 Wed, 05/30/2007 - 07:49

it means if i will have full mesh IBGP then i donot need to go for redistrubution, is that correct?

and i have to use the next-hop self comand on the BGP router which are pearing with the IBGP router, right!!!

Amit Singh Wed, 05/30/2007 - 08:08

Dan,

Yes you have to use full mesh connectivity. If you have a large network then making the full mesh BGP is not a practical approach. We have tools like Route reflectors and BGP confiderations to manage the iBGP internal peerings.

It is not necessary to use " BGP next-hop self " command to be used everytime. Its only in case if you want to use an AS router as prefred exit point out an AS.

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

HTH,Please rate if it does.

-amit singh

mohammedmahmoud Wed, 05/30/2007 - 10:29

Hi Dan,

Yes you should either do full mesh iBGP, or use RR (Route Reflector) or confederation (this need is due to that a route learned via iBGP is never sent to another iBGP neighbor).

You only need "next-hop self" when the PE router is doing eBGP, when it advertise the eBGP routes to its iBGP neighbors you should do 1 of 2 things in order for the next-hop of the route to be reachable, either use next-hop self in front of the iBGP neighbor or redistribute the ip of the eBGP neighbor into the IGP in order to be reachable for all the internal routers.

HTH, please do rate all helpful replies,

Mohammed Mahmoud.

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