BGP questions

[ericng1983]

Hi there,

I have a scenario that I require to have 2 PE routers and 2 CE routers interconnected in a way that using eBGP and iBGP protocol. Let me describe how it should be connected in GNS topology.

CE1 fa0/0 connects PE1 fa0/0

CE1 fa1/0 connects PE2 fa0/0

CE2 fa0/0 connects PE1 fa1/0

CE2 fa1/0 connects PE2 fa1/0

Both PE router are running iBGP with AS no. 1234

Both CE1 & CE2 router are running eBGP with AS no. 1000 and 2000 respectively

My question is, iBGP that the PE router are running are being separated by the CE router and this kind of setup is workable?

Will both the PE iBGP router synchronize it's routing table via the connectivity of the CE router?

PE

eBGP eBGP

CE iBGP CE

eBGP eBGP

PE

Above is the physical setup in the lab practice. Sorry not able to screenshot the topology as my GNS in on my PC and I currently using IPad App to write. Hope my output is clear to you guys and appreciate your expert explanation.

Cheers,

Eric

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[Marwan ALshawi]

i am not sure if i understood you question 100%

but from your topology if you have PE to PE you will need a P in between and you run MPLS between PEs over the P

and each CE has to be in differnt VRF in the PEs

routing of each PE will be in its own vrf routing table PE reach other PE over IGP

even thorugh they run iBGP between them

the ibgpo between PEs used for MPBGP L3VPNv4 to carry CEs/VRFs routing over MPLS

not sure if you are aware about this type of setup which is basic typical service provider MPLS network

if you dont want to use MPLS you can use vrf lites ( each PE has itw one vrf ) but you need back to back link of each vrf between the PEs

if you are not looking for isolation of the routing tables then you will be runing in a complex BGP polices and filtering

for example PE recive route from CE1 with AS 1000

you need to have a polcy route-map that deny any route with AS 1000 in the outbound direction in each PE toward CE2

to make sure CE1 route will not go to CE2

same for CE2 route to be denied from going to CE1

the route of both CEs will be exchanged between PEs over iBGP

HTH

if helpful Rate

[milan.kulik]

Hi,

IMHO, forget any VRFs, MPLS and other complicated things.

Just look at this topology as: AS 1000 connected via eBGP to AS 1234 connected to AS 2000 via eBGP.

And AS 1234 is running iBGP inside.

My understanding of the original question is: "Couldn't there be some routing loops created due to both PE routers peering to the same CE router and exchanging routing info via iBGP?"

The answer is: No, as any prefix not originated from the CE AS would have to include 1234 in the AS-PATH and would be rejected by any of the PE routers.

HTH,

Milan

[bhornilesh]

Hi Eric,

As per your requirement & Diagram, before implementing EBGP & IBGP on CE & PE Router. You need completely IGP reachability on all Router..After that you can do EBGP & IBGP peering both CE & PE router….

Please let me know if you have any confusion.

Regards

Nilesh

[Marwan ALshawi]

Nilesh,

EBGP dose not need IGP unless you do the BGP Peering using loopback interfaces as you need the loopback interface to be reachable thorugh IGP or Static route

other than this only ibgp require IGP for fully meshed and redundant ibgp sessions

HTH

[bhornilesh]

Yaa....i'm considering Loopback interface peering only & if directly connected link cable..we don't need use IGP peering...

[ericng1983]

Hi all,

First of all appreciate the response.

Am I right to say that with my current setup is workable unless I configure an IGP protocol such as static routing to make the all network routable before peering can be done to sync the iBGP routing table between the 2 PE router?

Please look at the photo. Sry my drawing cmi as I am using free finger to draw.

Thank you

Eric

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[bhornilesh]

Hi Eric,

First of all, r u able to ping your PE1 Router from PE2 & vice versa..? if Yes, then you don't need use any IGP routing or Static Routing for peering IBGP....If not able to ping, then you have to use any IGP routing protocol or Static Routing in PE1 & PE2 Router.. then only you can peering IBGP in 2 between the PE router....

If you want to verify then you can make it Same topologoy on GNS3 & try to do..you will get right output...

Please revert if you have any confusion or Problem,..

Regards,

Nilesh

[Marwan ALshawi]

i can see from the drawing there is router between PE1 and PE2

is this router runing iBGP ?

and in any way as long as those PEs not directly connected then you need IGP in between and you need the BGP of the CEs to be redistributed in PEs BGP (iBGP) for end to end reachability

HTH