ibgp with private ip and ebgp with public ip

[sjovanovic]

Dear all,

I need your help about a discussion about ebgp session in my public AS, the objective is to save the usage of my public ip range.

I plan to establish ibgp session between several internal routers with private ip address.

R3 and R4 establish ebgp with public ip provided by each ISP.

R1, R2 and R4 announce some prefix to each router of my AS. R3 and R4 announce all my public prefix to ISP. Each router of my AS have a publique loopback.

Could you tell me if is a good way or not ?

Thx

[sjovanovic]

a schema of the topology

[Jon Marshall]

Stevan

It's not clear exactly what your question is.

If you own the AS with R1, R2, R3 and R4 then you are free to use any addressing you want. However you say -

R1, R2 and R4 announce some prefix to each router of my AS

What about R3, do you not want routes from your AS to be advertised to R3 as well and routes received via EBGP on R3 to be advertised within your AS ?

It's also not clear from your diagram where the IBGP sessions are. If each router is peering with only directly connected routers using IBGP (within your AS) then you will run into the IBGP restriction of a route learnt from an IBGP peer cannot be advertised to another IBGP peer.

So R2 could learn a route from R1 via IBGP but it would not be able to advertise this to R4 via IBGP. Similarly R4 advertises a route via IBGP to R2, R2 cannot then advertise that route to R1.

The way round this is either -

1) create IBGP session between each router ie. a full mesh. Ideally this would be done over dedicated links but you can do it via other routers but if you do this a failure of one router can disrupt more than one IBGP session.

2) use route reflectors and clients eg. R2 could be a route reflector and R1 and R4 clients and then R2 could receive a route from R4 and advertise to R1 and vice versa. 

Jon

[sjovanovic]

Jon,

Thank you for your response. I will try to clarify my situation.

I own the AS and a /21 prefix. R3 and R4 announce the aggregated prefix to each ISP.

R3 and R4 advertise to internal router all prefix learned from each ISP. I use local-preference and prepending to optimize inbound/outbound trafic.

R1, R2, R3 and R4 advertise to the AS each prefix who are directly connected. R3 receive full table from R4 through R1/R2 and R4 receveive full table from R3.

R1, R2, R3 and R4 are full meshed with dedicated link. R3 and R4 are not directly connected because they are on a different location.

Session is established with loopback. Loopback and /30 network are advertise between routers of the AS with OSPF.

If R4 loose ISP B and C, he can reach internet through R1, R2 and R3.

I would use private IP address for connecting routers between them to reduce the public ip address loss. Technically it will work, but I don't know if it's a good practice or not.

Below a corrected diagram.

Stevan

[Jon Marshall]

Stevan

I would use private IP address for connecting routers between them to reduce the public ip address loss. Technically it will work, but I don't know if it's a good practice or not.

To be honest i don't know whether there is a best practice for this. If you need to preserve IP public IP addressing then it makes perfect sense to me to use private addressing for internal connectivity that you are not advertising anyway.

As long as it is working i cannot see the advantage of using up public IPs when you do not need to.

Jon

[viswa sai]

Hi,

i'm searching for iBGP config and saw your post. I am in similar situation and would like to know, how have you achieved it.

Could please help with the steps and any challenges faced?