Showing results for 
Search instead for 
Did you mean: 

iBGP with Public or Private IP's

Working on a new datacenter deployment with 2 eBGP routers, each connected to a separate ISP where both will give us full routes


My plan for iBGP and the lateral connections between our routers:

1x10Gb layer 2 connection

2 x 2Gbps LACP port channels using 4 ports on each router

A public /30 on each port channel


My question is do I need to use publicly routed /30's on these lateral links or can I use private IP's in the design for B.B.B.X and Y and D.D.D.X and Y?


See graphic here



VIP Mentor

You can use any one depends on availability of the IP address space(if you have spare public IP you can)


Private also works since this is iBGP and reside internall, either case as long as you have ip reachability you can do.


Again how is your eBGP configured, in the same Router ?

*** Rate All Helpful Responses ***
VIP Advocate


As mentioned that it depends on your EBGP configuration and IP address availability. But I will go with a Private IP address if really not required. Because those two IP addresses are not going publicly anymore. 



Deepak Kumar 

Deepak Kumar,
Don't forget to vote and accept the solution If this comment will make help you!
VIP Mentor


If you use public addressing from which isp are you going to use it from?  Also you dont want to be advertising isp addressing from within your internal network possibly incurring transient traffic between through your wan rtr's


Go with internal addressing, no need for public - your isps will be aware of your internal network anyway.

kind regards

Please rate and mark posts accordingly if you have found any of the information provided useful.
It will hopefully assist others with similar issues in the future

There are 2 ISP's providing the northern link on each router. Per the picture, one ISP is A.A.A.A and the second ISP is C.C.C.C. So I expect that both ISP's are going to give an external public IP to use to terminate each of their links on my routers.


My ask to them was going to be to each provide a /30 or /31 that I would use on the lateral links between the routers 1 ISP would provide B.B.B.X and the other ISP would provide D.D.D.X


Then I would build a 2 port port channel for each /30 or /31 and put the two IP's on each side of each port channel. Then use OSPF to distribute traffic across both

The ibgp peer on each router would use a loopback private IP, and then OSPF would be configured to route traffic between the two loopback private IP's over the 2 port channels with a public IP on each port channel

Like this: is subnetted, 2 subnets
O [110/21] via, 7w0d, Port-channel3
[110/21] via, 7w0d, Port-channel6


All of the above would be using ISP provided IP space.

I have my own provider independent /23 that is separate and will use a public ASN. I would advertise to the first ISP the /23 and 1 /24 that is part of the /23. Then I would advertise the /23 to the second ISP and the other /24 that is part of the /23

I'll be using HSRP on my /23 addresses

both routers would be doing Full Routes from each respective ISP


We would receive traffic from the internet inbound from both ISP's and the traffic would travel laterally if the destination is hanging off the adjacent external router or it would just proceed south if the destination is hanging off the router the traffic it came in on. 


I don't have the public IP space to do the lateral links, because I don't want to break off 2 x /30's from my /23. I was planning to try to get the /30 or /31 thrown in by each ISP to do this


We have a similar setup in another datacenter we have, but I didn't set it up. I guess I am taking for granted that I need to use public ip's for those lateral links as that is how this other datacenter is setup. 


In light of this it is still fine to just use private IP's on the lateral links for purposes of routing between the loopback IBGP peers over 2 L3 portchannels with OSPF?



You can use private IP addressing. 


The destination IP inbound will never be those IPs on the lateral links and the ISPs are peering on public IPs on the northern links so they have no need to know them either ie. they will just be used for IBGP peering and routing across the link if the destination is reachable via the other switch. 


I inherited a similar setup where the interconnect is using public IPs and I keep meaning to change them but never get the downtime :)