BGP Tutorial for ISP customers – Controlling inbound traffic

   July 13, 2009

Controlling inbound traffic is the most important consideration when you are configuring BGP. icon_BGP.pngIf you have more than one ISP links, definetely you need to make sure your BGP announcement is propagate to Internet. If you have only one link to the ISP, again you don’t really need BGP. Simple static route statement should be taking care of your needs.
Redundancy, failover and load-balancing traffic those are few major reasons that you have multiple links to ISP. Your BGP announcement is directly affecting your inbound traffic. Yes, it kinds of confuse if you are not dealing with BGP daily basis. How you annouce your routes bring traffic on one of your uplinks.

 

 

Don’t let an ISP controls your traffic

Time to time, people ask me why one link take more traffic than others. Most of case, root of the problem would be the ISP routing policy changes. WHAT~ How could they.

In these days, ISP routing policy is being modifed to avoid loosing their customers traffic. ISP tags high local-pref on their customers routes that is announcing thru their service. It make traffic force to flow thru their service link over the alternate path is learned from peer ISP.  Further more, Tier2 ISPs are tagging higher local-pref on their customers route when they announce it to peers or upstream providers(Tier1 ISPs) using BGP community string.

Do not let ISP control your BGP announcement.
Do control your inbound traffic.
Do not allow one of ISP suching all your traffic.
Get involve more on traffic engineering.
Ask BGP community string to your ISP

Check your BGP announcement thru public route server periodically

 

ISPs are not accepting smaller than /24, c class IP block

To reduce BGP routing table size, most of the ISP is not advertising smaller than /24 IP block to peers.  In other words, if you have your own IP address /24, and split into two /25s. Announcing first half of /24 to ISP A and second half of /24 to ISP B, then you will have
reachability issue. Because, ISP A wont announce first half of /24 to ISP "B" vise versa. You must announce bigger than /24 IP block if its your own.

If you have two ISP links with owned /24 IP block that is allocated from InterNIC directly(but not from your ISP), it is no fun to control inbound traffic. B/C you cannot split it in half for redundancy plan. Well, actually you can but not sure you want it.

See below example,

Let’s assume your public IP address is 172.16.1.0/24 and two ISP link. If you announce route with below plan, it would cover redundancy(failover) and load-sharing

ISP "A"

Announcing 172.16.1.0/24 and 172.16.1.0/25

ISP"B"

Announcing 172.16.1.0/24 and 172.16.1.128/25

ISP "A" will take most of traffic that is destined to 172.16.1.0/25 and guaranteed within ISP "A", but not outside of ISP "A". ISP "B" will take most of traffic that is destined to 172.16.1.128/25 and guaranteed within ISP "B", but not outside of ISP "B". This is just example. There are quite a bit ways to control inbound traffic.
Look around our BGP sample configurations and article for discussing inbound traffic control

See our BGP sample configurations click here

Author: Chris Yoon

IT professional