Load Balance 3 internet Link with Full internet Routes

Unanswered Question
Mar 16th, 2009


I have 3 bgp peering with different ISP. I would like to load share all these 3 link.

What are the best method to do this?



I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
JamesLuther Mon, 03/16/2009 - 02:56


If you setup iBGP between the three internet routers then BGP will select the best path via each ISP. Of course remember to setup a filter list to stop you acting as a transit AS.

However using this method you will see that one ISP will probably be used more than another (depending on how good there peering is). This is because it is the best path.

You can use Cisco OER which will use all of your links more evenly, however your path to each route on the internet will be suboptimal


there are other design choices as well, it depends on whether you're advertising any PI ranges or NATing behind each providers IPs?


Joseph W. Doherty Mon, 03/16/2009 - 05:19

"You can use Cisco OER which will use all of your links more evenly, however your path to each route on the internet will be suboptimal "

Perhaps true if you only enable link load balancing. However, if you also enable flow performance monitoring, performance (in theory) should be the best possible since a flow can be directed to the best performing path (for that flow). Works best for outbound, not so well (or at all) inbound unless you run PfR (OER v2) and do some very complex cooperation with your ISPs.


An interesting aspect of using actual flow performance, you may no longer need to accept BGP routes.

ccsam Mon, 03/16/2009 - 18:53


Thx for the information. In terms of transit AS, are there any ways to verify whether my environment become a transit AS?

The normal practice is to received routes from direct peer with ^$, and the direct peer customer routes ^123_ and peer's routes ^123$.

By doing this will it cover all the internet routes? and prevent from become transit AS?



Joseph W. Doherty Mon, 03/16/2009 - 19:21

To avoid being a transit AS (unrelated to OER/PfR), insure only your network's public address blocks are advertised to your ISP peers. Additionally you can drop any inbound traffic or outbound traffic that doesn't contain your public IP addresses.

JamesLuther Fri, 03/20/2009 - 12:58


When peering with multiple ISPs towards the internet I always use the following

router bgp 179

neighbor filter-list 99 out


ip as-path access-list 99 permit ^$

This will advertise any routes originating from your local AS (blank AS path), but filter out anything else.



This Discussion