Data Center ISP Load Sharing Part 3 – Route Selection

Part 2 of the posting shows the advantage of simple default route based Internet load sharing design. This part further optimize the design.

Using the entire Internet routing table for outbound load sharing proves to be resource intensive, and ineffective for load balancing. Default route only provides simplicity and better load balancing. To further optimize, a subset of Internet routes, when selected according to the unique environment, can complement the default route design very well.

Route Selection

Route selection refers to filtering and allow a subset of the Internet routing table to be introduced into the data center. The desired effect is to take the shorter path to content that is directly attached to specific ISPs, while the rest of the traffic load share equally to both ISPs.

The effectiveness of the design is largely based on route selection techniques applicable to the specific data center environment. In the example shown below, BGP regular expression is used to select a subset of Internet destinations adjacent to each ISP.

ISP1 is a tier one, therefore has more directly attached networks. BGP expression is used to select those directly attached networks, with the objective that traffic destined for those networks will exit on this ISP for optimal path.

ISP2 is a tier two, with less number of directly attached networks. BGP expression is used to select those directly attached networks as well as those one additional hop away, with the objective that roughly equal number of specific target networks will prefer ISP2 as the exit point, thus achieving load sharing with both ISPs.

Implementation

On the respective internet router connected to each ISP, AS path filtering is applied on a route map, which is then applied to BGP inbound route filtering. As a result, the default route, as well as a subset of Internet routes is received from each ISP, in order to optimize outbound traffic to take the more direct path to destinations.

ip as-path access-list 1 permit ^3549_[0-9]*$

route-map ISP1in permit 10 

 match ip address prefix default 

route-map ISP1in permit 20 

 match as-path 1 

router bgp

 neighbor … route-map ISP1in in

Verification and Tuning

At the planning stage, counting number of routes using BGP regular expression filter may serve to arrive at the initial route selection design. By filtering in similar amount of specific routes from each ISP, the desired load sharing can usually be achieved. 

However, equivalent number of routes does not always result in equivalent amount of traffic. Over time, actual load on the respective ISP connections will provide more accurate information about traffic in the particular data center. ISP specific characteristics may also factor in. Part 4 will show why fine-tuning may be necessary.