BGP load Balancing using a single router w/multiple links

[mpetrella]

I have a situation where we have a single router with multiple ISP links. A link is to ISP1 the other is to ISP2. The ISPs are static routing to us. They both want us to default route to them. The topic is how do we load blanace between the links to gain the fullest use of the links. I was thinking of adjusting the BGP weights to make them equal hoping that the router would "see" equal weights and thus load balance. I have not found any documentaion on the subject and decided to try this forum. Thanks in advance for any responses.

[mmikhail]

You don't need to use BGP. Just static default to each. The IOS would load balance between the two routes by default. If you are using NAT to each of their address spaces, that would take care then of balancing return traffic as well. Otherwise, if you're using your own public address space, balancing return traffic becomes a complex issue.

[MickPhelps]

You need to be careful with load balancing to two ISPs and using NAT in this fashion. It can be done, but you must take care that each session follows the same path out or destination devices will disgard packets that weren't NATed from initial interface. In other words, don't do packet based load sharing. Even destintation based load sharing could cause problems if forwarding cache timeouts occured and your session was still open.

Mick.

[mmoscoso]

Hi, The idea is to set a better metric for the routes in the range 1.0.0.0 to 128.0.0.0 learned from isp1 and a better metric for the rest of the routes learned from isp2. (You can choose any range)

When your router receives a packet for networks between 1.0.0.0 to 128.0.0.0, it will forward those packets to routerISP1.

When your router receives any packets destined for networks above 128.0.0.0 it will forward those packets to routerISP2.

To do this you need a combination of access list and route maps.

If you need more help with the configuration, let me know.

Hope this help,

Mauricio

[msheehan]

Cisco has a good write up about this on their website. Here's the link ..

http://www.cisco.com/warp/public/459/40.html

[ramesh.krishnan]

hi,

you could do this with route-maps. I hope both the ISPs have given you IPs and you just need to load balance your output.

For this just create 2 access-lists (extended) mactching the Ips of each ISPs.

Then put "ip policy route-map " in your router's ethernet interface.

then

route-map ISP1

[ashok_boin]

Hello,

Packet based load sharing which is by default with default routes pointing to your ISP's is not recommendable. For every load balancing scenarios, we have to consider both INPUT and OUTPUT balancing separately.

For your OUTPUT, segmenting total received networks into two parts and manually increasing the Local preference will work.

For your INPUT, assuming your networks as 10.1.1.0/24, 10.1.2.0/24, 10.1.3.0/24. Then advertise first two networks as it is and for another network use any AS-path prepending technique to one ISP A to make inbound path from external to 10.1.3.0/24 through A less preferred. Just do the reverse process to ISP B. Consider the bandwidth ratios of A & B.

Regards...

--Ashok.