Router Load Balance

Unanswered Question
Jul 24th, 2007

Hi all,

I have 3 T1 lines run in a 2621 box. (WIC 0-1 serial port, WIC 1-2 serial ports).One Public IP configured in a FastEthernet port - Fa0/0. Fa 0/0 is the gateway for internal users to access the Internet. Can I configure in such way: user 1 access thru T1(A), user 2 access thru T1(B), user 3 access thru T1(C) and user 4 back to T1(A) (round robin)?

If can, how to configure it ?If can't, what is the another option ?

I have this problem too.
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Are the 3 T1 bundled (mlpp, mfr) or just using layer 3 load balancing? If you use layer 3 load balancing then the default is per-destination load sharing. You have option to change it to per packet load sharing (only recommended if the 3 links have the same delay). You can configure policy routing to achieve what you described by means of configuring different next hop for the particular host.


erichiew Tue, 07/24/2007 - 21:38

Hi Krisztan,

I better to illustrated it. My ISP provide 3 serial IPs 68.xx.xx.xx/30, 56.xx.xx.xx/30, 58.xx.xx.xx/30 and share a set of Public IPs 204.xx.xx.xx/27. (I'm not sure whether the 3 T1 lines are bundled or not but if they configure separately using same Public IP, user can access internet thru these T1 lines.). 204.xx.xx.20 is connect to Fa 0/0 and F/W IP is 204.xx.xx.21. All packets access to internet will route from 204.xx.xx.21 (f/w)to 204.xx.xx.21 using static route. From ISP's traffic monitoring tool, I only saw 68.xx.xx.xx got heavy traffic flow but others may have little or totally no traffic flow thru. How to configure Cisco 2621 router to make these 3 T1 lines have equal/balance traffic in their network ?


So you have 3 independent layer 3 connection.

Ok. It is getting clear :)

As long as all your traffic comming wih source IP 204.xx.xx.21 (I guess your FW doing NAT) you cannot distinguish the traffic based on source address.

Is there any routing protocol run between you and the ISP? If so then you may have to tune that. If not you can use equal cost static route through the 3 circuit.

like this

ip route serial0

ip route serial1

ip route serial2

And the ISP have to configure it for your subnet on the Fa in order to achive load balancing in the reverse direction.

Hope it helps,


erichiew Sat, 08/04/2007 - 08:38


I don't understand about "ISP have to configure it for your subnet on the Fa in order to achive load balancing". Whose "Fa"? My site or ISP site? if my site, how to configure it.



Let me clarify.

You probably have a public IP subnet configured on the interface connected to the LAN (Fa).

Your ISP configure an IP route on their router to reach that subnet through the leased line. If you have 3 leased lines and you want that the traffic to be load balanced using both link they have to configure to reach that subnet through not only one link but both 3 and you have to also configure to reach "everything" through both links.

This can be achieved by

ip route serialx

ip route serialy

ip route serialz

And the ISP will configure something like this

ip route serialv

ip route serialy

ip route serialw

Another choice is to use dynamic routing protocol, but the static route is more simple and unnecessary routing protocol traffic will not eat the bandwidth.

Please post your config to see what you have.

Hope it helps


erichiew Wed, 03/26/2008 - 20:01

Did configure through dynamic routing protocol involved ISP? If not, how to configure using dynamic routing protocol?

Thank you



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