Load Balancing Two Circuits

[encarter]

So i have two circuits that term to another site. Both circuits term to seperate routers

Site1R1----------Site2R1
| |
| |
Site1R2----------Site2R2

The IGP is OSPF and we want to load balance the circuits in a bi-directional fashion, how would one go about that?

 

[unclerico]

what are you using as the gateway at each site?? what are the link speeds?? ospf will do equal-cost load balancing by default if both circuits have, well, an equal cost.

I hate all Uppercase... I don't want my groups to seem angry at me all the time! =)
- ColdFlame (vbscript forum)

 

[Quadratic]

Is the intent to have traffic from within Site 1 to load balance between R1 and R2 when going out to Site 2?

What is the A to B path for traffic within a site to reach its own R1 or R2? What is the default gateway of individual workstations? Speaking generally, you might use something like HSRP or GLBP to redundantly load-balance LAN-side traffic towards R1 and R2, then just have your site's R1 direct all inter-site traffic to the other site's R1, and do the same for the R2's.

For the common HSRP method, you'd assign (for simplicity) all traffic for VLAN 10 into one HSRP group, and all traffic for vlan 20 into another HSRP group, where R1 is routing for group A and R2 is routing for group B. They'll failover to a single path if one router fails, but otherwise load-balancing can be done on a per-vlan basis.

You might use policy-based routing to direct inter-site traffic, but there are a few options there. The bigger goal from what I understand of your question though is in directing traffic flow LAN-side towards the R1 and R2 routers, not in load-balancing traffic that gets to R1 in site 1 towards both R1 and R2 at site 2.

CCNP, CCDP

 

[encarter]

The intent is from Site-1 one to send traffic over both links even if one circuit has a better metric than the other.
When it gets to Site-2 then it can take the preferred path.

 

[unclerico]

you really should run EIGRP instead if that's the case. with OSPF you can tweak the costs on the links to make them equal. the problem is that you could completely saturate the slower link. EIGRP will allow you to do unequal cost load sharing and it will take into account the slower link and distribute the traffic proportionally. you could also use OER/Pfr

I hate all Uppercase... I don't want my groups to seem angry at me all the time! =)
- ColdFlame (vbscript forum)

 

[Quadratic]

Is the network contiguous such that Site 2 can be defined within a single subnet range? Unless there's a specific reason for it I don't recommend this kind of load balancing (far more efficient to direct traffic flow to R1 and R2 LAN-side rather than to have them send traffic across to each other and then onto Site 2), but one way is to cut Site 2's IP scheme in half, and use route maps to push all traffic for the first half out Interface A, and the second half out Interface B.

Here's an example config file snip to make a routing policy that puts anything destined for the first half of the 172.16.0.0/16 range onto one interface, and the second half onto the other (replace the loopbacks with relevant interfaces in your configuration):

interface Loopback1
no ip address
ip policy route-map Example
!
access-list 100 permit ip any 172.16.0.0 0.0.127.255
access-list 101 permit ip any 172.16.128.0 0.0.127.255
!
route-map Example permit 10
match ip address 100
set interface FastEthernet0/0
!
route-map Example permit 20
match ip address 101
set interface Loopback0

Since your routing protocol is OSPF, you can't do unequal cost load balancing without hard-coding your metrics, and even then the cost changes on each given interface is globally significant, so it would be difficult to do it for both R1 and R2, for both sites, unless you get really fanciful.

Also if you DO go with EIGRP and do unequal-cost load balancing by changing the variance, I strongly suggest adding "mls ip cef load-sharing full" for per-flow load-balancing. Much better than the default, although I think that syntax has changed a few times. Also assuming you're running CEF.

CCNP, CCDP