EIGRP unequal load balancing scenario

PJ · ‎01-07-2010

Hi All,

I have a network scenario involving EIGRP routing between Cisco 6500 and two 3800 routers. The Internet T1s are connected on the 3800 with two T1s on Rt1 and one T1 on Rt2. I have setup EIGRP unequal load balancing on 6500 with variance 2 so that two connections go to Rt1 and one connection goes out to Rt2 and the routing works fine. PAT is running on both 3800s to get out to the internet. However, since the internet T1s have different public serial IP addresses, the connections going out to the internet servers might be taking any T1 path out to the internet and we have users complaining about some websites not fully loading pages with partial pages getting displayed. Is there a better way to define load balancing. CEF is turned on and default connection based load balancing is occuring. I have attached the diagram to be more clear.

Example; From a client PC behind the 6500, I go to ford.com and click on several links on the website. As supposed to work, I see NAT translations on both 3800 routers sourced from the client PC, going to same website ford.com. Is it causing problems because the Ford website sees two different public IP addresses (of the internet t1s) for the same connection ?

Any suggestions will be appreciated.

Thanks.

Giuseppe Larosa · ‎01-07-2010

Hello Praful,

this happens because actually also the website has resources associated to different ip addresses.

CEF works on the pair IP SA, IP DA so until the IP SA (client) and IP DA (destination) are the same the same path is chosen.

In your case you should consider the following options:

change the routing so that RT1 is used if at least one T1 is alive.

AND you can use PBR policy based so that a subset of possible users are always routed to one router on the ip source base.

This would solve this issue.

PBR has to be applied inbound on the interface that receives user traffic on the C6500.

access-list 111 permit ip 10.10.10.128 0.0.0.127 any

route-map PBR_toRT2 permit 10

match ip address 111

set ip next-hop RT2:lan:ipaddress

int vlan 55

desc internal vlan to user

ip policy-map route-map PBR_toRT2

you may need to apply PBR on multiple client vlans if they are terminated directly on the C6500

Hope to help

Giuseppe

Mohamed Sobair · ‎01-07-2010

Hi Praful,

There is no diagram attached, However, This setup is not appropriate.

The best approach is to loadhare the traffic by having the servers takes a single path utilizing the first T1 while leaving the clients takes the second path.

HTH

Mohamed

PJ · ‎01-07-2010

Thanks for your input. Sorry, I missed the attachment. You should be able to see it now.

Jon Marshall · ‎01-07-2010

praful_jaded wrote:

Thanks for your input. Sorry, I missed the attachment. You should be able to see it now.

Looking at the diagram i agree with the other guys. I would suggest using one of the links for servers and the other for user connectivity to the internet. PBR can be used for this.

Jon

Marwan ALshawi · ‎01-07-2010

Hi guys

Although PBR is a valid option for this topology, but i found it not redundant for example if the Internet link connected to RT2 then the PBR will blackhole the traffic !!! unless you configure PBR with IP SLA in this case you can change the PBR path if the link goes down !!

i think one of the best option for this topology is the use of performance routing

you can configure one of routers as BR and MC and the other router as BR

you assign utilization percent to each link and you use static routing with a tag number all

internally you can keep the eigrp as the routing protocol and in the border routers you redistribute static with only for the taged route assigned

chose a long prefix for the automatically created routes like /29 or /32 in this case the routing will prefer the path based on logiest match and you will have lodbalncing over the links based on links utilization

bellow is a document will take through a simple example of configuring this technology

https://supportforums.cisco.com/docs/DOC-8353

good luck

if helpful Rate