configure load balanced

[erodrig]

Hi

i have a router with 3 links to ethernet all of the same isp, and i want to configure load balanced over the 3 links, i dont have any routing protocol, i only have a default static to the isp router, how can i configure load balanced? do i need to configure any routing protocol?

thanks

[Harold Ritter]

You simply need to configure one static default route for each one of the ethernet interfaces and let CEF do the rest. You provider needs to do the same but they probably already know that. CEF will do per-flow (src and dst IP addresses) load-sharing by default.

Hope this helps,

[erodrig]

hi

the question it has a mistake, the router have 3 internet links (not ethernet) and just one fastethernet interface, this change anything?

or just need the static default route for each link to isp? do i need to configure cef or is enabled by default?

regards

[Harold Ritter]

No, it doesn't change a thing. Just make sure that your default routes point to the next-hop IP address not the physical interface itself.

CEF is not enabled by default in all releases. Do a "show ip cef" to see if it is enabled. If it is not enabled, just configure it using the "ip cef" command in global configuration mode.

Hope this helps,

[gaurav_saxena]

Hi Hritter,

This will deifinately going to affect the design.If you have three links with teo serial and one fastethernet than in case the link connected on FE port goes down, it will result in dropping of the packets as line protocol will never go down of FE link. In that case you need to remove the one default route poiting to FE port/IP manually.You need to have a routing protocol run between ISP.

Regards

Gaurav

[Harold Ritter]

Gaurav,

I agree that you could have circumstances where the interface could be up but not forwarding traffic. This could happen with serial links as well (Frame-relay for instance). This has always been the drawback of using static routes.

The best way to protect against this is to use a feature called Reliable Static Routing. For more information about this feature, refer to the following URL:

http://www.cisco.com/en/US/products/sw/iosswrel/ps5413/products_feature_guide09186a00801d862d.html

Hope this helps,

[mazlan.alatif]

Hi, what is the different if i point to the interface instead of the next hop address?

[gaurav_saxena]

Since one link is Fastethernet link so its not going to make any difference either point to interface or next hop.

[Richard Burts]

It is NOT true that it is not going to make any difference whether you point to the interface or to the next hop. Especially for interfaces like FastEthernet a static route pointing to the interface is less desirable than a static route pointing to a next hop. There are several factors that cause this including:

- if the static route points to the interface then the router must ARP for every destination address. This will drive up CPU utilization on the router.

- if the router must ARP for every destination it must create an ARP entry for every destination. This will increase memory utilization on the router.

- if the router must ARP for every destination it depends on the next hop router (the ISP router) to answer each ARP and the next hop router may not be willing to do this.

if the router must ARP for every destination it depends on the next hop router responding to the ARP request. If the next hop router does not enable proxy ARP it can only respond to ARP for connected subnet addresses. This will make traffic undeliverable.

So an a LAN type interface like FastEthernet it is much better to do static routes that point to next hop addresses.

HTH

Rick

[mazlan.alatif]

Hi Rburts,

Thank you for the useful tips. How about POS interface? Are this 'rule' also applied to POS?

[Richard Burts]

Some of the disadvantages I discussed are specific to LAN interfaces where the router must ARP to learn the MAC address of the next hop. Since you do not ARP on POS some of these do not apply. But in general I believe that it is appropriate to point static routes at interfaces only where the interface is a point to point link. If the interface is multi-point or multi-access I believe that it is best to point at next hop addresses.

HTH

Rick

[gaurav_saxena]

Hi Rburts,

Your inputs are fine.But requirement of erodrig is something different. He has three Internet links to ISP and want to load share on these three links.If all three links are Serial/point to point link than there is no problem and simple static default routes will solve his purpose. Problem here is b'coz of one fastethernet link. So there are two options left..

1) To run routing protocol with ISP

2) To use reliable static routing as mentioned by hritter.

Regards

Gaurav