CEF in presence of two T1s...

[abatson]

I'm about to get a dual-T1 solution from a major telco. vendor; a connection to the Internet. They offer per-destination & per-packet options on CEF configuration. Question on per-destination: Let's say I have 100 people with 100 TCP connections open to CNN.COM. Will each of the 100 separate TCP connections go to a different T1, or will the router ensure that ANY traffic destined for CNN.COM goes to a *single* T1?

-Alex

[Georg Pauwen]

Hello,

with per-destination load sharing, your 100 users will get evenly distributed over both T1 links, but the source-destination traffic stream will always stay on the same link. So if one of your users downloads large files, that traffic will not be load-shared over both links, which can lead to, in effect, unbalanced load-sharing.

With per-packet load sharing you can achieve true load-sharing, since the router sends successive data packets over paths without regard to individual hosts or user sessions. It uses the round-robin method to determine which path each packet takes to the destination.

Check out this document for the detailed explanantion from Cisco:

Troubleshooting Load Balancing Over Parallel Links Using Cisco Express Forwarding

http://www.cisco.com/en/US/partner/tech/tk827/tk831/technologies_tech_note09186a0080094806.shtml#knowniss

Regards,

Georg

[thisisshanky]

CEF by default uses a hash algorithm, which calculates a hash based on source and destination ip address pair. This hash is used to select the route (in your case, default routes to the ISP). So if 100 users are accessing CNN.com, each user will alternate between the T1s.

Like said, in the other post, if one user downloads a large file, you wont be able to share the load between the T1s. So its better that you enable per packet load sharing.

ip cef

int s0

ip load-sharing per-packet.

int s1

ip load-sharing per-packet.

Hope that helps!

[abatson]

Original poster here. Has CEF overcome this issue, or would per-packet balancing via CEF consume more CPU cycles than per-destination, since there are more decisions to be made per time-period..

-alex

[tbaranski]

It will generally use more CPU time, though how much more seems to depends on IOS version and router model.

[abatson]

Original Poster here. I'll be doing the dual-T1 config on a 2611XM with one of the K9 IOS's from the 12.1 train I think. Two WIC-T1-DSU cards in it. 2611XM is an 80Mhz CPU(?) I also have a 1751 available as well, which is better?

-Alex

[tbaranski]

The 2611XM is somewhat faster than the 1751, though both should be fine for dual-T1s.

[Georg Pauwen]

Hello,

according to Cisco there is no performance degradation:

Load balancing with CEF for IP

In IOS software 11.1CC a new forwarding mechanism for IP packets was introduced: Cisco Express Forwarding (CEF). The design of Cisco Express Forwarding includes enhancements that allow to use load balancing without sacrificing forwarding performance even when using per packet load balancing. Previously per packet load balancing required disabling of route-caching mechanisms like fast switching or optimum switching (see above paragraph). CEF is available in IOS software version 11.1CC for Cisco 7200 and 7500 series routers only. CEF currently supports the following encapsulations: ATM/AAL5snap, ATM/AAL5mux, ATM/AAL5nlpid, Frame Relay, Ethernet, FDDI, PPP, HDLC, and tunnels.

Check out this link:

Load Balancing with CEF

http://www.cisco.com/en/US/partner/products/hw/modules/ps2033/prod_technical_reference09186a00800afeb7.html

Regards,

Georg

HTH,

Georg