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2621CEF and NAT

Unanswered Question
Sep 8th, 2002
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I am trying to provide load balancing over two serial (HDLC) T1 links on a Cisco 2621 router using NAT. The documentation I've been able to glean from CCO is a bit ambiguous on the subject, so I thought I would try this forum.

1. Is CEF supported on the C2600 platform (32MB DRAM, 16MB Flash)?

2. Will CEF work with NAT over dual T1's (dual WIC-1T-DSU modules)?

3. Should CEF be enabled for this config globally or on a per interface basis?

Any assistance would be greatly appreciated.



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steve.barlow Sun, 09/08/2002 - 06:46
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1. Yes. http://www.cisco.com/warp/public/63/2600_architecture_23852.html

2. Cisco IOS NAT supports Cisco Express Forwarding (CEF) switching, Fast-switching and of course Process switching.

3. Enable CEF globally when your router has interface processors that do not support dCEF. See link:


Hope it helps


vmiller Mon, 09/09/2002 - 09:36
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be a bit cautious with CEF and NAT. It has been my experience that with a moderate to heavy nat load (1000+ translations) we were getting some partial failures until we turned CEF off . We were attempting a load share load balance config also

v.lui Wed, 09/11/2002 - 23:25
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With the Dual T1, and NAT in place, instead of CEF, i have been using MLPPP to bundle the two T1s together. My 2600 has 2 serial interface, 1 FE, and 1 Eth. WHen i try to nat overload the FE(with public IP), this seems togive me major problems with the nat'ed address not able to pass certain types of TCP traffic, as well as the interface cannot be telneted into from outside.

When i turn off the MLPPP and only use one T1 one serial, everything seems ok. Can anyone explain? Is it because NAT doesn't support or work with MLPPP?

i have been troubled by this for so long that i decided to take the NATing functions out of the router and have another device do the job. This is not clean, but works. Would appreciate anyone who can shed some lights.

please drop me a line at [email protected]

daniel.kline Thu, 09/12/2002 - 03:18
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Unfortunately, I did not have the option of using PPP as my WAN encapsulation type, I had to use HDLC. But I did manage to get CEF to work properly with NAT. I did not see it in any of the sample configs I viewed, but I had to point my default interface at both S0/0 and S0/1 to get load balancing to work. I have no ACL prohibiting it, but I too cannot telnet into the router Serial CEF interfaces externally.



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