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I have answered my own question so for the interested here is/are the answer(s):<br />
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1) There are several versions of the VPN card for the 1720/1721. The two I have are 73-4586-02A0 and 73-4586-02B0. &nbsp;The 02A0 will work in the 170 and 1721, the 02B0 will not work in the 1720 but will work in the 1721. &nbsp;So if you buy on eBay make sure you check.<br />
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2) The 12.3 images supporting IDS FW IPSec etc will not run in the 1721 - the release notes reveal this but are tricky. &nbsp;12.2 images work well in both the 1720 and 1721.<br />
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Cool I am my own expert.
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I have answered my own question so for the interested here is/are the answer(s):

1) There are several versions of the VPN card for the 1720/1721. The two I have are 73-4586-02A0 and 73-4586-02B0.  The 02A0 will work in the 170 and 1721, the 02B0 will not work in the 1720 but will work in the 1721.  So if you buy on eBay make sure you check.

2) The 12.3 images supporting IDS FW IPSec etc will not run in the 1721 - the release notes reveal this but are tricky.  12.2 images work well in both the 1720 and 1721.

Cool I am my own expert.

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I have a Cisco 1721 router and have tried to install a VPN module MOD1700 - 3DES and a show diag indicates the module is not working. &nbsp;The module works fine in a 1720. &nbsp;I think I have loaded the correct IOS image c1700-k9o3sy7-mz.123-9.4.b<wbr />in<br />
I also tried c1700-k9o3sy7-mz.122-4.T5.<wbr />bin - the same image as in the 1720.<br />
<br />
The 1721 has 96 meg of RAM and 16 meg of flash, an SDSL WIC and an Ethernet WIC.<br />
<br />
From what I have read the VPN module should work in a 1721, have I missed something?
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I have a Cisco 1721 router and have tried to install a VPN module MOD1700 - 3DES and a show diag indicates the module is not working.  The module works fine in a 1720.  I think I have loaded the correct IOS image c1700-k9o3sy7-mz.123-9.4.bin
I also tried c1700-k9o3sy7-mz.122-4.T5.bin - the same image as in the 1720.

The 1721 has 96 meg of RAM and 16 meg of flash, an SDSL WIC and an Ethernet WIC.

From what I have read the VPN module should work in a 1721, have I missed something?

VPN Module problem in a Cisco 1721 router

Networking is the process of connecting computing devices, peripherals and terminals together through a system that uses wiring, cabling or radio waves that enable their users to communicate, share information and interact over distances. Often associated are issues regarding operating systems, hardware and equipment, cloud and virtual networking, protocols, architecture, storage and management.

Networking

Hardware-based firewalls provide more sophisticated protection for inbound and outbound traffic than the simple Windows software firewall or the basic NAT firewalls found in routers. These devices implement techniques such as stateful packet inspection, deep packet inspection, and content filtering; and may include built-in antivirus and anti-malware protection.

Hardware Firewalls

Internet Protocol Security (IPsec) is a protocol suite for secure Internet Protocol (IP) communications by authenticating and encrypting each IP packet of a communication session. IPsec includes protocols for establishing mutual authentication between agents at the beginning of the session and negotiation of cryptographic keys to be used during the session. IPsec can be used in protecting data flows between a pair of hosts (host-to-host), between a pair of security gateways (network-to-network), or between a security gateway and a host (network-to-host). IPsec is an end-to-end security scheme operating in the Internet Layer of the Internet Protocol Suite.