meaning of one-arm setup and src nat

[AJAZ NAWAZ]

I've worked previously on CSS platform and recall deploying one-arm mode, which simply meant connecting the appliance via single physical trunk link.

In terms of the ace some docos and ANM seem to suggest that one-arm requires src nat, if true why is that unless one-arm now translates to one-vlan?.

btw i know about asymetric routing and src nat, but what i'm failing to get is how that relates to one-arm.

thanks

[Francesco Casotto · Accepted solution]

Hello Ajaz,

generally the convention is to call one arm those setups where both client and servers, for a certain loadbalanced service (so VIP), belong to the same VLAN, see for example how it's defined here:

http://docwiki.cisco.com/wiki/Cisco_Application_Control_Engine_%28ACE%29_Configuration_Examples_--_Routing_and_Bridging_Configuration_Examples

not sure whether the definition has changed over time, I would guess that it can be intended in the physical sense (single link) so as you do, or in the logical sense, where 2 VLANs would represent 2 arms even if the physical connectivity is provided through just one link. From my experience, in the LB field the logical interpretation is prevalent.

Thanks,

Francesco

[AJAZ NAWAZ]

Thanks for the reply Francesco

I perfomed a lookup on the defunct (i.e. CSS) platform, to understand meaning of 'One-armed' and it would appear its application and definition has changed with the introduction of ACE.

http://www.cisco.com/en/US/products/hw/contnetw/ps789/products_configuration_example09186a0080093dff.shtml

I understand now why SrcNAT is required when df-gw is pointing to a router as opposed to LB. 

best regards