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DS-3 card compability on 2921/2911 routers

To use a DS-3 card on 29xx routers, we have two options:


Option 1 (old NM module): Use "NM-1T3/E3" with "SM-NM-ADPTR" adapter.

Option 2: (new SM module): Use "SM-X-1T3/E3"


However, these two options seem to have different compatibility on 2921/2911 routers.


Option 1 is NOT compatible with 2921/2911 routers per

Option 2 is compatible with 2921/2911 routers per


Though option 1 was proved to work on 2921/2911 router per, we're afraid it's not officially supported by Cisco.


So now the question is: what's the difference between SM-X-1T3/E3 and NM-1T3/E3 that makes them have different compatiblity on 2921/2911 routers?


Since the router chassis is the same, the only explanation is that the new SM module put less load on the router processing power.  But how come?  We all know that the SM or NM itself handles packetization.  Router CPU does not participate in it.  If a 2921 router is capable of handling the line rate of SM-X-1T3/E3, why wouldn't it handle the NM-1T3/E3?



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