2911 is compatible with these cards:
http://www.cisco.com/c/dam/en/us/products/collateral/routers/1900-series-integrated-services-routers-isr/aag_c07_563807.pdf?bcsi_scan_242f14fdc4f33f94=cYmNLn0qbIh64bnzRw4xQijbskMFAAAA6aICDw==&bcsi_scan_filename=aag_c07_563807.pdf
HTH
Rajan
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