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Assuming your hand-off is Ethernet, 100 Mbps (full duplex), for 64 byte size packets, requires 297.6 Kpps. Going by this alone, all the 2900 series routers are documented as having more than 300 Kpps in a RFC 2544 test (see Table 1, 2nd page of attachment). However, for 100 Mbps, Cisco recommends minimally a 3925 (see figure 1, on last page of attachment), but that guarantees performance for multiple services while running at 100 Mbps (duplex) bandwidth. If you expect your service feature needs to be less and/or average traffic utilization less, you should be able to downsize accordingly. I've often found you can downsize one or two sizes from the Cisco recommendation. (Again, router sizing depends much on your expected usage.)
