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You normally size routers based on the transit traffic they need to process. There is workload due to supporting a dynamic routing protocol, such as OSPF, but how demanding that might be depends much on the logical topology and physical link stability. I don't recall reading any case studies providing guidelines for sizing for this aspect, but to worry you (laugh), I've seem link instability cause large chassis core routers to go into OSPF meltdown. (NB: they were brand X, which didn't have some of the OSPF stability features Cisco OSPF generally offers.)
