It should be MIB II. The SNMP object identifier (OID) is .1.3.6.1.2.1.4.21, which translates to ip.ipRouteTable . (This works for me on Windows, so I see no reason why it should be different for your Linksys / openWRT system.)
Here is the hierarchy table for the routing table sheet:
1 - ISO assigned OIDs 1.3 - ISO Identified Organization 1.3.6 - US Department of Defense 1.3.6.1 - OID assignments from 1.3.6.1 - Internet 1.3.6.1.2 - IETF Management 1.3.6.1.2.1 - SNMP MIB-2 1.3.6.1.2.1.4 - ip 1.3.6.1.2.1.4.21 - ipRouteTable
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