[mrtg] MRTG SNMPv2c cannot walk router

[Ed LaFrance]

Hello all -

I've got Mrtg v2.15.0 running on an old RH 9 box, and also on more 
recent CentOS 4.x and 5.x boxes.

I've just set up a couple of new RouterBoard routers and want to 
query them with MRTG. When doing so with SNMPv1, all works great. But 
when I select SNMPv2c, SNMPwalk fails on all three monitoring stations:

--base: Get Device Info on public3 at r1.0:::::2
SNMP Error:
no response received
SNMPv2c_Session (remote host: "r1.0" [xxx.xxx.xxx.xxx].161)
                    community: "public3"
                   request ID: 1430634338
                  PDU bufsize: 8000 bytes
                      timeout: 2s
                      retries: 5
                      backoff: 1)
  at /usr/local/mrtg-2/bin/../lib/mrtg2/SNMP_util.pm line 627
SNMPWALK Problem for 1.3.6.1.2.1.1 on public3 at r1.0:::::2:v4only
  at /usr/local/mrtg-2/bin/cfgmaker line 915
WARNING: Skipping public3 at r1.0:::::2 as no info could be retrieved

I also have an snmpwalk binary on the RH box, and that works fine 
when querying against the router using v2c:

snmpwalk -c public -v 2c r1.0 | more
SNMPv2-MIB::sysDescr.0 = STRING: router
SNMPv2-MIB::sysObjectID.0 = OID: SNMPv2-SMI::enterprises.14988.1
SNMPv2-MIB::sysUpTime.0 = Timeticks: (135716300) 15 days, 16:59:23.00
SNMPv2-MIB::sysContact.0 = STRING: admin at colocationonline.info
SNMPv2-MIB::sysName.0 = STRING: LIS.R.FMT.1.0
SNMPv2-MIB::sysLocation.0 = STRING: H.E. Fremont I
SNMPv2-MIB::sysServices.0 = INTEGER: 78
...etc

What's extra odd is that I also monitor some managed switches with 
the same MRTG, and I can query against those using v2c just fine.

Anyone have any ideas on steps to troubleshoot? There is no way to 
turn on/off SNMPv2c support in the router, it is on by default.

Thanks in advance!

Ed