SNMP queries time out, but tcpdump shows a response received

I have in my environment nine mostly identical Extreme Networks 
switches. I'm monitoring them over SNMPv3. One of them, for reasons 
unknown, times out on any SNMP query:

pfrost@nagios:~$ snmpget switch06 -v 3 -u initial SNMPv2-MIB::sysDescr.0
snmpget: Timeout (plaintext scopedPDU header type 00: s/b 30)

tcpdump indicates the switch is sending a reply (and the SNMP 
query/response counters on the switch confirm):

pfrost@nagios:~$ sudo tcpdump -s 0 -i eth0 -vvv -i eth0 port snmp and 
host switch06
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 
65535 bytes
11:08:22.717981 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto 
UDP (17), length 92)
     nagios.macprofessionals.lan.52771 > 
switch06.macprofessionals.lan.snmp: [udp sum ok]  { SNMPv3 { F=r } { USM 
B=0 T=0 U= } { ScopedPDU E=  C= { GetRequest(14) R=98826882  } } }
11:08:22.719715 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto 
UDP (17), length 136)
     switch06.macprofessionals.lan.snmp > 
nagios.macprofessionals.lan.52771: [udp sum ok]  { SNMPv3 { F= } { USM 
B=-838860580 T=147167 U= } { ScopedPDU E= 
0x800x000x070x7C0x030x000x040x960x520x4F0xBF C= { Report(32) R=98826882 
S:snmpUsmMIB.usmMIBObjects.usmStats.usmStatsUnknownEngineIDs.0=7351 } } }
[this exchange repeats a few times]

I've compared this against a working reply from a different switch, and 
couldn't see any relevant differences:

pfrost@nagios:~$ sudo tcpdump -s 0 -i eth0 -vvv -i eth0 port snmp and 
host switch05
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 
65535 bytes
11:19:42.913805 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto 
UDP (17), length 92)
     nagios.macprofessionals.lan.58019 > 
switch05.macprofessionals.lan.snmp: [udp sum ok]  { SNMPv3 { F=r } { USM 
B=0 T=0 U= } { ScopedPDU E=  C= { GetRequest(14) R=699839334  } } }
11:19:42.917389 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto 
UDP (17), length 136)
     switch05.macprofessionals.lan.snmp > 
nagios.macprofessionals.lan.58019: [udp sum ok]  { SNMPv3 { F= } { USM 
B=13619213 T=835949 U= } { ScopedPDU E= 
0x800x000x070x7C0x030x000x040x960x520x510x39 C= { Report(32) R=699839334 
S:snmpUsmMIB.usmMIBObjects.usmStats.usmStatsUnknownEngineIDs.0=5372 } } }
11:19:42.917586 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto 
UDP (17), length 140)
     nagios.macprofessionals.lan.58019 > 
switch05.macprofessionals.lan.snmp: [udp sum ok]  { SNMPv3 { F=r } { USM 
B=13619213 T=835949 U=initial } { ScopedPDU E= 
0x800x000x070x7C0x030x000x040x960x520x510x39 C= { GetRequest(28) 
R=699839333  system.sysDescr.0 } } }
11:19:42.927783 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto 
UDP (17), length 238)
     switch05.macprofessionals.lan.snmp > 
nagios.macprofessionals.lan.58019: [udp sum ok]  { SNMPv3 { F= } { USM 
B=13619213 T=835949 U=initial } { ScopedPDU E= 
0x800x000x070x7C0x030x000x040x960x520x510x39 C= { GetResponse(125) 
R=699839333  system.sysDescr.0="ExtremeXOS version 12.6.2.10 
v1262b10-patch1-3 by release-manager on Mon Dec 12 08:50:30 EST 2011" } } }

Notably, if I enable SNMPv2c on switch06, it does work, though 
ultimately I'd want to stick with v3.

NET-SNMP version 5.4.3, from Debian stable. Anyone have some idea what's 
going on here?


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