Você pode tentar enviar sua saída através deste script sed
:
$ snmpwalk ... | sed -n '/MIB::/!{H;$g};/MIB::/{$!x;s/\n/\n/g;ta;${x;/^$/!p;x};:a;/^$/!p}'
SNMPv2-MIB::sysDescr.0 Cisco Internetwork Operating System Software\nIOS (tm) Catalyst 4000 L3 Switch Software (cat4000-I5S-M), Version 12.2(18)EW4, RELEASE SOFTWARE (fc1)\nTechnical Support: http://www.cisco.com/techsupport\nCopyright (c) 1986-2005 by cisco Systems, Inc.\nCom
SNMPv2-MIB::sysObjectID.0 SNMPv2-SMI::enterprises.9.1.502
DISMAN-EVENT-MIB::sysUpTimeInstance 122:19:10:46.16
SNMPv2-MIB::sysContact.0 012345\nfoo\nbar
SNMPv2-MIB::sysName.0 name
SNMPv2-MIB::sysLocation.0 London,uk,emea
Aqui está uma versão do AWK mais legível, mas um pouco mais longa:
$ snmpwalk ... | awk 'function printit() {printf prev; if (accum) printf "\n" accum; print ""} !/MIB::/{accum=accum "\n" $0} /MIB::/{if (NR>1) {printit()}; accum=""; prev=$0} END{printit()}'