[SNMP][IPv6]: Fix SNMP IPv6 reachability issue in certain scenarios (#15487) (#15874)

dockers/docker-snmp/snmpd.conf.j2

@@ -13,12 +13,36 @@
 #  AGENT BEHAVIOUR
 #
 
-#  Listen for connections on all ip addresses, including eth0, ipv4 lo
+# Listen for connections on all ip addresses, including eth0, ipv4 lo for multi-asic platform
+# Listen on managment and loopback0 ips for single asic platform
 #
+{% macro protocol(ip_addr) %}
+{%- if ip_addr|ipv6 -%}
+{{ 'udp6' }}
+{%- else -%}
+{{ 'udp' }}
+{%- endif -%}
+{% endmacro %}
+
 {% if SNMP_AGENT_ADDRESS_CONFIG %}
 {% for (agentip, port, vrf) in SNMP_AGENT_ADDRESS_CONFIG %}
-agentAddress {{ agentip }}{% if port %}:{{ port }}{% endif %}{% if vrf %}%{{ vrf }}{% endif %}{{ "" }}
+agentAddress {{ protocol(agentip) }}:[{{ agentip }}]{% if port %}:{{ port }}{% endif %}{% if vrf %}%{{ vrf }}{% endif %}{{ "" }}
 {% endfor %}
+{% elif NAMESPACE_COUNT is not defined or NAMESPACE_COUNT|int <= 1 %}
+{% if MGMT_INTERFACE is defined %}
+{% for if, ip in MGMT_INTERFACE %}
+{% set agentip = ip.split('/')[0] %}
+agentAddress {{ protocol(agentip) }}:[{{ agentip }}]:161
+{% endfor %}
+{% endif %}
+{% if LOOPBACK_INTERFACE is defined %}
+{% for lo in LOOPBACK_INTERFACE %}
+{% if lo | length == 2 %}
+{% set agentip = lo[1].split('/')[0] %}
+agentAddress {{ protocol(agentip) }}:[{{ agentip }}]:161
+{% endif %}
+{% endfor %}
+{% endif %}
 {% else %}
 agentAddress udp:161
 agentAddress udp6:161

dockers/docker-snmp/start.sh

@@ -16,11 +16,14 @@ mkdir -p /etc/ssw /etc/snmp
 # Parse snmp.yml and insert the data in Config DB
 /usr/bin/snmp_yml_to_configdb.py
 
+ADD_PARAM=$(printf '%s {"NAMESPACE_COUNT":"%s"}' "-a" "$NAMESPACE_COUNT")
+
 SONIC_CFGGEN_ARGS=" \
     -d \
     -y /etc/sonic/sonic_version.yml \
     -t /usr/share/sonic/templates/sysDescription.j2,/etc/ssw/sysDescription \
     -t /usr/share/sonic/templates/snmpd.conf.j2,/etc/snmp/snmpd.conf \
+    $ADD_PARAM \
 "
 
 sonic-cfggen $SONIC_CFGGEN_ARGS