diff --git a/docs/source/guides/admin-guides/references/man1/switchdiscover.1.rst b/docs/source/guides/admin-guides/references/man1/switchdiscover.1.rst
index e84f3e74d..e219a5ea8 100644
--- a/docs/source/guides/admin-guides/references/man1/switchdiscover.1.rst
+++ b/docs/source/guides/admin-guides/references/man1/switchdiscover.1.rst
@@ -15,7 +15,7 @@ SYNOPSIS
 
 \ **switchdiscover [-v| -**\ **-version]**\ 
 
-\ **switchdiscover**\  [\ *noderange*\  | \ **-**\ **-range**\  \ *ip_ranges*\ ] \ **[-V] [-w][-r|-x|-z][-s**\  \ *scan_methods*\  \ **-**\ **-setup**\ ]
+\ **switchdiscover**\  [\ *noderange*\  | \ **-**\ **-range**\  \ *ip_ranges*\ ] \ **[-V] [-w][-r|-x|-z][-s**\  \ *scan_methods*\ ] [\ **-**\ **-setup**\ ] [\ **-c**\  \ *community*\ ]
 
 
 ***********
@@ -31,7 +31,7 @@ To view all the switches defined in the xCAT database use \ **lsdef -w "nodetype
 
 For lldp method, make sure that lldpd package is installed and lldpd is running on the xCAT management node. lldpd comes from xcat-dep package or you can get it from http://vincentbernat.github.io/lldpd/installation.html.
 
-For snmp method, make sure that snmpwalk command is installed and snmp is enabled for switches. To install snmpwalk, "yum install net-snmp-utils" for redhat and sles,  "apt-get install snmp" for Ubuntu.
+For snmp method, make sure that snmpwalk command is installed and snmp is enabled for switches. To install snmpwalk, "yum install net-snmp-utils" for redhat and sles,  "apt-get install snmp" for Ubuntu. The switchdiscover command only check the switches with default community string, if user already configured switch with other commmunity string, need to pass in with \ **-c**\  option for switchdiscover command to be able to discover.
 
 
 *******
@@ -118,6 +118,12 @@ OPTIONS
  
 
 
+\ **-c**\ 
+ 
+ User defined community string for snmp scan.
+ 
+
+
 
 ************
 RETURN VALUE
diff --git a/perl-xCAT/xCAT/Usage.pm b/perl-xCAT/xCAT/Usage.pm
index efa72327e..6b1dab41c 100755
--- a/perl-xCAT/xCAT/Usage.pm
+++ b/perl-xCAT/xCAT/Usage.pm
@@ -328,7 +328,7 @@ my %usage = (
        pdudiscover [<noderange>|--range ipranges] [-r|-x|-z] [-w] [-V|--verbose] [--setup]",
     "switchdiscover" =>
       "Usage: switchdiscover [-h|--help|-v|--version]
-       switchdiscover [<noderange>|--range ipranges] [-s scan_methods] [-r|-x|-z] [-w] [-V|--verbose] [--setup]",
+       switchdiscover [<noderange>|--range ipranges] [-s scan_methods] [-c community] [-r|-x|-z] [-w] [-V|--verbose] [--setup]",
     "switchprobe" =>
       "Usage: switchprobe [<noderange>] [-V|--verbose | -c|--check]",
     "makentp" =>
diff --git a/xCAT-client/pods/man1/switchdiscover.1.pod b/xCAT-client/pods/man1/switchdiscover.1.pod
index ec16fcaad..20f8e56e1 100755
--- a/xCAT-client/pods/man1/switchdiscover.1.pod
+++ b/xCAT-client/pods/man1/switchdiscover.1.pod
@@ -10,7 +10,7 @@ B<switchdiscover [-h| --help]>
 
 B<switchdiscover [-v| --version]>
 
-B<switchdiscover> [I<noderange> | B<--range> I<ip_ranges>] B<[-V] [-w][-r|-x|-z][-s> I<scan_methods> B<--setup>]
+B<switchdiscover> [I<noderange> | B<--range> I<ip_ranges>] B<[-V] [-w][-r|-x|-z][-s> I<scan_methods>] [B<--setup>] [B<-c> I<community>]
 
 
 
@@ -24,7 +24,7 @@ To view all the switches defined in the xCAT database use B<lsdef -w "nodetype=s
 
 For lldp method, make sure that lldpd package is installed and lldpd is running on the xCAT management node. lldpd comes from xcat-dep package or you can get it from http://vincentbernat.github.io/lldpd/installation.html. 
 
-For snmp method, make sure that snmpwalk command is installed and snmp is enabled for switches. To install snmpwalk, "yum install net-snmp-utils" for redhat and sles,  "apt-get install snmp" for Ubuntu. 
+For snmp method, make sure that snmpwalk command is installed and snmp is enabled for switches. To install snmpwalk, "yum install net-snmp-utils" for redhat and sles,  "apt-get install snmp" for Ubuntu. The switchdiscover command only check the switches with default community string, if user already configured switch with other commmunity string, need to pass in with B<-c> option for switchdiscover command to be able to discover. 
 
 
 =head1 OPTIONS
@@ -87,6 +87,10 @@ Stanza formatted output.
 
 Process switch-based switch discovery. Update discovered switch's ip address, hostname and enable snmpv3 configuration based on the predefined switch. 
 
+=item B<-c>          
+
+User defined community string for snmp scan.
+
 =back
 
 =head1 RETURN VALUE
diff --git a/xCAT-server/lib/xcat/plugins/switchdiscover.pm b/xCAT-server/lib/xcat/plugins/switchdiscover.pm
index bf3ca0a76..40a9fd640 100644
--- a/xCAT-server/lib/xcat/plugins/switchdiscover.pm
+++ b/xCAT-server/lib/xcat/plugins/switchdiscover.pm
@@ -21,6 +21,7 @@ no strict;
 use Data::Dumper;
 use Socket;
 use Expect;
+use SNMP;
 use xCAT::data::switchinfo;
 
 #global variables for this module
@@ -144,7 +145,7 @@ sub parse_args {
     # Process command-line flags
     #############################################
     if (!GetOptions( \%opt,
-            qw(h|help V|verbose v|version x z w r n range=s s=s setup pdu))) {
+            qw(h|help V|verbose v|version x z w r n range=s s=s c=s setup pdu))) {
         return( usage() );
     }
 
@@ -252,6 +253,13 @@ sub parse_args {
         $globalopt{n} = 1;
     }
 
+    #########################################################
+    # Accept the community string from user
+    #########################################################
+    if ( exists( $opt{c} )) {
+        $community=$opt{c};
+    }
+
     #########################################################
     # setup discover switch
     #########################################################
@@ -892,13 +900,6 @@ sub snmp_scan {
     }
     my @lines = split /\n/, $result;
   
-    #set community string for switch
-    $community = "public";
-    my @snmpcs = xCAT::TableUtils->get_site_attribute("snmpc");
-    my $tmp    = $snmpcs[0];
-    if (defined($tmp)) { $community = $tmp }
-
-
     foreach my $line (@lines) {
         my @array = split / /, $line;
         if ($line =~ /\b(\d{1,3}(?:\.\d{1,3}){3})\b/)
@@ -954,20 +955,47 @@ sub get_snmpvendorinfo {
     my $request = shift;
     my $ip  = shift;
     my $snmpwalk_vendor;
+    my $result;
+    my $rc=1;
 
+    #checking snmp connection
+    #during discovery phase, xCAT will not know the community string for snmp
+    #so, will try community string from following way:
+    #  1) -c options from user input
+    #  2) snmpc attribute defined in the site table
+    #  3) default community string for the switches.
 
-    #Ubuntu only takes OID
-    #get sysDescr.0";
-    my $ccmd = "snmpwalk -Os -v1 -c $community $ip 1.3.6.1.2.1.1.1";
-    if (exists($globalopt{verbose}))    {
-       send_msg($request, 0, "Process command: $ccmd\n");
+    my @comm_list;
+    my $snmpver = '1';
+
+    my @snmpcs = xCAT::TableUtils->get_site_attribute("snmpc");
+    my $snmp_site    = $snmpcs[0];
+
+    if ($community) {
+        push @comm_list, $community;
     }
+    if ($snmp_site) {
+        push @comm_list, $snmp_site;
+    }
+    push @comm_list, 'public';
 
-    my $result = xCAT::Utils->runcmd($ccmd, 0);
-    if ($::RUNCMD_RC != 0)
-    {
+    foreach $comms(@comm_list) {
+        #get sysDescr.0";
+        my $ccmd = "snmpwalk -Os -v1 -c $comms $ip 1.3.6.1.2.1.1.1";
         if (exists($globalopt{verbose}))    {
-            send_msg($request, 1, "Could not process this command: $ccmd" );
+           send_msg($request, 0, "Process command: $ccmd\n");
+        }
+        $result = xCAT::Utils->runcmd($ccmd, 0);
+        if ($::RUNCMD_RC == 0)
+        {
+            $community = $comms;
+            $rc=0;
+            last;
+        }
+    }
+    if ($rc == 1) {
+        if (exists($globalopt{verbose}))    {
+            send_msg($request, 1, "Could not process snmpwalk command to get sysDescr, please verify community string" );
         }
         return $snmpwalk_vendor;
     }