I converted the script I wrote to monitor BGP on Cisco routers via SNMP into Nagios plugin. I did not like check_bgp plugin which comes with Nagios, because it requires username and password to access a router to be stored in plain text.

Here is how to configure Nagios to use my plugin:
copy check_bgp_snmp.pl into nagios/libexec directory. Then, add the following to Nagios config

define command{       
      command_name    check_bgp_snmp
      command_line    $USER1$/check_bgp_snmp.pl -r $HOSTADDRESS$ -c $ARG1$
      }



define service{
      host_name               router1
      service_description     BGP CHECK
      check_command           check_bgp_snmp!public
      max_check_attempts      2
      normal_check_interval   5
      retry_check_interval    2
      check_period            24x7
      notification_interval   10
      notification_period     24x7
      notification_options    w,u,c,r
      notifications_enabled   0
      contact_groups          admins
}

replace "router1" with your router IP address and "public" with appropriate 
SNMP community string

Download check_bgp_snmp.pl