Accessing Router via SSH.Net Library

Question

I'm trying to create a C# program using SSH.Net lib to change some config in a SSH enabled cisco router. But I'm getting "Permission denied" Error while trying to establish connection. But I'm able to access the router through putty/plink using the username and password.

            using (var client = new SshClient(routerIPAddr, 22, routerUserName, routerPassword))
            {
                _logger.Write("SSH client created");
                try
                {
                    client.Connect();
                    if (client.IsConnected)
                    {
                        _logger.Write("Connected to the router.");

                        // Execute router commands
                        ExecuteRouterCommands(client, commands);

                        client.Disconnect();
                        _logger.Write("Disconnected from the router.");
                    }
                    else
                    {
                        _logger.Write("Failed to connect to the router.");
                    }
                }

On executing i got this error

 Permission denied (password).    at Renci.SshNet.ClientAuthentication.Authenticate(IConnectionInfoInternal connectionInfo, ISession session)
   at Renci.SshNet.ConnectionInfo.Authenticate(ISession session, IServiceFactory serviceFactory)
   at Renci.SshNet.Session.Connect()
   at Renci.SshNet.BaseClient.CreateAndConnectSession()
   at Renci.SshNet.BaseClient.Connect()

I also tried keyboardInteractiveAuthenticationMethod

var authMethod = new KeyboardInteractiveAuthenticationMethod(routerUserName);
authMethod.AuthenticationPrompt += (sender, e) =>
{
    e.Prompts.Single().Response = routerPassword;
};
var connectionInfo = new ConnectionInfo(routerIPAddr, 22, routerUserName, authMethod);
using (var client = new SshClient(connectionInfo))
//////////////
......
client.Connect();
......
same code
/////////////

On executing I got similar error again.

 Permission denied (keyboard-interactive).    at Renci.SshNet.ClientAuthentication.Authenticate(IConnectionInfoInternal connectionInfo, ISession session)
   at Renci.SshNet.ConnectionInfo.Authenticate(ISession session, IServiceFactory serviceFactory)
   at Renci.SshNet.Session.Connect()
   at Renci.SshNet.BaseClient.CreateAndConnectSession()
   at Renci.SshNet.BaseClient.Connect()

Is there anyway to fix this authentication issue.