Product Documentation

Configuring Modes of Packet Forwarding

Jun 24, 2013

The NetScaler appliance can either route or bridge packets that are not destined for an IP address owned by the appliance (that is, the IP address is not the NSIP, a MIP, a SNIP, a configured service, or a configured virtual server). By default, L3 mode (routing) is enabled and L2 mode (bridging) is disabled, but you can change the configuration. The following flow chart shows how the appliance evaluates packets and either processes, routes, bridges, or drops them.

Figure 1. Interaction between Layer 2 and Layer 3 Modes


An appliance can use the following modes to forward the packets it receives:

  • Layer 2 (L2) Mode
  • Layer 3 (L3) Mode
  • MAC-Based Forwarding Mode

Enabling and Disabling Layer 2 Mode

Updated: 2013-09-13

Layer 2 mode controls the Layer 2 forwarding (bridging) function. You can use this mode to configure a NetScaler appliance to behave as a Layer 2 device and bridge the packets that are not destined for it. When this mode is enabled, packets are not forwarded to any of the MAC addresses, because the packets can arrive on any interface of the appliance and each interface has its own MAC address.

With Layer 2 mode disabled (which is the default), the appliance drops packets that are not destined for one of its MAC address. If another Layer 2 device is installed in parallel with the appliance, Layer 2 mode must be disabled to prevent bridging (Layer 2) loops. You can use the configuration utility or the command line to enable Layer 2 mode.

Note: The appliance does not support spanning tree protocol. To avoid loops, if you enable L2 mode, do not connect two interfaces on the appliance to the same broadcast domain.

To enable or disable Layer 2 mode by using the command line interface

At the command prompt, type the following commands to enable/disable Layer 2 mode and verify that it has been enabled/disabled:

  • enable ns mode <Mode>
  • disable ns mode <Mode>
  • show ns mode

    Examples

    > enable ns mode l2 
     Done 
    > show ns mode 
     
          Mode                   Acronym          Status 
          -------                -------          ------ 
     1)   Fast Ramp              FR               ON 
     2)   Layer 2 mode           L2               ON 
     . 
     . 
     . 
     Done 
    > 
     
    > disable ns mode l2 
     Done 
    > show ns mode 
     
          Mode                     Acronym          Status 
          -------                  -------          ------ 
     1)   Fast Ramp                FR               ON 
     2)   Layer 2 mode             L2               OFF 
     . 
     . 
     . 
     Done 
    >

To enable or disable Layer 2 mode by using the configuration utility

  1. In the navigation pane, expand System, and then click Settings.
  2. In the details pane, under Modes and Features, click Configure modes.
  3. In the Configure Modes dialog box, to enable Layer 2 mode, select the Layer 2 Mode check box. To disable Layer 2 mode, clear the check box.
  4. Click OK. The Enable/Disable Mode(s)? message appears in the details pane.
  5. Click Yes.

Enabling and Disabling Layer 3 Mode

Updated: 2013-09-13

Layer 3 mode controls the Layer 3 forwarding function. You can use this mode to configure a NetScaler appliance to look at its routing table and forward packets that are not destined for it. With Layer 3 mode enabled (which is the default), the appliance performs route table lookups and forwards all packets that are not destined for any appliance-owned IP address. If you disable Layer 3 mode, the appliance drops these packets.

To enable or disable Layer 3 mode by using the command line interface

At the command prompt, type the following commands to enable/disable Layer 3 mode and verify that it has been enabled/disabled:

  • enable ns mode <Mode>
  • disable ns mode <Mode>
  • show ns mode

    Examples

    > enable ns mode l3 
     Done 
    > show ns mode 
     
         Mode                          Acronym          Status 
         -------                       -------          ------ 
     1)  Fast Ramp                     FR               ON 
     2)  Layer 2 mode                  L2               OFF 
     . 
     . 
     . 
     9)  Layer 3 mode (ip forwarding)  L3               ON 
     . 
     . 
     . 
     Done 
    >  
     
    > disable ns mode l3 
     Done 
    > show ns mode 
     
         Mode                          Acronym          Status 
         -------                       -------          ------ 
     1)  Fast Ramp                     FR               ON 
     2)  Layer 2 mode                  L2               OFF 
     . 
     . 
     . 
     9)  Layer 3 mode (ip forwarding)  L3               OFF 
     . 
     . 
     . 
     Done 
    >

To enable or disable Layer 3 mode by using the configuration utility

  1. In the navigation pane, expand System, and then click Settings.
  2. In the details pane, under Modes and Features, click Configure modes.
  3. In the Configure Modes dialog box, to enable Layer 3 mode, select the Layer 3 Mode (IP Forwarding) check box. To disable Layer 3 mode, clear the check box.
  4. Click OK. The Enable/Disable Mode(s)? message appears in the details pane.
  5. Click Yes.

Enabling and Disabling MAC-Based Forwarding Mode

Updated: 2013-09-13

You can use MAC-based forwarding to process traffic more efficiently and avoid multiple-route or ARP lookups when forwarding packets, because the NetScaler appliance remembers the MAC address of the source. To avoid multiple lookups, the appliance caches the source MAC address of every connection for which it performs an ARP lookup, and it returns the data to the same MAC address.

MAC-based forwarding is useful when you use VPN devices because the appliance ensures that all traffic flowing through a particular VPN passes through the same VPN device.

The following figure shows the process of MAC-based forwarding.

Figure 2. MAC-Based Forwarding Process

When MAC-based forwarding is enabled, the appliance caches the MAC address of:

  • The source (a transmitting device such as router, firewall, or VPN device) of the inbound connection.
  • The server that responds to the requests.

When a server responds through an appliance, the appliance sets the destination MAC address of the response packet to the cached address, ensuring that the traffic flows in a symmetric manner, and then forwards the response to the client. The process bypasses the route table lookup and ARP lookup functions. However, when an appliance initiates a connection, it uses the route and ARP tables for the lookup function. To enable MAC-based forwarding, use the configuration utility or the command line.

Some deployments require the incoming and outgoing paths to flow through different routers. In these situations, MAC-based forwarding breaks the topology design. For a global server load balancing (GSLB) site that requires the incoming and outgoing paths to flow through different routers, you must disable MAC-based forwarding and use the appliance’s default router as the outgoing router.

With MAC-based forwarding disabled and Layer 2 or Layer 3 connectivity enabled, a route table can specify separate routers for outgoing and incoming connections. To disable MAC-based forwarding, use the configuration utility or the command line.

To enable or disable MAC-based forwarding by using the command line interface

At the command prompt, type the following commands to enable/disable MAC-based forwarding mode and verify that it has been enabled/disabled:

  • enable ns mode <Mode>
  • disable ns mode <Mode>
  • show ns mode

    Example

    > enable ns mode mbf 
     Done 
    > show ns mode 
     
         Mode                        Acronym           Status 
         -------                     -------           ------ 
     1)  Fast Ramp                   FR                ON 
     2)  Layer 2 mode                L2                OFF 
     . 
     . 
     . 
     6)  MAC-based forwarding        MBF               ON 
     . 
     . 
     . 
     Done 
    >  
     
    > disable ns mode mbf 
     Done 
    > show ns mode 
     
         Mode                        Acronym           Status 
         -------                     -------           ------ 
     1)  Fast Ramp                   FR                ON 
     2)  Layer 2 mode                L2                OFF 
     . 
     . 
     . 
     6)  MAC-based forwarding        MBF               OFF 
     . 
     . 
     . 
     Done 
    >

To enable or disable MAC-based forwarding by using the configuration utility

  1. In the navigation pane, expand System, and then click Settings.
  2. In the details pane, under Modes and Features group, click Configure modes.
  3. In the Configure Modes dialog box, to enable MAC-based forwarding mode, select the MAC Based Forwarding check box. To disable MAC-based forwarding mode, clear the check box.
  4. Click OK. The Enable/Disable Mode(s)? message appears in the details pane.
  5. Click Yes.