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ADC
Current Release 13.1 13.0 12.1
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Packet forwarding modes

March 10, 2022
Contributed by:
C S

Citrix ADC设备可以路线或缩短”e 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

Network interaction between layer2 and layer3

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

Enable and disable layer 2 mode

Layer 2 mode controls the Layer 2 forwarding (bridging) function. You can use this mode to configure a Citrix ADC 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 the 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 CLI

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

  • disable ns 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 GUI

  1. In the navigation pane, expandSystem, and then clickSettings.
  2. In the details pane, underModesandFeatures, clickConfigure modes.
  3. In theConfigure Modesdialog box, to enable Layer 2 mode, select theLayer 2 Modecheck box. To disable Layer 2 mode, clear the check box.
  4. ClickOK. TheEnable/Disable Mode(s)?message appears in the details pane.
  5. ClickYes.

Enable and disable layer 3 mode

Layer 3 mode controls the Layer 3 forwarding function. You can use this mode to configure a Citrix ADC 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.

Enable or disable Layer 3 mode by using the CLI

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

  • disable ns mode

  • show ns mode

    Examples

> >启用ns模式l3做秀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 >

Enable or disable Layer 3 mode by using the GUI

  1. In the navigation pane, expandSystem, and then clickSettings.
  2. In the details pane, underModes and Features, clickConfigure modes.
  3. In theConfigure Modesdialog box, to enable Layer 3 mode, select theLayer 3 Mode (IP Forwarding)check box. To disable Layer 3 mode, clear the check box.
  4. ClickOK. TheEnable/Disable Mode(s)?message appears in the details pane.
  5. ClickYes.

Enable and disable MAC based forwarding mode

You can use MAC-based forwarding to process traffic more efficiently and avoid multiple-route or ARP lookups when forwarding packets, because the Citrix ADC 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

Working MAC-based forwarding

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

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.

Enable or disable MAC-based forwarding by using the CLI

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

  • Example

    ``` pre codeblock

    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 >  ```

Enable or disable MAC-based forwarding by using the GUI

  1. In the navigation pane, expandSystem, and then clickSettings.
  2. In the details pane, underModes and Featuresgroup, clickConfigure modes.
  3. In theConfigure Modesdialog box, to enable MAC-based forwarding mode, select theMAC Based Forwardingcheck box. To disable MAC-based forwarding mode, clear the check box.
  4. ClickOK. TheEnable/Disable Mode(s)?message appears in the details pane.
  5. ClickYes.
Packet forwarding modes
March 10, 2022
Contributed by:
C S
March 10, 2022
Contributed by:
C S

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