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Fabric Grant Bypass

Understanding Fabric Grant Bypass

Modular Port Concentrators (MPCs) contain one, two, or four Packet Forwarding Engines. Each Packet Forwarding Engine handles its forwarding decisions independently. Also, each Packet Forwarding Engine implements fabric queuing and flow control features required to communicate with other Packet Forwarding Engines on the same chassis. Transmitting a packet from a Packet Forwarding Engine to another involves a fabric request and grant process. As per this, the ingress Packet Forwarding Engine first sends a fabric request to the egress Packet Forwarding Engine across an active fabric plane. And when it receives the fabric grant in response, it sends the packets to the egress Packet Forwarding Engine.

However, the MX2010 and 2020 routers in some configurations are set to bypass the fabric request and grant process by default. The fabric grant bypass configuration is required to support MPC1 (MX-MPC1-3D), MPC2 (MX-MPC2-3D), and 16-port 10-Gigabit Ethernet MPC (MPC-3D-16XGE-SFPP) on MX2020 and MX2010 platforms. On the MX Series routers with the fabric grant bypass enabled, the switch fabric takes in the fabric requests from the ingress Packet Forwarding Engine and provides fabric grants; and the ingress Packet Forwarding Engine sends the packet to the egress Packet Forwarding Engine. In this case, the switch fabric forwards the fabric request to the egress Packet Forwarding Engine, but discards the fabric grants it receives from the egress Packet Forwarding Engine.

Table 1 describes the fabric grant bypass behavior on MX2010 and MX2020 routers.

Table 1: Fabric Grant Bypass Behavior on MX2010 and MX2020 Routers

MX Series Routers

Switch Control Board

Switch Fabric Board

Default Fabric Grant Bypass Behavior
       

MX2010 and MX2020

-

SFB

Enabled for all MPCs.

MX2010 and MX2020

-

SFB2

Enabled for MPC1 (MX-MPC1-3D), MPC2 (MX-MPC2-3D), and 16-port 10-Gigabit Ethernet MPC (MPC-3D-16XGE-SFPP). Disabled for all other MPCs.

Disabling Fabric Grant Bypass to Control Congestion and Improve Performance

You can disable fabric grant bypass on the MX2020 and MX2010 routers with SFBs. Disabling the default fabric grant bypass behavior controls congestion and thus improves system behavior and performance on MX2010 and MX2020 routers. After disabling fabric grant bypass, you must reboot the router for the changes to take effect.

Note:

After you disable fabric grant bypass and reboot the router, the existing MPCs on the router where fabric grant bypass is enabled by default—such as MPC1 (MX-MPC1-3D), MPC2 (MX-MPC2-3D), and the 16-port 10-Gigabit Ethernet MPC (MPC-3D-16xGE-SFPP)—do not power on.

To disable fabric grant bypass to control congestion and improve system behavior and performance:

  1. Disable fabric grant bypass by including the fabric disable-grant-bypass statement at the [edit chassis] hierarchy level.
  2. After disabling fabric grant bypass, commit the configuration.
    Note:

    After you disable fabric grant bypass and commit the configuration, the router displays the following warning message:[edit] ’chassis’ WARNING: Chassis configuration for fabric grant bypass has been changed. A system reboot is mandatory. Please reboot the system NOW. Continuing without a reboot might result in unexpected system behavior. commit complete

  3. Reboot the router for the configuration to take effect.

Re-Enabling Fabric Grant Bypass

After you disable fabric grant bypass, you can re-enable it on the MX2020 and MX2010 routers with SFBs.

Note:

  • By default, fabric grant bypass is enabled on the MX2010 and MX2020 routers.

  • After you enable fabric grant bypass feature and reboot the router, the existing MPCs on the router where fabric grant bypass is enabled by default—such as MPC1 (MX-MPC1-3D), MPC2 (MX-MPC2-3D), and the 16-port 10-Gigabit Ethernet MPC (MPC-3D-16XGE-SFPP)—power on.

To re-enable fabric grant bypass:

  1. Use the delete statement with the fabric disable-grant-bypass statement at the [edit chassis] hierarchy level to enable fabric grant bypass.
  2. After enabling fabric grant bypass, commit the configuration.
    Note:

    After you enable fabric grant bypass and commit the configuration, the router displays the following warning message: [edit] ’chassis’ WARNING: Chassis configuration for fabric grant bypass has been changed. A system reboot is mandatory. Please reboot the system NOW. Continuing without a reboot might result in unexpected system behavior. commit complete

  3. Reboot the router for the configuration to take effect.

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