Forwarding Engine

The forwarding engine is that part of a router that receives packets and forwards them from one interface to another. In the case of XORP, the forwarding engine may be the kernel forwarding path on UNIX, the Click forwarding path, or it may reside in external forwarding hardware. On any particular router, it might be desirable to enable or disable different parts of the forwarding functionality. For example, a router might only be intended to forward IPv6 packets but not IPv4 packets, or it might be intended to forward unicast packets but not multicast packets. Thus XORP provides the ability to enable and configure various forwarding functionality. In XORP, the term fea refers to Forwarding Engine Abstraction and the term mfea refers to Multicast Forwarding Engine Abstraction. The term “abstraction” here refers to a high-level configuration interface that should be the same irrespective of whether the forwarding engine is provided in software in the operating system kernel or in external forwarding hardware.

On a XORP router, forwarding functionality must be explicitly enabled or no packets will be forwarded. Forwarding can be separately enabled for unicast and multicast, and for IPv4 and IPv6. In addition, multicast interfaces/vifs need to be explicitly enabled individually, and certain special-purpose forwarding functionality can also be enabled for multicast.

fea {
  targetname: txt
  unicast-forwarding4 {
    disable: bool
    table-id: u32
    forwarding-entries {
      retain-on-startup: bool
      retain-on-shutdown: bool
    }
  }
  unicast-forwarding6 {
    disable: bool
    table-id: u32
    forwarding-entries {
      retain-on-startup: bool
      retain-on-shutdown: bool
    }
  }
  click {
    disable: bool
    duplicate-routes-to-kernel: bool
    kernel-click {
      disable: bool
      install-on-startup: bool
      kernel-click-modules: text
      mount-directory: text
      kernel-click-config-generator-file: text
    }
    user-click {
      disable: bool
      command-file: text
      command-extra-arguments: text
      command-execute-on-startup: bool
      control-address: IPv4-addr
      control-socket-port: uint(1..65535)
      startup-config-file: text
      user-click-config-generator-file: text
    }
  }
}
plumbing {
  mfea4 {
    disable: bool
    interface text {
      vif text {
        disable: bool
      }
    }
    interface register vif {
      vif register vif {
        disable: bool
      }
    }
    traceoptions {
      flag all {
        disable: bool
      }
    }
  }
  mfea6 {
    disable: bool
    interface text {
      vif text {
        disable: bool
      }
    }
    interface register vif {
      vif register vif {
        disable: bool
      }
    }
    traceoptions {
      flag {
        all {
          disable: bool
        }
      }
    }
  }
}

fea {
  unicast-forwarding4 {
    disable: false
  }
  unicast-forwarding6 {
    disable: true
  }
}
plumbing {
  mfea4 {
    disable: false
    interface dc0 {
      vif dc0 {
        disable: false
      }
    }
    interface register_vif {
      vif register_vif {
        /* Note: this vif should be always enabled */
        disable: false
      }
    }
    traceoptions {
      flag all {
        disable: false
      }
    }
  }
  
  mfea6 {
    disable: false
    interface dc0 {
      vif dc0 {
        disable: false
      }
    }
    interface register_vif {
      vif register_vif {
        /* Note: this vif should be always enabled */
        disable: false
      }
    }
  }
}

The configuration above enables unicast IPv4 forwarding, but disables IPv6 unicast forwarding. In addition, it enables multicast forwarding for IPv4 and IPv6 on interface/vif dc0/dc0, and enables the register vif for use by PIM-SM multicast routing.

interfaces {
  interface eth0 {
    description: "control interface"
    vif eth0 {
      address 10.10.10.10 {
        prefix-length: 24
        broadcast: 10.10.10.255
      }
    }
    mac: aa:bb:cc:dd:ee:ff
    mtu: 1500
  }
}

fea {
  unicast-forwarding4 {
    disable: false
  }
  
  click {
    disable: false
    duplicate-routes-to-kernel: false
    
    kernel-click {
      disable: true
      install-on-startup: true
      kernel-click-modules: "/path/to/proclikefs.o:/path/to/click.o";
      mount-directory: "/click"
      kernel-click-config-generator-file: "/path/to/kernel click config generator"
    }
    
    user-click {
      disable: false
      command-file: "/path/to/click"
      command-extra-arguments: "-R"
      command-execute-on-startup: true
      control-address: 127.0.0.1
      control-socket-port: 13000
      startup-config-file: "/dev/null"
      user-click-config-generator-file: "/path/to user click config generator"
    }
  }
}

The configuration above enables configures both kernel-level and user-level Click (eventually on Linux given that it contains two kernel Click modules), but enables only user-level Click.

The show mfea dataflow command can be used to display information about MFEA IPv4 dataflow filters:

user@hostname> show mfea dataflow
Group                                       Source
224.0.1.20                                  10.2.0.1
  Measured(Start|Packets|Bytes) Type Thresh(Interval|Packets|Bytes)     Remain
  1091667269.982158|0|?         <=         210.0|0|?                      202.434319
  1091667269.984406|?|0         >=         100.0|?|102400                 92.436567

Note that the above information is shown only if the filters are kept at user-space. If the filters are kept at kernel-space (e.g., in case of UNIX system with advanced multicast API support), then currently xorpsh cannot be used to show the information. In that case, the appropriate system command should be used instead (e.g., the UNIX netstat -gn command).

The show mfea interface command can be used to display information about MFEA IPv4 interfaces:

user@hostname> show mfea interface
Interface      State     Vif/PifIndex    Addr         Flags
dc0            UP        0/6             10.4.0.1     MULTICAST BROADCAST KERN_UP
dc2            UP        1/8             10.3.0.2     MULTICAST BROADCAST KERN_UP
register_vif   UP        2/6             10.4.0.1     PIM_REGISTER KERN_UP

The show mfea interface address command can be used to display information about MFEA IPv4 interface addresses:

user@hostname> show mfea interface address
Interface     Addr      Subnet         Broadcast      P2Paddr
dc0           10.4.0.1  10.4.0.0/24    10.4.0.255     0.0.0.0
dc2           10.3.0.2  10.3.0.0/24    10.3.0.255     0.0.0.0
register_vif  10.4.0.1  10.4.0.1/32    10.4.0.1       0.0.0.0

The equivalent commands for IPv6 multicast forwarding are:

• show mfea6 dataflow
• show mfea6 interface
• show mfea6 interface address