Configuring IP Forwarding Parameters

Various IP packet forwarding behaviors can be enabled or disabled.

The following configurable parameters control the forwarding behavior of RUCKUS Layer 3 devices:

  • Time-To-Live (TTL) threshold: The time to live (TTL) threshold prevents routing loops by specifying the maximum number of router hops an IP packet originated by the Layer 3 device can travel through. Each device capable of forwarding IP that receives the packet decrements (decreases) the packet TTL by one. If a device receives a packet with a TTL of 1 and reduces the TTL to zero, the device drops the packet.
  • Forwarding of directed broadcasts: A directed broadcast is an IP broadcast to all devices within a single directly-attached network or subnet. A net-directed broadcast goes to all devices on a given network. A subnet-directed broadcast goes to all devices within a given subnet.
    Note: A less common type, the all-subnets broadcast, goes to all directly-attached subnets. Forwarding for this broadcast type also is supported, but most networks use IP multicasting instead of all-subnet broadcasting.
  • Forwarding of source-routed packets: A source-routed packet specifies the exact router path for the packet. The packet specifies the path by listing the IP addresses of the router interfaces through which the packet must pass on its way to the destination. The Layer 3 device supports both types of IP source routing:
    • Strict source routing: Requires the packet to pass through only the listed routers. If the Layer 3 device receives a strict source-routed packet but cannot reach the next hop interface specified by the packet, the Layer 3 device discards the packet and sends an ICMP Source-Route-Failure message to the sender.
    • Loose source routing: Requires that the packet pass through all of the listed routers but also allows the packet to travel through other routers, which are not listed in the packet.
  • Ones-based and zero-based broadcasts: Most IP hosts are configured to receive IP subnet broadcast packets with all ones in the host portion of the address. However, some older IP hosts instead expect IP subnet broadcast packets that have all zeros instead of all ones in the host portion of the address. To accommodate this type of host, you can enable the Layer 3 device to treat IP packets with all zeros in the host portion of the destination IP address as broadcast packets.

All these parameters are optional and can be configured globally; they affect all IP interfaces configured on the Layer 3 device. The IP directed broadcast parameter can also be configured for a specific interface.

  1. Enter global configuration mode. 
    device# configure terminal
  2. (Optional) Change the TTL threshold. 
    device(config)# ip ttl 25
  3. (Optional) Enable forwarding of IP directed broadcasts. 
    device(config)# ip directed-broadcast
    You can also configure the forwarding of IP directed broadcasts in interface configuration mode for a specific interface.
  4. (Optional) IP source-routed packets forwarding is enabled by default. To disable the forwarding of IP source-routed packets, enter the no ip source-route command. 
    device(config)# no ip source-route
    You cannot enable or disable strict or loose source routing separately.
  5. (Optional) To permit zero-based IP subnet broadcasts in addition to ones-based IP subnet broadcasts, use the ip broadcast-zero command. 
    device(config)# ip broadcast-zero
    If the ip broadcast-zero command is entered, you must save the configuration using the write memory command and use the reload command to reload the software. Refer to the following second example for all the steps.
    Note: The zero-based IP subnet broadcast feature applies only to IP subnet broadcasts, not to local network broadcasts. The local network broadcast address is still expected to be all ones.

The following example changes the TTL threshold, enables IP directed broadcasts, and disables IP source routing. 

device# configure terminal
device(config)# ip ttl 25
device(config)# ip directed-broadcast
device(config)# no ip source-route

The following example permits zero-based IP subnet broadcasts, saves the configuration, and reloads the software. 

device# configure terminal
device(config)# ip broadcast-zero
device(config)# write memory
device(config)# end
device# reload
Parent topic: IP Addressing