Foundry Configuration Guide for the FESX, FSX, and FWSX
16 - 30 © Foundry Networks, Inc. December 2005
NOTE: A less common type, the all-sub-nets broadcast, goes to all directly-attached sub-nets. Forwarding for
this broadcast type also is supported, but most networks use IP multicasting instead of all-sub-net broadcasting.
Forwarding for all types of IP directed broadcasts is disabled by default. You can enable forwarding for all types if
needed. You cannot enable forwarding for specific broadcast types.
To enable forwarding of IP directed broadcasts, enter the following command:
FESX424 Router(config)# ip directed-broadcast
Syntax: [no] ip directed-broadcast
Foundry software makes the forwarding decision based on the router's knowledge of the destination network
prefix. Routers cannot determine that a message is unicast or directed broadcast apart from the destination
network prefix. The decision to forward or not forward the message is by definition only possible in the last hop
router.
To disable the directed broadcasts, enter the following command in the CONFIG mode:
FESX424 Router(config)# no ip directed-broadcast
To enable directed broadcasts on an individual interface instead of globally for all interfaces, enter commands
such as the following:
FastIron SuperX Router(config)# interface ethernet 1/1
FastIron SuperX Router(config-if-1/1)# ip directed-broadcast
Syntax: [no] ip directed-broadcast
Disabling Forwarding of IP 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 Switch supports both types of IP source routing:
• Strict source routing – requires the packet to pass through only the listed routers. If the Layer 3 Switch
receives a strict source-routed packet but cannot reach the next hop interface specified by the packet, the
Layer 3 Switch discards the packet and sends an ICMP Source-Route-Failure message to the sender.
NOTE: The Layer 3 Switch allows you to disable sending of the Source-Route-Failure messages. See
“Disabling ICMP Messages” on page 16-31.
• 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.
The Layer 3 Switch forwards both types of source-routed packets by default. To disable the feature, use either of
the following methods. You cannot enable or disable strict or loose source routing separately.
To disable forwarding of IP source-routed packets, enter the following command:
FESX424 Router(config)# no ip source-route
Syntax: [no] ip source-route
To re-enable forwarding of source-routed packets, enter the following command:
FESX424 Router(config)# ip source-route
Enabling Support for Zero-Based IP Sub-Net Broadcasts
By default, the Layer 3 Switch treats IP packets with all ones in the host portion of the address as IP broadcast
packets. For example, the Layer 3 Switch treats IP packets with 209.157.22.255/24 as the destination IP address
as IP broadcast packets and forwards the packets to all IP hosts within the 209.157.22.x sub-net (except the host
that sent the broadcast packet to the Layer 3 Switch).
Most IP hosts are configured to receive IP sub-net broadcast packets with all ones in the host portion of the
address. However, some older IP hosts instead expect IP sub-net broadcast packets that have all zeros instead of
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