The following configurable parameters control the forwarding behavior of HP routing switches: • Time-To-Live (TTL) threshold
• Forwarding of directed broadcasts • Forwarding of source-routed packets • Ones-based and zero-based broadcasts
All these parameters are global and thus affect all IP interfaces configured on the routing switch. To configure these parameters, use the procedures in the following sections.
Changing the TTL Threshold
The TTL threshold prevents routing loops by specifying the maximum number of router hops an IP packet originated by the routing switch can travel through. Each device capable of forwarding IP that receives the packet decrements (decreases) the packet’s 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.
The default TTL is 64. You can change the TTL to a value from 1– 255. To modify the TTL, use either of the following methods.
USING THE CLI
To modify the TTL threshold to 25, enter the following commands: HP9300(config)# ip ttl 25
Syntax: ip ttl <1-255>
USING THE WEB MANAGEMENT INTERFACE
1. Log on to the device using a valid user name and password for read-write access. The System configuration panel is displayed.
2. Click on the plus sign next to Configure in the tree view to display the list of configuration options. 3. Click on the plus sign next to IP to display the list of IP configuration options.
4. Select the General link to display the IP configuration panel. 5. Enter a value from 1 – 255 into the TTL field.
6. Click the Apply button to save the change to the device’s running-config file.
7. Select the Save link at the bottom of the dialog. Select Yes when prompted to save the configuration change to the startup-config file on the device’s flash memory.
Enabling Forwarding of Directed Broadcasts
A directed broadcast is an IP broadcast to all devices within a single directly-attached network or sub-net. A net directed broadcast goes to all devices on a given network. A sub-net-directed broadcast goes to all devices within a given sub-net.
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, use either of the following methods.
USING THE CLI
HP9300(config)# ip directed-broadcast
Syntax: [no] ip directed-broadcast
HP software makes the forwarding decision based on the routing switch’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: HP9300(config)# no ip directed-broadcast
USING THE WEB MANAGEMENT INTERFACE
1. Log on to the device using a valid user name and password for read-write access. The System configuration panel is displayed.
2. Click on the plus sign next to Configure in the tree view to display the list of configuration options. 3. Click on the plus sign next to IP to display the list of IP configuration options.
4. Select the General link to display the IP configuration panel. 5. Select Enable or Disable next to Directed Broadcast Forward.
6. Click the Apply button to save the change to the device’s running-config file.
7. Select the Save link at the bottom of the dialog. Select Yes when prompted to save the configuration change to the startup-config file on the device’s flash memory.
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 routing switch supports both types of IP source routing:
• Strict source routing – requires the packet to pass through only the listed routers. If the routing switch receives a strict source-routed packet but cannot reach the next hop interface specified by the packet, the routing switch discards the packet and sends an ICMP Source-Route-Failure message to the sender.
NOTE: The routing switch allows you to disable sending of the Source-Route-Failure messages. See “Disabling ICMP Messages” on page 6-34.
• 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 routing 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.
USING THE CLI
To disable forwarding of IP source-routed packets, enter the following command: HP9300(config)# no ip source-route
Syntax: [no] ip source-route
HP9300(config)# ip source-route
USING THE WEB MANAGEMENT INTERFACE
1. Log on to the device using a valid user name and password for read-write access. The System configuration panel is displayed.
2. Click on the plus sign next to Configure in the tree view to expand the list of configuration options. 3. Click on the plus sign next to IP in the tree view to expand the list of IP option links.
4. Click on the General link to display the IP configuration panel. 5. Select the Disable or Enable radio button next to Source Route.
6. Click the Apply button to save the change to the device’s running-config file.
7. Select the Save link at the bottom of the dialog. Select Yes when prompted to save the configuration change to the startup-config file on the device’s flash memory.
Enabling Support for Zero-Based IP Sub-Net Broadcasts
By default, the routing switch treats IP packets with all ones in the host portion of the address as IP broadcast packets. For example, the routing 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 routing 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 all ones in the host portion of the address. To accommodate this type of host, you can enable the routing switch to treat IP packets with all zeros in the host portion of the destination IP address as broadcast packets.
NOTE: When you enable the routing switch for zero-based sub-net broadcasts, the routing switch still treats IP packets with all ones the host portion as IP sub-net broadcasts too. Thus, the routing switch can be configured to support all ones only (the default) or all ones and all zeroes.
NOTE: This feature applies only to IP sub-net broadcasts, not to local network broadcasts. The local network broadcast address is still expected to be all ones.
To enable the routing switch for zero-based IP broadcasts, use either of the following methods.
USING THE CLI
To enable the routing switch for zero-based IP sub-net broadcasts in addition to ones-based IP sub-net broadcasts, enter the following command.
HP9300(config)# ip broadcast-zero
Syntax: [no] ip broadcast-zero
USING THE WEB MANAGEMENT INTERFACE
You cannot enable zero-based IP sub-net broadcasting using the Web management interface.