Configuring Forwarding Parameters; Enabling Forwarding Of Directed Broadcasts - HP 2920 Series Multicast And Routing Manual

To obtain the MAC address required for forwarding a datagram, the routing switch does the
following:
First, the routing switch looks in the ARP cache (not the static ARP table) for an entry that lists
the MAC address for the IP address. The ARP cache maps IP addresses to MAC addresses.
The cache also lists the port attached to the device and, if the entry is dynamic, the age of
the entry. A dynamic ARP entry enters the cache when the routing switch receives an ARP
reply or receives an ARP request (which contains the sender's IP address and MAC address.)
A static entry enters the ARP cache from the static ARP table (which is a separate table) when
the interface for the entry comes up.
To ensure the accuracy of the ARP cache, each dynamic entry has its own age timer. The timer
is reset to zero each time the routing switch receives an ARP reply or ARP request containing
the IP address and MAC address of the entry. If a dynamic entry reaches its maximum allowable
age, the entry times out and the software removes the entry from the table. Static entries do
not age-out and can be removed only by you.
If the ARP cache does not contain an entry for the destination IP address, the routing switch
broadcasts an ARP request out all of its IP interfaces. The ARP request contains the IP address
of the destination. If the device with the IP address is directly attached to the routing switch,
the device sends an ARP response containing its MAC address. The response is a unicast
packet addressed directly to the routing switch. The routing switch places the information from
the ARP response into the ARP cache.
ARP requests contain the IP address and MAC address of the sender, so all devices that receive
the request learn the MAC address and IP address of the sender and can update their own
ARP caches accordingly.
Note that the ARP request broadcast is a MAC broadcast, which means the broadcast goes
only to devices that are directly attached to the routing switch. A MAC broadcast is not routed
to other networks. However, some routers, including HP routing switches, can be configured
to reply to ARP requests from one network on behalf of devices on another network.
NOTE: If the routing switch receives an ARP request packet that it is unable to deliver to the final
destination because of the ARP time-out, and no ARP response is received (the routing switch knows
of no route to the destination address), the routing switch sends an ICMP Host Unreachable message
to the source.

Configuring forwarding parameters

The following configurable parameters control the forwarding behavior of HP routing switches:
Time-To-Live (TTL) threshold
The configuration of this parameter is covered in the chapter "Configuring IP Addressing" in
the Management and Configuration Guide for your routing switch.
Forwarding of directed 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.

Enabling 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.
Configuring IP parameters for routing switches 43