Configuring Ip Parameters For Routing Switches; Configuring Arp Parameters; How Arp Works - HP 2920 Series Multicast And Routing Manual

Table 4 IP interface parameters — routing switches (continued)
Parameter
ICMP Router Discovery
Protocol (IRDP)
IP helper address
1
See the chapter "Configuring IP Addressing" in the Management and Configuration Guide for your switch.

Configuring IP parameters for routing switches

The following sections describe how to configure IP parameters. Some parameters can be configured
globally while others can be configured on individual VLAN interfaces. Some parameters can be
configured globally and overridden for individual VLAN interfaces.
NOTE: For IP configuration information when routing is not enabled, see the chapter "Configuring
IP Addressing" in the Management and Configuration Guide for your routing switch.

Configuring ARP parameters

ARP is a standard IP protocol that enables an IP routing switch to obtain the MAC address of
another device's interface when the routing switch knows the IP address of the interface. ARP is
enabled by default and cannot be disabled.

How ARP works

A routing switch needs to know a destination's MAC address when forwarding traffic, because
the routing switch encapsulates the IP packet in a Layer 2 packet (MAC layer packet) and sends
the Layer 2 packet to a MAC interface on a device directly attached to the routing switch. The
device can be the packet's final destination or the next-hop router toward the destination.
The routing switch encapsulates IP packets in Layer 2 packets regardless of whether the ultimate
destination is locally attached or is multiple router hops away. Since the routing switch's IP route
table and IP forwarding cache contain IP address information but not MAC address information,
the routing switch cannot forward IP packets based solely on the information in the route table or
forwarding cache. The routing switch needs to know the MAC address that corresponds with the
IP address of either the packet's locally attached destination or the next-hop router that leads to
the destination.
For example, to forward a packet whose destination is multiple router hops away, the routing
switch must send the packet to the next-hop router toward its destination, or to a default route or
default network route if the IP route table does not contain a route to the packet's destination. In
each case, the routing switch must encapsulate the packet and address it to the MAC address of
a locally attached device, the next-hop router toward the IP packet's destination.
42 IP Routing Features
Description
parameter applies only to
RIP routes.
Locally overrides the global
IRDP settings. See Table 3
(page 38) for global IRDP
information.
The IP address of a UDP
application server (such as
a BootP or DHCP server) or
a directed broadcast
address. IP helper addresses
allow the routing switch to
forward requests for certain
UDP applications from a
client on one subnet to a
server on another subnet.
Default
Disabled
None configured
See page
61
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