User`s guide

XSR User’s Guide 67
Chapter 5 General IP Features
Configuring IP
The XSR supports directed broadcast using the ip directed-broadcast
command. For security purposes, restrictions can be set by defining and
applying an ACL and by restricting the protocols. There are two types of
directed broadcasts, described as follows:
 A net-directed broadcast specifies a destination address with a host ID
of all 1s. For example, a Class A net-directed broadcast destination
address is
netid.255.255.255 where the netid is the Class A
network ID. The XSR forwards it by default.
 A subnet-directed broadcast also specifies a destination address with a
host ID of all 1s, but with a specific subnet ID. For example, a Class A
subnet-directed broadcast destination address is
netid.subnetid.255.255 where netid is the Class A network ID
and subnetid is the subnet. The XSR forwards it by default.
Local Broadcast
A local broadcast is a broadcast to a destination address of all ones -
255.255.255.255. This broadcast should not be forwarded. It may be:
 Consumed by the router, or,
 Forwarded using UDP broadcast forwarding. UDP broadcast
forwarding is a feature that allows XSR to forward a UDP local
broadcast to one or more new destinations if the UDP port of the
datagram matches the configured one. The destination address is
replaced by a configured unicast address, and there is no change in
the source IP address (except BOOTP/DHCP relay). A total of 50
UDP broadcast forwarding entries is allowed in the table with
standard memory installed.
ICMP
The Internet Control Message Protocol (ICMP) communicates error messages
and other conditions that require attention as defined by RFC-792.
ICMP messages are transmitted in IP datagrams and are usually acted on by
the IP layer or higher layer protocols (TCP/UDP). The XSR supports these
message types: ICMP router discovery, destination unreachable, time exceeded,
parameter problem, redirect, echo or echo reply.
The XSR also supports the ICMP Router Discovery Protocol (IRDP) which
dynamically discovers routes to other networks, as defined by RFC-1256.