Broadcast; Directed Broadcast; Local Broadcast; Icmp - Enterasys Security Router X-PeditionTM User Manual

When a BOOTP/DHCP response is received, the packet is sent to the requester as a unicast IP
packet, according to RFC-951, with clarifications in RFC-1532.
The source addresses of the relayed BOOTP/DHCP packets can be selected using ip dhcp relay-
source gateway command. By default, IP stack selects the outgoing interface address as the source
address.

Broadcast

A broadcast is a packet destined for all hosts on a given network as defined by RFC-919 and
RFC-922.

Directed Broadcast

An IP directed broadcast is a datagram sent to the broadcast address of a subnet to which the
sending device is not directly attached. The directed broadcast is routed through the network as a
unicast packet until it arrives at the target subnet, where it is converted into a link-layer broadcast.
The XSR supports directed broadcast using the
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
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
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, a feature which 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 with no
change in the source address (except BOOTP/DHCP relay).

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.
IRDP allows hosts to locate routers and can also infer router locations by checking RIP updates.
When the XSR operates as a client, router discovery packets are generated. When the device
operates as a host, router discovery packets are received. The IRDP client/server implementation
ip directed-broadcast
where netid is the Class A network ID and subnetid is the subnet.
command. For security
netid.255.255.255 where the netid is
XSR User's Guide 5-5
General IP Features