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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter22 Configuring IP Unicast Routing Configuring IP Addressing
GigabitEthernet0/3 has router discovery enabled
Advertisements will occur between every 450 and 600 seconds.
Advertisements are sent with broadcasts.
Advertisements are valid for 1800 seconds.
Default preference will be 0.
GigabitEthernet0/4 has router discovery disabled
Port-channel1 has router discovery disabled
Configuring Broadcast Packet Handling
After configuring an IP interface address, you can c hoos e to e nab le rout ing a nd co nfi g ure o ne or m ore
routing protocols, or you can configure the way the switch responds to n etwork broadcasts. A broadcast
is a data packet destined for all hosts on a physical network. The swi tch suppor ts two ki nds of
broadcasting:
•A directed broadcast packet is sent to a specific network or series of networks. A directed broa dcast
address includes the network or subnet fields.
•A flooded broadcast packet is sent to every network.
Note You can also limit broadcast, unicast, and multicast traffic on Layer 2 interfaces by using the
switchport broadcast, switchport unicast, and switchport multicast interface configuration
commands. For more information, see Chapter 12, “Configuring Port-Based Traffic Control.”
Routers provide some protection from broadcast storms by limiting their extent to the local cable.
Bridges (including intelligent bridges), because they are Layer 2 devices, forward broadcasts to all
network segments, thus propagating broadcast storms. The best solution to the broadca st storm problem
is to use a single broadcast address scheme on a network. In most mo dern IP implementations, you can
set the address to be used as the broadcast address. Many impleme nta tions, inc ludi ng t he one in the
Catalyst 3550 switch, support several addressing schemes for forwarding broadcast messages.
Perform the tasks in these sections to enable these schemes:
•Enabling Directed Broadcast-to-Physical Broadcast Translation, pag e 22- 17
•Forwarding UDP Broadcast Packets and Protocols, page 22-18
•Establishing an IP Broadcast Address, page 22-20
•Flooding IP Broadcasts, page 22-20

Enabling Directed Broadcast-to-Physical Broadcast Translation

By default, IP directed broadcasts are dropped; they are not forwa rded. Dropping IP-directed broadcasts
makes routers less susceptible to denial-of-service attacks.
You can enable forwarding of IP-directed broadcasts on an interface where the broadcast becomes a
physical (MAC-layer) broadcast. Only those protocols confi gure d b y using the ip for war d-protocol
global configuration command are forwarded.
You can specify an access list to control which broadcasts are forwarded. When an access list is
specified, only those IP packets permitted by the access list are eligible to be translated from directed
broadcasts to physical broadcasts. For more information on access lists, see Chapter19, “Configuring
Network Security with ACLs.”