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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 22 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 choose to enable routing and configure one or more
routing protocols, or you can configure the way the switch responds to network broadcasts. A broadcast
is a data packet destined for all hosts on a physical network. The switch supports two kinds of
broadcasting:
• A directed broadcast packet is sent to a specific network or series of networks. A directed broadcast
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 broadcast storm problem
is to use a single broadcast address scheme on a network. In most modern IP implementations, you can
set the address to be used as the broadcast address. Many implementations, including the 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, page 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 forwarded. 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 configured by using the ip forward-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 Chapter 19, “Configuring
Network Security with ACLs.”