Cisco 3560G 48TS Software Configuration Guide - Page 621

Chapter 30 Configuring IP Unicast Routing Configuring IP Addressing If you change the maxadvertinterval value, the holdtime and minadvertinterval values also change, so it is important to first change the maxadvertinterval value, before manually changing either the holdtime or minadvertinterval values. Use the no ip irdp interface configuration command to disable IRDP routing. Configuring Broadcast Packet Handling After configuring an IP interface address, you can 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 storm-control interface configuration command to set traffic suppression levels. For more information, see Chapter 20, "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 3560 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 30-13 • Forwarding UDP Broadcast Packets and Protocols, page 30-14 • Establishing an IP Broadcast Address, page 30-15 • Flooding IP Broadcasts, page 30-16 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 27, "Configuring Network Security with ACLs." 78-16156-01 Catalyst 3560 Switch Software Configuration Guide 30-13