Appendix C Switch Port Functionality

Storm Control

(132.206.72.28, 224.2.136.89), 00:14:31/00:01:40, flags:CJT Incoming interface:GigabitEthernet4/8, RPF nbr 10.15.1.20, RPF-MFD

Outgoing interface list:Null Router#

Note The RPF-MFD flag indicates that the flow is completely hardware switched. The H flag indicates that the flow is hardware-switched on the outgoing interface.

Storm Control

A packet storm occurs when a large number of broadcast, unicast, or multicast packets are received on a port. Forwarding these packets can cause the network to slow down or to time out. Storm control is configured for the switch as a whole, although it operates on a per-interface basis. By default, storm control is disabled.

Storm control prevents switch ports on a LAN from being disrupted by a broadcast, multicast, or unicast storm on one of the interfaces. A LAN storm occurs when packets flood the LAN, creating excessive traffic and degrading network performance. Errors in the protocol-stack implementation or in the network configuration can cause a storm.

Storm control monitors incoming traffic statistics over a time period and compares the measurement with a predefined suppression level threshold. The threshold represents the percentage of the total available bandwidth of the port. If the threshold of a traffic type is reached, further traffic of that type is suppressed until the incoming traffic falls below the threshold level.

The graph in Figure C-6shows broadcast traffic patterns on an interface over a given period of time. In this example, the broadcast traffic exceeded the configured threshold between time intervals T1 and T2 and between intervals T4 and T5. When the amount of specified traffic exceeds the threshold, all traffic of that kind is dropped. Therefore, broadcast traffic is blocked during those intervals. At the next time interval, if broadcast traffic does not exceed the threshold, it is again forwarded.

Figure C-6 Broadcast Suppression Example

Total

number of

Threshold broadcast

packets or bytes

0

T1

T2

T3

T4

T5

Time

Forwarded traffic

Blocked traffic

46651

When storm control is enabled, the switch monitors the packets that are passing from an interface to the switching bus and determines whether the packet is unicast, multicast, or broadcast. The switch monitors the number of broadcast, multicast, or unicast packets received within the 1-second time interval, and

Cisco 3200 Series Router Hardware Reference

 

OL-5816-10

C-13

 

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Cisco Systems 3200 manual Appendix C Switch Port Functionality Storm Control

3200 specifications

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