BVI Flooding Enhancement for the Catalyst 2948G-L3 and the Catalyst 4908G-L3

Document ID: 10606

Introduction
Before You Begin
      Conventions
      Prerequisites
      Components Used
Problem
      Conditions
Solution
Related Information

Introduction

This document provides information on Bridge-Group Virtual Interface (BVI) flooding enhancements for the Catalyst 2948G-L3 and the Catalyst 4908G-L3.

Before You Begin

Conventions

For more information on document conventions, see the Cisco Technical Tips Conventions.

Prerequisites

The BVI flooding enhancments are not availalbe until Cisco IOS® software release 12.0(10)W5(18e) or later.

Components Used

The information in this document is based on the software and hardware versions below.

Cisco IOS software release 12.0(10)W5(18e) and later
The Catalyst 2948G-L3 and the Catalyst 4908G-L3

Problem

When Integrated Routing and Bridging (IRB) is configured on the Catalyst 2948G-L3 and 4908G-L3, routing might be performed in the CPU path instead of in the hardware (microcode).

When Layer 3 (L3) traffic arrives on a routed interface, the Cisco IOS code running in the CPU takes the incoming packet and routes it to the BVI interface. The BVI interface then forwards the L3 traffic to the destination host or router.

If the Cisco IOS code does not know the actual outgoing bridge-group member interface to which the packet needs to be sent, the packet is flooded to all the interfaces in the bridge-group. This is determined by checking to see if the bridge-table contains the MAC address for the destination device. The assumption is that the Address Resolution Protocol (ARP) entry is complete, with all the IP addresses.

All this activity takes place in process-level code and thus when the incoming rate of traffic is high, the CPU utilization goes up to an alarming rate. High CPU utilization makes the system unstable, and it has a wide range of negative side-effects.

Conditions

The condition that triggers traffic to be sent to the CPU is when the MAC address of the host connected to the bridge-group ages out of the bridge table. This makes the L3 (IP/IPX) adjacency for the host INVALID.

This information is propagated to all the L3 Content Addressable Memory (CAM) tables for all the routed ports. This is done because the L3 adjacency is no longer valid. Therefore the Layer 2 (L2) MAC information for the adjacency can no longer be guaranteed.

Once this condition happens, traffic received on any routed port destined to the host behind a BVI interface will be sent to the CPU. The Cisco IOS code running in the CPU will perform L3 routing of the traffic to the BVI interface.

Next, it will do a bridge-table lookup to find out the destined outgoing interface for this traffic. If it does not find any entry in the bridge table for the destination, it will flood the traffic to all the members of the bridge-group. Therefore the throughput will be significantly lower and the CPU utilization will be higher.

Solution

To resolve the high CPU issues associated with BVI flooding on the Catalyst 2948G-L3 and the Catalyst 4908G-L3 switches, upgrade to code version 12.0(10)W5(18e) or higher. Enhancements to this code allow the BVI flooding to be done in hardware, rather then the CPU intensive software method.

Therefore, beginning in Cisco IOS software release 12.0(10)W5(18e), once a L3 adjacency becomes invalid, microcode floods the traffic destined to a host behind a BVI to all the members of the bridge-group, thereby offloading the CPU from this strenuous job.

Note: For IP, the ARP must be complete for flooding in microcode to occur.

Related Information

Updated: Aug 30, 2005

Document ID: 10606

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