Cisco Switch Operation


Cisco Switch Operation

MAC address forwarding

The second example shows that the switch receives a broadcast message on port 1. The frame will be forwarded through all ports within the same VLAN, except port 1. The frame was received at port 1 and is therefore forwarded to all ports that are in the same VLAN except port 1.

Next, we will discuss Layer 2 switch operation from the mechanics perspective.

Switch Operation

A switch places a frame in an ingress queue when it receives it. Multiple ingress queues can be created for a port. These queues can be used to service frames in different ways (e.g., apply QoS). A simplified view of it is that when a switch selects a frame to transmit from a queue, it must answer some questions.

To which address should the frame go?

Is there any restriction on the forwarding?

Does the frame require prioritization?

These three questions can be answered in the following way:

Layer 2 forwarding tables: Also known as the MAC table or the MAC table, the Layer 2 forwarding tables contain information about where to forward a frame. It contains destination ports and MAC addresses. Buy refurbished firewall switch fortigate 110c price online in India.

The MAC addresses of destination ports are referenced by switches. The frame will be flooded through all ports of the same VLAN if the MAC address cannot be found.


Access control list (ACLs), do not apply only to routers. ACLs that are based on IP addresses and MAC addresses can be applied to switches. 


Incoming frames may be classified according QoS parameters. Traffic can be then marked, prioritized or rate-limited.

To house the MAC table and ACL lookup data as well as QoS lookup information, switches use specialized hardware. Switches use content-addressable memories (CAM) for the MAC table. ACL and QoS tables, on the other hand, are stored in ternary content addressable memory (TCAM). Both TCAM and CAM allow for fast switching and enable line-rate access. CAM only supports two results: 0 and 1. CAM can be used for Layer 2 forwarding tables.

TCAM returns three results: 0, 1 and don’t matter. TCAM is best used to build tables that search for longest matches. For example, IP routing tables organized according to IP prefixes. TCAM tables store ACL, QoS and other information that is generally associated with upper layer processing. TCAM is a way to ensure that the switch performs well.

This section covers only the details and implementations of CAM/TCAM required for CCNP certification. has the following support document:

Multilayer Switch Operation

Multilayer switches can perform Layer 2 switching, as well as forward frames based upon Layer 3 and 4. Multilayer switches combine the functions of a router and switch, but also include a flow cache component.

Multilayer Switch Operation

A FIB table is the associated table for Layer 3 lookups. The FIB table includes VLAN information and egress ports. It also contains information about MAC rewrite. ACL and QoS parallel looksups are the same as Layer cisco 1900 router. However, Layer 3 ACLs may be supported and QoS prioritization may be possible.

A Layer 2 switch might not be able apply to rate-limiting frame based on destination or source MAC addresses. However, a multilayer switch can generally support rate-limiting frames on IP/MAC addresses.

Cisco switches come in different configurations, so some Layer 2-only switches may not be able to support Layer 3 ACLs or QoS lookups. For detailed information on the features of your switch, it is a good idea to refer to’s product documentation. For the purposes of CCNP Switch, Layer 2 switches can support ACLs or QoS based upon MAC addresses. Layer 3 switches can support ACLs or QoS based either on IP addresses or MAC addresses.

Here are some useful commands to view and edit Catalyst Switch’s MAC Address Tables

The command to view the Layer 2 forwarding tables on Nexus switches and Catalyst is show mac address-table. There are many options to reduce the output of the table to make it easier to manage in large networks. count

Mac Address Table

Type of Vlan Mac Address Ports

1 0000:0c00.9001 DYNAMIC Et0/1

This criterion has 3 Total Mac Addresses

Type of Vlan Mac Address Ports

1 0000:0c00.9001 DYNAMIC Et0/1

This criterion uses Total Mac Addresses: 1.

include 9001

1 0000:0c00.9001 DYNAMIC Et0/1

Frame Rewrite

You know from your CCNA studies that packets must be rewritten every time they are routed between subnets. These fields include the source and destination MAC addresses as well as the IP header checksum and trailer checksum (Ethernet-CRC). For an example, see Chapter 1, “Fundamentals Review.

Distributed Hardware Forwarding

At least three operations are required for network devices:

Management plane

Fly control

Forwarding plane

The management plane handles network management such as SSH access, SNMP, and may also operate on an out-of-band (OOB) port. The protocols and routing decisions are handled by the control plane, while the forwarding plane handles the actual routing or switching of most packets.

Multilayer switches need to achieve high line rates across large numbers of ports. Multilayer switches use independent control and forwarding aircrafts to achieve this. The control plane will then program the forwarding aircraft on how to route packets.

Multiple forwarding planes may be used in multilayer switches. A Catalyst 6800, for example, uses forwarding planes on each module of the line, and a central control plane on its supervisor module.

As in the Catalyst 6800 example, each line module has a microcoded process that handles packet forwarding

Distributed Hardware Forwarding

These are the main functions of the control layer protocol between forwarding and control planes:

Managing internal data and control circuits of the packet-forwarding/control functions

Extracting the other routing and packet-forwarding-related control information from the Layer 2 and Layer 3 bridging and routing protocols and the configuration data, and then conveying the information to the interface module for control of the data path

Collecting data path information such as traffic statistics from the interface module to route processor

Handling some data packets sent from the Ethernet interface module to the route processor (e.g., DCHP requests and broadcast packets, routing protocol messages)

By 12disruptors Admin

Leave a Reply

Your email address will not be published. Required fields are marked *