Catalyst 4500 Series Switch Software Configuration Guide, IOS XE 3.7.xE and IOS 15.2(3)Ex

Layer 2 Ethernet Switching

Catalyst 4000 family switches support simultaneous, parallel connections between Layer 2 Ethernet segments. Switched connections between Ethernet segments last only for the duration of the packet. New connections can be made between different segments for successive packets.

Note With Cisco IOS Release 12.1(13)EW, the Catalyst 4000 family switches can handle packets of 1600 bytes, rather than treat them as “oversized” and discard them. This size is larger than the usual IEEE Ethernet Maximum Transmission Unit (MTU) (1518 bytes) and 802.1q MTU (1522 bytes). The ability to handle larger packets is required to support two nested 802.1q headers and Multiprotocol Label Switching (MPLS) on a network.

The Catalyst 4006 switch with Supervisor Engine III solves congestion problems caused by high-bandwidth devices and a large number of users by assigning each device (for example, a server) to its own 10-, 100-, or 1000-Mbps segment. Because each Ethernet interface on the switch represents a separate Ethernet segment, servers in a properly configured switched environment achieve full access to the bandwidth.

Because collisions are a major bottleneck in Ethernet networks, an effective solution is full-duplex communication. Normally, Ethernet operates in half-duplex mode, which means that stations can either receive or transmit. In full-duplex mode, two devices can transmit and receive at the same time. When packets can flow in both directions simultaneously, effective Ethernet bandwidth doubles to 20 Mbps for 10-Mbps interfaces and to 200 Mbps for Fast Ethernet interfaces. Gigabit Ethernet interfaces on the Catalyst 4006 switch with Supervisor Engine III are full-duplex mode only, providing 2-Gbps effective bandwidth.

Switching Frames Between Segments

Each Ethernet interface on a Catalyst 4500 series switch can connect to a single workstation or server, or to a hub through which workstations or servers connect to the network.

On a typical Ethernet hub, all ports connect to a common backplane within the hub, and the bandwidth of the network is shared by all devices attached to the hub. If two devices establish a session that uses a significant level of bandwidth, the network performance of all other stations attached to the hub is degraded.

To reduce degradation, the switch treats each interface as an individual segment. When stations on different interfaces need to communicate, the switch forwards frames from one interface to the other at wire speed to ensure that each session receives full bandwidth.

To switch frames between interfaces efficiently, the switch maintains an address table. When a frame enters the switch, it associates the MAC address of the sending station with the interface on which it was received.

Building the MAC Address Table

The Catalyst 4006 switch with Supervisor Engine III builds the MAC address table by using the source address of the frames received. When the switch receives a frame for a destination address not listed in its MAC address table, it floods the frame to all interfaces of the same VLAN except the interface that received the frame. When the destination device replies, the switch adds its relevant source address and interface ID to the address table. The switch then forwards subsequent frames to a single interface without flooding to all interfaces.

The address table can store at least 32,000 address entries without flooding any entries. The switch uses an aging mechanism, defined by a configurable aging timer, so if an address remains inactive for a specified number of seconds, it is removed from the address table.

VLAN Trunks

A trunk is a point-to-point link between one or more Ethernet switch interfaces and another networking device such as a router or a switch. Trunks carry the traffic of multiple VLANs over a single link and allow you to extend VLANs across an entire network.

Only 802.1Q trunking encapsulations is available on all Catalyst 4500 Ethernet interfaces.

Note Ports 3 to 18 are blocking Gigabit ports on the WS-X4418-GB module. Ports 1 to 12 are blocking Gigabit ports on the WS-X4412-2GB-T module.

You can configure a trunk on a single Ethernet interface or on an EtherChannel bundle. For more information about EtherChannel, see Chapter26, “Configuring EtherChannel and Link State Tracking”

Layer 2 Interface Modes

Table 18-1 lists the Layer 2 interface modes and describes how they function on Ethernet interfaces.

Note DTP is a point-to-point protocol. However, some internetworking devices might forward DTP frames improperly. To avoid this problem, ensure that interfaces connected to devices that do not support DTP are configured with the access keyword if you do not intend to trunk across those links. To enable trunking to a device that does not support DTP, use the nonegotiate keyword to cause the interface to become a trunk without generating DTP frames.

Configuring an Ethernet Interface as a Layer 2 Trunk

Note The default for Layer 2 interfaces is switchport mode dynamic auto. If the neighboring interface supports trunking and is configured to trunk mode or dynamic desirable mode, the link becomes a Layer 2 trunk.

To configure an interface as a Layer 2 trunk, perform this task:

This example shows how to configure the Fast Ethernet interface 5/8 as an 802.1Q trunk. This example assumes that the neighbor interface is configured to support 802.1Q trunking and that the native VLAN defaults to VLAN 1:

This example shows how to verify the running configuration:

This example shows how to verify the switch port configuration:

This example shows how to verify the trunk configuration:

Configuring an Interface as a Layer 2 Access Port

Note If you assign an interface to a VLAN that does not exist, the interface is not operational until you create the VLAN in the VLAN database (see the “Configuring VLANs in Global Configuration Mode” section).

To configure an interface as a Layer 2 access port, perform this task:

This example shows how to configure the Fast Ethernet interface 5/6 as an access port in VLAN 200:

This example shows how to verify the running configuration:

This example shows how to verify the switch port configuration:

Clearing Layer 2 Configuration

To clear the Layer 2 configuration on an interface, perform this task:

This example shows how to clear the Layer 2 configuration on the Fast Ethernet interface 5/6:

This example shows how to verify that the Layer 2 configuration was cleared:

This example shows how to verify the switch port configuration: