This chapter describes how to use the command-line interface (CLI) to configure Fast Ethernet and Gigabit Ethernet interfaces for Layer 2 switching on Catalyst 4500 series switches. It also provides guidelines, procedures, and configuration examples. The configuration tasks in this chapter apply to Fast Ethernet and Gigabit Ethernet interfaces on any module, including the uplink ports on the supervisor engine.
This chapter includes the following major sections:
Catalyst 4500 series 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 4500 series 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 4500 series 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 4500 series switch 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 4500 series 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.
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.
Table 1-1 lists the Layer 2 interface modes and describes how they function on Ethernet interfaces.
Table 1-1 Layer 2 Interface Modes
switchport mode access
Puts the interface into permanent nontrunking mode and negotiates to convert the link into a nontrunking link. The interface becomes a nontrunk interface even if the neighboring interface does not change.
Makes the interface actively attempt to convert the link to a trunking link. The interface becomes a trunk interface if the neighboring interface is set to
switchport mode dynamic auto
Makes the interface convert the link to a trunking link if the neighboring interface is set to
mode. It is the default mode for all Ethernet interfaces.
switchport mode trunk
Puts the interface into permanent trunking mode and negotiates to convert the link into a trunking link. The interface becomes a trunk interface even if the neighboring interface does not change.
Puts the interface into permanent trunking mode but prevents the interface from generating DTP frames. You must configure the neighboring interface manually as a trunk interface to establish a trunking link.
NoteDTP 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.
Default Layer 2 Ethernet Interface Configuration
Table 1-2 shows the Layer 2 Ethernet interface default configuration.
Layer 2 Interface Configuration Guidelines and Restrictions
When using (or configuring) Layer 2 interfaces, consider these guidelines and restrictions:
In a network of Cisco switches connected through 802.1Q trunks, the switches maintain one instance of spanning tree for each VLAN allowed on the trunks. Non-Cisco 802.1Q switches maintain only one instance of spanning tree for all VLANs allowed on the trunks.
When you connect a Cisco switch to a non-Cisco device through an 802.1Q trunk, the Cisco switch combines the spanning tree instance of the native VLAN of the trunk with the spanning tree instance of the non-Cisco 802.1Q switch. However, spanning tree information for each VLAN is maintained by Cisco switches separated by a cloud of non-Cisco 802.1Q switches. The non-Cisco 802.1Q cloud separating the Cisco switches is treated as a single trunk link between the switches.
Make sure the native VLAN for an 802.1Q trunk is the same on both ends of the trunk link. If the VLAN on one end of the trunk is different from the VLAN on the other end, spanning tree loops might result.
Disabling spanning tree on any VLAN of an 802.1Q trunk can cause spanning tree loops.
Configuring Ethernet Interfaces for Layer 2 Switching
The following sections describe how to configure Layer 2 switching on a Catalyst 4500 series switch:
Configuring an Ethernet Interface as a Layer 2 Trunk
NoteThe 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:
(Optional) Specifies the access VLAN, which is used if the interface stops trunking. The access VLAN is not used as the native VLAN.
Note The vlan_num parameter is either a single VLAN number from 1 to 1005 or a range of VLANs described by two VLAN numbers, the lesser one first, separated by a dash. Do not enter any spaces between comma-separated vlan parameters or in dash-specified ranges.
(Optional) Configures the list of VLANs allowed to be pruned from the trunk (see the “VLAN Trunking Protocol” section). The default list of VLANs allowed to be pruned contains all VLANs, except for VLAN 1.
Activates the interface. (Required only if you shut down the interface.)
Displays the trunk configuration of the interface.
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:
Switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.