Catalyst 4500 Series Switch Cisco IOS Software Configuration Guide, 12.2(20)EWA

Configuring Layer 2 Ethernet Interfaces

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:

•Overview of Layer 2 Ethernet Switching

•Default Layer 2 Ethernet Interface Configuration

•Layer 2 Interface Configuration Guidelines and Restrictions

•Configuring Ethernet Interfaces for Layer 2 Switching

Note To configure Layer 3 interfaces, see Chapter 21, "Configuring Layer 3 Interfaces."

Note For complete syntax and usage information for the switch commands used in this chapter, look at the Cisco Catalyst 4500 Series Switch Command Reference and related publications at this location:

http://www.cisco.com/en/US/products//hw/switches/ps4324/index.html

If the command is not found in the Catalyst 4500 Command Reference, it will be found in the larger Cisco IOS library. Refer to the Catalyst 4500 Series Switch Cisco IOS Command Reference and related publications at this location:

http://www.cisco.com/en/US/products/ps6350/index.html

Overview of Layer 2 Ethernet Switching

The following sections describe how Layer 2 Ethernet switching works on Catalyst 4500 series switches:

•Understanding Layer 2 Ethernet Switching

•Understanding VLAN Trunks

•Layer 2 Interface Modes

Understanding Layer 2 Ethernet Switching

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 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.

Understanding 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.

Two trunking encapsulations are available on all Ethernet interfaces:

•Inter-Switch Link (ISL) Protocol—ISL is a Cisco-proprietary trunking encapsulation.

Note The blocking Gigabit ports on the WS-X4418-GB and WS-X4412-2GB-T modules do not support ISL. Ports 3 to 18 are blocking Gigabit ports on the WS-X4418-GB module. Ports 1to 12 are blocking Gigabit ports on the WS-X4412-2GB-T module.

•802.1Q—802.1Q is an industry-standard trunking encapsulation.

You can configure a trunk on a single Ethernet interface or on an EtherChannel bundle. For more information about EtherChannel, see Chapter 15, "Understanding and Configuring EtherChannel."

Ethernet trunk interfaces support different trunking modes (see Table 10-2). You can specify whether the trunk uses ISL encapsulation, 802.1Q encapsulation, or if the encapsulation type is autonegotiated.

To autonegotiate trunking, make sure your interfaces are in the same VTP domain. Use the trunk or nonegotiate keywords to force interfaces in different domains to trunk. For more information on VTP domains, see Chapter 25, "Understanding and Configuring VTP."

Trunk negotiation is managed by the Dynamic Trunking Protocol (DTP). DTP supports autonegotiation of both ISL and 802.1Q trunks.

Encapsulation Types

Table 10-1 lists the Ethernet trunk encapsulation types.

The trunking mode, the trunk encapsulation type, and the hardware capabilities of the two connected interfaces determine whether a link becomes an ISL or 802.1Q trunk.

Layer 2 Interface Modes

Table 10-2 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.

Default Layer 2 Ethernet Interface Configuration

Table 10-3 shows the Layer 2 Ethernet interface default configuration.

Layer 2 Interface Configuration Guidelines and Restrictions

Keep the following guidelines and restrictions in mind when you configure Layer 2 interfaces:

•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

•Configuring an Interface as a Layer 2 Access Port

•Clearing Layer 2 Configuration

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. By default, trunks negotiate encapsulation. If the neighboring interface supports ISL and 802.1Q encapsulation and both interfaces are set to negotiate the encapsulation type, the trunk uses ISL encapsulation.

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

Switch(config)# interface {fastethernet | 
gigabitethernet} slot/port 

Switch(config-if)# shutdown 

Switch(config-if)# switchport trunk 
encapsulation {isl | dot1q | negotiate} 

Switch(config-if)# switchport mode 
{dynamic {auto | desirable} | trunk} 

Switch(config-if)# switchport access vlan 
vlan_num 

Switch(config-if)# switchport trunk 
native vlan vlan_num 

Switch(config-if)# switchport trunk 
allowed vlan {add | except | all | 
remove} 
vlan_num[,vlan_num[,vlan_num[,....]]

Switch(config-if)# switchport trunk 
pruning vlan {add | except | none | 
remove} 
vlan_num[,vlan_num[,vlan_num[,....]] 

Switch(config-if)# no shutdown 

Switch(config-if)# end 

Switch# show running-config interface 
{fastethernet | gigabitethernet} 
slot/port 

Switch# show interfaces [fastethernet | 
gigabitethernet] slot/port switchport 

Switch# show interfaces [{fastethernet | 
gigabitethernet} slot/port] trunk 

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.

Switch(config)# interface fastethernet 5/8 

Switch(config-if)# shutdown 

Switch(config-if)# switchport mode dynamic desirable 

Switch(config-if)# switchport trunk encapsulation dot1q 

Switch(config-if)# no shutdown 

Switch(config-if)# end 

Switch# exit 

This example shows how to verify the running configuration:

Switch# show running-config interface fastethernet 5/8 

Building configuration...

Current configuration:

!

interface FastEthernet5/8

 switchport mode dynamic desirable

 switchport trunk encapsulation dot1q

end

This example shows how to verify the switch port configuration:

Switch# show interfaces fastethernet 5/8 switchport 

Name: Fa5/8

Switchport: Enabled

Administrative Mode: dynamic desirable

Operational Mode: trunk

Administrative Trunking Encapsulation: negotiate

Operational Trunking Encapsulation: dot1q

Negotiation of Trunking: Enabled

Access Mode VLAN: 1 (default)

Trunking Native Mode VLAN: 1 (default)

Trunking VLANs Enabled: ALL

Pruning VLANs Enabled: 2-1001

This example shows how to verify the trunk configuration:

Switch# show interfaces fastethernet 5/8 trunk 

Port      Mode         Encapsulation  Status        Native vlan

Fa5/8     desirable    n-802.1q       trunking      1

Port      Vlans allowed on trunk

Fa5/8 1-1005

Port      Vlans allowed and active in management domain

Fa5/8 1-6,10,20,50,100,152,200,300,303-305,349-351,400,500,521,524,570,801-8

02,850,917,999,1002-1005

Port      Vlans in spanning tree forwarding state and not pruned

Fa5/8 1-6,10,20,50,100,152,200,300,303-305,349-351,400,500,521,524,570,801-8

02,850,917,999,1002-1005

Switch# 

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 on page 8-5).

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

Switch(config)# interface {fastethernet | 
gigabitethernet} slot/port 

Switch(config-if)# shutdown 

Switch(config-if)# switchport 

Switch(config-if)# switchport mode access 

Switch(config-if)# switchport access vlan 
vlan_num 

Switch(config-if)# no shutdown 

Switch(config-if)# end 

Switch# show running-config interface 
{fastethernet | gigabitethernet} slot/port 

Switch# show interfaces [{fastethernet | 
gigabitethernet} slot/port] switchport 

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

Switch# configure terminal 

Enter configuration commands, one per line.  End with CNTL/Z.

Switch(config)# interface fastethernet 5/6 

Switch(config-if)# shutdown 

Switch(config-if)# switchport mode access 

Switch(config-if)# switchport access vlan 200 

Switch(config-if)# no shutdown 

Switch(config-if)# end 

Switch# exit 

This example shows how to verify the running configuration:

Switch# show running-config interface fastethernet 5/6

Building configuration...

!

Current configuration :33 bytes

interface FastEthernet 5/6

 switchport access vlan 200

 switchport mode access

end

This example shows how to verify the switch port configuration:

Switch# show running-config interface fastethernet 5/6 switchport

Name:Fa5/6

Switchport:Enabled

Administrative Mode:dynamic auto

Operational Mode:static access

Administrative Trunking Encapsulation:negotiate

Operational Trunking Encapsulation:native

Negotiation of Trunking:On

Access Mode VLAN:1 (default)

Trunking Native Mode VLAN:1 (default)

Administrative private-vlan host-association:none

Administrative private-vlan mapping:none

Operational private-vlan:none

Trunking VLANs Enabled:ALL

Pruning VLANs Enabled:2-1001

Switch# 

Clearing Layer 2 Configuration

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

Switch(config)# default interface {fastethernet | 
gigabitethernet} slot/port 

Switch(config-if)# end 

Switch# show running-config interface 
{fastethernet | gigabitethernet} slot/port 

Switch# show interfaces [{fastethernet | 
gigabitethernet} slot/port] switchport 

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

Switch# configure terminal 

Enter configuration commands, one per line.  End with CNTL/Z.

Switch(config)# default interface fastethernet 5/6 

Switch(config)# end 

Switch# exit 

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

Switch# show running-config interface fastethernet 5/6 

Building configuration...

Current configuration:

!

interface FastEthernet5/6

end

This example shows how to verify the switch port configuration:

Switch# show interfaces fastethernet 5/6 switchport 

Name: Fa5/6

Switchport: Enabled

Switch#