Catalyst Supervisor Engine 32 PISA IOS Software Configuration Guide, 12.2ZY

Configuring LAN Ports for Layer 2 Switching

This chapter describes how to use the command-line interface (CLI) to configure Ethernet, Fast Ethernet, Gigabit Ethernet, and 10-Gigabit Ethernet LAN ports for Layer 2 switching on the Catalyst 6500 series switches. The configuration tasks in this chapter apply to LAN ports on LAN switching modules and to the LAN ports on the supervisor engine.

Note•For complete syntax and usage information for the commands used in this chapter, refer to the Catalyst Supervisor Engine 32 PISA Cisco IOS Command Reference, Release 12.2ZY, at this URL:

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2ZY/command/reference/cmdref.html

•To configure Layer 3 interfaces, see Chapter 19 "Configuring Layer 3 Interfaces."

This chapter consists of these sections:

•Understanding How Layer 2 Switching Works

•Default Layer 2 LAN Interface Configuration

•Layer 2 LAN Interface Configuration Guidelines and Restrictions

•Configuring LAN Interfaces for Layer 2 Switching

Understanding How Layer 2 Switching Works

These sections describe how Layer 2 switching works on the Catalyst 6500 series switches:

•Understanding Layer 2 Ethernet Switching

•Understanding VLAN Trunks

•Layer 2 LAN Port Modes

Understanding Layer 2 Ethernet Switching

These sections describe Layer 2 Ethernet switching:

•Layer 2 Ethernet Switching Overview

•Switching Frames Between Segments

•Building the Address Table

Layer 2 Ethernet Switching Overview

Catalyst 6500 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 the next packet.

Catalyst 6500 series switches solve congestion problems caused by high-bandwidth devices and by a large number of users by assigning each device (for example, a server) to its own 10-, 100-, or 1000-Mbps collision domain. Because each LAN port connects to a separate Ethernet collision domain, servers in a properly configured switched environment achieve full access to the bandwidth.

Because collisions cause significant congestion 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 stations can transmit and receive at the same time. When packets can flow in both directions simultaneously, the effective Ethernet bandwidth doubles.

Switching Frames Between Segments

Each LAN port on a Catalyst 6500 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 stations 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 considers each LAN port to be an individual segment. When stations connected to different LAN ports need to communicate, the switch forwards frames from one LAN port to the other at wire speed to ensure that each session receives full bandwidth.

To switch frames between LAN ports efficiently, the switch maintains an address table. When a frame enters the switch, it associates the MAC address of the sending network device with the LAN port on which it was received.

Building the Address Table

Catalyst 6500 series switches build the 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 address table, it floods the frame to all LAN ports of the same VLAN except the port that received the frame. When the destination station replies, the switch adds its relevant source address and port ID to the address table. The switch then forwards subsequent frames to a single LAN port without flooding to all LAN ports.

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

These sections describe VLAN trunks on the Catalyst 6500 series switches:

•Trunking Overview

•Encapsulation Types

Trunking Overview

Note For information about VLANs, see Chapter 12 "Configuring VLANs."

A trunk is a point-to-point link between the switch and another networking device. 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 ports:

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

Note The following switching modules do not support ISL encapsulation:

· WS-X6502-10GE
· WS-X6548-GE-TX, WS-X6548V-GE-TX, WS-X6548-GE-45AF
· WS-X6148-GE-TX, WS-X6148V-GE-TX, WS-X6148-GE-45AF

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

You can configure a trunk on a single Ethernet port or on an EtherChannel. For more information about EtherChannel, see Chapter 10 "Configuring EtherChannels."

Ethernet trunk ports support several trunking modes (see Table 8-2). You can specify whether the trunk uses ISL or 802.1Q encapsulation, and if the encapsulation type is autonegotiated.

Note You can configure LAN ports to negotiate the encapsulation type. You cannot configure WAN interfaces to negotiate the encapsulation type.

The Dynamic Trunking Protocol (DTP) manages trunk autonegotiation on LAN ports. DTP supports autonegotiation of both ISL and 802.1Q trunks.

To autonegotiate trunking, the LAN ports must be in the same VTP domain. Use the trunk or nonegotiate keywords to force LAN ports in different domains to trunk. For more information on VTP domains, see Chapter 11 "Configuring VTP."

Encapsulation Types

Table 8-1 lists the Ethernet trunk encapsulation types.

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

Layer 2 LAN Port Modes

Table 8-2 lists the Layer 2 LAN port modes and describes how they function on LAN ports.

Note DTP is a point-to-point protocol. However, some internetworking devices might forward DTP frames improperly. To avoid this problem, ensure that LAN ports 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 LAN port to become a trunk but not generate DTP frames.

Default Layer 2 LAN Interface Configuration

Table 8-3 shows the Layer 2 LAN port default configuration.

Layer 2 LAN Interface Configuration Guidelines and Restrictions

When configuring Layer 2 LAN ports, follow these guidelines and restrictions:

•The following switching modules do not support ISL encapsulation:

–WS-X6502-10GE

–WS-X6548-GE-TX, WS-X6548V-GE-TX, WS-X6548-GE-45AF

–WS-X6148-GE-TX, WS-X6148V-GE-TX, WS-X6148-GE-45AF

•The following configuration guidelines and restrictions apply when using 802.1Q trunks and impose some limitations on the trunking strategy for a network. Note these restrictions when using 802.1Q trunks:

–When connecting Cisco switches through an 802.1q trunk, make sure the native VLAN for an 802.1Q trunk is the same on both ends of the trunk link. If the native VLAN on one end of the trunk is different from the native VLAN on the other end, spanning tree loops might result.

–Disabling spanning tree on the native VLAN of an 802.1Q trunk without disabling spanning tree on every VLAN in the network can cause spanning tree loops. We recommend that you leave spanning tree enabled on the native VLAN of an 802.1Q trunk. If this is not possible, disable spanning tree on every VLAN in the network. Make sure your network is free of physical loops before disabling spanning tree.

–When you connect two Cisco switches through 802.1Q trunks, the switches exchange spanning tree BPDUs on each VLAN allowed on the trunks. The BPDUs on the native VLAN of the trunk are sent untagged to the reserved IEEE 802.1d spanning tree multicast MAC address (01-80-C2-00-00-00). The BPDUs on all other VLANs on the trunk are sent tagged to the reserved Cisco Shared Spanning Tree (SSTP) multicast MAC address (01-00-0c-cc-cc-cd).

–Non-Cisco 802.1Q switches maintain only a single instance of spanning tree (the Mono Spanning Tree, or MST) that defines the spanning tree topology for all VLANs. When you connect a Cisco switch to a non-Cisco switch through an 802.1Q trunk, the MST of the non-Cisco switch and the native VLAN spanning tree of the Cisco switch combine to form a single spanning tree topology known as the Common Spanning Tree (CST).

–Because Cisco switches transmit BPDUs to the SSTP multicast MAC address on VLANs other than the native VLAN of the trunk, non-Cisco switches do not recognize these frames as BPDUs and flood them on all ports in the corresponding VLAN. Other Cisco switches connected to the non-Cisco 802.1q cloud receive these flooded BPDUs. This allows Cisco switches to maintain a per-VLAN spanning tree topology across a cloud of non-Cisco 802.1Q switches. The non-Cisco 802.1Q cloud separating the Cisco switches is treated as a single broadcast segment between all switches connected to the non-Cisco 802.1q cloud through 802.1q trunks.

–Make certain that the native VLAN is the same on all of the 802.1q trunks connecting the Cisco switches to the non-Cisco 802.1q cloud.

–If you are connecting multiple Cisco switches to a non-Cisco 802.1q cloud, all of the connections must be through 802.1q trunks. You cannot connect Cisco switches to a non-Cisco 802.1q cloud through ISL trunks or through access ports. Doing so causes the switch to place the ISL trunk port or access port into the spanning tree "port inconsistent" state and no traffic will pass through the port.

Configuring LAN Interfaces for Layer 2 Switching

These sections describe how to configure Layer 2 switching on the Catalyst 6500 series switches:

•Configuring a LAN Port for Layer 2 Switching

•Configuring a Layer 2 Switching Port as a Trunk

•Configuring a LAN Interface as a Layer 2 Access Port

•Configuring a Custom IEEE 802.1Q EtherType Field Value

Note Use the default interface {ethernet | fastethernet | gigabitethernet | tengigabitethernet} slot/port command to revert an interface to its default configuration.

Configuring a LAN Port for Layer 2 Switching

To configure a LAN port for Layer 2 switching, perform this task:

After you enter the switchport command, the default mode is switchport mode dynamic desirable. If the neighboring port supports trunking and is configured to allow trunking, the link becomes a Layer 2 trunk when you enter the switchport command. By default, LAN trunk ports negotiate encapsulation. If the neighboring port supports ISL and 802.1Q encapsulation and both ports are set to negotiate the encapsulation type, the trunk uses ISL encapsulation (10-Gigabit Ethernet ports do not support ISL encapsulation).

Note When using the switchport command, if a port configured for Layer 3 is now configured for Layer 2, the configuration for Layer 3 is retained in the memory but not in the running configuration and is applied to the port whenever the port switches back to Layer 3. Also, if a port configured for Layer 2 is now configured for Layer 3, the configuration for Layer 2 is retained in the memory but not in the running configuration and is applied to the port whenever the port switches back to Layer 2. To restore the default configuration of the port in the memory and in the running configuration, use the default interface command. To avoid potential issues while changing the role of a port using the switchport command, shut down the interface before applying the switchport command.

Configuring a Layer 2 Switching Port as a Trunk

These section describe configuring a Layer 2 switching port as a trunk:

•Configuring the Layer 2 Switching Port as an ISL or 802.1Q Trunk

•Configuring the Layer 2 Trunk to Use DTP

•Configuring the Layer 2 Trunk Not to Use DTP

•Configuring the Access VLAN

•Configuring the 802.1Q Native VLAN

•Configuring the List of VLANs Allowed on a Trunk

•Configuring the List of Prune-Eligible VLANs

•Completing Trunk Configuration

•Verifying Layer 2 Trunk Configuration

•Configuration and Verification Examples

Configuring the Layer 2 Switching Port as an ISL or 802.1Q Trunk

Note•Complete the steps in the "Configuring a LAN Port for Layer 2 Switching" section before performing the tasks in this section.

•When you enter the switchport command with no other keywords (Step 3 in the previous section), the default mode is switchport mode dynamic desirable and switchport trunk encapsulation negotiate.

To configure the Layer 2 switching port as an ISL or 802.1Q trunk, perform this task:

When configuring the Layer 2 switching port as an ISL or 802.1Q trunk, note the following information:

•The switchport mode trunk command (see the "Configuring the Layer 2 Trunk Not to Use DTP" section) is not compatible with the switchport trunk encapsulation negotiate command.

•To support the switchport mode trunk command, you must configure the encapsulation as either ISL or 802.1Q.

•The following switching modules do not support ISL encapsulation:

–WS-X6502-10GE

–WS-X6548-GE-TX, WS-X6548V-GE-TX, WS-X6548-GE-45AF

–WS-X6148-GE-TX, WS-X6148V-GE-TX, WS-X6148-GE-45AF

Note Complete the steps in the "Completing Trunk Configuration" section after performing the tasks in this section.

Configuring the Layer 2 Trunk to Use DTP

Note Complete the steps in the "Configuring a LAN Port for Layer 2 Switching" section before performing the tasks in this section.

To configure the Layer 2 trunk to use DTP, perform this task:

When configuring the Layer 2 trunk to use DTP, note the following information:

•Required only if the interface is a Layer 2 access port or to specify the trunking mode.

•See Table 8-2 for information about trunking modes.

Note Complete the steps in the "Completing Trunk Configuration" section after performing the tasks in this section.

Configuring the Layer 2 Trunk Not to Use DTP

Note Complete the steps in the "Configuring a LAN Port for Layer 2 Switching" section before performing the tasks in this section.

To configure the Layer 2 trunk not to use DTP, perform this task:

When configuring the Layer 2 trunk not to use DTP, note the following information:

•Before entering the switchport mode trunk command, you must configure the encapsulation (see the "Configuring the Layer 2 Switching Port as an ISL or 802.1Q Trunk" section).

•To support the switchport nonegotiate command, you must enter the switchport mode trunk command.

•Enter the switchport mode dynamic trunk command. See Table 8-2 for information about trunking modes.

•Before entering the switchport nonegotiate command, you must configure the encapsulation (see the "Configuring the Layer 2 Switching Port as an ISL or 802.1Q Trunk" section) and configure the port to trunk unconditionally with the switchport mode trunk command (see the "Configuring the Layer 2 Trunk to Use DTP" section).

Note Complete the steps in the "Completing Trunk Configuration" section after performing the tasks in this section.

Configuring the Access VLAN

Note Complete the steps in the "Configuring a LAN Port for Layer 2 Switching" section before performing the tasks in this section.

To configure the access VLAN, perform this task:

Note Complete the steps in the "Completing Trunk Configuration" section after performing the tasks in this section.

Configuring the 802.1Q Native VLAN

Note Complete the steps in the "Configuring a LAN Port for Layer 2 Switching" section before performing the tasks in this section.

To configure the 802.1Q native VLAN, perform this task:

When configuring the native VLAN, note the following information:

•The vlan_ID value can be 1 through 4094, except reserved VLANs (see Table 12-1).

•The access VLAN is not automatically used as the native VLAN.

Note Complete the steps in the "Completing Trunk Configuration" section after performing the tasks in this section.

Configuring the List of VLANs Allowed on a Trunk

Note Complete the steps in the "Configuring a LAN Port for Layer 2 Switching" section before performing the tasks in this section.

To configure the list of VLANs allowed on a trunk, perform this task:

When configuring the list of VLANs allowed on a trunk, note the following information:

•The vlan parameter is either a single VLAN number from 1 through 4094, 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.

•All VLANs are allowed by default.

•You can remove VLAN 1. If you remove VLAN 1 from a trunk, the trunk interface continues to send and receive management traffic, for example, Cisco Discovery Protocol (CDP), VLAN Trunking Protocol (VTP), Port Aggregation Protocol (PAgP), and DTP in VLAN 1.

Note Complete the steps in the "Completing Trunk Configuration" section after performing the tasks in this section.

Configuring the List of Prune-Eligible VLANs

Note Complete the steps in the "Configuring a LAN Port for Layer 2 Switching" section before performing the tasks in this section.

To configure the list of prune-eligible VLANs on the Layer 2 trunk, perform this task:

When configuring the list of prune-eligible VLANs on a trunk, note the following information:

•The vlan parameter is either a single VLAN number from 1 through 4094, except reserved VLANs (see Table 12-1), 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.

•The default list of VLANs allowed to be pruned contains all VLANs.

•Network devices in VTP transparent mode do not send VTP Join messages. On Catalyst 6500 series switches with trunk connections to network devices in VTP transparent mode, configure the VLANs used by the transparent-mode network devices or that need to be carried across the transparent-mode network devices as pruning ineligible.

Note Complete the steps in the "Completing Trunk Configuration" section after performing the tasks in this section.

Completing Trunk Configuration

To complete Layer 2 trunk configuration, perform this task:

Verifying Layer 2 Trunk Configuration

To verify Layer 2 trunk configuration, perform this task:

Configuration and Verification Examples

This example shows how to configure the Fast Ethernet port 5/8 as an 802.1Q trunk. This example assumes that the neighbor port is configured to support 802.1Q trunking:

Router# configure terminal 

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

Router(config)# interface fastethernet 5/8 

Router(config-if)# shutdown 

Router(config-if)# switchport 

Router(config-if)# switchport mode dynamic desirable 

Router(config-if)# switchport trunk encapsulation dot1q 

Router(config-if)# no shutdown 

Router(config-if)# end 

Router# exit 

This example shows how to verify the configuration:

Router# show running-config interface fastethernet 5/8 

Building configuration...

Current configuration:

!

interface FastEthernet5/8

 no ip address

 switchport

 switchport trunk encapsulation dot1q

end

Router# 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: ALL

Router# 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

Router# 

Configuring a LAN Interface as a Layer 2 Access Port

Note If you assign a LAN port to a VLAN that does not exist, the port is shut down until you create the VLAN in the VLAN database (see the "Creating or Modifying an Ethernet VLAN" section).

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

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

Router# configure terminal 

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

Router(config)# interface fastethernet 5/6 

Router(config-if)# shutdown 

Router(config-if)# switchport 

Router(config-if)# switchport mode access 

Router(config-if)# switchport access vlan 200 

Router(config-if)# no shutdown 

Router(config-if)# end 

Router# exit 

This example shows how to verify the configuration:

Router# show running-config interface fastethernet 5/6 

Building configuration...

!

Current configuration:

interface FastEthernet5/6

 no ip address

 switchport access vlan 200

 switchport mode access

end

Router# show interfaces fastethernet 5/6 switchport 

Name: Fa5/6

Switchport: Enabled

Administrative Mode: static access

Operational Mode: static access

Administrative Trunking Encapsulation: negotiate

Operational Trunking Encapsulation: native

Negotiation of Trunking: Enabled

Access Mode VLAN: 200 (VLAN0200)

Trunking Native Mode VLAN: 1 (default)

Trunking VLANs Enabled: ALL

Pruning VLANs Enabled: ALL

Router# 

Configuring a Custom IEEE 802.1Q EtherType Field Value

You can configure a custom EtherType field value on a port to support network devices that do not use the standard 0x8100 EtherType field value on 802.1Q-tagged or 802.1p-tagged frames.

To configure a custom value for the EtherType field, perform this task:

When configuring a custom EtherType field value, note the following information:

•To use a custom EtherType field value, all network devices in the traffic path across the network must support the custom EtherType field value.

•You can configure a custom EtherType field value on trunk ports, access ports, and tunnel ports.

•You can configure a custom EtherType field value on the member ports of an EtherChannel.

•You cannot configure a custom EtherType field value on a port-channel interface.

•Each port supports only one EtherType field value. A port that is configured with a custom EtherType field value does not recognize frames that have any other EtherType field value as tagged frames. For example, a trunk port that is configured with a custom EtherType field value does not recognize the standard 0x8100 EtherType field value on 802.1Q-tagged frames and cannot put the frames into the VLAN to which they belong.

Caution A port that is configured with a custom EtherType field value considers frames that have any other EtherType field value to be untagged frames. A trunk port with a custom EtherType field value places frames with any other EtherType field value into the native VLAN. An access port or tunnel port with a custom EtherType field value places frames that are tagged with any other EtherType field value into the access VLAN. If you misconfigure a custom EtherType field value, frames might be placed into the wrong VLAN.

•See the Release Notes for Cisco IOS Release 12.2ZY on the Supervisor Engine 32 PISA for a list of the modules that support custom IEEE 802.1Q EtherType field values.

This example shows how to configure the EtherType field value to 0x1234:

Router (config-if)# switchport dot1q ethertype 1234

Router (config-if)#