Different Types of Switch Ports

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•Access Ports: belong to and carry the traffic of only one VLAN

•Trunk Ports: carry the traffic of multiple VLANs and by default is a member of all VLANs in the VLAN database. Both ISL and 802.1Q trunk ports are supported

•Tunnel Ports: designed for service providers who carry traffic of multiple customers across their networks and are required to maintain the VLAN and Layer 2 protocol configurations of each customer without impacting the traffic of other customers. Both 802.1Q tunneling and Layer 2 protocol tunneling are supported

© 2002, Cisco Systems, Inc. All rights reserved. Cisco CCIE Prep v1.0—Module 5-23

Switch ports are Layer 2-only interfaces associated with a physical port. A switch port can be an access port, a trunk port, or a tunnel port. You can manually configure a port as an access port or trunk port or let the Dynamic Trunking Protocol (DTP) operate on a per-port basis to determine if a switch port should be an access port or a trunk port by negotiating with the port on the other end of the link. You must manually configure tunnel ports as part of an asymmetric link connected to an 802.1Q trunk port. Switch ports are used for managing the physical interface and associated Layer 2 protocols and do not handle routing or bridging.

Access Ports

An access port belongs to and carries the traffic of only one VLAN. Traffic is received and sent in native formats with no VLAN tagging. Traffic arriving on an access port is assumed to belong to the VLAN assigned to the port.

Trunk Ports

A trunk port carries the traffic of multiple VLANs and by default is a member of all VLANs in the VLAN database. Two types of trunk ports are supported:

■ ISL trunk port - All received packets are expected to be encapsulated with an ISL header, and all transmitted packets are sent with an ISL header. Native (non-tagged) frames received from an ISL trunk port are dropped.

■ IEEE 802.1Q trunk port - Supports simultaneous tagged and untagged traffic. An 802.1Q trunk port is assigned a default Port VLAN ID (PVID), and all untagged traffic travels on the port default PVID. All untagged traffic and tagged traffic with a NULL VLAN ID are assumed to belong to the port default PVID. A packet with a VLAN ID equal to the outgoing port default PVID is sent untagged. All other traffic is sent with a VLAN tag.

Tunnel Ports

Tunnel ports are used in 802.1Q tunneling to segregate the traffic of customers in a service provider network from other customers who appear to be on the same VLAN. You configure an asymmetric link from a tunnel port on a service provider edge switch to an 802.1Q trunk port on the customer switch. Packets entering the tunnel port on the edge switch, already 802.1Q-tagged with the customer VLANs, are encapsulated with another layer of 802.1Q tag (called the metro tag) containing a VLAN ID unique in the service provider network, for each customer. The double-tagged packets go through the service-provider network keeping the original customer VLANs separate from those of other customers. At the outbound interface, also a tunnel port, the metro tag is removed, and the original VLAN numbers from the customer network are retrieved.

Tunnel ports cannot be trunk ports or access ports and must belong to a VLAN unique for each customer.

Note Switch ports are configured using the switchport interface configuration command.

This section discusses the configuration of Access Ports on the Catalyst 3550.

This section discusses the configuration of Access Ports on the Catalyst 3550.

You can manually assign access ports to a VLAN without having VTP globally propagate VLAN configuration information.

Note If you assign an interface to a VLAN that does not exist, a new VLAN is created.

Use the steps outlined in the following table to manually assign an access port to a VLAN.

Table 4-10: Assign Ports to a VLAN

Command

Purpose

3550(config)# interface interface-id

Enter the interface to be added to the VLAN.

3550(config-if)# switchport mode access

Define the VLAN membership mode for the port (Layer 2 access port).

3550(config-if)# switchport access vlan vlan-id

Assign the port to a VLAN. Valid VLAN IDs are 1 to 4094; do not enter leading zeros.

Note To return an interface to its default configuration, use the default interface interface-id interface configuration command.

This section discusses the configuration of Trunk Ports on the Catalyst 3550.

This section discusses the configuration of Trunk Ports on the Catalyst 3550.

3550(config) 3550(config-3550(config-3550(config-3550(config-3550ft

3550(config) 3550(config-3550(config-3550(config-3550(config-3550ft

# interface fastEthernet 0/11 if)# switchport trunk encapsulation isl if)# switchport mode trunk if)# switchport access vlan 1

# interface fastEthernet 0/11 if)# switchport trunk encapsulation isl if)# switchport mode trunk if)# switchport access vlan 1

3550(config) ft 3550(config-i 3550(config-i 3550(config-i 3550(config-i 3550(config-i 3550ft interface fastEthernet 0/12 r)ft switchport trunk encapsulation dotlq r)ft switchport mode trunk r)ft switchport access vlan 1 r)ft switchport trunk native vlan 1 ")# end

3550(config) ft 3550(config-i 3550(config-i 3550(config-i 3550(config-i 3550(config-i 3550ft interface fastEthernet 0/12 r)ft switchport trunk encapsulation dotlq r)ft switchport mode trunk r)ft switchport access vlan 1 r)ft switchport trunk native vlan 1 ")# end

ISL Trunk

802.1Q Trunk

• Make sure the native vlan is set on 802.1Q trunks and that it matches on both sides of the trunk link

© 2002, CiscoSystems, Inc. All rights reserved.

Cisco CCIE Prep v1.0—Mot

5-25

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. Fast Ethernet and Gigabit Ethernet trunks carry the traffic of multiple VLANs over a single link, and you can extend the VLANs across an entire network.

Two trunking encapsulations are available on all Ethernet interfaces on the Catalyst 3550:

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

■ 802.1Q—802.1Q is industry-standard trunking encapsulation.

You can set an interface as trunking or nontrunking or to negotiate trunking with the neighboring interface. To autonegotiate trunking, the interfaces must be in the same VTP domain. Trunk negotiation is managed by the Dynamic Trunking Protocol (DTP), which is a Point-to-Point Protocol. DTP supports autonegotiation of both ISL and 802.1Q trunks.

Use the steps outlined in the following table to configure a port as an ISL or 802.1Q trunk port:

Table 4-11: Configure a Port

Command

Purpose

3550(config)# interface interface-id

Enter the interface configuration mode and the port to be configured for trunking.

The default mode for Layer 2 interfaces is switchport mode dynamic desirable. If the neighboring interface supports trunking and is configured to allow trunking, the link is a Layer 2 trunk or, if the interface is currently in Layer 3 mode, it becomes a Layer 2 trunk when you enter the switchport interface configuration command.

3550(config-if)# switchport trunk encapsulation {isl | dotlq | negotiate}

Configure the port to support ISL or 802.1Q encapsulation or to negotiate (default) with the neighboring interface for encapsulation type. You must configure each end of the link with the same encapsulation type.

3550(config-if)# switchport mode {dynamic {auto | desirable} | trunk}

Configure the interface as a Layer 2 trunk (required only if the interface is currently a Layer 2

access port or tunnel port, or to specify the trunking mode).

■ dynamic auto—Set the interface to a trunk link if the neighboring interface is set to trunk or desirable mode.

■ dynamic desirable—Set the interface to a trunk link if the neighboring interface is set to trunk, desirable, or auto mode.

■ trunk—Set the interface in permanent trunking mode and negotiate to convert the link to a trunk link even if the neighboring interface is not a trunk interface.

3550(config-if)# switchport access vlan vlan-id

(Optional) Specify the default VLAN, which is used if the interface stops trunking.

3550(config-if)# switchport trunk native vlan vlan-id

Specify the native VLAN for 802.1Q trunks.

Note To reset all trunking characteristics of a trunking interface to the defaults, use the no switchport trunk interface configuration command. To disable trunking, use the switchport mode access interface configuration command to configure the port as a static-access port.

Defining the List of Allowed VLANs on a Trunk

355O(config)# interface fastEthernet O/ll

3550(config-if)# switchport trunk

allowed vlan except

5Ol-lOOl

355O(config-if)# exit

355O(config)# interface fastEthernet O/l2

3550(config-if)# switchport trunk

allowed vlan remove

2-5OO

355O(config-if)# end

35 5O#

show interfaces trunk

Port

Mode Encapsulat

ion Status Native vlan

FaO/11

on isl

trunking 1

FaO/12

on 802.lq

trunking 1

FaO/24

desirable n-isl

trunking 1

Port

Vlans allowed on trunk

FaO/11

1-500,1002-4094

FaO/12

1,501-4094

FaO/24

1-4094

Port

Vlans allowed and active in management domain

FaO/11

1-5,20,30

FaO/12

1

FaO/24

1-5,20,30

Port

Vlans in spanning tree

forwarding state and

not pruned

FaO/11

1-5,20,30

FaO/12

1

FaO/24

1-4,30

© 2002, Cis

co Systems, Inc. All rights reserved.

Only VLANs 2-500 should be carried across this trunk

Only VLANs 2-500 should be carried across this trunk

Only VLANs 501 -1001 should be carried across this trunk

Cisco CCIE Prep v1.0—Module 5-26

By default, a trunk port sends traffic to and receives traffic from all VLANs. All VLAN IDs, 1 to 4094, are allowed on each trunk. However, you can remove VLANs from the allowed list, preventing traffic from those VLANs from passing over the trunk. To restrict the traffic a trunk carries, use the switchport trunk allowed vlan remove vlan-list interface configuration command to remove specific VLANs from the allowed list.

A trunk port can become a member of a VLAN if the VLAN is enabled, if VTP knows of the VLAN, and if the VLAN is in the allowed list for the port. When VTP detects a newly enabled VLAN and the VLAN is in the allowed list for a trunk port, the trunk port automatically becomes a member of the enabled VLAN. When VTP detects a new VLAN and the VLAN is not in the allowed list for a trunk port, the trunk port does not become a member of the new VLAN.

Use the steps outlined in the following table to restrict the VLANs that are carried on a trunk port:

Table 4-12: Restrict VLANs

Command

Purpose

3550(config-if)# switchport trunk allowed vlan {add | all | except | remove} vlan-list

Configure the list of VLANs allowed on the trunk.

The vlan-list parameter is either a single VLAN number from 1 to 4094 or a range of VLANs described by two VLAN numbers, the lower one first, separated by a hyphen. Do not enter any spaces between comma-separated VLAN parameters or in hyphen-specified ranges.

All VLANs are allowed by default. You cannot remove any of the default VLANs (1 or 1002 to 1005) from a trunk.

Note To return to the default allowed VLAN list of all VLANs, use the no switchport trunk allowed vlan interface configuration command.

Note To return to the default allowed VLAN list of all VLANs, use the no switchport trunk allowed vlan interface configuration command.

Configuring the Prune Elig Pruning ble List for VTP

Cisco.com

3550(config)# interface fastEthernet 0/11 3550(config-if)# switchport trunk pruning vlan 2-500

3550(config-if)# exit

3550(config)# interface fastEthernet 0/12 3550(config-if)# switchport trunk pruning vlan 501-1001

3550(config-if)# end

• Extended-range VLANs (VLAN IDs 1006 to 4094) cannot be pruned.

© 2002, Cisco Systems, Inc. All rights reservec

Cisco CCIE Prep v1.0—MoCule 5-27

Pruning increases available bandwidth by restricting flooded traffic to those trunk links that the traffic must use to access the destination devices. You can only enable VTP pruning on a switch in VTP server mode. Only VLANs included in the pruning-eligible list can be pruned. By default, VLANs 2 through 1001 are pruning eligible on trunk ports. The pruning-eligible list applies only to trunk ports. Each trunk port has its own eligibility list. VTP pruning must be enabled for this procedure to take effect.

Use the steps outlined in the following table to remove VLANs from the pruning-eligible list on a trunk port:

Table 4-13: Remove VLANs from Pruning

Command

Purpose

3550(config-if)# switchport trunk pruning vlan {add | except | none | remove} vlan-list [,vlan[,vlan[,,,]]

Configure the list of VLANs allowed to be pruned from the trunk.

Separate nonconsecutive VLAN IDs with a comma and no spaces; use a hyphen to designate a range of IDs. Valid IDs are from 2 to 1001. Extended-range VLANs (VLAN IDs 1006 to 4094) cannot be pruned.

VLANs that are pruning-ineligible receive flooded traffic.

The default list of VLANs allowed to be pruned contains VLANs 2 to 1001.

Note To return to the default pruning-eligible list of all VLANs, use the no switchport trunk pruning vlan interface configuration command.

Note To return to the default pruning-eligible list of all VLANs, use the no switchport trunk pruning vlan interface configuration command.

This section discusses the configuration of Tunnel Ports on the Catalyst 3550. Tunneling is a feature designed for service providers who carry traffic of multiple customers across their networks and are required to maintain the VLAN and Layer 2 protocol configurations of each customer without impacting the traffic of other customers. The Catalyst 3550 switch supports 802.1Q tunneling and Layer 2 protocol tunneling.

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    Which types of ports can belong to only one vlan at a time?
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