Introduction
This document illustrates how to create a switch-to-switch Inter-Switch Link (ISL) trunk. Trunk ports enable connections between switches to carry traffic from more than one VLAN. If trunking is not enabled, the link that connects the two switches only carries traffic from the VLAN that you have configured on the port. Trunking is not necessary in very simple switched networks with only one VLAN (broadcast domain). In most LANs, a small portion of traffic consists of special protocols that manage the network. (A few examples are Cisco Discovery Protocol [CDP], VLAN Trunk Protocol [VTP], Dynamic Trunking Protocol [DTP], Spanning Tree Protocol [STP], and Port Aggregation Protocol [PAgP].) You also use the management VLAN when you ping or establish a Telnet directly to or from the switch. (If you use Catalyst OS [CatOS], you define the VLAN and the IP address of the switch when you configure the sc0 interface. The Step-by-Step Instructions for CatOS section of this document explains this process.) In a multi-VLAN environment, many network administrators advocate the restriction of this management traffic to a single VLAN. The VLAN is normally VLAN 1. The administrators then configure user traffic to flow in VLANs other than this default VLAN. ISL (Cisco proprietary) is one of two possible trunking protocols for Ethernet. The other protocol is the IEEE 802.1Q standard.
This document covers the procedure to configure ISL trunking between Catalyst 5500/5000 and Catalyst 6500/6000 series switches. The CatOS configuration applies to both Catalyst 5500/5000 and 6500/6000 series switches. However, you can only apply the Cisco IOS® Software configuration to a Catalyst 6500/6000 series switch.
Prerequisites
Requirements
There are no specific prerequisites for this document.
Components Used
The information in this document is based on these software and hardware versions:
- at least one terminal.
- at least one console cable that is suitable for the Supervisor Engine in your switches. (Refer to the document Connecting a Terminal to the Console Port on Catalyst Switches for more information.)
- two Catalyst 5500/5000 or Catalyst 6500/6000 switches (that run CatOS) or two Catalyst 6500/6000 switches (that run Cisco IOS Software) in a lab1 environment with cleared configurations2.
- two Ethernet interfaces that are able to support ISL.
- one 10BASE-T crossover cable.
1The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.
2 For CatOS, the issue of the clear config all command ensured that there was a default configuration. For Cisco IOS Software, the write erase command cleared the configuration.
Conventions
For more information on document conventions, refer to the Cisco Technical Tips Conventions.
Creation of a Switch-to-Switch ISL Trunk
Switches A and B in this topology represent either two Catalyst 5500/5000 switches that run CatOS or two Catalyst 6500/6000 switches that run Cisco IOS Software.
Switches A and B, by default, have the 2/1 ports in VLAN 1. This configuration allows traffic from other ports in VLAN 1 to flow between the switches without the need to configure trunking. References to "the management VLAN" apply to VLAN 1.
Note: VLAN 1 is the default VLAN for all types of Ethernet interfaces, as well as FDDI. VLAN 1 is also the default VLAN for the management interface (sc0).
In the example that follows, you have configured ports 2/2 on both switches in VLAN 2. Traffic from devices that you have attached to the ports in VLAN 2 do not cross over the link between the switches. Therefore, PCs A and B are not able to communicate.
The solution is to enable ISL trunking on the link between switches A and B. Trunking adds a VLAN header to each frame during transmit (multiplex) onto a trunk link. This addition allows the switch at the other end of the link to demultiplex the frames; the switch then forwards the frames to the appropriate VLAN ports.
Tasks
These steps guide you through this configuration:
- Connect a terminal to the switches.
- Verify ISL support on the ports.
- Connect the switches.
- Verify that the ports are operational.
- Assign IP addresses to the management ports.
- Verify that the switches are not trunking over the link.
- Ping from switch to switch.
- Create a VLAN 2 in each switch.
- Move the management interface (sc0) to VLAN 2 (for CatOS).
- Verify that you cannot ping from switch to switch.
- Configure the same VTP domain name in each switch.
- Enable trunking between the switches.
- Verify that the switches are trunking over the link.
- Ping from switch to switch.
Step-by-Step Instructions for CatOS
Follow these steps:
- Connect a terminal to the console ports of the switches.
For more information, refer to this document:
- Make certain that the ports that you have decided to use support ISL trunking.
There are several types of Ethernet interfaces that support ISL trunking. The 10BASE-T (common Ethernet) ports do not support trunking, while most 100BASE-T (Fast Ethernet) ports do support trunking.
Issue the show port capabilities module_number | module_number/port_number command on both switches to determine if the ports you use support ISL.
Note: In this example, the command specifies the port designator 2/1. This limits the response to the information that is directly applicable.
cat5000> (enable) show port capabilities 2/1
Model WS-X5234
Port 2/1
Type 10/100BaseTX
Speed auto,10,100
Duplex half,full
Trunk encap type 802.1Q,ISL
Trunk mode on,off,desirable,auto,nonegotiate
Channel 2/1-2,2/1-4
Broadcast suppression percentage(0-100)
Flow control receive-(off,on),send-(off,on)
Security yes
Membership static,dynamic
Fast start yes
QOS scheduling rx-(none),TX(1q4t)
COs rewrite yes
ToS rewrite IP-Precedence
Rewrite yes
UDLD yes
AuxiliaryVlan 1..1000,1025..4094,untagged,dot1p,none
SPAN source,destination
- Connect the two switch ports together with the Ethernet crossover cable.
In this example, the Switch A 2/1 port connects to the Switch B 2/1 port. - To verify that the ports are operational, issue the show port 2/1 command on Switch A.
Switch-A> (enable) show port 2/1
Port Name Status VLAN Level Duplex Speed Type
----- ------------------ ---------- ---------- ------ ------ ----- ------------
2/1 connected 1 normal a-full a-100 10/100BaseTX
Switch-A> (enable)
- Issue the set interface sc0 172.16.84.17 255.255.255.0 172.16.84.255 command on Switch A and the set interface sc0 172.16.84.18 255.255.255.0 172.16.84.255 command on Switch B.
These commands assign IP addresses from the same subnet to the management ports on both switches. You may also need to specify in the command the VLAN for sc0 (the management VLAN). Include this VLAN if the VLAN is different than the default (VLAN 1).
If you have the output of a show interfaces command from your Cisco device, you can use the Output Interpreter ToolSwitch-A> (enable) set interface sc0 172.16.84.17 255.255.255.0 172.16.84.255Interface sc0 IP address, netmask, and broadcast set.
Switch-A> (enable)
(registered customers only) to display potential issues and fixes. - To verify that the link between switches A and B is not trunking, issue the show trunk 2/1 command on Switch A.
Note: The term Native VLAN in this output indicates the VLAN placement of this port when the port is not in trunking mode. If you have configured the port for 802.1Q trunking, the Native VLAN field also indicates the VLAN for which the frames have no tags; all others have tags. (Conversely, with ISL trunking, every data frame has the appropriate VLAN identifier.)Switch-A> (enable) show trunk 2/1
Port Mode Encapsulation Status Native VLAN
-------- ----------- ------------- ------------ -----------
2/1 auto isl not-trunking 1
Port VLANs allowed on trunk
-------- ---------------------------------------------------------------------
2/1 1-1005 Port VLANs allowed and active in management domain
-------- ---------------------------------------------------------------------
2/1 1 Port VLANs in spanning tree forwarding state and not pruned
-------- ---------------------------------------------------------------------
2/1 1
Switch-A> (enable)
The trunking status should be not-trunking because the default mode for the DTP is auto. DTP is the strategic replacement for Dynamic ISL (DISL) because DTP incorporates support for 802.1Q trunking negotiation. DTP is available in Catalyst software version 4.x and later, as well as in certain hardware modules. There are five different modes to which you can configure DTP. Cisco Technical Support recommends the configuration of desirable trunking mode on the ports of the trunk link. Step 12 discusses this information in more detail. - Ping Switch B from Switch A to verify that the switches can communicate over the link.
Switch-A> ping 172.16.84.18 172.16.84.18 is alive Switch-A>
- To create VLAN 2 in Switch A, issue the set vlan 2 command on Switch A.
Switch B learns about VLAN 2 after the establishment of the VTP domain in Step 11.
Switch-A> (enable) set vlan 2 Vlan 2 configuration successful Switch-A> (enable)
- Move the management interface in switches A and B to VLAN 2, which you created in Step 8.
To change the interface, issue the set interface sc0 2 command. This output shows the issue of the command on Switch A:
Issue the show interfaces command to view the change you just made. This output shows the issue of the command on Switch A. The output shows the new association of interface sc0 and VLAN 2:Switch-A> (enable) set interface sc0 2 Interface sc0 vlan set. Switch-A> (enable)
Switch-A> (enable) show interfaces sl0: flags=51
slip 0.0.0.0 dest 0.0.0.0 sc0: flags=63 vlan 2 inet 172.16.84.17 netmask 255.255.255.0 broadcast 172.16.84.255 Switch-A> (enable) - Attempt to ping Switch B from Switch A.
The ping should fail because the management ports are now in VLAN 2 while the link between the switches is in VLAN 1.
Switch-A> (enable) ping 172.16.84.18 no answer from 172.16.84.18 Switch-A> (enable)
- Establish the same VTP domain for both switches.
Issue the set vtp domain Cookbook command on both switches.
Note: The name of the VTP domain is Cookbook.
If you have the output of a show vtp domain command from your Cisco device, you can use the Output Interpreter ToolSwitch-A> (enable) set vtp domain Cookbook VTP domain Cookbook modified Switch-A> (enable)
(registered customers only) to display potential issues and fixes. - Turn on trunking between the switches.
To configure port 2/1 on Switch A for desirable mode, issue the set trunk 2/1 desirable isl command on Switch A. Switch B is in auto mode. Switch B automatically places the port 2/1 in trunking mode after completion of the DTP negotiation between the two switches.
Note: Cisco Technical Support recommends the configuration of desirable trunking mode on the ports of the trunk link.
If you have the output of a show trunk command from your Cisco device, you can use the Output Interpreter ToolSwitch-A> (enable) set trunk 2/1 desirable isl Port(s) 2/1 trunk mode set to desirable. Port(s) 2/1 trunk type set to Isl. Switch-A> (enable)
(registered customers only) to display potential issues and fixes.
This list describes the five different states for which you can configure DTP:
- auto: The port listens for DTP frames from the neighbor switch. If the neighbor switch indicates that the switch would like to be a trunk, or that the switch is a trunk, auto state creates the trunk with the neighbor switch. The auto state does not propagate any intent to become a trunk; auto state is solely dependent on the neighbor switch to make the trunking decision.
- desirable: DTP is spoken to the neighbor switch to which you want to establish an ISL trunk. The switch with desirable configuration communicates that the switch is able to be an ISL trunk and wants the neighbor switch to also be an ISL trunk. Cisco Technical Supportrecommends the configuration of desirable trunking mode on the ports of the trunk link.
- on: DTP is spoken to the neighbor switch. The on state automatically enables ISL trunking on the port, regardless of the state of the neighbor switch. The port remains an ISL trunk unless the port receives an ISL packet that explicitly disables the ISL trunk.
- nonegotiate: DTP is not spoken to the neighbor switch. The nonegotiate state automatically enables ISL trunking on the port, regardless of the state of the neighbor switch.
- off: There can be no use of ISL on this port, regardless of the configuration of DTP mode on the other switch port.
You may see other messages that relate to changes to the state of STP on the switch. These messages are not relevant to this document. Refer to the document Understanding and Configuring Spanning Tree Protocol (STP) on Catalyst Switches for more information on this protocol. Because you cleared the configurations in the switches to start, you have the defaults for the STP parameters. The default parameters of STP should provide the connectivity necessary for this document to succeed.Switch A Port/Interface 2/1 Switch B Port/Interface 2/1 ISL Trunk Status DTP mode auto DTP mode auto not-trunking DTP mode desirable DTP mode auto trunking DTP mode on DTP mode auto trunking DTP mode nonegotiate DTP mode auto not-trunking DTP mode off DTP mode auto not-trunking DTP mode desirable (Cisco Technical Support recommended configuration) DTP mode desirable (Cisco Technical Support recommended configuration) trunking DTP mode on DTP mode desirable trunking DTP mode nonegotiate DTP mode desirable not-trunking DTP mode off DTP mode desirable not-trunking DTP mode on DTP mode on trunking DTP mode nonegotiate DTP mode on trunking DTP mode off DTP mode on not-trunking DTP mode nonegotiate DTP mode nonegotiate trunking DTP mode off DTP mode nonegotiate not-trunking DTP mode off DTP mode off not-trunking - To verify the trunk link, issue the show trunk 2/1 command at the prompt on Switch A.
You should now see that trunking is operational.Switch-A> (enable) show trunk 2/1 Port Mode Encapsulation Status Native VLAN -------- ----------- ------------- ------------ ----------- 2/1 desirable isl trunking 1 Port VLANs allowed on trunk -------- --------------------------------------------------------------------- 2/1 2,1002-1005 Port VLANs allowed and active in management domain -------- --------------------------------------------------------------------- 2/1 2,1002-1005 Port VLANs in spanning tree forwarding state and not pruned -------- --------------------------------------------------------------------- 2/1 2,1002-1005 Switch-A> (enable)
Note: VLANs 1–1005 are allowable on all trunk ports by default. You can clear VLAN 1 from the list of allowable VLANs. If you remove VLAN 1 from a trunk, the trunk interface continues to send and receive management traffic, for example, CDP, VTP, PAgP, and DTP in VLAN 1. You cannot remove VLANs 1002–1005. In CatOS versions earlier than 5.4(x), you cannot remove VLAN 1 from the trunk.
To limit the VLANs on a trunk in CatOS, clear them. Issue the clear trunk 2/1 1-1001 command. To establish the allowable VLANs on the trunk, issue the set trunk 2/1 2 command on Switch A.
In this output, notice the permission of only VLANs 1 and 2 on this trunk link now:Switch-A>(enable) clear trunk 2/1 1-1001 Removing Vlan(s) 1-1001 from allowed list. Port 2/1 allowed vlans modified to 4. Switch-A> (enable) Switch-A>(enable) set trunk 2/1 2 Adding vlans 2 to allowed list. Port(s) 2/1 allowed vlans modified to 5. Switch-A>(enable)
Switch-A> (enable) show trunk 2/1 Port Mode Encapsulation Status Native VLAN -------- ----------- ------------- ------------ ----------- 2/1 desirable isl trunking 1 Port VLANs allowed on trunk -------- --------------------------------------------------------------------- 2/1 1-1005 Port VLANs allowed and active in management domain -------- --------------------------------------------------------------------- 2/1 1-2 Port VLANs in spanning tree forwarding state and not pruned -------- --------------------------------------------------------------------- 2/1 1-2 Switch-A> (enable)
- Ping Switch B from Switch A to verify that the switches can communicate with each other over the trunk link.
Switch-A> ping 172.16.84.18 172.16.84.18 is alive Switch-A>
Troubleshoot the Result
Commands to Use to Troubleshoot CatOS
- show port capabilities mod/port —To see the physical status of a port and the port capabilities.
- show trunk mod/port —To see the trunking information for a particular port.
- show vtp domain—To display VTP information.
- show vlan vlan_number —To see information on a particular VLAN.
- show spantree vlan_number —To see the status of the spanning tree for a particular VLAN.
- show interfaces—To display the configuration of sc0 and sl0.
- ping—To send an Internet Control Message Protocol (ICMP) echo message to another IP host.
Note: In switches with several interfaces and VLANs, include the module/port or VLAN number with the show command to restrict the command output. Use of the ? argument with show commands displays which commands allow use of the mod/port argument. For example, the show trunk ? command indicates that show trunk mod/port is allowable.
Step-by-Step Instructions for Cisco IOS Software
Follow these steps:
- Connect a terminal to the console ports of the switches.
For more information, refer to this document:
- Make certain that the ports that you have decided to use support ISL trunking.
There is currently no command available to view the module or port capabilities in Cisco IOS Software. All 10/100BASE-T, 1000BASE-TX, 100BASE-FX, and 1000BASE-SX/LX/ZX-type Ethernet modules for the Catalyst 6500/6000 series switches support ISL trunking.
Note: The 10-Gigabit Ethernet switching module (WS-X6501-10GEX4) does not support ISL encapsulation. - Connect the two switch ports together with the Ethernet crossover cable.
In this example, the Switch A 2/1 port connects to the Switch B 2/1 port. - To verify that the ports are operational, issue the show interfaces fastethernet 2/1 status command.
The command displays this information:
Switch-A# show interfaces fastethernet 2/1 status Port Name Status VLAN Duplex Speed Type Fa2/1 connected 1 a-full a-100 10/100BaseTX Switch-A#
- Configure a VLAN interface on both switches A and B.
In the global configuration mode, issue the command interface vlan 1. When you configure the IP address, issue the command ip address 172.16.84.17 255.255.255.0 on Switch A and ip address 172.16.84.17 255.255.255.0 on Switch B. To configure the switchport 2/1 as a Layer 2 (L2) interface and a member of VLAN 1, issue the commands interface fastethernet 3/1, switchport, and switchport access vlan 1. For more information on the configuration of L2 interfaces in Cisco IOS Software, refer to this document:
Switch-A(config)# interface vlan 1 Switch-A(config-if)# ip address 172.16.84.17 255.255.255.0 Switch-A(config-if)# Switch-A(config)# interface fastethernet 2/1 Switch-A(config-if)# switchport Switch-A(config-if)# switchport access vlan 1 Switch-A(config-if)#
- To verify that the link between switches A and B is not trunking, issue the command show interfaces fastethernet 2/1 trunk.
Note: The term Native VLAN in this output indicates the VLAN placement of this port when the port is not in trunking mode. If you have configured the port for 802.1Q trunking, the Native VLAN field also indicates the VLAN for which the frames have no tags; all others have tags. (Conversely, with ISL trunking, every data frame has the appropriate VLAN identifier.)Switch-A# show interfaces fastethernet 2/1 trunk Port Mode Encapsulation Status Native VLAN Fa2/1 desirable negotiate not-trunking 1 Port VLANs allowed on trunk Fa2/1 none Port VLANs allowed and active in management domain Fa2/1 none Port VLANs in spanning tree forwarding state and not pruned Fa2/1 none Switch-A#
- Ping Switch B from Switch A to verify that the switches can communicate over the link.
Switch-A> ping 66.123.210.122 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.16.84.18, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 4/21/92 ms Switch-A>
- To create VLAN 2 in Switch A, issue the vlan database command and the vlan 2 command on Switch A.
Switch B learns about VLAN 2 after the establishment of the VTP domain in Step 11. For more information on the configuration of VLANs in Cisco IOS Software code, refer to this document:
- Configuring VLANs
Switch-A# vlan database Switch-A(vlan)# vlan 2 VLAN 2 added: Name: VLAN0002 Switch-A(vlan)#exit APPLY completed. Exiting.... Switch-A#
- Configuring VLANs
- Move the management interface on switches A and B to VLAN 2, which you created in Step 8.
In Cisco IOS Software mode, there is no sc0 management interface. Therefore, configure the ports, which connect to host A on Switch A and host B on Switch B, to VLAN 2. Issue the commands switchport and switchport access vlan 2. Then, use the hosts to perform the ping tests. For this example, configure IP addresses 172.16.85.1/24 on host A and 172.16.85.2/24 on host B.
Issue the show interfaces command to view the change you just made. This output shows the issue of the command on Switch A. The output shows the new association of interface 2/2 and VLAN 2:Switch-A(config)# interface fastethernet 2/2 Switch-A(config-if)# switchport Switch-A(config-if)# switchport access vlan 2
Switch-A# show interfaces fastethernet 2/2 switchport Name: Fa2/2 Switchport: Enabled Administrative Mode: dynamic desirable Operational Mode: up Administrative Trunking Encapsulation: negotiate Negotiation of Trunking: On Access Mode VLAN: 2 (VLAN0002) Switch-A# show interfaces fastethernet 2/2 switchport Name: Fa2/2 Switchport: Enabled Administrative Mode: dynamic desirable Operational Mode: up Administrative Trunking Encapsulation: negotiate Negotiation of Trunking: On Access Mode VLAN: 2 (VLAN0002) !--- Output suppressed.
- Attempt to ping the host that connects to the alternate switch.
The ping should fail because the ports are now in VLAN 2 while the link between the switches is in VLAN 1.
C:\> ping 172.16.85.1 Pinging 172.16.85.1 with 32 bytes of data: Request timed out. Request timed out. Request timed out. Request timed out. Ping statistics for 172.16.85.1: Packets: Sent = 4, Received = 0, Lost = 4 (100% loss), Approximate round trip times in milli-seconds: Minimum = 0ms, Maximum = 0ms, Average = 0ms C:\>
- Establish the same VTP domain for both switches.
Issue the vtp domain Cookbook command in global configuration mode.
Note: The name of the VTP domain is Cookbook.
For more information on the configuration of VTP parameters with Cisco IOS Software, refer to this document:
- Configuring VTP
Switch-A(config)# vtp domain Cookbook Changing VTP domain name from NULL to cookbook Switch-A(config)#
- Configuring VTP
- Turn on trunking between the switches.
Configure port 2/1 on Switch A for desirable mode. To configure the encapsulation type and the mode, issue the switchport trunk encapsulation isl command and the switchport mode dynamic desirable command. Switch B is in auto mode. Switch B automatically places the port 2/1 in trunking mode after completion of the DTP negotiation between the two switches.
Note: Cisco Technical Support recommends the configuration of desirable trunking mode on the ports of the trunk link.
This list describes the five different states for which you can configure DTP:Switch-A(config)# interface fastethernet 2/1 Switch-A(config-if)# switchport trunk encapsulation isl Switch-A(config-if)# switchport mode dynamic desirable
- auto: The port listens for DTP frames from the neighbor switch. If the neighbor switch indicates that the switch would like to be a trunk, or that the switch is a trunk, auto state creates the trunk with the neighbor switch. The auto state does not propagate any intent to become a trunk; auto state is solely dependent on the neighbor switch to make the trunking decision.
- desirable: DTP is spoken to the neighbor switch to which you want to establish an ISL trunk. The switch with desirable configuration communicates that the switch is able to be an ISL trunk and wants the neighbor switch to also be an ISL trunk.
- on: DTP is spoken to the neighbor switch. The on state automatically enables ISL trunking on the port, regardless of the state of the neighbor switch. The port remains an ISL trunk unless the port receives an ISL packet that explicitly disables the ISL trunk. Cisco Technical Support recommends the configuration of desirable trunking mode on the ports.
- nonegotiate: DTP is not spoken to the neighbor switch. The nonegotiate state automatically enables ISL trunking on the port, regardless of the state of the neighbor switch.
- off: There can be no use of ISL on this port, regardless of the configuration of DTP mode on the other switch port.
You may see other messages that relate to changes to the state of STP on the switch. These messages are not relevant to this document. Refer to the document Understanding and Configuring Spanning Tree Protocol (STP) on Catalyst Switches for more information on this protocol. Because you cleared the configurations in the switches to start, you have the defaults for the STP parameters. The default parameters of STP should provide the connectivity necessary for this document to succeed.Switch A Port/Interface 2/1 Switch B Port/Interface 2/1 ISL Trunk Status DTP mode auto DTP mode auto not-trunking DTP mode desirable DTP mode auto trunking DTP mode on DTP mode auto trunking DTP mode nonegotiate DTP mode auto not-trunking DTP mode off DTP mode auto not-trunking DTP mode desirable (Cisco Technical Support recommended configuration) DTP mode desirable (Cisco Technical Support recommended configuration) trunking DTP mode on DTP mode desirable trunking DTP mode nonegotiate DTP mode desirable not-trunking DTP mode off DTP mode desirable not-trunking DTP mode on DTP mode on trunking DTP mode nonegotiate DTP mode on trunking DTP mode off DTP mode on not-trunking DTP mode nonegotiate trunking trunking DTP mode off DTP mode nonegotiate not-trunking DTP mode off DTP mode off not-trunking - To verify the trunk, issue the show interfaces fastethernet 2/1 trunk command.
You should now see that trunking is operational.Switch-A# show interfaces fastethernet 2/1 trunk Port Mode Encapsulation Status Native VLAN Fa2/1 desirable isl trunking 1 Port VLANs allowed on trunk Fa2/1 1-1005 Port VLANs allowed and active in management domain Fa2/1 1-2,1002-1005 Port VLANs in spanning tree forwarding state and not pruned Fa2/1 1-2,1002-1005
Note: VLANs 1–1005 are allowable on all trunk ports by default. You can clear VLAN 1 from the list of allowable VLANs. If you remove VLAN 1 from a trunk, the trunk interface continues to send and receive management traffic, for example, CDP, VTP, PAgP, and DTP in VLAN 1.
To limit or remove VLANs on a trunk, issue the switchport trunk allowed vlan remove 1-1001 command in interface configuration mode. The command removes all removable VLANs from the trunk. Issue the switchport trunk allowed vlan add 2 command to add VLAN 2 to the list of allowable VLANs on the trunk.
In this output, notice the permission of only VLANs 2 and 1002–1005 on the trunk:Switch-A(config-if)# switchport trunk allowed vlan remove 1-1001 Switch-A(config-if)# Switch-A(config-if)# switchport trunk allowed vlan add 2 Switch-A(config-if)#
Switch-A# show interfaces fastethernet 2/1 trunk Port Mode Encapsulation Status Native VLAN Fa2/1 desirable isl 2,1002-1005 1 Port VLANs allowed on trunk Fa2/1 2,1002-1005 Port VLANs allowed and active in management domain Fa2/1 2,1002-1005 Port VLANs in spanning tree forwarding state and not pruned Fa2/1 2,1002-1005
- To verify that the trunk link is operational, attempt to ping across the trunk link.
Ping host A from host B to verify connectivity over VLAN 2.
C:\> ping 172.16.85.1 Pinging 172.16.85.1 with 32 bytes of data: Reply from 172.16.85.1: bytes=32 time<10ms TTL=255 Reply from 172.16.85.1: bytes=32 time<10ms TTL=255 Reply from 172.16.85.1: bytes=32 time<10ms TTL=255 Reply from 172.16.85.1: bytes=32 time<10ms TTL=255 Ping statistics for 172.16.85.1: Packets: Sent = 4, Received = 4, Lost = 0 (0% loss), Approximate round trip times in milliseconds: Minimum = 0ms, Maximum = 0ms, Average = 0ms C:\>
Troubleshoot the Result
Commands to Use to Troubleshoot Cisco IOS Software
- show interfaces fastethernet port/mod trunk—To see the trunking information for a particular interface.
- show vtp status—To display VTP information.
- show vlan vlan_number —To see information on a particular VLAN.
- show spanning-tree vlan vlan_number —To see the status of the spanning tree for a particular VLAN.
- show interfaces fastethernet port/mod switchport —To see L2 interface information.
- ping —To send an ICMP echo message to another IP host.
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