3.4.5 Packet Tracer - Configure Trunks (Instructor Version)
Instructor Note: Red font color or gray highlights indicate text that appears in the instructor copy only
3.4.5 Packet Tracer - Configure Trunks
Addressing Table
| Device | Interface | IP Address | Subnet Mask | Switch Port | VLAN |
|---|---|---|---|---|---|
| PC1 | NIC | 172.17.10.21 | 255.255.255.0 | S2 F0/11 | 10 |
| PC2 | NIC | 172.17.20.22 | 255.255.255.0 | S2 F0/18 | 20 |
| PC3 | NIC | 172.17.30.23 | 255.255.255.0 | S2 F0/6 | 30 |
| PC4 | NIC | 172.17.10.24 | 255.255.255.0 | S3 F0/11 | 10 |
| PC5 | NIC | 172.17.20.25 | 255.255.255.0 | S3 F0/18 | 20 |
| PC6 | NIC | 172.17.30.26 | 255.255.255.0 | S3 F0/6 | 30 |
Objectives
- Part 1: Verify VLANs
- Part 2: Configure Trunks
Background
Trunks are required to pass VLAN information between switches. A port on a switch is either an access port or a trunk port. Access ports carry traffic from a specific VLAN assigned to the port. A trunk port by default is a member of all VLANs. Therefore, it carries traffic for all VLANs. This activity focuses on creating trunk ports and assigning them to a native VLAN other than the default.
Instructions
Part 1: Verify VLANs
Step 1: Display the current VLANs.
a. On S1, issue the command that will display all VLANs configured. There should be ten VLANs in total. Notice that all 26 access ports on the switch are assigned to VLAN 1.
S1#show vlan brief
VLAN Name Status Ports
---- -------------------------------- --------- -------------------------------
1 default active Fa0/1, Fa0/2, Fa0/3, Fa0/4
Fa0/5, Fa0/6, Fa0/7, Fa0/8
Fa0/9, Fa0/10, Fa0/11, Fa0/12
Fa0/13, Fa0/14, Fa0/15, Fa0/16
Fa0/17, Fa0/18, Fa0/19, Fa0/20
Fa0/21, Fa0/22, Fa0/23, Fa0/24
Gig0/1, Gig0/2
10 Faculty/Staff active
20 Students active
30 Guest(Default) active
88 Management active
99 Native active
1002 fddi-default active
1003 token-ring-default active
1004 fddinet-default active
1005 trnet-default activeb. On S2 and S3, display and verify that all the VLANs are configured and assigned to the correct switch ports according to the Addressing Table.
Step 2: Verify loss of connectivity between PCs on the same network.
Ping between hosts on the same the VLAN on the different switches. Although PC1 and PC4 are on the same network, they cannot ping one another. This is because the ports connecting the switches are assigned to VLAN 1 by default. In order to provide connectivity between the PCs on the same network and VLAN, trunks must be configured.
Part 2: Configure Trunks
Step 1: Configure trunking on S1 and use VLAN 99 as the native VLAN.
a. Configure G0/1 and G0/2 interfaces on S1 for trunking.
S1(config)# interface range g0/1 - 2 S1(config-if)# switchport mode trunk
b. Configure VLAN 99 as the native VLAN for G0/1 and G0/2 interfaces on S1.
S1(config-if)# switchport trunk native vlan 99
The trunk port takes about a short time to become active due to Spanning Tree Protocol. Click Fast Forward Time to speed the process. After the ports become active, you will periodically receive the following syslog messages:
%CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on GigabitEthernet0/2 (99), with S3 GigabitEthernet0/2 (1). %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on GigabitEthernet0/1 (99), with S2 GigabitEthernet0/1 (1).
You configured VLAN 99 as the native VLAN on S1. However, S2 and S3 are using VLAN 1 as the default native VLAN as indicated by the syslog message.
Although you have a native VLAN mismatch, pings between PCs on the same VLAN are now successful.
Explain.
Pings are successful because trunking has been enabled on S1. Dynamic Trunking Protocol (DTP) has automatically negotiated the other side of the trunk links. In this case, S2 and S3 have now automatically configured the ports attached to S1 as trunking ports.
Step 2: Verify trunking is enabled on S2 and S3.
On S2 and S3, issue the show interface trunk command to confirm that DTP has successfully negotiated trunking with S1 on S2 and S3. The output also displays information about the trunk interfaces on S2 and S3.
You will learn more about DTP later in the course.
Which active VLANs are allowed to cross the trunk?
1,10,20,30,88,99
Step 3: Correct the native VLAN mismatch on S2 and S3.
a. Configure VLAN 99 as the native VLAN for the appropriate interfaces on S2 and S3.
S2
S2(config)#interface g0/1 S2(config-if)#switchport mode trunk S2(config-if)#switchport trunk native vlan 99
S3
S3(config)#interface g0/2 S3(config-if)#switchport mode trunk S3(config-if)#switchport trunk native vlan 99
b. Issue show interface trunk command to verify the correct native VLAN configuration.
Step 4: Verify configurations on S2 and S3.
a. Issue the show interface interface switchport command to verify that the native VLAN is now 99.
b. Use the show vlan command to display information regarding configured VLANs.
Why is port G0/1 on S2 no longer assigned to VLAN 1?
Port G0/1 is a trunk port and trunks ports are not displayed.
Device Configs - Final
Switch S1
! ============================================================== !--- 3.4.5 Packet Tracer - Configure Trunks !--- ANSWER SCRIPT FOR SWITCH S1 !--- Usage: from the console (or a Telnet/SSH session, if already reachable) on S1, enter privileged EXEC mode with "enable", then paste this whole file. Every line beginning with "!" is a comment; IOS ignores it, so pasting a comment by accident is harmless. !--- S1 is the core switch with no directly-connected PCs - only uplinks to S2 (G0/1) and S3 (G0/2). All VLANs (10/20/30/88/99 and the defaults) and the S2/S3 access ports already exist from a previous activity; this lab is entirely about trunking. This script trunks both uplinks and sets VLAN 99 as the native VLAN on each. ! ============================================================== enable configure terminal ! -------------------------------------------------------------- !--- Part 2, Step 1: Configure trunking on S1, native VLAN 99 ! -------------------------------------------------------------- !--- Both uplinks get the same treatment in one range command: hard-set to trunk, then native VLAN 99 instead of the default VLAN 1. Expect transient "%CDP-4-NATIVE_VLAN_MISMATCH" syslog messages against S2 and S3 until their own native VLAN is corrected (see their scripts) - that mismatch does NOT block the trunk from passing traffic, it's just a configuration warning. interface range GigabitEthernet0/1 - 2 switchport mode trunk switchport trunk native vlan 99 end ! -------------------------------------------------------------- !--- Operational habit (not explicitly required by this lab): save to NVRAM. (Press Enter when prompted for the destination filename.) ! -------------------------------------------------------------- copy running-config startup-config ! ============================================================== !--- Verification (Parts 1-2, from S1 and its neighbors): !--- S1# show interfaces trunk -> Gig0/1 and Gig0/2 both trunking, native VLAN 99 !--- S1# show vlan brief -> S1 has no access ports to show; trunk ports never appear per-VLAN !--- From PC1, ping PC4 (VLAN 10, S2<->S3) -> success once S2 and S3 also correct their native VLAN !--- From PC2, ping PC5 (VLAN 20, S2<->S3) -> success once S2 and S3 also correct their native VLAN !--- From PC3, ping PC6 (VLAN 30, S2<->S3) -> success once S2 and S3 also correct their native VLAN ! ==============================================================
Switch S2
! ============================================================== !--- 3.4.5 Packet Tracer - Configure Trunks !--- ANSWER SCRIPT FOR SWITCH S2 !--- Usage: from the console (or a Telnet/SSH session, if already reachable) on S2, enter privileged EXEC mode with "enable", then paste this whole file. Every line beginning with "!" is a comment; IOS ignores it, so pasting a comment by accident is harmless. !--- S2 connects PC1/PC2/PC3 (already assigned to VLANs 10/20/30 from a previous activity) and uplinks to S1 over G0/1 ONLY - S2 has no link on G0/2 (see the discrepancy note: the source answer key mistakenly ranges g0/1-2 here). DTP already brought this port up as a trunk automatically once S1 was configured; this script only needs to correct the native VLAN to 99 to clear the mismatch. ! ============================================================== enable configure terminal ! -------------------------------------------------------------- !--- Part 2, Step 3: Correct the native VLAN mismatch on S2 ! -------------------------------------------------------------- !--- G0/1 is the only uplink S2 actually has (to S1's G0/1, per the topology and the CDP mismatch message naming "S2 GigabitEthernet0/1"). It's already trunking via DTP negotiation - this only moves its native VLAN from the default (1) to 99, to match S1 and clear the "%CDP-4-NATIVE_VLAN_MISMATCH" warning. interface GigabitEthernet0/1 switchport trunk native vlan 99 end ! -------------------------------------------------------------- !--- Operational habit (not explicitly required by this lab): save to NVRAM. (Press Enter when prompted for the destination filename.) ! -------------------------------------------------------------- copy running-config startup-config ! ============================================================== !--- Verification (Parts 1-2, from S2 and its neighbors): !--- S2# show interfaces trunk -> Gig0/1 trunking, native VLAN 99, allowed VLANs include 1,10,20,30,88,99 !--- S2# show interface g0/1 switchport -> native VLAN now 99 !--- S2# show vlan -> G0/1 no longer listed under any VLAN (trunk ports aren't shown there) !--- From PC1, ping PC4 (VLAN 10, on S3) -> success once S3 also corrects its native VLAN !--- From PC2, ping PC5 (VLAN 20, on S3) -> success once S3 also corrects its native VLAN ! ==============================================================
Switch S3
! ============================================================== !--- 3.4.5 Packet Tracer - Configure Trunks !--- ANSWER SCRIPT FOR SWITCH S3 !--- Usage: from the console (or a Telnet/SSH session, if already reachable) on S3, enter privileged EXEC mode with "enable", then paste this whole file. Every line beginning with "!" is a comment; IOS ignores it, so pasting a comment by accident is harmless. !--- S3 connects PC4/PC5/PC6 (already assigned to VLANs 10/20/30 from a previous activity) and uplinks to S1 over G0/2 ONLY - S3 has no link on G0/1 (see the discrepancy note: the source answer key mistakenly ranges g0/1-2 here). DTP already brought this port up as a trunk automatically once S1 was configured; this script only needs to correct the native VLAN to 99 to clear the mismatch. ! ============================================================== enable configure terminal ! -------------------------------------------------------------- !--- Part 2, Step 3: Correct the native VLAN mismatch on S3 ! -------------------------------------------------------------- !--- G0/2 is the only uplink S3 actually has (to S1's G0/2, per the topology and the CDP mismatch message naming "S3 GigabitEthernet0/2"). It's already trunking via DTP negotiation - this only moves its native VLAN from the default (1) to 99, to match S1 and clear the "%CDP-4-NATIVE_VLAN_MISMATCH" warning. interface GigabitEthernet0/2 switchport trunk native vlan 99 end ! -------------------------------------------------------------- !--- Operational habit (not explicitly required by this lab): save to NVRAM. (Press Enter when prompted for the destination filename.) ! -------------------------------------------------------------- copy running-config startup-config ! ============================================================== !--- Verification (Parts 1-2, from S3 and its neighbors): !--- S3# show interfaces trunk -> Gig0/2 trunking, native VLAN 99, allowed VLANs include 1,10,20,30,88,99 !--- S3# show interface g0/2 switchport -> native VLAN now 99 !--- S3# show vlan -> G0/2 no longer listed under any VLAN (trunk ports aren't shown there) !--- From PC4, ping PC1 (VLAN 10, on S2) -> success once S2 also corrects its native VLAN !--- From PC5, ping PC2 (VLAN 20, on S2) -> success once S2 also corrects its native VLAN ! ==============================================================
