(interesting for a router!) in order to verify the encapsulations you have in place on your subinterfaces.
Example 8.4 shows the verification on SW1. EXAMPLE 8.4 Verification on SW1
Click here to view code image
SW1#show interface trunk
Port Mode Encapsulation Status Native vlan
Gi0/1 on 802.1q trunking 1
Port Vlans allowed on trunk Gi0/1 1-4094
Port Vlans allowed and active in management domain Gi0/1 1,10,20
pruned
Gi0/1 1,10,20 SW1#
My verification on SW1 is super straightforward. I just like to check the trunking status of my trunk port with show interface trunk. This, of course, assumes I have already properly verified my VLANs and interfaces and all of the other infrastructure stuff from earlier chapters.
ExamAlert
In a production environment, or in a robustly constructed exam simulation, you might have the opportunity to move to properly configured host workstations in each VLAN and ensure they can ping each other. Remember, the point of inter- VLAN routing is to permit devices in one VLAN and IP subnet to communicate with devices in another, so always test this if it is possible.
If you are curious like me, you might be wondering about the Native VLAN, that one VLAN that is not tagged with an 802.1Q tag by default. The preceding configurations assume that no Native VLAN is in use by the router. Let me demonstrate two separate methods you can use to handle a Native VLAN. Example 8.5 demonstrates placing the IP address that coordinates with the subnet of the Native VLAN on the physical interface. This makes sense because this traffic is not tagged and requires no recognition of an encapsulated tag, since the encapsulated tag does not exist on the native VLAN! EXAMPLE 8.5 The Native VLAN on the Physical Interface
Click here to view code image
R1#
R1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z. R1(config)#interface gi0/1
R1(config-if)#no shutdown
R1(config-if)#
%LINK-3-UPDOWN: Interface GigabitEthernet0/1, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/1,
changed state to up
R1(config-if)#! Notice now the IP address on the physical interface!
R1(config-if)#ip address 10.1.0.1 255.255.255.0
R1(config-if)#interface gi0/1.10
R1(config-subif)#encapsulation dot1q 10
R1(config-subif)#ip address 10.1.10.1 255.255.255.0
R1(config)#interface gi0/1.20
R1(config-subif)#encapsulation dot1q 20
R1(config-subif)#ip address 10.1.20.1 255.255.255.0
R1(config-subif)#end
R1#
%SYS-5-CONFIG_I: Configured from console by console R1#
Example 8.6 show another way to handle the use of a Native VLAN on your ROAS configuration. This time, a subinterface is used for the Native VLAN. Because there is no tagging to identify the VLAN, the native keyword must be used in the encapsulation command.
EXAMPLE 8.6 The Native VLAN on a Subinterface
Click here to view code image
R1#
R1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z. R1(config)#interface gi0/1
R1(config-if)#no shutdown
R1(config-if)#
%LINK-3-UPDOWN: Interface GigabitEthernet0/1, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/1,
changed state to up
R1(config-if)#! Notice no IP address on the physical interface
R1(config-if)#interface gi0/1.10
R1(config-subif)#encapsulation dot1q 10
R1(config-subif)#ip address 10.1.10.1 255.255.255.0
R1(config-subif)#exit
R1(config)#interface gi0/1.20
R1(config-subif)#encapsulation dot1q 20
R1(config-subif)#ip address 10.1.20.1 255.255.255.0
R1(config-subif)#exit
R1(config)#interface gi0/1.777
R1(config-subif)#encapsulation dot1q 777 native
R1(config-subif)#ip address 10.1.0.1 255.255.255.0
R1(config-subif)#end
R1#
%SYS-5-CONFIG_I: Configured from console by console R1#
1. Examine the configuration of R1 following the depicted network topology. Why is
ROAS not functioning?
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Building configuration...
Current configuration : 3056 bytes !
version 15.6
service timestamps debug datetime msec service timestamps log datetime msec no service password-encryption ! hostname R1 ! boot-start-marker boot-end-marker ! ! ! no aaa new-model ethernet lmi ce ! ! ! mmi polling-interval 60 no mmi auto-configure no mmi pvc mmi snmp-timeout 180 ! ! ! no ip routing ! ! ! no ip cef no ipv6 cef !
multilink bundle-name authenticated ! ! ! redundancy ! !
! interface GigabitEthernet0/0 ip address 10.255.0.14 255.255.0.0 no ip route-cache shutdown duplex auto speed auto media-type rj45 ! interface GigabitEthernet0/1 no ip address no ip route-cache duplex auto speed auto media-type rj45 no shutdown ! interface GigabitEthernet0/1.10 encapsulation dot1Q 10 ip address 10.1.10.1 255.255.255.0 no ip route-cache ! interface GigabitEthernet0/1.20 encapsulation dot1Q 20 ip address 10.1.20.1 255.255.255.0 no ip route-cache ! ip forward-protocol nd ! ! no ip http server no ip http secure-server ! ! ! control-plane ! ! ! line con 0 exec-timeout 0 0 logging synchronous line aux 0 line vty 0 4 login
transport input none !
no scheduler allocate !
A. The IP address on the gi0/1.20 interface is the subnet ID. B. Interface gi0/1 has no IP address assigned.
C. AAA is disabled. D. IP routing is disabled.
2. Examine the configuration of SW1 following the depicted network topology in the
figure. Why is ROAS not functioning?
Click here to view code image
Building configuration...
Current configuration : 2688 bytes !
version 15.2
service timestamps debug datetime msec service timestamps log datetime msec no service password-encryption service compress-config ! hostname SW1 ! boot-start-marker boot-end-marker ! ! ! no aaa new-model ! ! ! ip cef no ipv6 cef ! ! !
spanning-tree mode rapid-pvst spanning-tree extend system-id !
vlan internal allocation policy ascending ! ! ! interface GigabitEthernet0/0 media-type rj45 negotiation auto
! interface GigabitEthernet0/1 no shutdown media-type rj45 negotiation auto ! ip forward-protocol nd ! no ip http server no ip http secure-server ! ! ! control-plane ! ! ! line con 0 exec-timeout 0 0 logging synchronous line aux 0 line vty 0 4 login ! ! end
A. The trunk is not going to form properly. B. The gi0/1 interface is shutdown.
C. The interface cannot be set to autonegotiate. D. The device cannot use CEF.
CramQuiz Answers
1. D is correct. On this router, someone has inserted the no ip routing command,
which disabled routing capabilities.
2. A is correct. The trunk interface is missing a required command of switch trunk encapsulation dot1q. This is required before the use of the command switch mode trunk.
Review Questions
1. An RE allows inter-VLAN communication. What is an RE?
A. Routing entity B. Routing entry C. Routing engine D. Register entry
2. What is the virtual interface that makes ROAS possible?
A. Loopback B. Subinterface C. TTY
D. VTY
3. What command allows you to verify the ROAS configuration on a router?
A. show interface trunk B. show vlan status C. show vlan
D. show subinterfaces status
4. Why might a ROAS configuration have an IP address on the physical interface?
A. The configuration is in error B. This is a required configuration
C. The IP address is associated with the Native VLAN D. The IP address is an RFC 1918 address and is not used
Answers to Review Questions
1. C is correct. A routing engine (RE) routes between IPv4 networks and their
associated VLANs. In a ROAS configuration, this is a physical router appliance. In modern times, this is a function integrated as part of a multilayer switch.
2. B is correct. Subinterfaces are the virtual interfaces that make ROAS able to
support multiple IP subnets and their associated VLANs.
3. C is correct. You can use the show vlan command on a router to help verify
ROAS.
4. C is correct. You might see an IP address on the physical interface for the Native
VLAN. Remember, this VLAN is not 802.1Q tagged.
Additional Resources
Configure Inter-VLAN Routing on Cisco Routers and Switches
—http://www.ajsnetworking.com/inter-vlan
Configure InterVLAN Routing on Layer 3 Switches
—http://bit.ly/1FELL8P (NOTE: The first character in the bit.ly URL is the numeric 1 and the later characters are lowercase l’s)
Chapter 9. Routing Technologies: Routing Methods
This chapter covers the following official CCNA 200-125 exam topics:
Compare and contrast static routing and dynamic routing
Compare and contrast distance vector and link state routing protocols Compare and contrast interior and exterior routing protocols
This chapter ensures you are ready for the listed topics from the Routing Technologies section of the overall exam blueprint from Cisco Systems. Remember, this is just a section of the Routing Technologies area. Chapters Seven, Eight, and Ten also exist in this grouping. These other chapters deal with general routing concepts, inter-VLAN routing, and the configuration of static and dynamic routing.
Essential Terms and Components Default Routing
Static Routing Dynamic Routing Distance Vector Link State
Interior Routing Protocols Exterior Routing Protocols