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Chapter6-Inter VLAN routing

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					            Inter-VLAN routing
            CCNA Exploration Semester 3
                              Chapter 6




30-Dec-12            S Ward Abingdon and Witney College   1
Topics
    Routing network traffic between VLANs
    Configuring a router for routing between
     VLANs
    Troubleshoot inter-VLAN connectivity issues




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Semester 3
                   LAN Design

    Basic Switch                                         Wireless
    Concepts

VLANs                             STP


VTP                       Inter-VLAN
                          routing
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Introducing Inter-VLAN
Routing
•    What is Inter-VLAN Routing?
          Each VLAN is a unique broadcast domain.
               Computers on separate VLANs are, by default, not
                able to communicate.
          Each VLAN is a unique IP subnetwork.
          To allow VLANs to communicate, we need a
           router to communicate among separate broadcast
           domains and unique IP subnetworks.
•    Inter-VLAN routing, then, is a process of forwarding
     traffic from one VLAN to another VLAN using a router.
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Inter VLAN routing
                                                 Switch keeps
                                                 VLANs
                                                 separate.
                                                 Router can
                                                 route between
                                                 VLANs
                                                 (subnets).




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Introducing Inter-VLAN
Routing
Methods:
  Traditional Inter-VLAN Routing.

     One router interface per VLAN

  Router-on-a-stick Inter-VLAN Routing.

     One router interface for all VLANs

  Switch Based Inter-VLAN Routing.

     Uses Switch Virtual Interfaces (SVI) to retag
      the frame

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Traditional inter-VLAN routing
    The router has one physical port for each
     VLAN.
    Each port has an IP address on its own
     VLAN.
    Routing is the same as routing between any
     subnets.



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Traditional inter-VLAN routing
                                                   Router sendsup
                                                   Router looks out
                                                   packet through F0/1 –
                                                   192.168.2.0 network to
                                                   port F0/1 F0/2
                                                   switchport
 ARP packet to
 Send requestto
 find of router
 F0/0 MAC address
                                                           Switchport F0/2 is
 of default gateway
                                                           on VLAN30 and
                                                           forwards to PC3.
PC1 has a
packet for
192.168.2.19
– different
subnet
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Traditional inter-VLAN routing

Trunks
between
switches.
No trunk to
router.
No tags on
frames to
router.


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Trunk to the router
    You soon run out of router ports.
    What about making a trunk link to the router?
    A trunk link can be used with several VLANs
     on the same physical port.
    But each VLAN needs a different IP address.
    So use subinterfaces, each with its own IP
     address.


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Router on a stick
R1 F0/0 has
subinterfaces,
one for each
VLAN.
Each has its
own IP
address.
VLAN tags on
trunk.


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Availability of trunking
    Not all routers allow subinterfaces for VLAN
     trunking on Ethernet ports.
    It depends on the IOS feature set.
    Some of ours do, others do not.




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Router-on-a-stick Inter-VLAN
Routing:
 Subinterfaces:
           Overcomes the hardware limitation of a router.
           Subinterfaces are software-based virtual interfaces
            that are assigned to physical interfaces.
           Each subinterface is configured with its own IP
            address, subnet mask, and unique VLAN
            assignment.
           Connected to a switch trunk link.
           Functionally the same as using the traditional
            routing model.

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Router-on-a-stick Inter-VLAN
Routing:
    Subinterfaces take the interface name
     followed by a dot and a number.
    It is normal to use the VLAN number. If this
     ties in with the IP address, even better.
    E.g. interface f0/0.10
    The subinterfaces each have an IP address.
    The physical interface has no IP address
    The physical interfaces needs no shutdown
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Configuring subinterfaces
    R1(config)#interface f0/0.10
    R1(config-subif)#encapsulation dot1q 10
    R1(config-subif)#ip address 172.17.10.1
     255.255.255.0
    (add any other subinterfaces)
    R1(config-subif)#interface f0/0
    R1(config-if)#no shutdown

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Routing table
    Subinterfaces are shown

  172.17.0.0/24 is subnetted, 2 subnets
C    172.17.10.0 is directly connected,
  FastEthernet0/0.10
C    172.17.30.0 is directly connected,
  FastEthernet0/0.30


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Subinterface considerations
    Routers have a limited number of interfaces,
     so subinterfaces help where there are many
     VLANs.
    Subinterfaces share the bandwidth on a
     physical interface so can cause bottlenecks.
    Cheaper to use subinterfaces.
    Simpler physical layout but more complex
     configuration with subinterfaces.

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Multilayer switch
    A multilayer switch can
     route between VLANs.
    But that’s CCNP.




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Interfaces and Subinterfaces




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Troubleshooting
    Check physical connections
    Check switchport VLAN assigment
    Check trunking on switches
    Check IP addressing and subnetting scheme
     in relation to VLANs
    Check router subinterface configuration



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                                           The End




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