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Routing Between VLANs & Layer 3 Switches

    Routing Between VLANs & Layer 3 Switches

    We wrote an article which covers Virtual Local Area Networks (VLANs) as a concept, and another article on configuring VLANs on Cisco switches. The remaining subjects to cover are the different options that exist for routing between VLANs. This will let us illustrate the concepts of inter-vlan routing, Router on a Stick (RoaS), and Layer 3 Switches (occasionally called MultiLayer Switches).

    Why do we need Routing Between VLANs?

    As we learned in a prior article, VLANs create a logical separation between Switch ports. Essentially, each VLAN behaves like a separate physical switch. To illustrate this, below are two topology pictures of the same environment – one Physical and one Logical.

    The Physical topology depicts a switch and four hosts in two different VLANs – Host A and Host B are in VLAN 20 and Host C and Host D are in VLAN 30. The logical topology reflects how the physical topology operates – the two VLANs essentially create two separate physical switches.

    PhysicalLogical

    Routing Between VLANs - Physical Topology

    Routing Between VLANs - Logical Topology

    Despite all four hosts being connected to the same physical switch, the logical topology makes it clear that the hosts in VLAN 20 are unable to speak with the hosts in VLAN 30. Notice since there is nothing connecting the two “virtual” switches, there is no way for Host A to speak to Host C.

    Since Host A and Host C are in different VLANs, it is also implied that they are in different Networks. Each VLAN will typically correspond to its own IP Network. In this diagram, VLAN 20 contains the 10.0.20.0/24 network, and VLAN 30 contains the 10.0.30.0/24 network.

    The purpose of a Switch is to facilitate communication within networks. This works great for Host A trying to speak to Host B. However, if Host A is trying to speak to Host C, we will need to use another device – one whose purpose is to facilitate communication between networks.

    If you’ve read the Packet Traveling series, then you know that the device which facilitates communication between networks is a Router.

    A router will perform the routing function necessary for two hosts on different networks to speak to one another. In the same way, a Router is what we will need in order for hosts in different VLANs to communicate with one another.

    There are three options available in order to enable routing between the VLANs:

    The remainder of this article will explore these three options and their configuration.

    Router with Separate Physical Interfaces

    The simplest way to enable routing between the two VLANs to simply connect an additional port from each VLAN into a Router.

    Routing Between VLANs - Physical Interfaces

    The Router doesn’t know that it has two connections to the same switch — nor does it need to. The Router operates like normal when routing packets between two networks.

    In fact, the process of a packet moving from Host A to Host D in this topology will work exactly as it does in this video. The only difference is since there is only one physical switch, there will only be one MAC address table – each entry includes the mapping of switchport to MAC address, as well as the VLAN ID number that port belongs to.

    Each switch port in this diagram is configured as an Access port, we can use the range command to configure multiple ports as once:

    Switch(config)# interface range eth2/0 - 2
    Switch(config-if-range)# switchport mode access
    Switch(config-if-range)# switchport access vlan 20
    
    Switch(config)# interface range eth3/0 - 2
    Switch(config-if-range)# switchport mode access
    Switch(config-if-range)# switchport access vlan 30
    


    Of course, before assigning the switchport to a VLAN, it is a good idea to create the VLAN in the VLAN Database.

    The Router interfaces also use a standard configuration — configuring an IP address and enabling the interface:

    Router(config)# interface eth0/2
    Router(config-if)# ip address 10.0.20.1 255.255.255.0
    Router(config-if)# no shutdown
    
    Router(config)# interface eth0/3
    Router(config-if)# ip address 10.0.30.1 255.255.255.0
    Router(config-if)# no shutdown
    


    Below you will find various show commands for the Router and the Switch, these can be used to understand and validate how the environment is functioning.

    Router Show Commands

    show runip int briefip routearpcdp neighbor

    Router# show running-config
    ...
    interface Ethernet0/2
     ip address 10.0.20.1 255.255.255.0
    !
    interface Ethernet0/3
     ip address 10.0.30.1 255.255.255.0
    




    Router# show ip interface brief
    Interface             IP-Address      OK? Method Status        Protocol
    ...
    Ethernet0/2           10.0.20.1       YES manual up            up
    Ethernet0/3           10.0.30.1       YES manual up            up
    ...
    




    Router# show ip route
    Codes: L - local, C - connected, ...
    
    Gateway of last resort is not set
    
          10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
    C        10.0.20.0/24 is directly connected, Ethernet0/2
    L        10.0.20.1/32 is directly connected, Ethernet0/2
    C        10.0.30.0/24 is directly connected, Ethernet0/3
    L        10.0.30.1/32 is directly connected, Ethernet0/3
    




    Router# show arp
    Protocol  Address          Age (min)  Hardware Addr   Type   Interface
    Internet  10.0.20.1               -   aabb.cc00.0220  ARPA   Ethernet0/2
    Internet  10.0.20.11              2   0050.7966.6800  ARPA   Ethernet0/2
    Internet  10.0.20.22              5   0050.7966.6801  ARPA   Ethernet0/2
    Internet  10.0.30.1               -   aabb.cc00.0230  ARPA   Ethernet0/3
    Internet  10.0.30.33              4   0050.7966.6802  ARPA   Ethernet0/3
    Internet  10.0.30.44              4   0050.7966.6803  ARPA   Ethernet0/3
    




    Router# show cdp neighbors
    Capability Codes: R - Router, S - Switch, I - IGMP, B - Source Route Bridge
    ...
    Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
    Switch           Eth 0/3           126             R S I  Linux Uni Eth 3/0
    Switch           Eth 0/2           126             R S I  Linux Uni Eth 2/0
    



    Switch Show Commands

    show runmac tablevlan briefcdp neighbor

    Switch# show running-config
    ...
    vlan 20
     name RED
    !
    vlan 30
     name BLUE
    ...
    interface Ethernet2/0
     switchport access vlan 20
     switchport mode access
    !
    interface Ethernet2/1
     switchport access vlan 20
     switchport mode access
    !
    interface Ethernet2/2
     switchport access vlan 20
     switchport mode access
    !
    interface Ethernet3/0
     switchport access vlan 30
     switchport mode access
    !
    interface Ethernet3/1
     switchport access vlan 30
     switchport mode access
    !
    interface Ethernet3/2
     switchport access vlan 30
     switchport mode access
    




    Switch# show mac address-table
              Mac Address Table
    -------------------------------------------
    
    Vlan    Mac Address       Type        Ports
    ----    -----------       --------    -----
      20    0050.7966.6800    DYNAMIC     Et2/1
      20    0050.7966.6801    DYNAMIC     Et2/2
      20    aabb.cc00.0220    DYNAMIC     Et2/0
      30    0050.7966.6802    DYNAMIC     Et3/1
      30    0050.7966.6803    DYNAMIC     Et3/2
      30    aabb.cc00.0230    DYNAMIC     Et3/0
    Total Mac Addresses for this criterion: 6
    




    Switch# show vlan brief
    
    VLAN Name                        Status    Ports
    ---- --------------------------- --------- --------------------
    ...
    20   RED                         active    Et2/0, Et2/1, Et2/2
    30   BLUE                        active    Et3/0, Et3/1, Et3/2
    ...
    




    Switch# show cdp neighbors
    Capability Codes: R - Router, S - Switch, I - IGMP, B - Source Route Bridge
    ...
    Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
    Router           Eth 3/0           152              R B   Linux Uni Eth 0/3
    Router           Eth 2/0           166              R B   Linux Uni Eth 0/2
    



    Router with Sub-Interfaces

    The previously described method is functional, but scales poorly. If there were five VLANs on the switch, then we would need five switchports and five router ports to enable routing between all five VLANs

    Instead, there exists a way for multiple VLANs to terminate on a single router interface. That method is to create a Sub-Interface.

    Routing Between VLANs - SubinterfacesA Sub-Interface allows a single Physical interface to be split up into multiple virtual sub-interfaces, each of which terminate their own VLAN.

    Sub-interfaces to a Router are similar to what Trunk ports are to a Switch – one link carrying traffic for multiple VLANs. Hence, each router Sub-interface must also add a VLAN tag to all traffic leaving said interface.

    The logical operation of the Sub-interface topology works exactly as the separate physical interface topology in the section before it. The only difference is with Sub-interfaces, only one Router interface is required to terminate all VLANs.

    Keep in mind, however, that the drawback with all VLANs terminating on a single Router interface is an increased risk of congestion on the link.

    The Sub-interface feature is sometimes referred to as Router on a Stick or One-armed Router. This is in reference to the single router terminating the traffic from each VLAN.

    The Switch’s port facing the router is configured as a standard Trunk:

    Switch(config)# interface eth1/1
    Switch(config-if)# switchport trunk encapsulation dot1q
    Switch(config-if)# switchport mode trunk
    


    The Router’s configuration of Sub-interfaces is fairly straight forward. First, we enable the physical interface:

    Router(config)# interface eth1/1
    Router(config-if)# no shutdown
    


    Next, we create and configure the first Sub-interface:

    Router(config)# interface eth1/1.20 
    Router(config-subif)# encapsulation dot1Q 20
    Router(config-subif)# ip address 10.0.20.1 255.255.255.0
    


    Apart from using the Sub-interface distinguisher (eth1/1.20) and using the encapsulation dot1q <VLAN#> command, the rest of the interface configuration is exactly the same as any other regular physical interface.

    Similarly, we will also configure the Sub-interface for VLAN 30:

    Router(config)# interface eth1/1.30
    Router(config-subif)# encapsulation dot1Q 30
    Router(config-subif)# ip address 10.0.30.1 255.255.255.0
    



    A point of clarity regarding the Sub-interface syntax. The number after the physical interface (fa0/3.20 and fa0/3.30) simply serves the purpose of splitting up the physical interfaces into Sub-interfaces. The number specified in the encapsulation dot1q vlan ## command is what actually specifies what VLAN ID# the traffic belongs to.

    These two values do not have to match, but often they do for the purpose of technician sanity.


    Below you will find various show commands for the Router and the Switch. These can be used to understand and validate how the environment is functioning.

    Router Sub-Interface Show Commands

    show runip int briefip routearpcdp neighbor

    Router# show running-config
    ...
    interface Ethernet1/1
     no ip address
    !
    interface Ethernet1/1.20
     encapsulation dot1Q 20
     ip address 10.0.20.1 255.255.255.0
    !
    interface Ethernet1/1.30
     encapsulation dot1Q 30
     ip address 10.0.30.1 255.255.255.0
    




    Router# show ip interface brief
    Interface             IP-Address      OK? Method Status        Protocol
    ...
    Ethernet1/1           unassigned      YES NVRAM  up            up
    Ethernet1/1.20        10.0.20.1       YES manual up            up
    Ethernet1/1.30        10.0.30.1       YES manual up            up
    ...
    




    Router# show ip route
    Codes: L - local, C - connected, ...
    
    Gateway of last resort is not set
    
          10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
    C        10.0.20.0/24 is directly connected, Ethernet1/1.20
    L        10.0.20.1/32 is directly connected, Ethernet1/1.20
    C        10.0.30.0/24 is directly connected, Ethernet1/1.30
    L        10.0.30.1/32 is directly connected, Ethernet1/1.30
    




    Router# show arp
    Protocol  Address          Age (min)  Hardware Addr   Type   Interface
    Internet  10.0.20.1               -   aabb.cc00.0211  ARPA   Ethernet1/1.20
    Internet  10.0.20.11              0   0050.7966.6800  ARPA   Ethernet1/1.20
    Internet  10.0.20.22              0   0050.7966.6801  ARPA   Ethernet1/1.20
    Internet  10.0.30.1               -   aabb.cc00.0211  ARPA   Ethernet1/1.30
    Internet  10.0.30.33              0   0050.7966.6802  ARPA   Ethernet1/1.30
    Internet  10.0.30.44              0   0050.7966.6803  ARPA   Ethernet1/1.30
    




    Router# show cdp neighbors
    Capability Codes: R - Router, S - Switch, I - IGMP, B - Source Route Bridge
    ...
    Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
    Switch           Eth 1/1           150             R S I  Linux Uni Eth 1/1
    



    Switch Trunk Show Commands

    show runmac tablevlan briefint trunkcdp

    Switch# show running-config
    ...
    vlan 20
     name RED
    !
    vlan 30
     name BLUE
    ...
    interface Ethernet1/1
     switchport trunk encapsulation dot1q
     switchport mode trunk
    !
    interface Ethernet2/1
     switchport access vlan 20
     switchport mode access
    !
    interface Ethernet2/2
     switchport access vlan 20
     switchport mode access
    !
    interface Ethernet3/1
     switchport access vlan 30
     switchport mode access
    !
    interface Ethernet3/2
     switchport access vlan 30
     switchport mode access
    




    Switch# show mac address-table
              Mac Address Table
    -------------------------------------------
    
    Vlan    Mac Address       Type        Ports
    ----    -----------       --------    -----
       1    aabb.cc00.0211    DYNAMIC     Et1/1
      20    aabb.cc00.0211    DYNAMIC     Et1/1
      30    aabb.cc00.0211    DYNAMIC     Et1/1
      20    0050.7966.6800    DYNAMIC     Et2/1
      20    0050.7966.6801    DYNAMIC     Et2/2
      30    0050.7966.6802    DYNAMIC     Et3/1
      30    0050.7966.6803    DYNAMIC     Et3/2
    Total Mac Addresses for this criterion: 7
    




    Switch# show vlan brief
    
    VLAN Name                        Status    Ports
    ---- --------------------------- --------- -------------------
    ...
    20   RED                         active    Et2/1, Et2/2
    30   BLUE                        active    Et3/1, Et3/2
    ...
    
    




    Switch# show interfaces trunk
    
    Port        Mode             Encapsulation  Status        Native vlan
    Et1/1       on               802.1q         trunking      1
    
    Port        Vlans allowed on trunk
    Et1/1       1-4094
    
    Port        Vlans allowed and active in management domain
    Et1/1       1,20,30
    
    Port        Vlans in spanning tree forwarding state and not pruned
    Et1/1       1,20,30
    




    Switch# show cdp neighbors
    Capability Codes: R - Router, S - Switch, I - IGMP, B - Source Route Bridge
    ...
    Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
    Router           Eth 1/1           136              R B   Linux Uni Eth 1/1
    



    Layer 3 Switch

    The last option for routing between VLANs does not involve a router at all. Nor does it involve using a traditional switch.

    Instead, a different device entirely can be used. This device is known as a Layer 3 Switch (or sometimes also as a Multilayer switch). But exactly what is a Layer 3 switch?

    Routing Between VLANs - L3 SwitchA Layer 3 Switch is different from a traditional Layer 2 Switch in that it has the functionality for routing between VLANs intrinsically. In fact, when considering how a L3 Switch operates, you can safely imagine that a Layer 3 Switch is a traditional switch with a built in Router.

    With regard to VLANs the Multilayer switch is configured mostly the same way as a regular L2 switch:

    MultilayerSwitch(config)# vlan 20
    MultilayerSwitch(config-vlan)# name RED 
    MultilayerSwitch(config)# vlan 30
    MultilayerSwitch(config-vlan)# name BLUE
    
    MultilayerSwitch(config)# interface range eth2/0 - 2
    MultilayerSwitch(config-if-range)# switchport mode access
    MultilayerSwitch(config-if-range)# switchport access vlan 20
    
    MultilayerSwitch(config)# interface range eth3/0 - 2
    MultilayerSwitch(config-if-range)# switchport mode access
    MultilayerSwitch(config-if-range)# switchport access vlan 30
    


    Then, for each VLAN that you want the Multilayer switch to route for, you have the option of configuring an IP address within what is known as an SVI, or a Switched Virtual Interface.

    An SVI serves as the L3 termination point for each VLAN – aka, the way in or out of each VLAN. Another way of looking at it is that the SVI serves as the interface on the built-in Router of the Multilayer switch, allowing traffic from one VLAN to reach the built-in Router and be routed to another VLAN as necessary.

    The configuration for an SVI involves two parts. First, enabling IP Routing; and Second, applying an IP address to the VLAN.

    To enable IP Routing, use the following command:

    MultilayerSwitch(config)# ip routing
    


    IP Routing only needs to be enabled once. Some L3 switches come with it enabled by default. Applying the command while it is already enabled will not cause any harm, so if in doubt as to whether it is already enabled or not, simply applying it again is safe.

    To apply an IP address to the VLANs, configure the SVI as follows:

    MultilayerSwitch(config)# interface vlan 20
    MultilayerSwitch(config-if)# ip address 10.0.20.1 255.255.255.0
    MultilayerSwitch(config-if)# no shutdown
    
    MultilayerSwitch(config)# interface vlan 30
    MultilayerSwitch(config-if)# ip address 10.0.30.1 255.255.255.0
    MultilayerSwitch(config-if)# no shutdown
    


    The two configurations above will enable routing between VLAN 20 and VLAN 30. The hosts in each VLAN can use the IP addresses 10.0.20.1 and 10.0.30.1 as their default gateway (respectively).

    Routing Between VLANs - L3 Switch Traffic Flow

    When Host A sends a packet to Host B, the packet will be switched within the same VLAN – no L3 processing will occur.

    When Host A sends a packet to Host C, the packet will be sent to the SVI to be routed to the other VLAN – all regular L3 processing will occur: the TTL will be decremented and the L2 header will be rewritten.

    Multilayer Switch Configuration

    show runmac address-tablevlan brief

    MultilayerSwitch# show running-config
    ...
    ip routing
    ...
    interface Vlan20
     ip address 10.0.20.1 255.255.255.0
    !
    interface Vlan30
     ip address 10.0.30.1 255.255.255.0
    




    MultilayerSwitch# show mac address-table
              Mac Address Table
    -------------------------------------------
    
    Vlan    Mac Address       Type        Ports
    ----    -----------       --------    -----
      20    0050.7966.6800    DYNAMIC     Et2/1
      20    0050.7966.6801    DYNAMIC     Et2/2
      30    0050.7966.6802    DYNAMIC     Et3/2
      30    0050.7966.6803    DYNAMIC     Et3/1
    Total Mac Addresses for this criterion: 4
    




    MultilayerSwitch# show vlan brief
    
    VLAN Name                        Status    Ports
    ---- --------------------------- --------- -------------------
    ...
    20   RED                         active    Et2/1, Et2/2
    30   BLUE                        active    Et3/1, Et3/2
    



    ip routearpip int brief

    MultilayerSwitch# show ip route
    Codes: L - local, C – connected, ...
    
    Gateway of last resort is not set
    
          10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
    C        10.0.20.0/24 is directly connected, Vlan20
    L        10.0.20.1/32 is directly connected, Vlan20
    C        10.0.30.0/24 is directly connected, Vlan30
    L        10.0.30.1/32 is directly connected, Vlan30
    




    MultilayerSwitch# show arp
    Protocol  Address          Age (min)  Hardware Addr   Type   Interface
    Internet  10.0.20.1               -   aabb.cc80.0200  ARPA   Vlan20
    Internet  10.0.20.11              0   0050.7966.6800  ARPA   Vlan20
    Internet  10.0.20.22              0   0050.7966.6801  ARPA   Vlan20
    Internet  10.0.30.1               -   aabb.cc80.0200  ARPA   Vlan30
    Internet  10.0.30.33              0   0050.7966.6803  ARPA   Vlan30
    Internet  10.0.30.44              0   0050.7966.6802  ARPA   Vlan30
    




    MultilayerSwitch# show ip interface brief
    Interface         IP-Address      OK? Method Status        Protocol
    ...
    Ethernet2/1       unassigned      YES unset  up            up
    Ethernet2/2       unassigned      YES unset  up            up
    ...
    Ethernet3/1       unassigned      YES unset  up            up
    Ethernet3/2       unassigned      YES unset  up            up
    ...
    Vlan20            10.0.20.1       YES manual up            up
    Vlan30            10.0.30.1       YES manual up            up
    




    Note: both sets of tabs and configuration above are from the same device. For the sake of organization, one set of tabs refer to the L3 functions and the other refers to the L2 functions.

    Summary

    This article discussed the three different options for Routing between VLANs. In each case, the hosts in communication behave exactly the same. In fact, the hosts have no visibility into how and what they are connected to.

    Each strategy above has its own benefits and limitations. Hopefully at this point you have a good idea of the options available to enable communication between hosts on different VLANs.

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    nice post – thanks for these nuggets!

    I really liked the detailed diagrams, the config examples and of course, the clear explanation. An excellent source for beginners in networking. Kudos!

    Hi! I’m from Portugal. Congrats for the way you teaching. Very well explained and easily to understand. Thanks!

    Can single host access multiple vlan using layer 2 switch without router ?

    It can in the scenario of virtualization. Imagine the host is a hypervisor and it has a virtual switch, in that case, the switch port on physical switch, that connects your virtual switch to physical switch needs to be configured as a trunk port.

    The answer Ed provided is true as long as the host has only a single interface and is connected to an access port for a single vlan.

    However, it is possible to have a host with multiple physical interfaces, each connecting to access ports on two vlans. Or if connected to a trunk port, if the host is vlan-aware, it can connect to multiple vlans on its sub-interfaces on the trunk port.

    Here’s an example using a Raspberry Pi with the the vlan package loaded. https://www.sbprojects.net/projects/raspberrypi/vlan.php

    Excellent post. The best ! Thank you.

    Well expained, however, just a few questions since I am stuck in this same scenario. My question is;
    How do I route between two different networks, I created subinterfaces in both routers and I can ping from network 10.0.0.0 to network 192.168.0.2 interface fa0/0 but I cannot ping the interface (which have subinterfaces) directly connected to the other LAN which is interface fa0/1. I used router rip, please help what am I missing in the configurations.

    Thank you

    Asi

    GOOOOOOD

    I really liked very nice

    Thank you kindly. Your method has assisted with really understanding this Vlan communication environment.
    I appreciate you a whole lot.

    Really nice explanation. I was having some trouble understanding the SVI part. Could you share an example with MAC addresses of the ports and SVIs and show how the L2 header changes?

    Thanks Ed.

    thank you for explanation.
    I have a question on the SVI.
    I could do the SVI connection as per ur explanation. however, when I trunk it with second layer three switch. then I couldn’t ping each other from another layer three switch. why could this happen?
    thank you

    What does it mean eth2/1, eth0/1 ?

    In Layer 3 Switch
    Add “ip routing” for enable the option of routing, this is important for test the communication of the VLANs.

    Didn’t see your comment until after I hit post. 🙂

    Thanks. I understand the routing of vlans well!

    wow.. Great article.. I have worked 4 years in networking domain and configured VLANs too.. But never understood the concept. This is an eyeopener for me. Thanks a lot for writing such a lovely article 🙂

    Very clear and detailed explanation 🙂

    thank you soooooo much

    Hi, I’m from Uruguay, very clear, detailed articles, it help me a lot, congrats !!

    Perfect, just one suggestion: It would be perfect to add also “show cdp neigh detail” to your article, because it would be very explanatory to see the output on switch (in the first scenario – router with sub-interfaces), how are IP addresses visible in the output. Also if L3 switch would have one access switch connected to it, it would be perfect to see “sh cdp neigh detail” on this access switch, how are IP addresses visible in the output. If someone know the answer, please post it :).
    There is another problem to discussion, what with the native vlan, and when, why and where to change configuration for that (router – switch, L3switch – L2switch).

    Hi Ed,
    I must say,you have done brilliant work , but I think there is a small error upper image logical and physical has to alternate 🙂

    Good informative post.
    We have a problem with a 2nd switch we have added to the network. Core switch is L3 and the additional one is L3 too. We cannot get this 2nd/new switch to route from its vlan20 across to vlan20 on the other switch. We have port going from vlan1 to vlan1 on the switches. tried tagging the ports in vlan20 but that doesnt work.
    How can we route the new switch vlan20 across to the other switch vlan20?
    Thanks

    Hi,
    Please tell me do i need to enable ip routing cmd in L3 switch to enable routing between vlan 20&v Vlan 30 ?

    Please update the HSRP topic with troubleshooting methods.

    One of the simplest description of how connecting VLANs for beginners

    Thank you

    Very nice explanation in general. But I got some specific questions. The first is why do we need a VLAN field in the entry of MAC table when VLAN has been set up in the switch? Is it only useful for blocking traffic when broadcasting within a domain? The second is, given a layer 3 switch scenario, if two hosts in the same VLAN domain, like A and B, would like to communicate with each other, only the MAC table will be used for forwarding, right? Although they go through a TCP/IP stack. Thanks : )

    Thanks for your reply : ) I am still a little bit confused about the first question. Since users in diff VLANs live in diff IP subnets, they should be able to judge that they are in diff domains before sending packets. Hence, in my point of view, the VLAN field seems useless in this scenario. And the only scenario in which this field would be useful should be preventing broadcast packets from leaking.

    An excellent counterexample. Thanks a lot. = )

    I have ever seen the configuration file of a layer 3 switch in which some interfaces are also configured to be sub-interfaces. Does it mean that a layer 3 switch can be in a mixed mode with sub-interface and SVI in use? Thanks = )

    Hi Ed,

    Great explanation, thanks. Have a quick question. For routing, hosts need to configure the SVI as default gateway, or we have to enable routing protocol on SVI and hosts. Would enabling proxy arp remove the need for turning on routing protocol or configuring default gateway on hosts?.
    Host A needs to talk to Host C, it sends out a arp request for host C’s IP address, can the layer 3 switch jump in and proxy for host C?
    Thanks,
    Rajesh

    I just want to say Thank you very much for such efforts, The graphics and method is amazing, please keep the good work.

    can I get explanation of encapsulation and decapsulation for HTTP or FTP request

    Thank you so much, this really helped me!!

    Hi! I am an aeronautical engineer trying to acquire a solid background in networking as well. This is undoubtedly the best explanation of communication among hosts belonging to different VLANs that I have seen so far. Many thanks!

    In scenario 1> where configured router on stick with 2 vlans on switch.
    switch connected with 1 host on each vlan.
    Switch vlan 10 – host 10.10.10.10 = Host A
    Switch vlan 20 – host 20.20.20.20 = Host B

    *Default gateway not configured on both hosts

    In Scenario 2> below is connectivity with 2 Switches instead 1 switch
    Host A (Vlan10) – Switch 1 – Router – Switch 2 – host B (Vlan 20)

    *The default gateway is not configured on switches
    ——————Question below for both topologies —————————————————

    What will happen when Host A sends packets (Broadcast) packet who is 20.20.20.20 Will router interface/interfaces receive that packet if yes what will be details of that packet and frame. will it reply with proxy-arp and connectivity will work?

    I am literally blown away how smooth your content flows. It’s really easy to follow and I’m so glad I’ve found your website. I am learning CCNA from zero understanding or experience. Slowly going through the CCNA 200 -301 book and was struggling with understanding Router on a stick and SVIs. I’ve now understood that they are are two separate options which can be used, but most importantly I get what the SVIs are used for.

    Thank you so so much for setting this website and providing this content.

    Ed, thanks so much for this. I am currently designing my home network around a refurbished Cisco Catalyst 3750. I want to set up several VLANs but only need communication between a small subset of that. I am planning on several SSIDs running to isolate the wireless devices even further (I hate Wi-fi). Anyway, how do I isolate the communications between VLANs without it bleeding over to other VLANs? Granted I am very new at this managed switch game.

    Ed, solid article and used it to great effect to build a few VLANs at home for a lab setting. Quick question on the topic, if the original purpose of VLANs is to provide a logical separation between networks (or subnets), why would you want to perform routing between the VLANs? I can’t find any good real world examples of a network that has multiple VLANs with routing in between them.

    Hi Ed Harmoush, First of all I want to say my special thanks for your article. It is clearly described how to build different VLANS in a Cisco switch it self. Using Cisco packet tracer I practically did it it went well for Multilayer Switches, Layer 3 Switches. I wonder that How I solve this problem in Below layer 3 Switches till example Cisco 2900 series or 2800 series or 1800 series switches. Your solution will be highly appreciated. Thank you in advance. Looking forward to hear from you soon../Sisira

    Thank you very much for the detailed explanation, i had a lot of doubts about vlans and svi, and with this post you made them clear to me. Really thank you so much <3

    awesome post! Thanks. The layer 3 switch routing answered a question I’ve had for ages.
    Love the diagrams AND the switch/router configs as well. makes it REALLY clear

    I really loved it!. I need not to look for any other 100’s of sites!. Its all in one best and complete book!. I would say a Bible for me!. Thank you very much!!. Appreciate your knowledge and wisdom!!.

    Hi,
    Thanks for this topic. I always gets confused but when i read is straight forward and clear… just one question if you enable ip routingo on l3 switch you enable all vlans communication but is there any command to route just like 2 vlan from 5 vlans actually?

    Impressive article Ed. Awesome explanatory skills !!
    In your example regarding router’s sub-interfaces, just one question to shed light:
    was this the Mac address : aabb.cc00.0211 of router’s physical port,
    or switchport’s virtual port ( ie switch’s trunk port) ;

    To be precise, if the Mac address aabb.cc00.0211 belonged to the Switch or the Router ?
    Many Thanks

    wow , thank you ED !
    Awesome youtubes , very bullet-sharp, that smashed right through my confusion barriers ;

    I had the feeling it was router’s own interface , because as a “parent” it could spawn those sub-interfaces;

    But I was doubtful, because the switch apparently could enable somebody else’ interface as a trunk port ( given aabb.cc00.0211 belonged to the router, not the switch ) ;

    Then I realised something fundamental: that the switchport interface could become a trunk interface, even if the neighboring interface is not a trunk interface.  Many thanks Ed for your time , and your extraordinary genius explanatory lessons. Always enjoy reading up your wisdom !!

    I really enjoyed reading your materials !!!

    So, how would I configure a fiber ports then if I have a 2921 router and 3560 switch and I want to use fiber to move data between the 2 devices?

    awesome and clear. Total newb question..but on the last pic of the show ip route….why does the subnet change to /32 on th SVI Gateway instead of /24. Thanks so much!!!

    Hi Ed,

    Great article, explained very well. Perhaps an idea for another article or series might be expanding on inter-vlan routing and RoaS with Firewalls.

    I’d be interested in hearing your views and experiences of the speed vs security trade-off, multiple DMZ type vlans and scenarios of filtering and protecting backend databases, middle-tier applications, and front end web services. That may also open a can of worms into “Load Balancers”, “Application Delivery Controllers” and “Reverse Proxies” too.

    This is a very nice post, easy to understand and clearly. It helps me so much. Thanks!

    Damn, this is good explanation.

    In the MLS scenario, I was looking at the ARP and MAC tables, and found myself wondering,…when the packet crosses vlans, don’t the SRC and DST MAC addresses have to be changed when the L2 header is rewritten? And if so, it seems there aren’t enough MACs known in the CAM table (like, the MACs that were previously representing the router interfaces are gone, for example) I’m confused about how this will work, with regard to that. (I’ll take a look at the hyperlink about L2 stuff that is posted)

    Hi, I must say I’m really impressed with your explanation, I read hundreds of articles to understand vlan but found nothing as detailed and simple as yours. But I have got a question, consider 3 esxi servers which needs to be connected to a physical switch through ethernet cables. There are three vlans, each esxi server is involved in all these three vlans. All three esxi servers and switch are on same network. How do I configure this in order to be able to ping between esxi servers?

    As of now I created 3 vlans on the physical switch in switchport mode access and assigned them to 9 different switchports then connected to the servers with ethernet cable. I also enabled IP routing and configured SVI on all three vlans with an IP address on the same network, but ping between server failed. Please advise on what can be done, I’m new to this.