Routing between LANs or Inter VLAN routing ... same fight! For a router to route a packet from one broadcast domain to another it must have an interface configured in each of them

I will try through this article to clarify this notion of "Inter-Vlan routing". To do this, we will take as a base a router connected to two broadcast domains, Subnet A (192.168.0.0/24) and Subnet B (192.168.1.0/24).

logical topology
logical topology

This is what we can call logical topology. It takes up the subnets (broadcast domains) and the equipment that interconnects them, the 
router

From there, there are several ways to realize Inter VLAN Routing

The physical method of  Inter VLAN Routing


Undoubtedly the most obvious method. It is enough to connect each interface of the router to a switch for example and to configure him the appropriate address.

Router(config)#interface fastEthernet 0/0
Router(config-if)#ip address 192.168.0.1 255.255.255.0
Router(config-if)#no shutdown
Router(config)#interface fastEthernet 0/1
Router(config-if)#ip address 192.168.1.1 255.255.255.0 
Router(config-if)#no shutdown

Without any suprise, one notices that two connected roads appear in the routing table ...

Router#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
 D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
 N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
 i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
 * - candidate default, U - per-user static route, o - ODR
 P - periodic downloaded static route

Gateway of last resort is not set

C 192.168.0.0/24 is directly connected, FastEthernet0/0
C 192.168.1.0/24 is directly connected, FastEthernet0/1
Router#
Advantages:
·         Simplicity of implementation
·         Does not use any advanced technology and therefore can be implemented on
      almost any equipment (a Linux router and two unmanaged switches for example).
Disadvantages:
·         It is difficult to adapt to a large number of subnets (one physical interface 
      per subnet)
·         High cost of hardware, each subnet also requires an additional switch.

Integration of VLANs

VLANs (Virtual LANs) allow, among other things, to divide the same switch into several broadcast domains. In the present case, therefore, the previous principle is recreated, namely two distinct scattering domains. In each of its VLANs one places a part of the interfaces of the switch, and then one connects each interface of the router to an
interface placed in the desired VLAN.
Switch Configuration:
Switch(config)#vlan 10
Switch(config-vlan)#name SUBNET-A
Switch(config-vlan)#exit
Switch(config)#vlan 20
Switch(config-vlan)#name SUBNET-B
Switch(config-vlan)#exit
Switch(config)#interface range fastEthernet 0/1 - 12
Switch(config-if-range)#switchport mode access 
Switch(config-if-range)#switchport access vlan 10
Switch(config-if-range)#exit
Switch(config)#interface range fastEthernet 0/13 - 24
Switch(config-if-range)#switchport mode access 
Switch(config-if-range)#switchport access vlan 20
Switch(config-if-range)#exit
Switch(config)#exit
Switch#
Router Configuration:
Router(config)#interface fastEthernet 0/0
Router(config-if)#ip address 192.168.0.1 255.255.255.0
Router(config-if)#no shutdown
Router(config)#interface fastEthernet 0/1
Router(config-if)#ip address 192.168.1.1 255.255.255.0
Router(config-if)#no shutdown
Result:
Router side nothing changes ... (the configuration is identical)
Router#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
 D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
 N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
 i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
 * - candidate default, U - per-user static route, o - ODR
 P - periodic downloaded static route

Gateway of last resort is not set

C 192.168.0.0/24 is directly connected, FastEthernet0/0
C 192.168.1.0/24 is directly connected, FastEthernet0/1
Router#
 
On the switch side, the interfaces are placed in two different VLANs. A machine connected to the VLAN10 will therefore not be able to communicate directly with a VLAN20 machine. It must go through the router ... as in the 100% physical version. 
Advantages:
·         Simplicity of positioning on the router side.
·         It is no longer necessary to have a switch per subnet.
Disadvantages:
·         It is difficult to adapt to a large number of subnets (it takes a physical interface by subnet) on the router side but also on the switch side!
·         You must use a manageable switch that supports VLAN
Router on-a-stick method:


The idea is that we want to limit the number of physical connections and share them among several broadcast domains. However on the switch side, it is impossible to place an interface in two VLANs This is where the concept of Trunk comes in!
A trunk is a link on which the Ethernet frames are tagged (or encapsulated according to the protocol used). This tag (or additional header) contains, among other things, the identifier of the VLAN from which the frame originates, so that the equipment which receives it can reassign it to the correct broadcast domain.
A switch can be configured to establish a trunk on an interface ... but not a router ... On this side, it is necessary to respect the basic principle ... one interface per domain of diffusion. From then on, the concept of sub-interface is used. To put it simply, it's a bit like splitting the physical interface into several pieces. Each of its pieces is then configured to accept frames tagged with a defined VLAN ID.

Switch Configuration:
Switch(config)#vlan 10
 Switch(config-vlan)#name SUBNET-A
 Switch(config-vlan)#exit
 Switch(config)#vlan 20
 Switch(config-vlan)#name SUBNET-B
 Switch(config-vlan)#exit
 Switch(config)#interface range fastEthernet 0/2 - 12
 Switch(config-if-range)#switchport mode access
 Switch(config-if-range)#switchport access vlan 10
 Switch(config-if-range)#exit
 Switch(config)#interface range fastEthernet 0/13 - 24
 Switch(config-if-range)#switchport mode access
 Switch(config-if-range)#switchport access vlan 20
 Switch(config-if-range)#exit
 Switch(config)#int fastEthernet 0/1
 Switch(config-if)#switchport mode trunk
 Switch(config-if)#^Z
 Switch#
Router Configuration:
Router(config)#interface fastEthernet 0/0
Router(config-if)#no shutdown
Router(config-if)#exit
Router(config)#interface fastEthernet 0/0.10
Router(config-subif)#encapsulation dot1Q 10
Router(config-subif)#ip address 192.168.0.1 255.255.255.0
Router(config-subif)#exit
Router(config)#interface fastEthernet 0/0.20
Router(config-subif)#encapsulation dot1Q 20
Router(config-subif)#ip address 192.168.1.1 255.255.255.0
Router(config-subif)#^Z
Router#
 
Result:
Without the slightest surprise, the router has an interface in each of the broadcast domains. However, this time it is virtual interfaces, subdivisions of the physical interface.
Note: the name of the sub-interface (Fa0 / 0.10 for example) has no link with the associated VLAN identifier, it is the "encapsulation dot1q xxx" command that associates the VLAN xxx with the sub-interface in question.

Router#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
 D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
 N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
 i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
 * - candidate default, U - per-user static route, o - ODR
 P - periodic downloaded static route
Gateway of last resort is not set
C 192.168.0.0/24 is directly connected, FastEthernet0/0.10
C 192.168.1.0/24 is directly connected, FastEthernet0/0.20
Router#

Advantages:
·         A single physical link for several broadcast domains
·         Easily expandable, just create a new sub-interface on the router and a new vlan on the switch.
Disadvantages:
·         All broadcast domains go through the same physical link that becomes the bottleneck
·         A manageable switch must be used that supports the dot1q trunks and a router capable of supporting the dot1q protocol as well. 
The multi-layer switch


Definitely the most used method for inter-vlan routing. To simplify to the limit of the cartoon, it is a bit like setting up a router in a switch and connecting them internally. Of course it is more complex than that, but the general idea is easily understandable.
A multi-layer switch is a switch (serious?). But capable of doing much more than its level 2 job. By configuring it can activate IP routing which turns it into a potential router.
For the configuration, the original principle remains valid, we need an interface per domain of diffusion ... but here, since everything is internal to the MLS (multi-layer switch), one no longer uses the physical interfaces at all. We use SVIs (Switched Virtual Interface) which are neither more nor less than "VLAN interface". The access doors of the switch to send frames in the different broadcast domains.
There is not much to do if not:
·         Creating VLANs
·         Place switch interfaces in VLANs
·         Enable IP routing
·         Create and configure IVRs
Switch(config)#vlan 10
Switch(config-vlan)#name SUBNET-A
Switch(config-vlan)#exit
Switch(config)#vlan 20
Switch(config-vlan)#name SUBNET-B
Switch(config-vlan)#exit
Switch(config)#interface range fastEthernet 0/1 - 12
Switch(config-if-range)#switchport mode access
Switch(config-if-range)#switchport access vlan 10
Switch(config-if-range)#exit
Switch(config)#interface range fastEthernet 0/13 - 24
Switch(config-if-range)#switchport mode access
Switch(config-if-range)#switchport access vlan 20
Switch(config-if-range)#exit
Switch(config)#ip routing
Switch(config)#interface vlan 10
Switch(config-if)#ip address 192.168.0.1 255.255.255.0
Switch(config-if)#no shutdown
Switch(config-if)#exit
Switch(config)#interface vlan 20
Switch(config-if)#ip address 192.168.1.1 255.255.255.0
Switch(config-if)#no shutdown
Switch(config-if)#^Z
Switch#

Finally, the MLS has an interface in each of the broadcast domains and its routing table takes up the two connected routes.

Switch#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
 D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
 N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
 i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
 * - candidate default, U - per-user static route, o - ODR
 P - periodic downloaded static route
Gateway of last resort is not set
C 192.168.0.0/24 is directly connected, Vlan10
C 192.168.1.0/24 is directly connected, Vlan20
Switch#

Note: For an SVI to be UP / UP (and consequently the associated connected route appears in the routing table, there must be at least one active interface in the VLAN concerned (or a Trunk that leaves Pass this VLAN).

Advantages:
·         Inter-vlan routing is no longer dependent on the bandwidth of a physical link.
·         Easily expandable, it is enough to create a new VLAN and a new SVI associated.
Disadvantages:
·         MLSs are much more expensive than a standard switch or router (or even both).
Conclusion: 
Whatever the method used, the equipment in charge of the routing MUST have a connected interface in each of the diffusion domains, be it physical or virtual!



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