Inter Vl an Routing

8/10/2019 Inter Vl an Routing

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Inter-VLAN routing

:44526754456830


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Topics

Routing network traffic between VLANs

Configuring a router for routing between

VLANs Troubleshoot inter-VLAN connectivity issues


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LAN Design

Basic Switch

Concepts

VLANs

VTP

STP

Inter-VLAN

routing

Wireless


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Inter VLAN routingSwitch keepsVLANs

separate.

Router can

route betweenVLANs

(subnets).


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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 ownVLAN.

Routing is the same as routing between any

subnets.


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PC1 has a

packet for

192.168.2.19

different

subnet

ARP request to

find MAC addressof default gateway

Send packet to

F0/0 of router

Router looks up

192.168.2.0 network

port F0/1

Router sends out

packet through F0/1 to

switchport F0/2

Switchport F0/2 is

on VLAN30 and

forwards to PC3.


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Trunks

between

switches.

No trunk to

router.

No tags on

frames to

router.


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Traditional Vs Router-on-stick (1)

Port Limits

Physical interfaces are configured to have oneinterface per VLAN on the network.

On networks with many VLANs, using a single router toperform inter-VLAN routing is not possible.

Instead, you could use multiple routers to perform inter-VLAN routing for all VLANs if avoiding the use ofsubinterfaces is a priority.

Subinterfaces allow a router to scale toaccommodate more VLANs than the physicalinterfaces permit..


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Performance

Because there is no contention for bandwidth on separate physical

interfaces, physical interfaces have better performance whencompared to using subinterfaces.

Traffic from each connected VLAN has access to the full bandwidthof the physical router interface connected to that VLAN for inter-VLAN routing.

When subinterfaces are used for inter-VLAN routing, the trafficbeing routed competes for bandwidth on the single physicalinterface.

On a busy network, this could cause a bottleneck forcommunication.


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Access Ports and Trunk Ports

Connecting physical interfaces for inter-VLAN

routing requires that the switch ports be configuredas access ports.

Subinterfaces require the switch port to be

configured as a trunk port so that it can accept

VLAN tagged traffic on the trunk link.


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Cost

Financially, it is more cost-effective to use subinterfacesoverseparate physical interfaces.

Routers that have many physical interfaces cost more thanrouters with a single interface.

Additionally, if you have a router with many physical interfaces,each interface is connected to a separate switch port, consumingextra switch ports on the network.

Switch ports are an expensive resource on high performanceswitches.


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Complexity

Using subinterfaces for inter-VLAN routing results in a less complexphysicalconfigurationthan using separate physical interfaces,

because there are fewer physical network cables interconnecting therouter to the switch.

On the other hand, using subinterfaces with a trunk port results in amore complex software configuration, which can be difficult totroubleshoot. If one VLAN is having trouble routing to other VLANs, you need to

check to see if the switch port is configured to be a trunk and verifythat the VLAN is not being filtered on any of the trunk links before itreaches the router interface.

You also need to check that the router subinterface is configured touse the correct VLAN ID and IP address for the subnet associatedwith that VLAN.


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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 VLANson the same physical port.

But each VLAN needs a different IP address.

So use subinterfaces, each with its own IPaddress.


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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 ontrunk.


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Subinterfaces

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 172.17.10.1255.255.255.0

(add any other subinterfaces)

R1(config-subif)#interface f0/0 R1(config-if)#no shutdown


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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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Configuring Traditional Inter VLAN routing


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Configure router-on-a-stick inter VLAN routing


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Multilayer switch

A multilayer switch can

route between VLANs.


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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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Troubleshooting inter VLAN

routing switch issues (1)

Incorrect VLAN


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routing switch issues (2)

Not configure as a trunk


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routing router issues

Incorrect VLAN on a stick


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routing IP addressing issues (1)

Incorrect IPAddressing


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routing IP addressing issues (2)

Incorrectsubnet mask


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Inter Vl an Routing