chap 4 - inplementing inter-vlan routing

8/13/2019 Chap 4 - Inplementing Inter-VLAN Routing

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1Chapter 4

Chapter 4 - Implementing Inter-VLAN RoutingObjectives

Configure inter-VLAN routing on a router to enablecommunications between end-user devices onseparate VLANs

Configure CEF-based Multi-layer switching

Troubleshoot common inter-VLAN connectivityissues.


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2Chapter 4

Inter-VLAN Routing

Fa0/1

S2

PC1

172.17.10.21/24(VLAN 10)

Fa0/11

PC2

172.17.20.22/24(VLAN 20)

PC3

172.17.30.23/24(VLAN 30)

Fa0/18

S3 S1Fa0/1

Fa0/6

Fa0/2 Fa0/2

Fa0/3

Fa0/1

Fa0/4Fa0/3Fa0/4

Fa0/4

Fa0/2 Fa0/3

Inter-VLAN routing can be performedby connecting differentphysical routerinterfacesto different physical switchports.

The switch ports connect to the router

in access mode, and different staticVLANs are assigned to each portinterface.

Each switch interface would be

assigned to a different static VLAN.Each router interface can then accepttraffic from the VLAN associated withthe switch interface that it is connectedto, and traffic can be routed to theother VLANs connected to the other

interfaces.

R1Link to VLAN 20

Link to VLAN 30

Link to VLAN 10

Computer Computer Computer


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3Chapter 4

Router-on-a -Stick

Fa0/1

S2

PC1

172.17.10.21/24(VLAN 10)

Fa0/11

Computer

PC2

172.17.20.22/24(VLAN 20)

Computer

PC3

172.17.30.23/24(VLAN 30)

Fa0/18

S3 S1Fa0/1

Fa0/6

Computer

Fa0/2 Fa0/2

Fa0/3

Fa0/1

Fa0/4Fa0/3Fa0/4

Fa0/4

Fa0/2 Fa0/3

R1R1 - Fa0/0 Sub-interfaces

Fa0/0.10 172.17.10.1 Default Gateway to VLAN 10



Fa0/0

"Router-on-a-stick" is a type of routerconfiguration in which a single physicalinterface routes traffic between multipleVLANs on a network.

Sub-interfaces are configured fordifferent subnets corresponding to theirVLAN assignment to allow logical routingbefore data frames are VLAN taggedand sent back out the physical interface.

Sub-interfaces are multiple virtualinterfaces, associated with onephysicalinterface. These sub-interfaces areconfigured with an IP address and VLANassignment to operate on a specific VLAN.


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4Chapter 4

Fa0/1

S2

PC1

172.17.10.21/24(VLAN 10)

Fa0/11

Computer

PC2

172.17.20.22/24(VLAN 20)

Computer

PC3

172.17.30.23/24(VLAN 30)

Fa0/18

S3 S1Fa0/1

Fa0/6

Computer

Fa0/2 Fa0/2

Fa0/3

Fa0/1

Fa0/4Fa0/3Fa0/4

Fa0/4

Fa0/2 Fa0/3

R1Fa0/0.10

172.17.10.1/24

Fa0/0.30

172.17.30.1/24

Configure Router Interfaces

To avoid confusion, name the sub-interface after the VLAN to whichit is attached e.g. Fa0/0.10 isconnected to VLAN 10

Fa0/5

Sub-Interface Configuration


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5Chapter 4

Sub-Interface Configuration

Fa0/1

S2

PC1

172.17.10.21/24(VLAN 10)

Fa0/11

Computer

PC2

172.17.20.22/24(VLAN 20)

Computer

PC3

172.17.30.23/24(VLAN 30)

Fa0/18

S3 S1Fa0/1

Fa0/6

Computer

Fa0/2 Fa0/2

Fa0/3

Fa0/1

Fa0/4Fa0/3Fa0/4

Fa0/4

Fa0/2 Fa0/3

R1Fa0/0.10

172.17.10.1/24

Fa0/0.30

172.17.30.1/24

Fa0/5

S1 Fa0/5 must be configured

as a trunkto allow it to carrytagged data from multiple

VLANs


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6Chapter 4

Interface and Sub-Interface Comparison


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8/168Chapter 4

Computer

Computer

Computer

Computer

Computer

ManagementVLAN 99

172.17.99.10/24

StudentVLAN 20

172.17.20.22/24

StudentVLAN 20

172.17.20.25/24

GuestVLAN 30

172.17.30.26/24

GuestVLAN 30

172.17.30.23/24

Fa0/1

Fa0/1 Fa0/3

Fa0/3Fa0/18 Fa0/18

Fa0/6 Fa0/6

Connecting VLANs Using L3 Switch

SVI VLAN20

SVI VLAN30

SVI VLAN99

Layer 3 Switch

Switch Virtual Interface(SVI) is a logical interface configured for a

specific VLAN, and is used by layer 3 switches to route betweenVLANs or to provide IP host connectivity to a switch.

A Layer 3 switch has theability to routetransmissions betweenVLANs.

The process is the sameas when using a separaterouter, except that theSVIs act as the routerinterfaces for routing thedata between VLANs.


9/169Chapter 4

Layer-3 Switch SVI Configuration

Fa0/1

S2

PC1

172.17.10.21/24(VLAN 10)

Fa0/11

Computer

PC2

172.17.20.22/24(VLAN 20)

Computer

PC3

172.17.30.23/24(VLAN 30)

Fa0/18

S3 S1Fa0/1

Fa0/6

Computer

Fa0/2 Fa0/2

Fa0/3

Fa0/1

Fa0/4Fa0/3Fa0/4

Fa0/4

Fa0/2 Fa0/3

S1(config)#int vlan 10S1(config-if)#ip add 172.17.10.1 255.255.255.0S1(config-if)#int vlan 20S1(config-if)#ip add 172.17.20.1 255.255.255.0S1(config-if)#int vlan 30S1(config-if)#ip add 172.17.30.1 255.255.255.0

S1(config)#ip routingS1(config)#exitS1#sh ip route

172.17.0.0/24 is subnetted, 3 subnetsC 172.17.10.0 is directly connected, Vlan10C 172.17.20.0 is directly connected, Vlan20C 172.17.30.0 is directly connected, Vlan30

Configure SVI Addresses:

Configure Routing:


10/1610Chapter 4

Layer-3 Switch Routed Port Configuration

Fa0/1

S2

PC1

172.17.10.21/24(VLAN 10)

Fa0/11

Computer

PC2

172.17.20.22/24(VLAN 20)

Computer

PC3

172.17.30.23/24(VLAN 30)

Fa0/18

S3 S1Fa0/1

Fa0/6

Computer

Fa0/2 Fa0/2

Fa0/3

Fa0/1

Fa0/4Fa0/3Fa0/4

Fa0/4

Fa0/2 Fa0/3

Configure Routed Port:

Fa0/0172.17.40.1/30

R1Fa0/5

172.17.40.2/30

S1(config)#int fa0/5S1(config-if)#no switchport

S1(config-if)#ip add 172.17.40.2 255.255.255.0S1(config-if)#no shS1(config-if)#exitS1(config)#router eigrp 1S1(config-router)#network 172.17.40.0 0.0.0.3

A routed port has the following characteristics and functions:

Physical switch port with Layer 3 capabilityNot associated with any VLANServes as the default gateway for devices out that switch portLayer 2 port functionality must be removed before it can beconfigured


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Layer 3 Switch Processing

Layer 3 switching software employs

a distributed architecture in whichthe control pathand data patharerelatively independent.

The control path code, such asrouting protocols, runs on the route

processor, whereas most of the datapackets are forwarded by theEthernet interface module and theswitching fabric.

Layer 3 switching uses one of these two methods, depending on the platform:

Route caching:Also known as flow-based or demand-based switching, a Layer 3route cache is built in hardware, since the switch seestraffic flow into theswitch.

Topology-based:Information from the routing table is used to populate the routecache regardless of traffic flow. The populated route cache is called the

forwarding information base (FIB). CEF builds the FIB.


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Multilayer Switch Packet Forwarding Process

CEF expediently switches data packets to their destination.It cachesinformation generated by the Layer 3 routingengine.

CEF caches routing information in the ForwardingInformation Base(FIB), and caches Layer 2 next-hopaddresses for all FIB entries in an adjacency table.

Because CEF maintains multiple tables for forwarding

information, parallel paths can exist and enable CEF to loadbalance per packet.


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When traffic cannot be processed in hardware, the traffic

must receive processing in software by the Layer 3 engine. Anumber of different packet types may force the Layer 3engine to process them:

1. IP packets that use IP header options. (Packets that useTCP header options are switched in hardware because

they do not affect the forwarding decision.)2. Packets that have an expiring IP Time to Live (TTL)

counter.3. Packets that are forwarded to a tunnel interface.4. Packets that arrive with non-supported encapsulation

types.5. Packets that are routed to an interface with non-

supported encapsulation types.6. Packets that exceed the maximum transmission unit

(MTU) of an output interface and must be fragmented.

Multilayer Switch Packet Forwarding Process


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CEF Based MLS switching

ARP x 1

ARP Throttling (2 seconds)


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Configure & Verify CEF

If CEF is enabled globally, it is automatically enabled on all interfaces aslong as IP routing is enabled on the device.

CEF can be enabled/disabled on a per interface basis.

Cisco recommends that CEF be enabled on all Layer 3 interfaces.

Configure CEF:

S1 (conf)#ip cefS1 (conf-if)#ip route-cache cef

Verify CEF:

S1#sh ip cefS1#sh ip cef fa0/1 detailS1#sh adjacency fa0/1 detailS1#show ip cef summaryS1#show ip cef vlan 10


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