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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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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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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
Fa0/0.20 172.17.20.1 Default Gateway to VLAN 20
Fa0/0.30 172.17.30.1 Default Gateway to VLAN 30
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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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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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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Interface and Sub-Interface Comparison
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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.
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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:
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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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Any
Questions?