Describing Routing
Between VLANs
Implementing Inter-VLAN Routing
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—4-1
Inter-VLAN Routing Using an External
Router
 Provide a single trunk link
from switch to router.
 Router physical interface is
divided into logical
subinterfaces.
 Each router subinterface acts
as gateway for one VLAN.
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—4-2
Router on a Stick
1. Traffic from PC1 enters
switch on VLAN 10.
2. Crosses trunk to router,
arriving on subinterface
FA0/0.10 (VLAN 10).
3. Router determines that
route to PC2 is through
subinterface FA0/0.20.
4. Crosses trunk to switch,
arriving on VLAN 20.
5. Switched to PC2.
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—4-3
External Router Configuration
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—4-4
External Router: Advantages and
Disadvantages
Advantages:
 Works with any switch, since
Layer 3 services are not
required on the switch.
 Implementation is simple.
 The router provides
communication between
VLANs.
© 2009 Cisco Systems, Inc. All rights reserved.
Disadvantages:
 The router is a single point of
failure.
 Single traffic path may
become congested.
 Latency may be introduced as
frames leave and reenter the
switch chassis multiple times,
and the router makes
software-based routing
decisions.
SWITCH v1.0—4-5
Routed vs. Switched Campus
Architecture
 When switching was fast and routing was slow, campus networks
were switched.
 Today routing is almost as fast as switching; routing solves Layer
2 loops issues and helps isolated VLANs.
 Switches require IP addresses.
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—4-6
Switch Virtual Interfaces
Routers use interfaces or
subinterfaces to
interconnect multiple
VLANs.
© 2009 Cisco Systems, Inc. All rights reserved.
Multilayer switches use
SVIs for routing between
VLANs.
SWITCH v1.0—4-7
Configuration of Inter-VLAN Routing on a
Multilayer Switch
Basic tasks:
 Identify which VLANs require a Layer 3 gateway.
 Create a VLAN on multilayer switch if it does not already exist.
 Create an SVI interface for each VLAN.
 Configure the SVI interface with an IP address.
 Enable the SVI interface.
 Enable IP routing on the multilayer switch.
 Determine whether a dynamic routing protocol is needed.
 Configure a dynamic routing protocol if needed.
 Identify any switch ports that require autostate exclude.
 Configure autostate exclude on identified switch ports.
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—4-8
SVI Configuration
 Enable IP routing.
 Configure an SVI for each
VLAN.
 Configure an IP address.
 Enable the SVI.
 Configure the routing
protocol.
switch(config)# ip routing
switch(config)# interface vlan10
switch(config-if)# ip address 10.1.10.1 255.255.255.0
switch(config-if)# no shutdown
switch(config)# interface vlan20
switch(config-if)# ip address 10.1.20.1 255.255.255.0
switch(config-if)# no shutdown
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—4-9
SVI autostate exclude Command
The line state of an SVI is in the up state when:
 The VLAN exists and is active in the VLAN database on the
switch
 The VLAN interface exists and is not administratively down
 At least one Layer 2 (access or trunk) port exists, has a link in the
up state on this VLAN, and is in the spanning-tree forwarding
state on the VLAN
SVI autostate exclude can be used to remove a port from linestate up-and-down calculation.
switch(config)# interface fastethernet 0/24
switch(config-if)# switchport auto-state exclude
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—4-10
Routed Ports on a Multilayer Switch
 Physical switch port with
Layer 3 capability
 Not associated with any
VLAN
 Requires removal of Layer 2
port functionality
 Configured like a router
interface but does not support
VLAN subinterfaces
 Used when a switch has one
port per VLAN or subnet only
 Useful for point-to-point Layer
3 switch links
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—4-11
Configuration of a Routed Interface
 Enable IP routing.
 Disable Layer 2 processing on interface.
 Configure IP address.
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—4-12
Layer 2 EtherChannel vs. Layer 3
EtherChannel
 Layer 2 EtherChannel
bundles access or trunk ports
between switches or other
devices (e.g., servers).
 Layer 3 EtherChannel
bundles routed ports between
switches.
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—4-13
Configuration of Layer 3 EtherChannel
The no switchport command is applied both on the
physical ports and on the EtherChannel interface.
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—4-14
Verification of SVIs and Routed
Interfaces
switch# show ip interface fastethernet0/24
FastEthernet0/24 is up, line protocol is up
Internet address is 10.1.10.1/24
Broadcast address is 255.255.255.255
Address determined by setup command
MTU is 1500 bytes
Helper address is not set
Directed broadcast forwarding is disabled
Multicast reserved groups joined: 224.0.0.10
Outgoing access list is not set
Inbound access list is not set
Proxy ARP is enabled
Local Proxy ARP is disabled
Security level is default
Split horizon is enabled
ICMP redirects are always sent
ICMP unreachables are always sent
ICMP mask replies are never sent
IP fast switching is enabled
IP CEF switching is enabled
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—4-15
Routing Protocol Configuration
 Enable IP routing.
 Configure routing process.
 Disable auto-summary.
 Configure routed networks.
 Configure active interfaces.
 Configure summarization.
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—4-16
Verification of Routing Protocol
switch# show ip route
Codes: C - connected, S - static, 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
i - IS-IS, su - IS-IS summary, 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
D
D
C
10.0.0.0/8 is variably subnetted, 13 subnets, 2 masks
10.1.3.0/24 [90/28416] via 10.1.10.10, 08:09:49, Vlan10
10.1.2.0/24 [90/28416] via 10.1.10.10, 08:09:49, Vlan10
10.1.10.0/24 is directly connected, Vlan10
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—4-17
DHCP Service
Clients in access VLANs need
DHCP service.
DHCP service can be provided
by the distribution switches,
acting as gateways, or external
DHCP server elsewhere in the
network.
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—4-18
About DHCP
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—4-19
DHCP Configuration
 Configure DHCP pool with network,
mask, and other parameters.
 Configure excluded addresses.
 Pool is selected when DHCP request
is received from matching subnet.
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—4-20
DHCP with the ip helper Command
 Used if the DHCP server is not in
the same broadcast domain as
the client.
 Configure ip helper command on
the incoming interface to forward
DHCP requests via unicast to
DHCP server.
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—4-21
Verification of the DHCP Operation
switch# show ip dhcp binding
Bindings from all pools not associated with VRF:
IP address Client-ID/
Lease expiration
Hardware address/
User name
10.1.10.21 0100.1bd5.132a.d2 Jun 25 2009 06:09 AM
10.1.10.22 0100.4096.a46a.90 Jun 25 2009 09:40 AM
10.1.10.23 0100.4096.aa98.95 Jun 25 2009 11:28 AM
Type
Automatic
Automatic
Automatic
switch# debug ip dhcp server packet
DHCPD: DHCPDISCOVER received from client 0100.1bd5.132a.d2 on
interface Vlan6.
DHCPD: Sending DHCPOFFER to client 0100.1bd5.132a.d2 (10.1.10.21).
DHCPD: broadcasting BOOTREPLY to client 001b.d513.2ad2.
DHCPD: DHCPREQUEST received from client 0100.1bd5.132a.d2.
DHCPD: Sending DHCPACK to client 0100.1bd5.132a.d2 (10.1.10.21).
DHCPD: broadcasting BOOTREPLY to client 001b.d513.2ad2.
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—4-22
Summary
 Inter-VLAN communication requires a Layer 3 device; an external
router can perform this function.
 SVI is a virtual interface providing Layer 3 processing for a
particular VLAN, similar to what a router interface does.
 To be active, the SVI number must match an existing VLAN on
the local switch.
 A routed interface has Layer 3 functionality.
 SVIs and routed interfaces are verified using the same
commands.
 On multilayer switches, you can aggregate Layer 3 links using
Layer 3 EtherChannels.
 Once you have configured a Layer 3 interface, you can enable
routing.
 DHCP functions can be configured with Cisco IOS Software.
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—4-23
© 2009 Cisco Systems, Inc. All rights reserved.
SWITCH v1.0—4-24