1. Implementing Inter-VLAN Routing
Describing Routing Between VLANs
Implementing Inter-VLAN Routing

2. 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.

3. Router on a Stick Traffic from PC1 enters switch on VLAN 10.
Crosses trunk to router, arriving on subinterface FA0/0.10 (VLAN 10).
Router determines that route to PC2 is through subinterface FA0/0.20.
Crosses trunk to switch, arriving on VLAN 20.
Switched to PC2.

4. External Router Configuration

5. External Router: Advantages and Disadvantages
Works with any switch, since Layer 3 services are not required on the switch.
Implementation is simple.
The router provides communication between VLANs.
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.

6. 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.

7. Switch Virtual Interfaces
Routers use interfaces or subinterfaces to interconnect multiple VLANs.
Multilayer switches use SVIs for routing between VLANs.

8. 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.

9. 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
switch(config-if)# no shutdown
switch(config)# interface vlan20
switch(config-if)# ip address

10. 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 line-state up-and-down calculation.
switch(config)# interface fastethernet 0/24
switch(config-if)# switchport auto-state exclude

11. 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

12. Configuration of a Routed Interface
Enable IP routing.
Disable Layer 2 processing on interface.
Configure IP address.

13. 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.

14. Configuration of Layer 3 EtherChannel
The no switchport command is applied both on the physical ports and on the EtherChannel interface.

15. Verification of SVIs and Routed Interfaces
switch# show ip interface fastethernet0/24
FastEthernet0/24 is up, line protocol is up
Internet address is /24
Broadcast address is
Address determined by setup command
MTU is 1500 bytes
Helper address is not set
Directed broadcast forwarding is disabled
Multicast reserved groups joined:
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

16. Routing Protocol Configuration
Enable IP routing.
Configure routing process.
Disable auto-summary.
Configure routed networks.
Configure active interfaces.
Configure summarization.

17. 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
/8 is variably subnetted, 13 subnets, 2 masks
D /24 [90/28416] via , 08:09:49, Vlan10
D /24 [90/28416] via , 08:09:49, Vlan10
C /24 is directly connected, Vlan10

18. 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.

19. About DHCP
Lesson Aim
<Enter lesson aim here.>

20. 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.

21. 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.

22. Verification of the DHCP Operation
switch# show ip dhcp binding
Bindings from all pools not associated with VRF:
IP address Client-ID/ Lease expiration Type
Hardware address/
User name
bd5.132a.d2 Jun :09 AM Automatic
a46a.90 Jun :40 AM Automatic
aa Jun :28 AM Automatic
switch# debug ip dhcp server packet
DHCPD: DHCPDISCOVER received from client bd5.132a.d2 on interface Vlan6.
DHCPD: Sending DHCPOFFER to client bd5.132a.d2 ( ).
DHCPD: broadcasting BOOTREPLY to client 001b.d513.2ad2.
DHCPD: DHCPREQUEST received from client bd5.132a.d2.
DHCPD: Sending DHCPACK to client bd5.132a.d2 ( ).

23. 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.