A router interface providing inter-VLAN routing on trunk link must
be configured with a subinterface for each VLAN that will be serviced across
the link. Each of the subinterfaces on the physical link must then be
configured with the same trunk encapsulation protocol. That protocol, either
ISL or 802.1Q, is typically determined by what was configured on the switch
side of the link.

Configuring an External Router using ISL Encapsulation
Use the
encapsulation isl vlan_id subinterface configuration
command to enable ISL trunking on a router subinterface.

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NOTE:
The subnets of the VLANs are directly connected to the router. Routing
between these subnets does not require a dynamic routing protocol. Routes to
the subnets associated with each VLAN will appear in the routing table on
directly connected interfaces.
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Configuring an External Router using 802.1Q
Use the
encapsulation dot1q subinterface configuration command to
enable 802.1Q encapsulation on a router subinterface.

The use of
the native keyword indicates that the un-tagged frames are associated with a
particular subinterface. The configuration for the other individual VLAN
subinterfaces is configured as it was with ISL. The subinterface number need
not match the dot1q VLAN number.
Verifying the Inter-VLAN Routing
Configuration Using Ping
After the router is properly configured and
connected to the network, the router can communicate with other nodes on the
network.

To test
connectivity to remote hosts, use the ping command from
privileged mode:
Router#pingdestination-ip-address
Step 1 From
the router, attempt to ping a host address on each VLAN to verify router
connectivity to each VLAN.
Step 2 From a host on
a particular VLAN, attempt to ping a host on another VLAN to verify routing
across the external router.
The ping command will return one of these responses:
-
Success rate is 100 percent or ip-address
is alive – This response occurs in one to ten
milliseconds, depending on network traffic and the number of Internet Control
Message Protocol (ICMP) packets sent.
-
Destination does not respond – No answer message is returned if the
host does not respond.
-
Unknown host – This response occurs if the targeted host cannot be
resolved.
-
Destination unreachable – This response occurs if the default
gateway cannot reach the specified network or is being blocked.
-
Network or host unreachable – This response occurs if the Time to
Live (TTL) times out. The TTL default is 2 seconds.
Verifying the Inter-VLAN Routing Configuration
Use the
show commands to display the current (running)
configuration, IP routing information, and IP protocol information. This will
verify if the routing table represents the subnets of all VLANs.

Example: Displaying Inter-VLAN Configuration Information
Router#show
vlan
Example: Displaying Routing Table Information
Router#show ip
route