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Chapter 11
Troubleshooting Switched Ethernet
VLAN Translation
Translation must occur in two scenarios. The first scenario occurs when VLAN
A uses a different VLAN protocol than VLAN B. For example, VLAN A uses
ISL for its VLAN protocol, whereas VLAN B uses 802.1q. In order for these two
VLANs to communicate, the router must perform protocol translation. This
occurs at Layer 2 and the frame headers are changed to accommodate the
change in protocol.
The second scenario is when a VLAN protocol must be translated into a non-
VLAN Layer 2 protocol. An example of this is when VLAN A (using ISL or
802.1q) needs to communicate with a Layer 2 destination that does not use any
VLAN protocol. The router then translates the VLAN header into a header such
as 802.10 so the two can communicate.
Routing
To enable communication between different VLANs or non-VLAN net-
works (Layer 3), routing must occur. The router maintains routes for the
subnets/networks that belong to each VLAN. When VLAN A needs to reach
VLAN B, a route lookup is performed and the packets are routed on Layer 3.
When a machine on a VLAN wants to communicate to a host on any
other destination not on a local VLAN, routing is performed as well. It is
important to realize that translation is not occurring because routing is a
Layer 3 function, whereas translation occurs at Layer 2.
Troubleshooting VLANs on Routers
Some commands are similar across the IOS for the routers and the software run-
ning on the switches. It is important, however, to know which commands provide
unique output and should be executed on a router rather than on a switch.
The following commands provide additional information regarding the
VLANs from the router:
show vlans
show spanning-tree
show arp
show interface
show cdp neighbor
debug vlan packet
debug spantree
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