Note: The information in this document is based on the Catalyst 5000 Family
of switches and Switch Software Release 4.4(5). The feature first appeared in
This document explains why you should never use VLAN Trunking Protocol
(VTP) pruning on Catalyst series switches attached to Emulated Local Area
Networks (ELANs). VTP pruning increases available bandwidth by restricting
flooded traffic to those trunk links that the traffic must use to access the
appropriate network devices. With VTP, Catalyst series switches exchange
messages using Layer 2 frames sent to a Cisco Multicast Media Access Controller
Note: The locations of the LAN Emulation Server (LES), the broadcast and
unknown server (BUS), and the LAN Emulation Configuration Server (LECS) are
unimportant for these examples.
There are no specific requirements for this document.
This document is not restricted to specific software and hardware
Technical Tips Conventions for more information on document
In this example, there are three IP stations in virtual LAN (VLAN) #500
(500 is an arbitrary number). The first station is connected to Catalyst-1, the
second to Catalyst-2, and the third to Catalyst-3. Any station pings any other
without a problem, and no other VLAN #500 stations exist in this network. If
you ping nonstop from the station on Catalyst-1 to the station on Catalyst-2,
this works fine. If you remove the station on Catalyst-3, there are no other
stations in VLAN #500 connected to Catalyst-3. If pruning is configured, the
Catalyst-3 advertises the lack of stations by broadcasting a multicast message.
In LAN emulation (LANE), this message is sent by the BUS and received by all of
the LAN Emulation Clients (LECs). Catalyst-1 and Catalyst-2 receive this
message and believe that there are no more stations in VLAN #500 in the ELAN.
As a result, all of the LECs stop forwarding any traffic on this VLAN. Thus, if
you unplug the station on Catalyst-3, you stop communication between stations
on Catalyst-2 and Catalyst-1.
Now, assume there are four IP stations, with the last one connected to
Catalyst-4. If you unplug the station on Catalyst-4, a multicast message is
broadcast over the ISL trunk and hits only the Catalyst-3. This stops traffic
from forwarding to Catalyst-4 for this VLAN, as expected.
Pruning is good for point-to-point links like
Link (ISL). With an ELAN-based network, pruning can be enabled on the
server but VLANs should not be pruning-eligible on a Catalyst connected to
LANE. You can make specific VLANs pruning ineligible with the
clear vtp pruneeligible
command. By default, VLANs
2-1000 are pruning-eligible.