Problems With VLAN Trunk Protocol (VTP) Pruning Over LAN Emulation (LANE)Printable Pdf
Document ID: 10552
Example 1: Three IP Stations Example 2: Four IP Stations Related Information
Note: The information in this document is based on the Catalyst 5000 Family of switches and Switch
Software Release 4.4(5). Please note that the feature first appeared in release 2.3.
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 exchanges messages using layer 2 frames sent to a Cisco
multicast Media Access Controller (MAC) address.
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.
Example 1: Three IP Stations
In this example, you have 3 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 we ping nonstop from
the station on Catalyst-1 to the station on Catalyst-2, this works fine. If we were to remove the station on
Catalyst-3, there would be no other stations in VLAN #500 connected to Catalyst-3. If pruning has been
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 will 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.
Example 2: Four IP Stations
Now, let's say we had 4 IP stations, with the last one connected to Catalyst-4. If we 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 Inter-Switch 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 vlan_range
command. By default, VLANs 2-1000 are pruning-eligible.
