VTP Pruning
Recall that by definition, a about-face charge advanced advertisement frames out all accessible ports in the
broadcast area because broadcasts are destined everywhere there is a listener. Multicast
frames, unless forwarded by added able means, chase the aforementioned pattern. (Multicast
switching is covered in detail in Chapter 11, “Configuring Multicast Networks.”)
In addition, frames destined for an abode that the about-face has not yet abstruse or has forgotten
(the MAC abode has age-old out of the abode table) charge be forwarded out all ports in an
attempt to acquisition the destination. These frames are referred to as alien unicast.
When forwarding frames out all ports in a advertisement area or VLAN, block ports are included.
By default, a block articulation transports cartage from all VLANs, unless specific VLANs are removed
from the block with the bright block command. Generally, in a arrangement with several switches,
trunk links are enabled amid switches and VTP is acclimated to administer the advancement of VLAN
information. This book causes the block links amid switches to backpack cartage from all
VLANs—not aloof from the specific VLANs created.
Consider the arrangement apparent in Figure 4-9. When end user HostPC in VLAN 3 sends a
broadcast, Catalyst about-face C assiduously the anatomy out all VLAN 3 ports, including the block link
to Catalyst A. Catalyst A, in turn, assiduously the advertisement on to Catalysts B and D over those
trunk links. Catalysts B and D advanced the advertisement out alone their admission links that accept been
configured for VLAN 3. If Catalysts B and D don’t accept any users in VLAN 3, forwarding that
broadcast anatomy to them would absorb bandwidth on the block links and processor resources
in both switches, alone to accept switches B and D abandon the frames.
Example 4-3 appearance vtp statistics Reveals VTP Bulletin and Error Counters
Switch> appearance vtp statistics
VTP statistics:
summary advts accustomed 8
subset advts accustomed 11
request advts accustomed 0
summary advts transmitted 1
subset advts transmitted 1
request advts transmitted 0
No of config afterlight errors 0
No of config abstract errors 0
VTP pruning statistics:
Trunk Join Transmitted Join Accustomed Summary advts accustomed from
non-pruning-capable device
-------- --------------- ------------- ---------------------------
3/1
Switch>
124 Chapter 4: VLANs and Trunking
Figure 4-9 Flooding in a Catalyst About-face Network
VTP pruning makes added able use of block bandwidth by abbreviation accidental flooded
traffic. Advertisement and alien unicast frames on a VLAN are forwarded over a block articulation only
if the about-face on the accepting end of the block has ports in that VLAN. VTP pruning occurs as
an addendum to VTP adaptation 1, application an added VTP bulletin type. When a Catalyst switch
has a anchorage associated with a VLAN, the about-face sends an advertisement to its acquaintance switches
that it has alive ports on that VLAN. The neighbors accumulate this information, enabling them to
decide if abounding cartage from a VLAN should use a block anchorage or not.
Figure 4-10 shows the arrangement from Figure 4-9 with VTP pruning enabled. Because Catalyst B
has not advertised its use of VLAN 3, Catalyst A will accept not to flood VLAN 3 cartage to it
over the block link. Catalyst D has advertised the charge for VLAN 3, so cartage will be flooded
to it.
Catalyst A
(VLANs 1-1000)
Catalyst B Catalyst C Catalyst D
VLAN 2 VLANs 3,4
VLAN 3
Host PC
VTP Pruning 125
Figure 4-10 Flooding in a Catalyst About-face Arrangement Application VTP Pruning
Catalyst A
(VLANs 1-1000)
Catalyst B Catalyst C Catalyst D
VLAN 2 VLANs 3,4
VLAN 3
Host PC