Routing between VLANs
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VTP Pruning
VTP provides a way for you to preserve bandwidth by configuring it to
reduce the amount of broadcasts, multicasts, and other unicast packets.
This is called pruning. VTP pruning only sends broadcasts to trunk links
that truly must have the information. Here's an example: If Switch A
doesn't have any ports configured for VLAN 5, and a broadcast is sent
throughout VLAN 5, that broadcast would not traverse the trunk link to
this Switch A. By default, VTP pruning is disabled on all switches.
When you enable pruning on a VTP server, you enable it for the entire
domain. By default, VLANs 2 through 1005 are pruning-eligible, but
VLAN 1 can never prune because it's an administrative VLAN.
Routing between VLANs
H
osts in a VLAN live in their own broadcast domain and can com-
municate freely. VLANs create network partitioning and traffic separation
at layer 2 of the OSI, and like I said when I told you why we still need routers,
if you want hosts or any other device to communicate between VLANs, a
layer-3 device is absolutely necessary.
For this, you can use a router that has an interface for each VLAN or a
router that supports ISL routing. The least expensive router that supports
ISL routing is the 2600 series router. The 1600, 1700, and 2500 series don't
support ISL routing.
As shown in Figure 6.7, if you only had a few VLANs (two or three),
you could get a router with two or three 10BaseT or Fast Ethernet con-
nections. And 10BaseT is okay, but I'd recommend Fast Ethernet--that
will work really well.
What we see in Figure 6.7 is that each router interface is plugged into an
access link. This means that each of the routers' interface IP addresses would
then become the default gateway address for each host in each VLAN.
If you have more VLANs available than router interfaces, you can either
run ISL routing on one Fast Ethernet interface, or buy a route switch module
(RSM) for a 5000 series switch. The RSM can support up to 1005 VLANs
and run on the backplane of the switch.
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