Information Technology Reference
In-Depth Information
Figure23-16Layer 3 Switching Makes a Design with No Blocking Port Possible
VLAN 10
Core B
Core A
VLAN 2
VLAN 3
VLAN 3
VLAN 2
Dist2
Dist3
Users in VLAN 3
Users in VLAN 2
In Figure 23-16, Core A and Core B are now some Layer 3 switches. Note that we are not bridging any
more VLAN 2 and VLAN 3 between Core A and Core B; thus, we no longer have a loop to cut by the
ways of the STP.
•
Redundancy is still there, relying on Layer 3 routing protocols (and ensuring a reconvergence even
faster that with STP).
•
There is no longer any single port blocked by the STP. This removes all the potential for a bridging
loop.
•
There is no speed penalty because leaving the VLAN via Layer 3 switching is as fast as bridging
inside the VLAN.
The only drawback is that migrating to that kind of design generally implies a rework of the addressing
scheme.
Keep STP Even If It Is Not Needed
Even if you have succeeded in removing all the blocked ports of your network, and even if you don't
have any physical redundancy, it is safer to keep STP enabled. STP is generally not too
processor-intensive (and, anyway, CPU is not involved in packet switching in most Cisco switches), and
the few BPDUs sent on each link do not significantly reduce the available bandwidth. On the other end,
a bridged network without STP can melt down in a fraction of second if an operator makes an error on
a patch panel, for instance. Generally, disabling the STP in a bridged network is not worth the risk.
Keep Traffic Off the Administrative VLAN, and Avoid Having a Single VLAN Spanning the Entire
Network
Keeping traffic of the administration VLAN and avoiding having a single VLAN spanning the entire
network are related points.
