Information Technology Reference
In-Depth Information
The easiest way to view the PortFast status is through the menu system. If you select (P) for Port
Configuration from the main menu and then select a port, the output will tell you whether port fast mode
is enabled. The following output is for port Fast Ethernet 0/26 (which is port “A” on this switch).
Catalyst 1900 - Port A Configuration
Built-in 100Base-FX
802.1d STP State: Blocking
Forward Transitions: 0
----------------------- Settings ---------------------------------------
[D] Description/name of port
[S] Status of port
Suspended-no-linkbeat
[I] Port priority (spanning tree)
128 (80 hex)
[C] Path cost (spanning tree)
10
[H] Port fast mode (spanning tree)
Enabled
[E] Enhanced congestion control
Disabled
[F] Full duplex / Flow control
Half duplex
----------------------- Related Menus ----------------------------------
[A] Port addressing
[V] View port statistics
[N] Next port
[G] Goto port
[P] Previous port
[X] Exit to Main Menu
Enter Selection:
Timing Tests on the Catalyst 1900
The timing values are harder to verify on a 1900/2820 because of the lack of debugging tools, so we just
started a
ping
from a PC connected to the switch directed to the switch itself. We disconnected and then
reconnected the cable, and recorded how long it took for the switch to respond to the
ping
with PortFast
on and with PortFast off. For an Ethernet port with PortFast on (the default state), the PC received a
response within 5 to 6 seconds. With PortFast off, the PC received a response in 34 to 35 seconds.
An Additional Benefit to PortFast
There is another spanning-tree-related benefit to using PortFast in your network. Every time a link
becomes active and moves to the forwarding state in spanning tree, the switch will send a special
spanning-tree packet called a Topology Change Notification (TCN). The TCN notification is passed up
to the root of the spanning tree, where it is propagated to all the switches in the VLAN. This causes all
the switches to age out their table of MAC addresses using the forward delay parameter, which is usually
set to 15 seconds. So, every time a workstation joins the bridge group, the MAC addresses on all the
switches will be aged out after 15 seconds instead of the normal 300 seconds.
When a workstation becomes active, it does not really change the topology to any significant degree, so
as far as all the switches in the VLAN are concerned, it is unnecessary for them to have to go through
the fast aging TCN period. If you turn on PortFast, the switch will not send TCN packets when a port
becomes active.
Commands to Use for Verifying That the Configuration Is Working
For the 4000/5000/6000 series, use these commands to verify the configuration:
•
show port spantree 2/1
—To see whether “Fast-Start” (PortFast) is enabled or disabled
