Information Technology Reference
In-Depth Information
The PortFast feature should
never
be used on switch ports that connect to other
switches,
hubs,
or
routers
. These connections may cause physical loops, and it is very important that
the spanning-tree process go through the full initialization procedure in these situations. A
spanning-tree loop can bring your network down. If PortFast is turned on for a port that
is
part of a physical loop, it can cause a window of time in which packets could possibly be
continuously forwarded (and even multiply) in such a way that the network can't recover.
In later Catalyst operating system software (5.4(1)), a feature called PortFast BPDU-Guard
detects the reception of BPDUs on ports having PortFast enabled. Because this should
never happen, BPDU-Guard puts the port into errDisable state.
Note
EtherChannel
Another feature that a switch may have is called EtherChannel (or Fast EtherChannel, or Gigabit
EtherChannel). This feature allows multiple links between the same two devices to work as if they were
one fast link, with traffic load balanced among the links. A switch can form these bundles automatically
with a neighbor using a protocol called Port Aggregation Protocol (PAgP). Switch ports that can run
PAgP usually default to a passive mode called auto, which means that they are
willing
to form a bundle
if the neighbor device across the link asks them to. Running the protocol in auto mode can cause a port
to delay for up to 15 seconds before passing control to the spanning-tree algorithm (PAgP runs on a port
before spanning tree does). There is no reason to have PAgP running on a port connected to a
workstation. Setting the switch port PAgP mode to off will eliminate this delay.
Trunking
Another switch feature is the capability of a port to form a trunk. A trunk is configured between two
devices when they need to carry traffic from multiple VLANs. A VLAN is something that switches
create to make a group of workstations appear to be on their own segment or broadcast domain. Trunk
ports make these VLANs extend across multiple switches so that a single VLAN can cover an entire
campus. They do this by adding tags to the packets, indicating which VLAN the packet belongs to.
Different types of trunking protocols exist. If a port can become a trunk, then it may also have the
capability to trunk automatically and, in some cases, even negotiate what type of trunking to use on the
port. This capability to negotiate the trunking method with the other device is called Dynamic Trunking
Protocol (DTP); the precursor to DTP is a protocol called Dynamic ISL (DISL). If these protocols are
running, they can delay a port on the switch becoming active.
Usually a port connected to a workstation belongs to only one VLAN and therefore does not need to
trunk. If a port has the capability to negotiate the formation of a trunk, it will usually default to the auto
mode. If the port is changed to a trunking mode of off, it will further reduce the delay of a switch port
becoming active.
Speed and Duplex Negotiation
Just turning on PortFast and turning off PAgP (if present) is usually enough to solve the problem, but if
you need to eliminate every possible second, you could also set the port speed and duplex manually on
the switch if it is a multispeed port (10/100). Autonegotiation is a nice feature, but turning it off could
save you 2 seconds on a Catalyst 5000 (it does not help much on the 2800 or 2900XL).
