Identifying Designated Ports (continued)
Identifying Designated Ports (continued)
Because two ports on this segment are eligible to become the designated port, Identifying Designated Ports STP recognizes that a loop exists. One of the ports must be elected as the designated port, and the other must be placed in a blocking state.
Normally, whichever switch has the lowest cumulative path cost will have its port become designated. The switch with the highest path cost will have its port blocked.
In the above example, there is a tie in cumulative path cost. Both SwitchD and SwitchE have a path cost of 12 to reach the Root Bridge on that segment.
The lowest Bridge ID is used as the tiebreaker. SwitchD has a priority of 100, and SwitchE has the default priority of 32,768.
Thus, the port on SwitchD will become the designated port. The port on SwitchE will be placed in a blocking state.
As with electing the Root Bridge, if there is a tie in priority, the lowest MAC address is used as the tie breaker.
Remember: Any port not elected as a root or designated port will be placed in a blocking state.
Port ID
When electing root and designated ports, it is possible to have a tie
in both
path cost and Bridge
ID. Consider the following example:
The bandwidth of both links is
equal, thus both ports on SwitchB have an equal path cost to the Root Bridge.
Which port will become the root port then? Normally, the lowest Bridge ID is
used as the tiebreaker, but that is not possible in this circumstance.
Port ID is used as the
final tiebreaker, and consists of two components:
·
4-bit port priority
·
12-bit port
number, derived from the physical port number
By default, the
port priority of an interface is 128,
and a lower priority is preferred. If
there is a tie in priority, the lowest port number is preferred.
The sender port ID determines the tie break, and not the local port ID. In the above example,
SwitchB must decide whether gi2/23 or
gi2/24 becomes the root port. SwitchB
will observe BPDU’s from SwitchA, which will contain the port ID’s for gi2/10 and gi2/11.
If priorities are equal, the
sender Port ID from gi2/10 is
preferred, due to the lower port number. Thus, gi2/23 on SwitchB will become the root port.
The port number
is a fixed value, but port priority can be changed on a per- interface basis:
Switch(config)#
int gi2/11
Switch(config-if)# spanning-tree vlan 101 port-priority 32
Port ID
Note: Some reference material may state that the Port ID is
comprised of an 8-bit priority and 8-bit port number. This was accurate in
the original 802.1D specification.
However, IEEE 802.1t revised the original specification to provide the
larger 12-bit port number field, to
accommodate modular switches with high port density.
Even more confusing – some whitepapers
on Cisco’s website will define the Port ID as a combination of port priority
and MAC address, instead of port
number. This is not accurate in modern STP implementations.
Remember: Port ID is the last
tiebreaker STP will consider. STP determines root and designated ports
using the following criteria, in order:
·
Lowest path cost to the Root Bridge·
Lowest bridge ID
·
Lowest sender port ID
Lowest Bridge ID is always used to determine the Root
Bridge.
Read First:
Identifying Designated Ports STP
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