Port showing as Edge Shr

[Patrick McHenry]

I have a UPS plugged into an fa1/0/48, an access port. This is the output from a show spanning-tree, see bold.

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    8192
             Address     0015.2c03.1001
             Cost        3004
             Port        1 (GigabitEthernet1/0/1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    49153  (priority 49152 sys-id-ext 1)
             Address     0021.1ba4.1c80
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300
  UplinkFast enabled but inactive in rapid-pvst mode

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/0/1          Root FWD 3004      128.1    P2p
Gi1/0/2          Altn BLK 3004      128.2    P2p
Fa1/0/48         Desg FWD 3100      128.52   Edge Shr

My question is, why does the UPS port, fa1/0/48 show as shared when it is only one device? I thought it would say shared only if a switch or hub was connected to that port.

Thanks, Pat

[cadet alain]

hi,

shr means half duplex.

regards.

[Patrick McHenry]

so if a switch was connected that was full duplex, it would not show as shared on an access port?

[cadet alain · Accepted solution]

yes you're right.

[Patrick McHenry]

Thanks, man.

[Peter Paluch · Accepted solution]

Hello Pat, Alain,

Let me add a few words.

A shared link in STP means that this link may possibly interconnect several machines, not a single device. For example, a shared port connects to hubs or wireless access points that indeed create a shared link. This distinction is very important in particular for RSTP because the rapid convergence is supported only on point-to-point links (and edge ports).

However, the RSTP has no reliable way of knowing whether a port connects to a single neighboring device (the point-to-point type) or whether it connects to a shared segment (the shared type). Therefore, Cisco switches try to make an educated guess. If the port is working in half duplex, it is assumed to be connected to a hub (because hub always work in half-duplex mode only), and Cisco switch concludes that the port is connected to a shared link. If the port is working in full-duplex mode then it is assumed that it is connected to a single device and is by default considered as a point-to-point port type.

So the half duplex or a full duplex is used only as a starting indication of the port type but it is still only a guess. For example, wireless access points are equipped with full-duplex wired ports but they still create a shared link, so assuming the port type basing only on the duplex setting is not reliable in all cases. You can also see that your UPS is a single device and the link type is essentially point-to-point, yet the Cisco switch thinks that the link is shared.

To sum it up - the shared or point-to-point link type in RSTP is concerned with the actual link topology connected to a port, and the duplex mode is used to make an estimation what the topology actually is.

Best regards,

Peter