Spanning-tree link-type point-to-point - design considerations -

r1127hyduk · ‎06-26-2019

All,

Checking in with a request for feedback regarding use of the spanning-tree link-type point-to-point command and its uses. For configuration samples, when and where is the command used and is it possible to see it only at one end of a switch to switch link without issue?

Example config - two switches

Switch 02

Int Gi1/0/26

Description - Link to CORE Building 1 Complex A Sw01 P1/0/26

switchport access vlan 126

switchport trunk allowed vlan 2,101,126

switchport mode trunk

switchport nonegotiate

spanning-tree portfast (*not in play due to trunk port*)

spanning-tree link-type point-to-point (Full duplex direct link assumption)

Switch 01

Int Gi1/0/26

Description - Link to CORE Bld2 Complex B xxxx Sw02 P1/0/26

switchport access vlan 126

switchport trunk allowed vlan 2,101,126

switchport mode trunk

switchport nonegotiate

spanning-tree portfast (*not in play due to trunk port*)

REM spanning-tree link-type point-to-point (Full duplex direct link assumption)

Example config question - What if this side of the link did not have 'Spanning-tree link-type point-to-point' - what symptoms or risks would be visible or if the link is designated as full duplex, no anomalies would be seen?

Thanks in advance for the reviews and comments

Giuseppe Larosa · ‎06-26-2019

Hello r1127hyduk,

the command should be used with Rapid PVST on half duplex link connecting only two switches, in order to allow the use of STP message handshakes (Proposal bit flag / Agreement bit flag) instead of relying on slower STP timers.

IF the link is full duplex and it is not edge (=portfast) Rapid PVST or Rapid MST instance will use STP message handshakes by default.

So I agree that if there is configuration mismatch of this command, there will be impact only on an half duplex link with one side attempting to use STP message handshakes and the other side (the one without the command) not allowed to answer to handshake.

After few attempts the first side should revert to use/wait STP timers like it happens on an access port not configured as edge (= portfast).

Hope to help

Giuseppe

r1127hyduk · ‎07-10-2019

Thanks Giuseppe,

Makes sense, all links are full duplex.