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[EX/QFX] Spanning tree port cost on aggregated interfaces

  [KB31861] Show Article Properties


Summary:

This article explains the spanning tree port cost on aggregated interfaces.

Solution:

In general, aggregated Ethernet (AE) interface that contains 1GE physical interfaces have port cost 10000 (remember that port cost for 1GE interface is 20000).
AE interface that contains 10GE physical interfaces have port cost 1000 (remember that port cost for 10GE interface is 2000).
 
This means that on ELS (EX4600, QFX5100, EX4300) devices 802.1t standard is implemented. This standard suggests not to change Spanning Tree Protocol (STP) cost for AE interfaces when there is a change in the number of physical interfaces that belongs to this AE interface. This is to prevent topology change in that case.
 
On Legacy devices, (EX4500, EX4200) STP port cost depends on the number of physical interfaces in AE bundle. For example, AE interface made of one 10G port has STP cost 2000, and AE made of four 10G ports has STP cost 500, and so on.

TEST RESULTS on QFX5100, EX4600, and EX4300 devices (ELS):
This behavior was tested with Junos 15.1R4.6

root@R2-R3# run show spanning-tree interface

Spanning tree interface parameters for instance 0
Interface       Port ID    Designated        Designated      Port     State  Role
                           port ID           bridge ID       Cost
ae8             128:11     128:13    32768.44f477b0f1c2      10000    FWD    ROOT
ae11            128:14     128:13    32768.ec13db825802      10000    BLK    ALT
 

{master:0}[edit]

root@R2-R3# run show interfaces terse | match ae
ge-0/0/1.0              up    up   aenet    --> ae11.0
ge-0/0/3.0              up    up   aenet    --> ae11.0
ge-0/0/93.0             up    up   aenet    --> ae8.0
ge-0/0/95.0             up    up   aenet    --> ae8.0
ae8                     up    up
ae8.0                   up    up   eth-switch
ae11                    up    up
ae11.0                  up    up   eth-switch

When one of the child interface from ae8 is disabled, the path cost remains same.

​root@R2-R3# set interfaces ge-0/0/95 disable

{master:0}[edit]
root@R2-R3# commit
configuration check succeeds
commit complete
root@R2-R3# run show spanning-tree interface
Spanning tree interface parameters for instance 0
Interface      Port ID    Designated    Designated            Port    State  Role
                          port ID       bridge ID             Cost
ae8            128:11     128:13        32768.44f477b0f1c2    10000    FWD    ROOT  <-- Cost did not change
ae11           128:14     128:13        32768.ec13db825802    10000    BLK    ALT

root@R2-R3# run show spanning-tree interface detail

Spanning tree interface parameters for instance 0
Interface name                 : ae8
Port identifier                : 128.11
Designated port ID             : 128.13
Port cost                      : 10000
Port state                     : Forwarding
Designated bridge ID           : 32768.44:f4:77:b0:f1:c2
Port role                      : Root
Link type                      : Pt-Pt/NONEDGE
Boundary port                  : NA
 
Interface name                 : ae11
Port identifier                : 128.14
Designated port ID             : 128.13
Port cost                      : 10000
Port state                     : Blocking
Designated bridge ID           : 32768.ec:13:db:82:58:02
Port role                      : Alternate
Link type                      : Pt-Pt/NONEDGE
Boundary port                  : NA

TEST RESULTS on EX4200, EX4500, and EX2200 devices (Legacy):
This behavior was tested with Junos 15.1R5.5 

​root@R2-R3# run show spanning-tree interface
Spanning tree interface parameters for instance 0
Interface            Port ID   Designated    Designated       Port   State Role
                                port ID      bridge ID        Cost
ae0.0                    128:1     128:2 32768.78fe3dd31501    10000 BLK   ALT
ae1.0                    128:2     128:2 32768.001f12370941    10000 FWD   ROOT
 
{master:0}[edit]
root@R2-R3# run show interfaces terse | match ae
ge-0/0/0.0              up    up   aenet    --> ae0.0
ge-0/0/1.0              up    up   aenet    --> ae0.0
ge-0/0/2.0              up    up   aenet    --> ae1.0
ge-0/0/3.0              up    up   aenet    --> ae1.0
ae0                     up    up
ae0.0                   up    up   eth-switch
ae1                     up    up
ae1.0                   up    up   eth-switch

When one of the child interface from ae1 is disabled, notice that the port cost for ae1 is changed.

root@R2-R3# set interfaces ge-0/0/2 disable
 
{master:0}[edit]
root@R2-R3# commit
configuration check succeeds
commit complete
 
{master:0}[edit]
root@R2-R3# run show spanning-tree interface

Spanning tree interface parameters for instance 0
 
Interface            Port ID   Designated    Designated       Port   State Role
                                port ID      bridge ID        Cost
ae0.0                    128:1     128:2 32768.78fe3dd31501    10000 BLK   ALT
ae1.0                    128:2     128:2 32768.001f12370941    20000 FWD   ROOT  <-- Cost got changed
 
{master:0}[edit]
root@R2-R3# run show interfaces terse ge-0/0/2
Interface               Admin Link Proto    Local                 Remote
ge-0/0/2                down  down
ge-0/0/2.0              up    down aenet    --> ae1.0
 
root@R2-R3# run show spanning-tree interface detail
Spanning tree interface parameters for instance 0
 
Interface name                 : ae0.0
Port identifier                : 128.1
Designated port ID             : 128.2
Port cost                      : 10000
Port state                     : Blocking
Designated bridge ID           : 32768.78:fe:3d:d3:15:01
Port role                      : Alternate
Link type                      : Pt-Pt/NONEDGE
Boundary port                  : NA
Edge delay while expiry count  : 1
Rcvd info while expiry count   : 0
 
Interface name                 : ae1.0
Port identifier                : 128.2
Designated port ID             : 128.2
Port cost                      : 20000
Port state                     : Forwarding
Designated bridge ID           : 32768.00:1f:12:37:09:41
Port role                      : Root
Link type                      : Pt-Pt/NONEDGE
Boundary port                  : NA
Edge delay while expiry count  : 0
Rcvd info while expiry count   : 0
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