Posted on September 14, 2015 2:11 pm
 |  Asked by Shailendra Patil PATIL
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Hi all,

I just wanted to know how does Stp works in MLAG. Anyone document would really help me.

Posted by coreyhines
Answered on September 14, 2015 2:26 pm


Please take a look at this EOSC article on basic MLAG. From a very high level the two switches that make up the MLAG domain look to STP like a single switch. This allows for active active utilization of the links without blocking that is encountered in traditional STP topologies.

Chapter 11 of the User Manual for EOS 4.15.0F is also a great resource for both the concept and configuration details of Arista EOS MLAG implementation.


Corey Hines

Posted by Alexis Dacquay
Answered on September 14, 2015 2:55 pm


Below are some examples to illustrate how STP works with MLAG

The output of ’show spanning-tree’ shows Peer (e.g., Pet4 and Ppo100) interfaces as well as local interfaces. Notice that the MLAG created with ’mlag 3’ shows up under its local Port-Channel name (Po3).

 Switch1#show spanning-tree MST0
 Spanning tree enabled protocol mstp Root ID Priority 32768
 Address 0011.2201.0301 This bridge is the root
 Bridge ID Priority 32768 (priority 32768 sys-id-ext 0) Address 0011.2201.0301
 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
 Interface Role State Cost Prio.Nbr Type
 --------------- ---------- ---------------- 
 Et4       designated forwarding 2000
 Et5       designated forwarding 2000
 PEt4      designated forwarding 2000
 PEt5      designated forwarding 2000
 Po3       designated forwarding 1999


’show spanning-tree’ (and some other bridging-related ’show’ commands), when run on the MLAG primary peer, will show ”PeerEthernet” and ”PeerPort-Channel” interfaces corresponding to interfaces on the secondary switch. The CLI ”short names” for PeerEthernet1 and PeerPort-Channel1 are ”PEt1” and ”PPo1”.


Spanning tree does not run on the peer link, and so it is not listed in the output of ’show spanning-tree’.

Switch2#show spanning-tree
 Spanning tree status is overridden.
 STP is running on the primary MLAG peer


Note that the spanning tree protocol is disabled on the secondary, so ’show spanning- tree’ does not show the spanning tree state; you have to use the CLI of the primary switch to see the spanning tree state.


STP Considerations with MLAG:

  • The global STP configuration comes from the primary peer; secondary parameters are ignored.
  • STP runs with the primary switch bridgeId while the switches are MLAG-active.
  • Port-specific spanning-tree configuration comes from the switch where the port physically resides. This includes spanning-tree portfast , bpduguard and bpdufilter






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