Posted on September 14, 2015 2:11 pm
 |  Asked by Shailendra Patil PATIL
 |  6850 views
Tags:
RESOLVED
0
0
Print Friendly, PDF & Email

Hi all,

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

0
Posted by coreyhines
Answered on September 14, 2015 2:26 pm

Hello,

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.

https://eos.arista.com/mlag-basic-configuration/

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.

Thanks,

Corey Hines

0
Posted by Alexis Dacquay
Answered on September 14, 2015 2:55 pm

Hi,

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

 

 

 

Regards,

Alexis

Post your Answer

You must be logged in to post an answer.