Posted on December 21, 2020 4:38 am
 |  Asked by Adam Abel
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Hello, I’m a bit new to Arista but I have 1 in my environment I work with.  For a long time we had a single Arista switch running a set of hosts.  Recently we procured a second one to add in HA with MLAGs.  I am going through the planning to add the new switch and hoping to avoid any outages but from what I tell theres really no way around an outage.

So currently we have a compute system connected in a LAG on port-channel 1 via 2 of its 40gb ports.

We have LAG on port-channel 2 to our SAN switches that the compute system uses via 2 10gb ports.

We have a LAG on port-channel 3 to another pair of switches that provide our connection to the firewall via 2 10gb ports.

Finally we have 2 ports going to a pair of F5 load balancers which are not lagged.

This is on a single Arista 7050 switch.

Now I understand the concept and work to form the MLAGs and peer links between the switches, but when I go to form say a new set of MLAGs on port-channel 20 with 2 net new 10gb ports 1 per Arista, for the SAN switches and leave the current LAGs up.  STP is going to kick in a block one of the M/LAGs to the SAN switches.  If I set it up so it blocks the MLAG and keeps the existing LAG going I’ll still have a brief outage when I shutdown the old LAG on the single Arista and no shut the MLAG from the 2 Aristas to the SAN switches.  The same goes for my connection out to the firewall.   Is there any way around this outage or am I at the mercy of STP here?



Posted by Howard
Answered on December 24, 2020 5:03 pm

Hey Adam, your assumption is correct regarding the impact of STP if not planned correctly.   For the MLAG you would disable it under the "mlag configuration'' using the shutdown command as shown below:

mlag configuration
domain-id mlag_01
heartbeat-interval 2500
local-interface Vlan4094
peer-link Port-Channel20
shutdown                             <<<<<<

This would keep the MLAG out of the topology until you are ready to switch over.  Until you enable the MLAG there should be no STP changes.   When you're ready to switch over you can enable the MLAG, which may trigger a topology change, then shutdown the existing LAG or vis-versa.  There would be some impact of course but should be minimal if done correctly.  Good luck.


Posted by Naveen Chandra
Answered on December 27, 2020 2:32 pm

Hi Adam,

Thanks for putting your query here. In a conventional topology, with dually-attaching devices to multiple switches for redundancy, Spanning Tree Protocol (STP) blocks half of the switch-device links. In the MLAG topology, STP does not block any portion because it views the MLAG Domain as a single switch and each MLAG as a single port. The MLAG protocol facilitates the balancing of device traffic between the peer switches.

When implementing MLAG in a spanning tree network, spanning tree must be configured globally and on port-channels configured with an MLAG ID. Port specific spanning tree configuration comes from the switch where the port physically resides.

We should also understand that, as a best practice and to get flawless results, both the switches (participating in mlag) should have identical hardware and EOS. Also, as Howard said, this will trigger an STP topology change and there would be some impact.

Below article would help you in understanding this topic better.

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