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LCAP bond from two switches that aren't MLAG'd together?

  • 1 August 2018
  • 2 replies
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Hi,

I have two switches on a common network, which I've been asked to configure so that one interface from both switches is configured as an LACP bond.

I'm unsure, so my question to the community is, can two switches which aren't in an MLAG group, both serve an LCAP bond without having an explicit relationship with one-another?

Do I simply configure each interface as part of a bond so that the other end of that bonded interface understands my switches connections to it are effectively as one single device?

Many Thanks,
Adam.
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Best answer by Scott Emery 1 August 2018, 20:01

The add to what @Eric Pulvino said... If you want both links to be active you'll need MLAG. But if you just want one link to be active, and the other to be a backup, then you don't have to use MLAG.

Without MLAG, the device at the other end of the switch's LACP bonds will figure out that it is connected to two different devices and thus will only use one of them to transmit and receive. The bond on that device will be up, but only one of the links will be used. This will cause LACP, running on one of the switches, to bring down its single-member bond, preventing any traffic from going in or out of that interface on the switch. Like with MLAG, you should have a peer link between the switches to prevent black-holing traffic on the switch with the backup bond.
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Userlevel 5
MLAG is used to synchronize MAC addresses from Link1 and Link2 of Bond1. With out MLAG the MAC address learning does not happen correctly meaning you can have all kinds of flooding and other bad traffic flows. The only way to avoid MLAG when dual-connecting a host to two different Top-of-rack switches is to use active/backup bonding (where only one link is active at at time) or to use L3 connectivity down to the host via Routing on the Host with BGP etc.

TLDR; You will need MLAG here.
Userlevel 3
The add to what @Eric Pulvino said... If you want both links to be active you'll need MLAG. But if you just want one link to be active, and the other to be a backup, then you don't have to use MLAG.

Without MLAG, the device at the other end of the switch's LACP bonds will figure out that it is connected to two different devices and thus will only use one of them to transmit and receive. The bond on that device will be up, but only one of the links will be used. This will cause LACP, running on one of the switches, to bring down its single-member bond, preventing any traffic from going in or out of that interface on the switch. Like with MLAG, you should have a peer link between the switches to prevent black-holing traffic on the switch with the backup bond.

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