Network Redundancy – Implementing LACP Trunking for Switches

ieee-802.1axredundancyspanning treeswitchswitching

I would like to set up a redundant link between two switches. I have four ports available to accomplish this.

Scenario A: Switch 1 has ports 1+2 trunked together via LACP. They are plugged into Switch 2 ports 1+2.

Scenario B: Switch 1 has ports 1+2 trunked together without LACP. They are plugged into Switch 2 Ports 1+2.

In Scenario B I can unplug one of the patch cables and the switches will still be linked, establishing redundancy. In Scenario A, what will happen when I unplug one of the patch cables? Will LACP allow the link over just one interface? Or will the link stop because an interface is missing.

Best Answer

Scenario A: Switch 1 has ports 1+2 trunked together via LACP. They are plugged into Switch 2 ports 1+2.

Both links will be used, but a single flow will only use one link. There is a hashing algorithm that determines which flow uses which link. If one of the links goes down, then all the traffic will be switched to the other link. This happens very rapidly.

Scenario B: Switch 1 has ports 1+2 trunked together without LACP. They are plugged into Switch 2 Ports 1+2.

They will not actually be trunked together. STP will block one link because it creates a single, loop-free path to the root bridge. When the active link goes down, STP will switch over to the redundant link, but this happens fairly slowly; a few seconds for RSTP, and up to 50 seconds for standard STP.

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Switch – Redundancy between router and servers

I agree with @network_ninja but will extend it a bit.

How I'd solve this

Router1--L3--Router2
|              |
|              |
Switch1--L2--Switch2
|    |         |
|    |         |
PC1 PC2--------+

Router1 and Router2 are running VRRP, HSRP, GLBP or CARP to produce virtual default-GW IP address to the LAN.
This protocol will converse over the Switch core to agree which of the routers is owning the default-GW IP address at any given time.

PC2 is redundant linux server, which is using 'bonding' to redundantly connect to the Switches, it should be configured so that if the the virtual default-gw IP address stops responding to ARP WHO HAS, it'll switch to backup connection. IP address itself is not on the physical interfaces, but on the virtual bonding interface.
Equivalent solution is available to other OS, but often not included in base OS package.

PC1 is non-redundant server.

Switches are not running anything special, no spanning tree (as there is no L2 loop) and no LACP. They can be from different vendors and can be taken down for maintenance separately.

Routers are not running any switching, IP addresses are configured directly in the L3 interfaces facing the switches.
If you choose VRRP as your first-hop-redundancy-protocols, routers can be from different vendor. Each router can be taken down for maintenance separately, by gracefully switching VRRP priority before work on the primary.

LACP – Configuring LACP from Server to Switch

Sounds like you're almost there.

You'll need to create a "team" on the server using two or more of the Intel ports. Use the Configure option in the NIC settings to choose 802.3ad.

Let's say you're using a team with two server NIC ports into two ports (47 and 48) on the same switch.

On the HP switch,

HP Procurve 2920> config
HP Procurve 2920(config)# int 47-48 lacp active
HP Procurve 2920(config)# show lacp

Check to be sure lacp status is "success"

Now patch in your server, each Intel NIC port to an HP port. Come back to the CLI and

HP Procurve 2920(config)# show lacp

to check if it's successful. Look for LACP partner = yes on both interfaces 47 and 48 and be sure all interfaces in your team are "up" Check your server to be sure traffic is flowing through both NIC ports. If so,

HP Procurve 2920(config)# write mem

to save configuration to memory.

Best Answer

Related Solutions

Switch – Redundancy between router and servers

LACP – Configuring LACP from Server to Switch

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