Girl: Hi da Selvam...
Selvam: Hi di..
Girl: what is that, you are staring so long in the laptop?
Selvam: I had been working on with Flex links, this week
Girl: Hmm, So another of your boring technical topics..
Selvam: Yes it is J, Don’t you want to know about Flex links?
Girl: Are you going to leave me if I say No.. L
Selvam: Definitely not J
Girl: Ok, Tell me.. What are Flex links?
Selvam: Flex Links are the means of achieving redundancy and load balancing at Layer 2 level, as an alternative to Spanning Tree Protocol (STP). Flex links are a pair of Layer 2 interfaces, [can be switch ports or port channels] where one interface is configured as a backup to the other.
Girl: Hmm, Where can they be applied?
Selvam: They can be configured on all switches that needs redundancy, but typical place for them are in service provider or enterprise networks where customers do not want to run STP on the switch.
Girl: Can you show me in drawings?
Selvam: Here they are.
Flex Link Examples:
Scenario 1: A Downlink Switch with one primary Uplink and a backup.
Scenario 2: Back to Back connected Switches.
Girl: How can we use them?
Selvam: It’s very simple, but requires some manual effort. You need to identify which interface is going to act as the primary active link and which is going to act as the backup link. Once it is identified, assign the backup layer 2 interface [also called Flex link] as the backup to the primary Layer 2 interface. That’s it, Flex links are enabled in your setup.
Selvam: Flex links are not enabled by default.
Girl: How do they work?
Selvam: The backup link will be in standby mode [ready to begin traffic forwarding] until the primary link is up and forwarding traffic. It will begin traffic forwarding if the other link shuts down. At any point of time only one interface will be in linkup state and will be actively forwarding traffic. If the primary link shuts down, the standby link takes up the duty and starts forwarding traffic. When the primary link comes back up active, it goes into standby mode and does not participate in traffic forwarding.
Selvam: Flex Links are supported only on Layer 2 ports and port channels, not on VLAN’s or on Layer 3 ports.
Girl: What if I want my primary link to be available at my service when its link is up?
Selvam: Yes you can have it, You can use the preemption mechanism to achieve this. Configure the Flex links pair with preemption mode. So when one of the primary link comes back up, it will directly take over traffic forwarding by putting itself as active link [with some preconfigured preemption delay]. The other link will be put back to standby [backup] mode.
Girl: How do Load balancing work in Flex Links?
Selvam: Load balancing in Flex links work at VLAN level. You can make both the ports in the Flex link pair to forward traffic simultaneously. One port in the flex links pair can be configured to forward traffic belonging to VLANs 1-50 and the other can forward traffic for VLANs 51-100. Mutually exclusive VLANs are load sharing the traffic between the Flex link pairs. If one of the ports fails, the other active link forwards all the traffic. This way we are achieving “Redundancy” & “Load Sharing” at the same time.
Girl: Will Flex links replace STP?
Selvam: Flex links are easy to configure, highly efficient and more controllable feature but they require manual effort which is not the case with Spanning Tree Protocol (STP). For larger networks with ever-changing link layer network, STP will be the only option. Flex links are more suitable and an opt choice for service provider and core enterprise networks.
Selvam: Spanning Tree Protocol is disabled on Flex Link interfaces.
Girl: Hmm, Sounds cool. That will be enough for the day.
Selvam: Hmm.. J
Girl: Bye da.
Selvam: Bye.
http://en.wikipedia.org/wiki/Flex_Links