Layer 2 Data Center Interconnect options

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By keeping unnecessary traffic away from data center interconnect we achieve better scalability and prevent bandwidth "waste". Proactive MAC learning is one of the unique OTV differentiators.

DCI Edge redundancy is achieved by having OTV Edge devices automatically elect an authoritative edge device (AED) on a per-VLAN basis, which allows traffic load-sharing while simplifying the deployment model. Only AED is responsible for sending VLAN traffic in and out of the data center, which guarantees loop-free topology across the DCI. Spanning Tree isolation between data centers is inherent within the protocol, which is really becoming standard best practice.

One additional OTV feature worth mentioning is regarding multicast traffic replication, which is performed by backbone routers, rather than by OTV DCI edge devices in the data center where multicast source resides, also known as head-end replication Subsequently, the load on those edge devices is significantly reduced, however the trade-off is that backbone routers now need to be aware of certain client multicast routing information.

OTV currently requires Nexus 7000 switches at the edge and multicast enabled core (for control plane protocol). Multicast requirement will be lifted in the subsequent Nexus 7000 NX-OS software releases.

By now you must see that extending VLANs across data centers is easier said than done, however we are not done yet. Having Layer 2 connection is only half of the story, we now need to provide the way to connect in and out of those VLANs and for that we need to bring Layer 3 connectivity into the mix.

The easiest method is to keep all Layer 3 functionality in one data center and extend Layer 2 to the other. This setup simplifies handling routing in and out of the extended VLANs, however, when traffic enters an extended VLAN in the data center where Layer 3 functionality for that VLAN is implemented, it will be sent across the DCI link if the destination server is in the other data center.

This solution increases the load on the data center interconnect and the latency for traffic that needs to traverse it. If the server's default gateway is in the other data center (remember, in this scenario there is only one Layer 3 entity for that VLAN), traffic leaving the extended VLAN will also cross DCI link with the same potential latency and bandwidth concerns.

To provide Layer 3 redundancy for extended VLANs, each one of the data centers needs to have a Layer 3 component for those VLANs. As advantageous as it sounds, the biggest concern in this setup is traffic symmetry, which is required for stateful devices, such as firewalls and load-balancers, which are often part of the setup.

Without symmetry, traffic entering the extended VLAN in Data Center A and then trying to leave through Data Center B will be dropped unless session state information is synchronized between firewalls/load-balancers in those data centers.

If such synchronization does not exist, you will need to make sure that returning traffic is forwarded back to the original data center, which the request came through. This is most frequently achieved by using Source Network Address Translation (SNAT) techniques. As inefficient as it can be from bandwidth and latency perspective, at least it works, unless of course using NAT breaks the application.

Other methods include injecting /32 host routes or using Global Load-Balancing to direct the traffic to the actual data center where the server is located. There are also more daring initiatives from vendors, such as Cisco, to perform workflow integration between multiple components to collectively deliver the solution that ties together Layer 2 and Layer 3 Data Center Interconnect extensions. Also, stay tuned for new technologies, such as LISP (Locator/ID Separation Protocol), that tackle the issue.

Data center interconnect is definitely morphing, but before you tear up what you have, you should thoroughly design the solution to be scalable and resilient by selecting the technology that addresses your connectivity requirements from all angles.

Read more about lan and wan in Network World's LAN & WAN section.

This story, "Layer 2 Data Center Interconnect options" was originally published by Network World.

Copyright © 2010 IDG Communications, Inc.

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