When I wrote about the Cisco Nexus 7000 NX-OS a few weeks back, I mentioned the pinnacle of new features in NX-OS was Virtual Device Contexts (VDCs). This is a feature I could've used a lot in the past during network design and is a long-time coming.
VDCs take a single physical switch and create many, unique virtual devices (switches), just as VMware does for servers. Each VDC is analogous to a virtual machine in VMware. NX-OS runs a single kernel and infrastructure layer to control the hardware and provide basic services. A single VDC Manager also runs and controls all the VDCs. It creates and deletes individual VDCs and launches necessary software services for the VDC.
Each VDC is its own switch. They have their own processes, VLANs, routing protocols, spanning-tree, management plane, and configuration. It's really a completely separate switch. Interfaces are assigned to the VDC to provide external connectivity.
Processes in each VDC are separate and run in their own protected memory space. Thus, if OSPF fails on VDC 12, it will not affect OSPF in the other VDCs. VDCs are created with the aid of a template, which specifies the resources a VDC can use. Inter-VDC communication is only via external interfaces, there is no internal switch like in VMware.
VDCs offer several direct benefits:
Furthermore, VDCs also provide an interesting way to scale hardware resources beyond their documented limits. Each line card in the Nexus 7000 can support:
- 128,000 MAC addresses
- 128,000 FIB entries
- 64,000 ACLs
- 512,000 NetFlow entries
Normally, with a single VDC, entries for each of these tables are copied across every linecard so distributed (local) switching can occur. So, the limit of the entire Nexus 7000 is the numbers listed above. But, now consider the case with VDCs. Each VDC may not necessarily have ports on every linecard. Without a port in a VDC, the line card does not store entries for its MAC addresses, ACLs, NetFlow entries, etc. So, that linecard does use up any resources, even though a MAC address has been stored on another linecard. The entire capability of the single Nexus 7000 has just risen because of virtualization.
With this resource breakdown using VDCs, hardware maximums can be exceeded. Notice in the table below the number of routes (FIB entries) supported in a single Nexus 7000 is no longer 128,000, it's now 180,000:
Finally, VDCs, along with VSS, are going to introduce radical changes to network design. I'll be covering that soon.
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Michael Morris is a communications engineering manager at a $3-billion high-tech company. His background is in enterprise WANs working with telcos and developing large-scale routing designs. He has worked on networks at government and corporate organizations, including networks at two Fortune 10 companies. In his current role, he leads a team of 10 engineers responsible for large-scale IT networking projects and architectural standards for data networks, storage area networks, IP telephony, contact centers, and security. Michael is CCIE #11733 and recently became one of the first three Cisco Certified Design Experts (CCDE) ever (#20080002). He has 11 years experience in networking and communications, including four years as a paratrooper in the U.S. Army. He has a bachelor's degree in MIS from the University at Buffalo and is working on his MBA from NC State University. In 2008, he was awarded the Network Professional Association (NPA) Professional Excellence and Innovation Award for his work on network architecture, templates and enterprise MPLS design.
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