Nexus 7000 Series VDC Concepts and Configuration: Notes on

Virtual Device Context

 Virtual Device Context (VDC) partitions a single physical device into multiple logical devices that provide fault isolation, management isolation, address allocation isolation, service differentiation domains, and adaptive resource management. VDC also virtualizes the control plane, which includes all those software functions that are processed by the CPU on the active supervisor module.

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Fiber Channel over Ethernet (FCoE): Notes On

Data Center Bridging Standards / Converged Enhanced Ethernet

Enhancements required for a successful Unified Fabric implementation, where network and storage traffic are carried over the same Ethernet links, are defined under the Data Center Bridging standards defined by the IEEE.

These enhancements make sure that Unified Ethernet fabric provides lossless, no-drop, in-order delivery of packets end-to-end.

This is also called Converged Enhanced Ethernet (CEE).

These enhancements include:

  • Priority Flow Control (PFC)
  • Enhanced Transmission Selection (ETS)
  • Quantized Congestion Notification (QCN)
  • Data Center Bridging Exchange (DCBX)

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Fibre Channel Concepts: A Not Very Brief Note On

Fibre Channel

Fibre Channel, or FC, is a high-speed network technology (commonly running at 1, 2, 4, 8, 16, and 32 gigabit per second rates) primarily used to connect computer data storage to servers. Fibre Channel is primarily used in storage area networks (SAN) in enterprise storage. Fibre Channel networks are known as a fabric because they operate in unison as one big switch. Fibre Channel mainly runs on optical fiber cables within and between data centers.

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Cisco 5508 Wireless LAN Controller Licenses Disappear After Software Upgrade!

We had a Cisco 5508 Wireless LAN Controller in our HQ building which was managing 26 Aironet 3501I access points. It was running controller software version 7.4.110 with 50 AP base-licenses.

We decided to add 20 more APs in order to improve the wireless coverage. So we got new Aironet 3702I access points.

According to Cisco Wireless Solutions Software Compatibility Matrix controller software version 7.4.110 does not support the 3702I access points. So we had to upgrade the controller software.

You will see a log message like this in case the controller software does not support the more recent access points: “Discarding primary discovery request in LWAPP from AP xx:xx:xx:xx:xx:xx supporting CAPWAP”

We downloaded the most recent version of software ( at the time of writing this post) and downloaded it to the controller. We rebooted the controller and licenses were gone!

We tried to re-install the license backup file we had made before upgrading but it FAILED!

We switched back to the previous software version immediately and rebooted the controller. The Licenses were back then.

We guessed that it was because we had made a major upgrade (from version 7.4.x. to 8.2.x) so we decided to upgraded to the very first version of controller software where Aironet 3702I APs were supported for the first time. We upgraded to controller software version 7.6.110 and this time it was successful.

I don’t know if Cisco guys approve this workaround but it worked for us.

A Big Picture of QoS Classification, Marking and Mapping

This document defines up to 13 classes of traffic. A brief table of these classes and their respective markings, mappings and different queuing systems is given.


ToS Byte

P2 P1 P0 T2 T1 T0 CU1 CU0
  • IP precedence—three bits (P2 to P0)
  • Delay, Throughput and Reliability—three bits (T2 to T0)
  • CU (Currently Unused)—two bits(CU1-CU0)

DiffServ Field

  • DSCP—six bits (DS5-DS0)
  • ECN—two bits


DS field in the IPv4/IPv6 headers replaced the IPv4 TOS field. (IETF RFC 2474)

Class Selectors in the DS field -> backward compatibility with the IP precedence in TOS field

PHBs -> define the packet-forwarding properties associated with a class of traffic

Expedited Forwarding (EF) PHB -> low-loss, low-latency traffic, low jitter, assured bandwidth

Assured Forwarding (AF) PHB -> gives assurance of delivery under prescribed conditions

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QoS Concepts


QoS is a set of techniques to manage bandwidth, delay, jitter, and packets loss for flows in a network. The Internet Engineering Task Force (IETF) defines two major models for QoS on IP-based networks: Integrated Services (Intserv) and Differentiated Services (Diffserv). 

The Intserv model integrates resource reservation and traffic control mechanisms to support special handling of individual traffic flows. The Diffserv model uses traffic control to support special handling of aggregated traffic flows.

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A Brief Comparison of Cisco Nexus Product Family

The Cisco Nexus Series switches are modular and fixed port network switches designed for the data center (

2000 3000 4000 5000
Fabric Extender Standalone Blade Switch
Rack Impl. TOR TOR Blade TOR
Performance 88 ~ 1440 Gbps 480Gbps ~ 6.4 Tbps 400 Gbps 1.4 ~ 7.68 Tbps
10G Density ~ 48 32 ~256 14 + 6 Uplink 48 ~ 384
40GDensity ~ 64 ~ 96
100G Density ~ 32 ~ 32
FEX Support Up to 48 (L2/L3)
FC ~ 160
FCoE Yes ? Yes Yes
VxLAN Some Models ?
FabricPath ? ? ? Yes
Programmability ? OpenFlow ? Python, TCL, OpenFlow, NX-API


6001 / 6004 7000 / 7700 9200 / 9300 9500
Standalone Standalone Standalone/ ACI Leaf
Standalone/ ACI
Rack Impl. TOR/EOR EOR TOR / Aggregation EOR
Performance 1.28 / 7.68 Tbps 1.92 ~ 83 Tbps 1.44 ~ 6.08 15 ~ 60 Tbps
10G Density 48 / 384 96 ~ 768 ~ 144 ~ 2304
40GDensity 4 / 96 24 ~ 384 ~ 36 ~ 576
100G Density 12 ~ 192 ~ 36 ~ 128
FEX Support Up to 48 32 / 64 – / ~ 16 Yes
FC ?
FCoE Yes Yes – / Yes ?
VxLAN ? Yes Yes Yes
FabricPath Yes Yes ? ?
ACI Yes Yes
Programmability ? ?/ Python, TCL, OpenFlow, OnePK Python , Puppet, Chef, Ansible, NX-API Python , Puppet, Chef, Ansible, NX-API


TCL Script for DHCP Snooping Configuration

The first script is used to configure 61 Cisco Catalyst 2960S access switches for DHCP Snooping in a way that every switch builds the DHCP Snooping database with its own hostname on the FTP server so that different databases do not overwrite each other.

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