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Troubleshooting MAC-Flushes on NX-OS

January 21, 2013

An interesting client problem in one of our multi-tenant data centers came to my attention the other day. A delay sensitive client noticed a slight increase in latency (20 ms) at very intermittent intervals from his servers in our data center to specific off-net destinations. The increase in latency was localized to the pair of Nexus 7000’s functioning as the core switch layer (CSW) and the layer3 edge for this particular data center. Beyond that all appeared normal on the N7K CSWs.

A TCP dump from a normal trunk interface attached to the N7Ks, showed unicast traffic on the N7K-2 device when the N7K-1 device was setup to receive internet traffic inbound and forward it into the data center client VLANs.  The N7Ks are setup using the Cisco VPC (Virtual Port Channels).

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Detecting Layer2 Loops

October 24, 2012

We all too familiar with the devastating impact a talented layer 2 loop could have on a data center lacking sufficient controls and processes. If you are using Cisco Nexus switches in your data center, you would be happy to know that NX-OS offers an interesting new tool you should add to your loop detection list. The somewhat undocumented feature is known as (for the lack of a better name)  FWM-Loop Detection. FWM refers to the NX-OS Forwarding Manager. In Syslog it is seen as:

%FWM-2-STM_LOOP_DETECT

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FEX Architectures

October 7, 2012

Here is an old post I never finished. With the benefits of the Nexus 2000 and the FEX architecture (a earlier post), scalability, simplified management, flexibility, Cisco extended its use further into the servers all the way up to the virtual hosts.This allows much greater control and flexibility. After all network guys should look after all aspects of networking, server guys should look after the servers and today virtual hosts.

A summary of the different FEX implementations:
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The Fabric ERA

May 25, 2012

“Fabric” is a loosely used term, which today creates more confusion instead of offering direction.

What exactly is a Fabric ? What is a Switch Fabric?

Greg Ferro did a post here explaining how Ethernet helped the layer 2 switch fabric evolve. Sadly the use of switch fabric did not stop there. And this is the part where the confusion trickles in.

The term fabric has been butchered (mostly by marketing people) to incorporate just about any function these days. The term ‘switch fabric’ today (in the networking industry) is broadly used to describe among others the following:

  • The structure of an ASIC, e.g., the cross bar silicon fabric.
  • The hardware forwarding architecture used within layer2 bridges or switches.
  • The hardware forwarding architecture used with routers, e.g., the Cisco CRS and its 3-stage Benes switch fabric.
  • Storage topologies like the fabric-A and fabric-B SAN architecture.
  • Holistic Ethernet technologies like TRILL, Fabric-Path, Short-Path Bridging, Q-Fabric, etc.
  • A port extender device that is marketed as a fabric extender (a.k.a. FEX) namely the Cisco Nexus 2000 series.

In short, a switch fabric is basically the interconnection of points with the purpose to transport data from one point to another. These points, as evolved with time, could represent anything from an ASIC, to a port, to a device, to an entire architecture.

Cisco added a whole new dimension to this by marketing a Port Extender device as a Fabric Extender and doing so with different FEX architectures namely VM-FEX and Adapter FEX…. More on that in the next post. :)

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What is a Fabric Extender

May 16, 2012

In this post I would like to cover the base of what is needed to know about the Cisco Fabric Extender that ships today as the Nexus 2000 series hardware.

The Modular Switch

The concept is easy to understand referencing existing knowledge. Everybody is familiar with the distributed switch architecture commonly called a modular switch:

Consider the typical components:

  • Supervisor module/s are responsible for the control and management plane functions.
  • Linecards or I/O modules, offers physical port termination taking care of the forwarding plane.
  • Connections between the supervisors and linecards to transport frames e.g., fabric cards, or backplane
    circuitry.
  • Encapsulating mechanism to identify frames that travel between the different components.
  • Control protocol used to manage the linecards e.g., MTS on the catalyst 6500.

Most linecards nowadays have dedicated ASICs to make local hardware forwarding decisions, e.g., Catalyst 6500 DFCs (Distributed Forwarding Cards). Cisco took the concept of removing the linecards from the modular switch and boxing them with standalone enclosures. These linecards could then be installed in different locations connected back to the supervisors modules using standard Ethernet. These remote linecards are called Fabric Extenders (a.k.a. FEXs). Three really big benefits are gained by doing this.

  1. The reduction of the number of management devices in a given network segment since these remote linecards are still managed by the supervisor modules.
  2. The STP footprint is reduced since STP is unaware of the co-location in different cabinets.
  3. Another benefit is the cabling reduction to a distribution switches. I’ll cover this in a later post. Really awesome for migrations.

Lets take a deeper look at how this is done.

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N5K Stuck in Boot Mode

May 8, 2012

Another trivial post. The upcoming posts following this one will take a more in-depth look at the Nexus technologies.

So you do an non-ISSU NX-OS upgrade on a Nexus 5000 switch and something goes wrong. After reload you get the following prompt:

...Loader Version pr-1.3
loader>

The switch did not successfully boot from the images it was suppose to. How to go about restoring it?

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Load-Sharing across ASICs

April 26, 2012

Port-channels have become an acceptable solution in data centers to both mitigate STP footprints and extend physical interface limits.

One of the biggest drawbacks with port-channels is the single point of failure.

Scenario 1- Failure of an ASIC on one switch, which could potentially bring the port-channel down, if all member interfaces were connected on one ASIC.

Scenario 2- Failure of one switch on either side. The obvious solution available today is multi-chassis port-channels which addresses the problem 95%.

Consider the following topology:

Even with multi-chassis port-channel there is the still the possibility of an ASIC failure.  Although not as detrimental as Scenario-1, there will still be some impact (depending on the traffic load) if both interfaces on one switch happen to connect to the same ASIC.

Thus it only makes sense that the ports used on the same switch, uses different ASICs. How would confirm this on the Nexus 5000 and Nexus 7000?

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