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Archives for July 2013

CSMA/CD :: Fundamental Switch Operation

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CSMA/CD, or Carrier Sense Multiple Access with Collision Detection, is an access method used in Ethernet.

When a device is about to send data over the network, it will sense if another device is already transmitting. If there is indeed another device transmitting, the device checking will back off from communicating for a random amount of time before retrying.

If two devices check the network at the same time and don’t see any other devices transmitting data, they will both transmit at the same time. When this occurs there is a collision. When a collision is detected, both devices use a random time to back off before retrying.

The back off time is random since if both devices talk on the network at the same time and back off at the same time they will have another collision.

TCAM :: Fundamental Switch Operations

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Two tables are used in switch operations, CAM and TCAM. We visited the CAM table in an earlier discussion. In this post we briefly talk about Ternary Content Addressable Memory, or TCAM. Why is it called Ternary? Because it provides three results: 0, 1, and “don’t care”.

The TCAM allows a packet to be evaluated against an entire ACL in a single table lookup. One swoop. This makes lookups ultra-fast. It is done in hardware.

There are two components of TCAM operation:

  • Feature Manager (FM)
    • Which compiles the access controle entries into the TCAM table.
  • Switch Database Manager (SDM) **Not to be confused with Security Device Manager
    • Which configures or tunes TCAM partitions, if needed.

Greg Ferro has some good information on TCAM and it’s impact with IPv6.

What is Cisco EtherChannel

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Cisco EtherChannel
Bundling switch ports

EtherChannel is a method used in Cisco switches to aggregate bandwidth from multiple switch ports. Another word for this method is bundling. On a Cisco switch you can bundle two to eight ports. An example is the photo above. Two switch ports have been bundled together to connect to another switch.

An example would be bundling eight fast ethernet ports. Each port is 100 Mbps. Multiply 100 Mbps by eight and you get 800 Mbps. If these were all full duplex ports you would get 100 Mbps of bidirectional traffic for each port totaling up to 1600 Mbps (8 x 200 Mbps) of aggregated bandwidth. If these were gigabit ports you could get up to 16 Gbps (8 x 2 Gbps).

When ports are put into the same EtherChannel they are considered bundled. To bundle ports, they must meet the following requirements:

  • Same Ethernet media (Fast Ethernet, Gigabit, etc)
  • Same VLAN
  • Same duplex and speed

If the bundle is going to be a trunk:

  • All bundled ports must be in a trunking mode
  • Same native VLAN
  • Pass the same set of VLANs
  • Same duplex and speed

Benefits

Aside from aggregating bandwidth, another benefit with EtherChannel is redundancy. If one of the ports in the bundle goes down, the other ports will have to pick up the slack. And because you are using EtherChannel, you are able to load balance the traffic across multiple ports.

Port Aggregation Protocols

When configuring EtherChannel you have the choice of using two protocols:

  • Port Aggregation Protocol (PAgP)
  • Link Aggregation Control Protocol

PAgP is a Cisco proprietary protocol. It isn’t used much anymore.

LACP is an alternative to PAgP and it is a standard (802.3ad).

Cisco Express Forwarding and the Forwarding Information Base :: Fundamental Switch Operat

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CEF, or Cisco Express Forwarding, is a layer 3 switching technology. It’s an improvement over fast switching and route processor switching.

The FIB, or Forwarding Information Base, is a direct copy of the routing table. If a change occurs on the routing table, it is also reflected in the FIB.

CEF and FIB are a team. Together they are fast. CEF is known as a topology-based switching operation, hence the FIB. Because the information CEF uses is stored in cache, it is able to switch subsequent traffic flows much faster.

References: Understanding Cisco Express Forwarding (CEF)

Tackling the CCNP SWITCH Exam

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As some of you know, I’ve been back and forth on the CCNP Routing and Switching certification. A little over a year ago I sat in for the ROUTE exam and failed.

This time around I’ve decided to take on the 642-813 SWITCH exam first. Currently, I’m doing more work on switches than routers so it only makes sense right?

Training Material

I’m determined to get a pass this time around so I’ve given myself plenty of resources:

CCNP SWITCH Official Certification Guide (OCG) – This is my primary text book. I am reading it from cover to cover (can’t forget the coffee.) The way I schedule out my reading is I keep it to one chapter per week. There’s 18 chapters so I should be done with the book in 18 weeks.

To be clear – I may not be ready for the exam in 18 weeks, I would be finished reading the text book. I may very well need to re-read some chapters. That’s fine. It’s better to know the material than to speed along just to pass an exam.. because you won’t pass if you don’t know it.

There are pre-chapter quizzes. I’ve opted to leave this process at the end of reading a chapter.

At the end of each chapter there is a section called “Exam Preparation Tasks”. Use this section to verify what you’ve just learned. Review the key topics listed and be sure you can define the key terms. What I actually do with this section is I turn it into a blog post. I write about the key topics and key terms but in my own words.

If I have trouble explaining those topics then I either:

  • Re-read the section in the OCG
  • Find the section in the CCNP SWITCH FLG
  • Find external material to help solidify the topic in my brain
    • Cisco’s documentation
    • Cisco Learning Network
    • Networking-forum.com
    • Other blogs

If you can teach it then you’ve done well. Only after doing the above I then take the chapter quiz. Didn’t do so well? Then spend more time reviewing.

Implementing Cisco IP Switched Networks (SWITCH) Foundation Learning Guide (FLG) – This is a supplemental text book to the OCG. I may not read it from cover to cover but it’s a nice to have if you want to read more details about a specific topic. You may even want to make this one your primary text book if you’re new to network engineering.

CBT Nuggets – For $99/mo you can have access to CCNP training videos. Jeremy Cioara is an excellent trainer. I don’t even fall asleep while watching or listening to these videos.

INE Videos – I’ve heard good things about INE’s CCNP series training videos. If anything, I would use this as secondary to CBT Nuggets if I find myself scoring lower than expected on a practice exam.

Forums, Blogs, etc – There are so many other resources out there. For example, Cisco has all their documentation online. If you’re having a hard time understanding HSRP, perform a Google search and find Cisco’s documentation on just that. Other people may have asked your question on forums. Don’t be afraid to ask on forums either. There are many blogs out there dedicated to passing certifications but it may be harder. INE has a good blog which discusses some certification topics.

Schedule

I don’t have a normal schedule. My wife and I work very hard and we have a 2 year old toddler who is very active. The majority of my studying gets started after my son goes to bed. Sometimes I have to make dinner. There’s grocery shopping to do beforehand. Don’t forget the laundry. Time management will be key here if you have a life similar to mine.

They say sticking to a schedule is key. What is more important is making it fit around your current schedule. Will it be 1-2 hours every night? Every other night? This is entirely your call.

Since I study in the evening, I can probably dedicate 2 hours max of studying. I wake up early to be at work by 6am. Some nights I just can’t study and I need sleep. If you can, get those training videos on MP3 and listen to them on your commute.

Write your notes in Evernote so you can review them anywhere. I have the Evernote app on my iPhone, iPad, and laptop.

Switch Unicast Flooding

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A unicast is a message being sent to a single destination. So what is unicast flooding then?

When a switch receives a frame destined to a host in which it does not have in it’s CAM table then the switch will flood the unicast frame to all ports with the same VLAN ID.

Basically, the unicast becomes a broadcast. The term for this is unknown unicast flooding.

In the example below, let’s assume Host A sends a frame to the switch destined to Host B. Because host B is not in the CAM table, the switch will flood the unicast frame to all ports with the same VLAN ID. This is what the CAM table looks like for the switch as Host A sends it’s frame:

[table id=3 /]

Switch Unicast Frame Flooding