Monday, September 24, 2012

ARP Cache/Table

The Address Resolution Protocol or ARP is very important in data network communication. It is used to translate Layer 3 address (IP) to Layer 2 address (MAC). To do so, host or switch/router will create one ARP table which contains the mapping between IP address and MAC address.

Like MAC table, ARP table could be statically configured and dynamically learned. ARP protocol is used for learning. Basically, when the host wants to send IP packets to 192.168.100;  It will first query whether corresponding entry in ARP table. If found,  the corresponding MAC address will be the L2 address used. Or else,  the host will broadcast one ARP request saying: whose IP address is, and what's your MAC address? Every host on the same LAN will compare its IP address with the request; If they matches, it will respond with its own MAC address. Then the host will update its local ARP table for future usage.
To prevent ARP table becoming too big, ARP table entry will age out. The aging time could be configured in
modern operating system or switch/routers.

Sunday, September 23, 2012

MAC Address

MAC address or Media Access control address is a 48-bit/6-byte address associate with a network interface card on a host or a network interface on a router/switch. Mostly, MAC address is permanent on the hardware. So sometimes it is called physical address or burn-in-address.
MAC address consists of 3-Byte of organization identifier and 3-byte of Network Interface Identifier. 

On switch, MAC address table is used to associate MAC address, VLAN to specific physical port so that Ethernet packet could be correctly forwarded by MAC address look-up.
MAC address table could be configured statically or learned dynamically. The entry will age out because of
MAC address table is a rare hardware resource and the number of entries are limited.
On host, MAC address is used to identify the specific physical network interface on the LAN network.
For further details, see MAC address wiki page.
To understand how MAC address works in switch, read this article.

Saturday, September 22, 2012

Virutal Output Queues

Virtual Output Queues(VOQ) is one buffer mechanism used to overcome head-of-line blocking in modern packet switches.  To understand the input and output queuing of switch, readers could refer to one good
blog article here.

Friday, September 21, 2012


On,  the following news just published a couple of days ago:

Cisco, Arista Unveil Low-Latency Ethernet Networking Switches 

What's the latency? How to measure it?  Google and found a couple of very good resources to explain latency.  Just list them here for reference.

  1. Understanding Switch Latency from Cisco Systems Inc
  2. Cut-Through and Store-and-Forward Ethernet Switching for Low-Latency Environments from Cisco Systems Inc
  3. It's time for low latency from Standford University

VN-Tag and VEPA

One good article about VN-Tag and VEPA is here:
 Also found a couple of resources on the Internet: 
  1. One blog article  is here
  2. VNTAG 101 slides from IEEE 802
  3. Another good link - IO Virtulization Overview
Both are network interface virtulization technologies,
which makes the switch be aware of VMs on the host,
so that traffic could be correctly forward.

VN-Tag is implemented on Cisco Nexus Series
Data Center Switches; while VEPA is implemented
by data center switching solutions from other vendors
like HP.

Thursday, September 20, 2012

Data Center Switching Products Overview

 1. Arista 7000 Family Switches
  • The Arista 7500 Series Modular Switches (4-Slot, 8-Slot)
  • The Arista 7150 Series  Fixed Switches
  • The Arista 7100 Series
  • The Arista 7050 Series
2. Cisco Nexus Series Switches
  •     Cisco Nexus 7000 Series  Modular Switches (4-Slot, 9-Slot, 10-Slot, 18-Slot)
  •     Cisco Nexus 5000 Series Fixed Switches 
  •     Cisco Nexus 3000 Series Fixed Switches (Ultra-low latency, High-density)
  •     Cisco Nexus 2000 Series Fabric Extenders
  •     Cisco Nexus 1000V Series Virtual Switches
 3. Dell Force10 Data Center Switches
  •       Dell Force10 Z-Series Core Switches 
  •       Dell Force10 E-Series Virtualized Core Switching
  •       Dell Force10 C150 Chassis-based Switches
 4. HP 7000 Family Switches
  • The HP 12500 Switch series(8-Slot, 18-Slot)
  • The HP 5900 Switch Series 
  • The HP 5920 Switch Series 
  • The HP 5800 Switch Series
  • The HP 5820 Switch Series
  • The HP 5830 Switch Series 

 5. IBM System Networking Switches
  • IBM G8000 Series RackSwitch
  • IBM Flex System Scalable Switches
  • IBM BladeCenter Series Switches