Tuesday, February 24, 2009

Selection Of Vlan In Local Switching Domain

Till now I have not bring out anything on switching design. In this post I will let you know how to select the vlans in switching domain especially if spanning tree is working in that domain. Have you ever thought during switching design which vlan number should be used for which type of vlans. Let say if we are having one vlan for data, one for voice and one for vlan. For this I select vlan 10 for data, vlan 20 for video and vlan 30 for voice. It’s so simple and easy. But what will happen if STP is running in that domain and the design is like Switch1-Switch2-Switch3-Switch1. One port will be in blocking mode and if the links fails another will be in forwarding mode. This is the way of spanning tree works. A question comes in mind what vlans have to do with this scenario. Definitely vlans plays a major role during the convergence from one link to another; during the convergence always small number vlans get preference over the bigger number vlans. As I mentioned earlier about the three vlans in that always data vlans will flow without any impact because it is shortest in number and voice vlan number is largest one and get to come in picture last. So think about the scenario where voice is always prioritized first during the convergence it will the least one. So if little change is made in the numbers than the voice vlan traffic is always preferred without any delay. So during lan switching design always take the suggest shortest number for critical applications.

shivlu jain

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