MLS basics

[vasista]

Hi

I am tring to understand the operation of multi layer switching. i am confused. according to the book its mentioned that traffic between the subnets is routed by a switch. the part that confuses me is why would some one connect 2 diffrent subnets to the same switch? the purpose of subnetting is to prevent broadcast traffic and if you are putting diffrent subnets on the same switch, broadcast trffic cannot be avoided.

can some one explain this to me

thanks

regards

vasista

[Georg Pauwen]

Hello,

MLS simply means that certain Catalyst switches, such as the 3550, the 4500, or the 6500, can also forward frames based on Layer 3 and Layer 4 information contained in packets. That effectively turns the switch into a router. Two subnets connected to the MLS switch/router then behave the same way as two subnets connected to a router, the broadcast domain is limited to the subnet.

HTH,

Georg

www.solutionfinders.nl

[pflunkert]

Hi Vasista,

in a modern network it's normal that different subnets are connected to one switch (or switching area). You can say that one subnet is one vlan. In a big company several vlans (subnets) are normal. The broadcast from one vlan can't reach other vlans, you prevent this. When you have several vlans on a switch you prevent the connection between vlan 1 (subnet 1) and other vlans (subnets). For this you need a layer 3 device. Several cisco catalyst switches have layer 3 capabilities. The disadvantage from a normal layer 3 device was the speed. So Cisco introduce MLS. When you want sent traffic from vlan 1 to vlan 2 (or subnet 1 to subnet 2) you need a router. In this case the switch as layer 3 device. The first packet is sent to the switch, the switch sent the packet to the internal router, the internal router routes the packet to subnet 2 (vlan2) and sent the packet to vlan 2 and the associated ports. Now the router sent a flow entry from the route processor to the switch processor. This means when the next packets arrive the switch knows the destination port for vlan 2. The switch sent the packet without contacting the route processor. The advantage from this is the performance. Normally you can switch packets with wire speed. Please remember that today a switch is a layer 2 and a layer 3 device (for example Cat 3550,3560,4500,3750,6500; but not 2950).

Regards

Peter

[vasista]

Hi Peter

Thank you very much for the reply. It didnt occur to me about Vlans. so for effective MLS design VLANS should be configured for every subnet

[glen.grant]

You apply a layer 3 virtual interface to each vlan at layer 2 for each subnet , this is usually done at the distribution layer of the network . MLS usually pertains to older cat 5500's with a rsm and catos supervisor card and also older 6500 using Hybrid software . Newer units use CEF for fast switching the packets .