Analysis Of Layer 2 Design

Traditional Enterprise and Campus deployments have been designed in such a way that allows for scalability, differentiated services and high availability. However in case of traditional layer 2 network design, there are many traps and dependencies. In our daily routine, we build these small LAN networks but hard to get time to analyze it. Mainly three issues can arise in Layer 2 networks - loops, traffic drop and excessive flooding.



The design of above depicted network is very simple with below listed design considerations:-
1.One Switch Per Subnet/Per VLAN.
2.It limits the Layer 2 broadcast domain size to the size of the access switch.
3.No Layer 2 Loops.
4.Easy to troubleshoot.

But above depicted network is well suited for small networks because per VLAN scale is very limited.
Lets see how does a LAN network looks like with a traditional Layer 2 loop.



Now the above network has layer 2 loop but still the VLAN scalability is very less, in fact it is restricted up to access switch only.



Lets analyze what could be the potential issues/risks can arise in above network.
1. Frames can replicate repeatedly.
2. Large number of multicast and broadcast frames.
3. High traffic drop.
4. There might be probability of BUM (broadcast, multicast and unknown unicast) traffic.

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WLAN subnet sizing recommendations or Best Practices

Excerpts from Mobility Design Guide-

"The default behavior of the WLC is to block broadcast and multicast traffic from being sent out the WLAN to other wireless client devices."

http://www.cisco.com/en/US/docs/solutions/Enterprise/Mobility/emob41dg/ch2_Arch.html#wp1028269

In most of the WLAN designs it is considered safe to use large subnet sizes if you are not allowing Bcast and not much of Mcast traffic.

As long as the WLC is not propagating Bcast/Mcast traffic and no spanning tree loops, the Mobility subnet sizing is not a constraint for WLAN. Usual recommendation to Large deployments is ---no subnet size more than /20 and /19.

Also it depends so much on the type of traffic specially Bcast/Mcast traffic. So it is NOT advisable to use large subnet size in case of Bcast/Mcast/STP loops in N/W.

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