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VLAN Design

Hello Fellow Experts,

Are there any Cisco documents that implicitily recommend the use of smaller subnets over larger ones? i.e. VLAN Design Best Practices

Aside from the obvious benifits, can anyone think of other advantages other then the following:

A. Smaller Broadcast / Bandwidth Domains

B. Less impact to STP BPDU on large (flat) L2 networks where timely receipt of BPDU's is important.

C. Less impact to wired and wireless client machines that must listen to broadcast/multicast traffic.

D. Increased granularity when defining ACL, QoS & Security Policies through increased VLAN segmentation (i.e. smaller subnets) 

E. Increased performance through VLAN segmentation of network traffic.

In addition, there might be alternate reasons why keeping a subnet/VLAN small. While reviewing some Cisco documentation, I discovered the following:

Examples of why the switches may not receive BPDUs include bad transceivers or Gigabit Interface Converters (GBICs), cabling issues, or hardware failures on the port, the linecard, or the Supervisor engine. One frequent reason for STP failures is a unidirectional link between the bridges. In such a condition, one bridge sends BPDUs, but the downstream bridge never receives them. STP processing can also be disrupted by an overloaded CPU (99 percent or more), because the switch is unable to process received BPDUs. BPDUs can be corrupted along the path from one bridge to the other, which also prevents proper STP behavior.

Aside from the forwarding loops, when no ports are blocked, there are situations when only certain packets are incorrectly forwarded through the blocking ports. In most cases, this is caused by software issues. Such behavior might cause “slow-loops.” This means some packets are looped, but the majority of the traffic is still flowing through the network, because the links are probably not congested.

STP Path Cost Automatically Changes When a Port Speed/Duplex Is Changed

STP calculates the path cost based on the media speed (bandwidth) of the links between switches and the port cost of each port forwarding frame. Spanning tree selects the root port based on the path cost. The port with the lowest path cost to the root bridge becomes the root port. The root port is always in the forwarding state.

If the speed/duplex of the port is changed, spanning tree recalculates the path cost automatically. A change in the path cost can change the spanning tree topology.

If auto negotiation fails, and STP re converges. If re-convergence does not occur rapidly enough, the segment will goe down until STP reconverges.  


In an effort to prove my point further, I would like to propse a risk management related question:


Q: Would you rather have an outage that affects 254 users or 1024?

A: You tell me?

Q: What are implications to wireless subnets that are 1024 or larger?

Q: What size subnet would you deploy for your WLAN's and WHY?

Aside from what I have already thought of, I would like to here from other experts 













Christian J. Estes, cwne #85, cciew #42615
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