• Aaron Magruder

Cisco Campus QoS for Metro and Long-Haul Ethernet


So you purchased a large ethernet circuit between your sites instead of a traditional TDM or optical ring. Metro and Long-Haul Ethernet circuits are offered on the cheap with some Tier 2/3 carriers so why not. 100M sounds pretty good vs a couple T-1's are a DS-3. But you haven't realized the full potential of the circuit, traffic is slower than expected and your end users are not happy. Been there. Let's look at some of the common causes of slow Metro Ethernet.

Distance = Latency

Did you know the longer the distance between Point A & B the longer the cable. Makes sense. Keep in mind that just because one carrier has a circuit from Missouri to New York, doesn't mean it doesn't route through Alaska first. The longer the cable, the longer a packet takes to get there. This is called Latency. The higher the lantency, the longer it takes for packets to reach the end host, and respond with an ACK so you can send more data. The higher the latency, the slower the throughput. Calculations to follow in another blog.

Buffers, Queues and Thresholds Oh My

Quality of Service on a Cisco Switch isn't the easiest concept to grab but lets dig into it. Cisco switches have buffers, queues and thresholds to hold certain types of data that you specify to go in each queue. Depending on what type of switch or blade you have, determines the number of input and output queues and thresholds available to allocate different types of traffic to. When I talk about types of traffic, I mean CoS and DSCP values. i.e. Real-Time Voice is CoS 5 and DSCP EF usually so those two values get mapped to their own queue and threshold so they get priority over all other traffic.

I will be referencing Cisco 2960 / 3560 / 3750 series. Cisco 4500 / 6500 have more QoS options that are more extensive than this discussion is meant to cover.

Buffers

If you leave the default buffers on a Cisco Switch, they are 25% to each of the 4 buffers. So if the interface is 100M, each buffer can send 25M. Now if the interface is 1G, each buffer gets 250M. Big difference. You can change the buffer % allocation for each queue which you will need to do if you have a 100M ethernet circuit and a 100M port. Otherwise you can't send 50M of data traffic in the data queue as you only have 25M allocated to that buffer. Exactly why you need to get a 1Gig port and a 100M access if you order Metro Ethernet so you have the ability to send 100M in each Queue 1,2,3,4 otherwise you have to pick how much bandwidth you want each buffer to use. This will waste your available bandwidth if you pick 10% for voice, 10% for Critical, 10% for Transactional and 70% for default queue. What if you need more for one or the other? The way to solve this is get a 1G port, then you can send 250M to each queue. Now it will be limited to 100M by the carrier, but you have the option to use any buffer to fill that 100M that you want. Hopefully this explains buffers on a switchport.

Queues & Thresholds

Queues and thresholds allow you to put certain CoS and DSCP values in different buckets to ensure one type of traffic doesn't consume all the bandwidth. You don't want your FTP transfers in the same queue as your business critical web applications. There are typically 4 Queues and 3 Thresholds on the Cisco 2960/3560/3750. A threshold allows you to start dropping traffic in a specific queue at a specific percentage of the used queue. Lets say you have Web Apps and your Corporate IM client in Queue 3. Map the DSCP value of Web Apps to threshold 1 and IM Client traffic to threshold 3. The switch can be configured to drop the Web App traffic at 80% of the queue, while the IM Client won't drop traffic until the queue is 100% full.

Other Options

You do not have to connect your Metro ethernet circuit to your switching infrastructure if the carrier wants to charge more money for a Gig Port. Put a router in the middle as long as it has Gig ports to connect to the LAN side. Let the router buffer and QoS the traffic to the 100M Metro Ethernet circuit.

Need Help?

If you need help analyzing your QoS policies or configurations, contact NonStop Networks at 816-846-0676 opt 2


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