Rate Limiting and QoS on 3750 WAN intf?

Answered Question
May 26th, 2009

All,

I'm currently using a 3750 in a co-lo environment as my co-lo backbone.

One of the interfaces goes to an MPLS cloud. Now, the co-lo delivers the circuit via Ethernet. As such, my interface is running 100Mbps. But, it's only a 3Mbps cloud.

How can I rate-limit this interface down to 3Mbps in such a way, to where I can apply my QoS policies, and QoS will use the rate-limited 3Mbps rather than the full 100Mbps.

According to the QoS Order of Operations, rate-limiting comes BEFORE queuing, and I'm afraid that my voice packets may get dropped.

I can police the individual voice traffic, but I want all other traffic to use up to 100% of the line, if voice is not there.

srr-queue bandwidth limit won't work by itself, as I can only go down to 10% (which, from 100Mbps is 10Mbps, and I need 3Mbps).

I guess I'm just getting hung up on the 3Mbps circuit. Since it's a 100Mbps interface, queuing will never really happen on my side, since the bandwidth will support it.

Thanks for any advice you have!

Correct Answer by Joseph W. Doherty about 7 years 8 months ago

Can you run 10 Mbps with the MPLS provider's Ethernet?

If not, place a "dumb" L2 switch between your 3750 and MPLS Ethernet. Your side, 10 Mbps; MPLS side, 100 Mbps.

For either, once your interface is at 10, set ssr-queue bandwidth for 30%.

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Correct Answer
Joseph W. Doherty Tue, 05/26/2009 - 16:38

Can you run 10 Mbps with the MPLS provider's Ethernet?

If not, place a "dumb" L2 switch between your 3750 and MPLS Ethernet. Your side, 10 Mbps; MPLS side, 100 Mbps.

For either, once your interface is at 10, set ssr-queue bandwidth for 30%.

apaxson Wed, 05/27/2009 - 07:16

josephdoherty,

As usual, I tend to make things more complicated than they need to be. :) I'm asking my provider for a 10Mb connection now.

Thanks for your assistance!

kejeusa@lycos.com Wed, 05/27/2009 - 07:54

you could just manually set the bandwidth on the interface that would used by the qos policy.

int fa0/1

no switchport

ip address x.x.x.x

bandwidth 3000

Paolo Bevilacqua Wed, 05/27/2009 - 09:28

The command "bandwidth" above is absolutely ineffective to set bandwidth as repeated countless times since the beginning of cisco.

apaxson Wed, 05/27/2009 - 10:00

Correct. I believe the "bandwidth" command is only useful for routing protocols only, or if you are getting an interface speed from snmp to base graphs off of.

Thanks!

Joseph W. Doherty Wed, 05/27/2009 - 12:08

Keenon, in this case, we're dealing with a 3750, so it doesn't work the same way it might on a router.

On routers (within platform/IOS limitations), we can use the bandwidth to control percentage based QoS.

e.g.

given this:

policy-map test

class class-default

shape average percent 50

On a Ethernet interface running at 100 Mbps, we get:

Rtr1#sh policy-map i o

FastEthernet0/0

Service-policy output: test

Class-map: class-default (match-any)

31 packets, 2076 bytes

5 minute offered rate 1000 bps, drop rate 0 bps

Match: any

Traffic Shaping

Target/Average Byte Sustain Excess Interval Increment

Rate Limit bits/int bits/int (ms) (bytes)

50 (%) 0 (ms) 0 (ms)

50000000/50000000 312500 1250000 1250000 25 156250

Adapt Queue Packets Bytes Packets Bytes Shaping

Active Depth Delayed Delayed Active

- 0 31 2076 0 0 no

But if we change the "bandwidth", e.g.:

Rtr1#sh run in fa 0/0

Building configuration...

Current configuration : 180 bytes

!

interface FastEthernet0/0

bandwidth 25000

We now get:

Rtr1#sh policy-map i o

FastEthernet0/0

Service-policy output: test

Class-map: class-default (match-any)

131 packets, 10654 bytes

5 minute offered rate 1000 bps, drop rate 0 bps

Match: any

Traffic Shaping

Target/Average Byte Sustain Excess Interval Increment

Rate Limit bits/int bits/int (ms) (bytes)

50 (%) 0 (ms) 0 (ms)

12500000/12500000 75000 300000 300000 24 37500

Adapt Queue Packets Bytes Packets Bytes Shaping

Active Depth Delayed Delayed Active

- 0 131 10654 0 0 no

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