Cisco QoS - Revision history

Casey at 21:07, 31 December 2013

2013-12-31T21:07:59Z

Casey at 21:06, 31 December 2013

2013-12-31T21:06:00Z

Casey at 20:57, 31 December 2013

2013-12-31T20:57:28Z

Casey: Created page with "There's lots of talk about Quality of Service (QoS). It lets you adjust traffic flow to give priority to some traffic, and limit others. It's not the magical cure to network..."

2013-12-31T20:32:52Z

Created page with "There's lots of talk about Quality of Service (QoS). It lets you adjust traffic flow to give priority to some traffic, and limit others. It's not the magical cure to network..."

New page

There's lots of talk about Quality of Service (QoS). It lets you adjust traffic flow to give priority to some traffic, and limit others. It's not the magical cure to network issues, that some people think it is. It's more like overdraft protection on your bank account: it's good for those little emergencies, but you do not want to depend on it. If your QoS is constantly adjusting packets to compensate for congestion or high latency, you need a better pipe.

That being said, here's how to set up a basic policy that will optimize various types of traffic.

QoS works on a two step process: classify packets then optimize their flow.

The policies like this get applied to the WAN interface of the router on each end of the connection. You need to know what the '''actual''' available bandwidth is on the link, not just what it's specified as. If you set the line rate in the policy to be higher than what the usable capacity is, the QoS policy will never kick in. '''With Cisco, the QoS policy is only activated during times of congestion.'''

A major problem with high-latency, low-bandwidth links like T1's is that small, time-sensitive VoIP packets get held up behind big data packets that take a long time to send. The solution is to break up the big packets, and interleave the voice packets. Part of the solution is to lower the IP MTU on the WAN interface of the router. Once this is done, big packets that come in to the LAN interface get broken up (fragmented) by the CPU, which mixes in the voice packets according to the priority statements in the QoS policy.

<PRE>
! Depending on the software revision, you can identify traffic by almost any attribute
class-map match-any DSCP-VOICE-CONTROL
match dscp af41
class-map match-any NETWORK-CONTROL
match protocol ospf
match protocol snmp
match protocol icmp
match protocol telnet
class-map match-any DSCP-VOICE
match dscp ef

! This policy takes the traffic classes and allocates bandwidth to them
policy-map CLASSIFY_TRAFFIC
class DSCP-VOICE
! "Priority" traffic is sent to the egress queue before all other traffic (in kbps)
! The priority is also a rate limiter, so during congestion, anything in excess of the vlaue will be dropped.
! You'll want to be sure to have this high enough that you won't cause any packet drops, but not so high that you
! wont have enough bandwidth for any other traffic classes.
priority 1100
class DSCP-VOICE-CONTROL
! Bandwidth values can be in kbps or percentages of the overall bandwidth (see below)
bandwidth 100
class NETWORK-CONTROL
bandwidth 8
! Any remaining bandwidth is given to any unclassified packets.

! This policy defines the overall bandwidth on the interface, as it may be less than the interface's physical bandwidth
policy-map WAN_POLICY
class class-default
! The total bandwidth values defined above cannot exceed the number below
shape average 1300000
! Embeds the policy above into this policy
service-policy CLASSIFY_TRAFFIC
!
!
!
interface FastEthernet1
description TO WAN CONNECTION
ip address 10.10.10.2 255.255.255.248
! 600 is a value that I found works generally well.
ip mtu 600
! These commands limit the number of packets that can be in the hardware transmit queue.
! This gives the CPU more control over which packets are sent over the wire first.
tx-ring-limit 3
tx-queue-limit 3
! Applies the policy to the WAN interface
service-policy output WAN_POLICY
!
</PRE>

@@ Line 46: / Line 46: @@
 ==The Outer Policy==
-This policy defines the overall usable bandwidth on the interface in kbps, as it may be less than the interface's physical bandwidth.  Note that the sum of the bandwidth values defined in the inner policy must not exceed this value.  If you used percentages in the inner policy, they are calculated against this number.
+This policy defines the overall usable bandwidth on the interface in kbps, as it may be less than the interface's physical bandwidth.  Note that the sum of the bandwidth values defined in the inner policy must not exceed this value.  If you used percentages in the inner policy, they are calculated against this number.  Note that it is a shaper, not a policer, so all packets that get queued ''should'' get transmitted instead of dropped.  It's possible that packets can still time out in the queue due to queue starvation.
 The inner policy is embedded in this policy.

@@ Line 6: / Line 6: @@
 The policies like this get applied to the outbound traffic on the WAN interface of the router on each end of the connection.  You need to know what the '''actual''' available bandwidth is on the link, not just what it's specified as.  If you set the line rate in the policy to be higher than what the usable capacity is, the QoS policy will never kick in.  '''With Cisco, the QoS policy is only activated during times of congestion.'''
@@ Line 48: / Line 46: @@
 ==The Outer Policy==
-This policy defines the overall usable bandwidth on the interface, as it may be less than the interface's physical bandwidth.  Note that the sum of the bandwidth values defined in the inner policy must not exceed this value.  If you used percentages in the inner policy, they are calculated against this number.
+This policy defines the overall usable bandwidth on the interface in kbps, as it may be less than the interface's physical bandwidth.  Note that the sum of the bandwidth values defined in the inner policy must not exceed this value.  If you used percentages in the inner policy, they are calculated against this number.
 The inner policy is embedded in this policy.
@@ Line 61: / Line 59: @@
 ==The Interface Config==
 <PRE>
 interface FastEthernet1
-  description TO WAN CONNECTION
+  description WAN CONNECTION
-  ip address 10.10.10.2 255.255.255.248
+  service-policy output WAN_POLICY
   ip mtu 600
   tx-ring-limit 3
   tx-queue-limit 3
 </PRE>

@@ Line 5: / Line 5: @@
 QoS works on a two step process:  classify packets then optimize their flow.
-The policies like this get applied to the WAN interface of the router on each end of the connection.  You need to know what the '''actual''' available bandwidth is on the link, not just what it's specified as.  If you set the line rate in the policy to be higher than what the usable capacity is, the QoS policy will never kick in.  '''With Cisco, the QoS policy is only activated during times of congestion.'''
+The policies like this get applied to the outbound traffic on the WAN interface of the router on each end of the connection.  You need to know what the '''actual''' available bandwidth is on the link, not just what it's specified as.  If you set the line rate in the policy to be higher than what the usable capacity is, the QoS policy will never kick in.  '''With Cisco, the QoS policy is only activated during times of congestion.'''
 A major problem with high-latency, low-bandwidth links like T1's is that small, time-sensitive VoIP packets get held up behind big data packets that take a long time to send.  The solution is to break up the big packets, and interleave the voice packets.  Part of the solution is to lower the IP MTU on the WAN interface of the router.  Once this is done, big packets that come in to the LAN interface get broken up (fragmented) by the CPU, which mixes in the voice packets according to the priority statements in the QoS policy.
 <PRE>
-! Depending on the software revision, you can identify traffic by almost any attribute
+class-map match-any DSCP-VOICE
-class-map match-any DSCP-VOICE-CONTROL
+ match dscp ef
-  match  dscp af41
+class-map match-any DSCP-VOICE-SIGNALLING
 class-map match-any NETWORK-CONTROL
   match protocol ospf
@@ Line 18: / Line 23: @@
   match protocol icmp
   match protocol telnet
-class-map match-any DSCP-VOICE
+</PRE>
-  match  dscp ef
-! This policy takes the traffic classes and allocates bandwidth to them
+<PRE>
 policy-map CLASSIFY_TRAFFIC
   class DSCP-VOICE
    priority 1100
   class DSCP-VOICE-CONTROL
-   ! Bandwidth values can be in kbps or percentages of the overall bandwidth (see below)
+   ! Bandwidth values can be in kbps or percentages of the overall bandwidth
    bandwidth 100
   class NETWORK-CONTROL
    bandwidth 8
-! Any remaining bandwidth is given to any unclassified packets.
+</PRE>
-! This policy defines the overall bandwidth on the interface, as it may be less than the interface's physical bandwidth
+==The Outer Policy==
 policy-map WAN_POLICY
   class class-default
    shape average 1300000
    service-policy CLASSIFY_TRAFFIC
+</PRE>
-−
-−
 interface FastEthernet1
   description TO WAN CONNECTION