Cisco: Mark voice packets at the network edge

You know how Cisco always advise to mark the packet as close to the your network edge as you can? Even more you can find a lot of example where Cisco show how to trust the packets directly on the access switch, but not all the time you can do this.

First because not everybody has devices that mark correct packets (like Cisco IP Phones) but we still have to deal somehow with packet marking as maybe your provider treat packets different on their backbone based on their marking.

In this idea what I’m taking care the most are the voice packets as usually this has to be prioritized on the network. Let’s face it, if you have a TCP connection and some FTP packet are retransmitted you don’t notice this too much, but if you have delay on your phone conversation with your boss, that it’s not so good.

Please have a look at the topology below:

In this scenario we have a Voice server and some IP Phones (I know they look like Cisco IP phone, but pretend they are not) connected to the access switch. Let’s assume that  we cannot trust marking on this packets as they arrive from this devices.

Here we run into one of the two issues. First if we trust the marking on the access port, than we don’t know what we are stuck with. If we don’t trust them, then the packets header DiffServ (TOS) bits are rewrite with a value of zero making no difference between voice packets and regular ones.

My solution is the following. I’m not saying that’s the only solution or the best, but it’s working:

On the access switch:

1. enable globally:

mls qos

2. configure and access-list that match the voice packet; this is a very general list:

access-list 101 permit udp any any range 16384 32767
access-list 101 permit udp any range 16384 32767 any
access-list 101 permit udp any any range 5060 5061
access-list 101 permit udp any range 5060 5061 any

3. match the access-list in a class-map

class-map match-all VOIP
match access-group 101

4.configure a policy-map with the class-map above and set the DSCP value to EF (decimal 46) or COS or whatever you need

policy-map ASTERISK
class VOIP
set dscp ef

5.on the access port configure the service-policy direction inbound

int x/y
service-policy input ASTERISK

6.on all trunks from your access switch to your first Layer 3 device trust this DSCP  value (or what you have set, as now we are sure what values we set) with:

mls qos trust dscp

Let me know if it’s working!

Cisco AutoQoS VoIP

QoS – one of the most interesting and challenging part of the network engineer’s life. I think I’m not wrong when I’m saying that most engineers that hear the word QoS, react somehow like “ah, that ugly stuff…” I have to be honest and say that QoS is not one of my preferred part, but as a network engineer you have to accomplish all task, not only the one that you like. With QoS you can achieve some great results, and once you have started to work with it, you’ll see that is not such a monster. The idea that QoS is not a friendly topic was develop due to historical ways to configure this service. In fact there were so many ways that without working with this topic everyday, you have no chance to remember how has to be implemented. Even among Cisco devices, there were multiple way to configure QoS from device to device. Nowadays Cisco is putting a lot of effort to standardize the implementation of QoS and in the same time to simplify it.

In this context AutoQoS appeared. First, AutoQoS VoIP was developed as the more QoS configuration is needed for Voice traffic over the network and then a more complex Auto QoS Enterprise was released. Today I plan to speak a little bit about AutoQoS VoIP and in one of the future articles about the Enterprise edition.

AutoQoS VoIP gives you the ability to deploy QoS features for converged IP telephony and data networks in a fast and reliable way, by simplifying the MQC (Modular QoS command-line interface). AutoQoS VoIP generate automatically traffic classes and policy map CLI templates. In more human terms, if AutoQoS VoIP is configured on an interface, the traffic that is passing by is receiving the required QoS treatment automatically. By doing this automatically AutoQoS spares you from having in-depth knowledge about the services policies, link  efficiency mechanism or best practice for Voice QoS. Everything is done by AutoQoS, which automatically discovers applications and provides appropriate QoS treatment, generate policies and provide configuration.

Of course that are some drawbacks like in any other technology, but this can be easy overcome if you have some knowledge about QoS configuration. It theory everything should run smoothly, but in real environment, you run into the situation that QoS should be fine tuned to reach the level of your expectation. This is easy done with AutoQoS, as after it enable and configure the QoS templates, you can easy configure them to fine tune parameters, add or delete policies. Basically you start from nice template, sourced by AutoQoS, and develop your own QoS. As I said before, you have only to do this is something is not working or if you feel the strong attraction to “make it your own way”.

AutoQoS VoIP is supported on the most popular Cisco routers and switches platform with the appropriate IOS or CatOS image. Before you configure AutoQoS VoIP you should know that:

– CEF is required to be configured, as AutoQoS use NBAR (Network-Based Application Recognition) which is based on Cisco Express Forwarding
– no QoS Policies are attached on the interface on which you are configuring AutoQoS
– correct bandwidth has to be configured on the interface; if there is no bandwidth statement, then this defaults to 1544 Mbps, which is not all the time accurate

Now a tricky part in regards to interface bandwidth. If this is 768kbps or below, you have to configure an IP address on the interface due to the fact the AutoQoS is enabling by default Multilink PPP and copy the  configured IP under the multilink interface. If you configure AutoQoS on a remote site, and the 768kbps (or lower) interface is the only connection there, please pay attention as configuring AutoQoS Voip may break your connection due to MLP situation explained above.

Another recommendation from Cisco and me, is that you should use no auto qos voip command when you stop using the interface on which AutoQoS VoIP is configured. In this way you assure that all the QoS configuration will be removed. If you just shutdown the interface or delete the PVC, without the above command, then the AutoQoS VoIP will not be completely removed.

Configuration is very simple:

router# configure terminal
router(config)# interface Serial x/y
router(config-if)# bandwidth 1540
router(config-if)# auto qos voip
router(config-if)# exit

I tried to put together a scenario like in the following topology, to generate some traffic and show you have AutoQoS behave when configuring policies:

Cisco AutoQoS VoIP topology

After applying the AutoQoS config you should see something like this in your running-config:

class-map match-any AutoQoS-VoIP-Remark
match ip dscp ef
match ip dscp cs3
match ip dscp af31
class-map match-any AutoQoS-VoIP-Control-UnTrust
match access-group name AutoQoS-VoIP-Control
class-map match-any AutoQoS-VoIP-RTP-UnTrust
match protocol rtp audio
match access-group name AutoQoS-VoIP-RTCP
policy-map AutoQoS-Policy-UnTrust
class AutoQoS-VoIP-RTP-UnTrust
priority percent 70
set dscp ef
class AutoQoS-VoIP-Control-UnTrust
bandwidth percent 5
set dscp af31
class AutoQoS-VoIP-Remark
set dscp default
class class-default
ip access-list extended AutoQoS-VoIP-Control
permit tcp any any eq 1720
permit tcp any any range 11000 11999
permit udp any any eq 2427
permit tcp any any eq 2428
permit tcp any any range 2000 2002
permit udp any any eq 1719
permit udp any any eq 5060
ip access-list extended AutoQoS-VoIP-RTCP
permit udp any any range 16384 32767

I will try to generate some traffic between this, so called, voice server and client and to let AutoQoS  VoIP do it’s job and configure automatically traffic flow policies.

Below you have a video tutorial about AutoQoS VoIP has to be configured and some monitoring commands that will output the result of traffic flow: