QOS on E1

Answered Question
Aug 14th, 2008
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Can i apologise in advance for the number of questions here but i have been searching for answers to the following and i cant seem to find anything that provides the answers:

I have a 3825 and 2821 router which have two VWIC-2MFT-E1 modules each. I have linked one interface from each module on the 3825 to the other modules on the 2821 for resilience and load balancing. I have OSPF running between the routers and i am hoping that they will automatically load balance. Is this the case or would i need to configure load-balancing in OSPF?

Can you point me in the direction of a document which provides config examples of how to configure WIC-2MFT-E1 modules to enable the transfer of both voice and data between the routers.

I have also set the following QOS config (With some help from SDM)on the routers and applied it to the serial interfaces that correspond to the Controllers using the 'service-policy output QOS-Policy-2' command:

class-map match-any Transactional

match dscp af21

match dscp af22

match dscp af23

class-map match-any signaling

match dscp cs3

match dscp af31

class-map match-any Routing

match dscp cs6

class-map match-any Voice

match dscp ef

class-map match-any Management

match dscp cs2


policy-map QOS-Policy-1

class Voice

priority percent 33

police cir 960000

conform-action transmit

exceed-action drop

class Signaling

bandwidth percent 5

police cir 100000

conform-action transmit

exceed-action drop

class Routing

bandwidth percent 5

police cir 100000

conform-action transmit

exceed-action drop

class Management

bandwidth percent 5

police cir 100000

conform-action transmit

exceed-action drop

class Transactional

bandwidth percent 5

police cir 100000

conform-action transmit

exceed-action drop

class class-default

fair-queue

random-detect

police cir 640000

conform-action transmit

exceed-action drop

policy-map QoS-Policy-2

class class-default

shape average 2000000

service-policy QoS-Policy-1


My question is:

Does the above look ok...especially the percentages and bandwidth that i have assigned? I have made the assumption that the CIR amounts need to equal 2000kbps for each link...is that correct?

Do i need to to configure anything else to make the QOS work correctly over each E1 link?

How does the router distinguish between different packets traversing the network for it to know what class to apply to the packet?

Is there something in the packet that is placed there automatically by the end device or application? For example i am also planning to connect a couple of 1861's to the routers via E1 and make use of the FXS ports for legacy phones and want to apply the same QOS config as above but how will the router know to treat the packets coming from the FXS ports as voice packets?


Any help with the above would be appreciated.


Thanks and regards,

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

"I am assuming that you are saying that the interfaces dont need shaping because it only needs to be on low bandwidth links and the serials are 2mbps...would that be correct?..."


No, it's more a question of whether you need to create a bottleneck slower than the physical interface. Doesn't make sense to shape at the speed of the link.


"but what about policing so that the router knows what to do with the packets when the link gets really busy?"


When the links gets really busy, packets normally (at least initially) queue, and with QoS, often in multiple queues. With multiple queues, your QoS policy can determine the sequence of dequeuing from the multiple queues.


Policing is a (very severe) method to cap traffic's bandwidth utilization. However, I believe it's better to prioritize differently.


The priority LLQ class is a good example of prioritization. If there is no LLQ traffic, other traffic can use all the bandwidth, but if there is LLQ, it goes first.


"I mentioned earlier that i will be connecting a couple of 1861's...and also some VG224's. If i am plugging analog phones into the FXS ports on these devices does the router automatically mark the packets coming from these interfaces as voice packets or do i need to do this manually?...if so i am not sure how i would because the interfaces are voice ports which i havent assigned ip addresses to...or would i need to apply an access list to mark the udp port number?"


I don't have first hand experience with either device, so not sure how they behave. As I noted before, such devices will often mark their traffic. If they don't, you might be able to identify is with ACLs, NBAR, or even source interfaces or VLANs.


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Joseph W. Doherty Fri, 08/15/2008 - 03:46
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With OSPF, it will use multiple paths as long at the cost to destination is the same for each path. I.e. normally, this would be the case from two equal bandwidth links connecting two routers.


If your routers are directly connected to serial interfaces, no need to use any shapers.


Your question about how does the router recognize VoIP packets is important. This depends on the packets being marked earlier (often the VoIP phone will do so) and looking for those markings, or by additional packet analysis within the router itself.


Although your policy is somewhat "text book", a much simpler policy might work well for you.


e.g.


class-map match-any signaling

match dscp cs3

match dscp af31


class-map match-any Voice

match dscp ef


policy-map QOS-Policy-1


class Voice

priority percent 33


class Signaling

priority percent 5


class class-default

fair-queue


interface Serial #

service-policy output QoS-Policy-1


The above example assumes the VoIP packets are already marked.


Some might comment that "best practice" would not have call signally in the LLQ, but I've found mixing class-default FQ with any non-LLQ classes often makes for issues. Signally traffic, for correct functioning, is as important as the voice payload and often is very light weight, so placing in LLQ ofters advantages without usaually causing any major issues.

michael.clark2 Fri, 08/15/2008 - 04:51
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Joseph,

Thankyou for the response and for the explanation...just a few more questions:


I am assuming that you are saying that the interfaces dont need shaping because it only needs to be on low bandwidth links and the serials are 2mbps...would that be correct?...but what about policing so that the router knows what to do with the packets when the link gets really busy? The reason why i ask is because when i used SDM to implement policing i had to specify the CIR for shaping as well even though i didnt want to apply shaping...??? not sure what that is about.

I mentioned earlier that i will be connecting a couple of 1861's...and also some VG224's. If i am plugging analog phones into the FXS ports on these devices does the router automatically mark the packets coming from these interfaces as voice packets or do i need to do this manually?...if so i am not sure how i would because the interfaces are voice ports which i havent assigned ip addresses to...or would i need to apply an access list to mark the udp port number?


Thanks also for the simplified QOS config...:0)


Regards,




Correct Answer
Joseph W. Doherty Fri, 08/15/2008 - 08:37
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  • Super Bronze, 10000 points or more

"I am assuming that you are saying that the interfaces dont need shaping because it only needs to be on low bandwidth links and the serials are 2mbps...would that be correct?..."


No, it's more a question of whether you need to create a bottleneck slower than the physical interface. Doesn't make sense to shape at the speed of the link.


"but what about policing so that the router knows what to do with the packets when the link gets really busy?"


When the links gets really busy, packets normally (at least initially) queue, and with QoS, often in multiple queues. With multiple queues, your QoS policy can determine the sequence of dequeuing from the multiple queues.


Policing is a (very severe) method to cap traffic's bandwidth utilization. However, I believe it's better to prioritize differently.


The priority LLQ class is a good example of prioritization. If there is no LLQ traffic, other traffic can use all the bandwidth, but if there is LLQ, it goes first.


"I mentioned earlier that i will be connecting a couple of 1861's...and also some VG224's. If i am plugging analog phones into the FXS ports on these devices does the router automatically mark the packets coming from these interfaces as voice packets or do i need to do this manually?...if so i am not sure how i would because the interfaces are voice ports which i havent assigned ip addresses to...or would i need to apply an access list to mark the udp port number?"


I don't have first hand experience with either device, so not sure how they behave. As I noted before, such devices will often mark their traffic. If they don't, you might be able to identify is with ACLs, NBAR, or even source interfaces or VLANs.


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