OSPF hello and LSA

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Apr 30th, 2008
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My OSPF neighbors reset them selves, I want to apply qos for HELLO and SLA traffic. I want to mark them but not sure how much bandwidth I need to allocate for it.

Can someone please advise or an approperaite estimate/config?


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mheusing Thu, 05/01/2008 - 00:45
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you can create a policy marking local traffic - originated by the router - with a specific IP preference value. Note, by default OSPF packets will be marked with IP preference 6. On the interfaces you can then include a class for this traffic in your policy-map. It is hard to tell which amount of bandwidth you should allocate, as this depends on parameters like interface bandwidth and also on how much local traffic the router generates. An example:

ip local policy route-map IPP6

route-map IPP6 permit 10

match ip address LocalPrioTraffic

set ip precedence 6

ip access-list extended LocalPrioTraffic

permit ospf any any

permit tcp any any eq telnet

permit tcp any eq telnet any

permit udp any any eq snmp


! Match your local traffic here, like telnet, SNMP, IPSLA, etc.


class-map match-any NMS

match ip precedence 6

policy-map MyQoS

class NMS

bandwidth percent 10

class class-default



interface Serial0

ip address

bandwidth 128

service-policy output MyQoS

For a detailed syntax description please have a look at


Be aware the service-policy will also "catch" all transit telnet traffic, respectively all traffic marked with IP precedence 6 and you need to allocate enough resources for the combined traffic (local+transit).

Hope this helps! Please use the rating system.

Regards, Martin

hadisharifi Thu, 05/01/2008 - 16:28
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thanks for the suggestion, do I need to set IP LOCAL-POLICY given that the ospf traffic are marked with IP precedence 6. what about the policy below? isn't it enough to treat the ospf traffic as desired.


match ip precedence 6

policy-map MY-POLICY


bandwidth percent 10

Given that OSPF traffics are marked with IP precedence 6 by default, I can just match them and give it 10% of the bandwidth.


Giuseppe Larosa Thu, 05/01/2008 - 01:10
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a reference document could be RFC 1245 "OSPF Protocol Analysis" where OSPF bandwidth usage is studied.

You have to take in account that even in a stable network LSAs are refreshed evry 30 minutes.

The bandwidth usage depends from the number of IP routes in the OSPF database.

OSPF packets have usually IP Prec = 6 so you can create a class-map for them and defines some bandwidth for it in a CBWFQ scheduler.

You don't need to mark OSPF packets.

Received OSPF packets are placed in a queue for process switching.

Because the bandwidth command in CBWFQ does not imply a fixed reservation don't be afraid of giving too much bandwidth to OSPF traffic.

Some platforms like a C3640 and other small ones have a system queue for routing traffic but other platforms like c7500 and 12000 haven't got one and you need to define one.

hope to help


hadisharifi Thu, 05/01/2008 - 16:32
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Thanks for the suggestion, this is what I am leaning towards as well and I have asked "mheusing" to confirm.

bvsnarayana03 Thu, 05/01/2008 - 13:21
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By default only 75% of the bandwidth configured on an ineterface is used by traffic. Rest 25% is reserved for use by protocol related traffic which include the Hello's & LSA's u were talking about. So if u are using default interface configurations, there seems no need to use any qos features.

hadisharifi Thu, 05/01/2008 - 16:35
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Can you please post us the link for reference as I am not quite sure what the default interface configuration is. Because by default an interface doesn't have a bandwidth statement but it it's configured I assume it changes the default behaviour and there are other similar configs...

Giuseppe Larosa Thu, 05/01/2008 - 23:13
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this is not true for all the router platforms.

Best Regards



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