From: gladston@br.ibm.com
Date: Mon May 30 2005 - 15:11:36 GMT-3
Hi,
QoS is always interesting, isn't it? (it can be as much interesting as confusing :))
========================
quote
> Is the bandwidth specified with the police command in MQC considered
> "reserved" and applicable to this max-reserved-bandwidth value?
>
> IOW, suppose this were the configuration:
>
> policy-map TEST
> class A
> police 500
>
> class B
> police 500
======================
No, it isn't. As you said, it will limit the transmission rate.
Using the construction "the bandwidth specified with the police" can really confuse us; there is no minimal
This is Cisco words:
"Importantly, neither mechanism provides a minimum bandwidth guarantee during periods of congestion. Use the bandwidth or priority command to provide such guarantees."
http://www.cisco.com/en/US/tech/tk543/tk545/technologies_tech_note09186a00800a3a25.shtml
This doc says it about Policing and Shaping.
========================
quoted
That being the case, does it make any sense to use the bandwidth command
under the default class? Will the IOS even accept the bandwidth command?
========================
Hope there is more feedback on this.
=========================
And, what about the other policy commands like police, priority, shape, etc.
I wonder because the default class is where important traffic such routing
updates and keepalives go, but if a separate class isn't defined for all
important traffic flows, the default class ends up with a bundle of traffic
that includes important traffic like routing updates and non-important like
p2p downloads. Since some of this traffic is important, do you think it's a
good to shape it or do something else or just leave it with the default
bandwidth of 25%?
========================
Tim, I am sure you have read this Cisco Doc. Just remembering:
There are two mechanisms used by Cisco IOS to prioritize control packets:
IP Precedence Bits Type of service byte in IP header and pak_priority (Internal packet label inside the router), assigned by interface driver.
Both mechanisms are designed to ensure that key control packets are not dropped or are dropped last by the router and the queuing system when an outbound interface is congested.
****Precedence 6****
IOS assigns an IP precedence of 6 to routing protocol packets on the control plane. Specifically, Cisco IOS marks the following IP-based control packets:
Open Shortest Path First (OSPF)
Routing Information Protocol (RIP)
Enhanced Interior Gateway Routing Protocol (EIGRP) hellos, and keepalives
Telnet packets to and from the router also receive an IP precedence value of 6.
****Pak_priority****
The RIP and OSPF routing processes mark all traffic they originate with both IP precedence 6 and pak_priority.
BGP instructs TCP to mark its traffic with IP precedence 6, but does not set pak_priority.
The following non-IP traffic is marked with pak_priority:
Intermediate System-to-Intermediate System (IS-IS) routing protocol messages.
PPP
HDLC keepalives on serial and POS interfaces.
Operations, administration, and maintenance (OAM) cells and ARP messages on ATM interfaces.
There is a different treatment between 7500 with VIP and other routers:
-----7200, 3600, 2600-----
IOS places pak_priority traffic in a separate set of queues other than class-default.
-----7500 with VIP-----
Places pak_priority traffic in the class-default class queue by default
The pak_priority flag is used to avoid dropping the high-priority packets
This is from http://www.cisco.com/warp/public/105/rtgupdates.html
So, answering your last question (about routing updates..), this is the way I see it:
Because on 7500 with VIP IOS use the class-default queue for control plane traffic, it is recommended to configure a queuing strategy for routing packets.
Now, for your second question, I see your point when you ask "does it make any sense to use the bandwidth command under the default class?".
I am not sure.
Considering the router is a 3600, IOS will by default use a special queue for control plane traffic. Them, only traffic that does not match any other class will go to default class.
Talking about each one of the options you mentioned:
Bandwidth
If you use bandwidth 10% under class-default, during congestion, instead of 25%, traffic under class-default will have only 10%.
The other classes can not use the reserved 25% if it was used by the class default.
So, it is a mind exercise what will occurs here
Police
If we police with 10%, then traffic that falls under class default will use only 10%, and be limited on this. The remaining bandwidth (15%) of reserved for class-default will be distributed among other classes
Shaping
If we shape the class default in a way it use less than the 25% of the bandwidth, there remaining will be distributed among other classes.
Does it make sense?
Just remembering, 12.2T changes the way % is used. It now refers to the total bandwidth.
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