03-15-2013 05:52 AM - edited 03-07-2019 12:16 PM
Hi All,
FiFo queueing is the default on high speed interfaces and does not prioritize traffic. Network traffic, like routing updates, are marked as high priority traffic.I always thought that this kind of traffic gets served first even on FiFo interfaces.
Can someone confirm this?
If it's not the case I mentioned, then I have to use priority queueing to get my routing updates served first?
03-15-2013 06:25 AM
Hi Richard,
SPD mechanism is there to prevent that also routing protocol packets (or control plane ones in general) are dropped if the interface is congested.
For details please see:
http://www.cisco.com/en/US/products/hw/routers/ps167/products_tech_note09186a008012fb87.shtml
regards,
Riccardo
03-15-2013 09:42 AM
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In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.
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By default, Cisco IOS software (in accordance with RFC 791 and RFC 2474) marks Interior Gateway Protocol (IGP) traffic such as Routing Information Protocol (RIP/RIPv2), Open Shortest Path First (OSPF), and Enhanced Interior Gateway Routing Protocol (EIGRP) to DSCP CS6. However, Cisco IOS software also has an internal mechanism for granting internal priority to important control datagrams as they are processed within the router. This mechanism is called PAK_PRIORITY.
As datagrams are processed though the router and down to the interfaces, they are internally encapsulated with a small packet header, referred to as the PAKTYPE structure. Within the fields of this internal header there is a PAK_PRIORITY flag that indicates the relative importance of control packets to the internal processing systems of the router. PAK_PRIORITY designation is a critical internal Cisco IOS software operation and, as such, is not administratively configurable in any way.
Note that Exterior Gateway Protocol (EGP) traffic such as Border Gateway Protocol (BGP) traffic is marked by default to DSCP CS6 but does not receive such PAK_PRIORITY preferential treatment and may need to be explicitly protected in order to maintain peering sessions.
When addressing the QoS needs of IP Routing traffic, Cisco recommends the following guidelines:
•IP Routing traffic should be marked to DSCP CS6; this is default behavior on Cisco IOS platforms.
•IGPs are usually adequately protected with the Cisco IOS internal PAK_Priority mechanism; Cisco recommends that EGPs such as BGP have an explicit class for IP routing with a minimal bandwidth guarantee.
•Cisco IOS automatically marks IP Routing traffic to DSCP CS6.
Additional information on PAK_PRIORITY can be found at:
http://www.cisco.com/warp/public/105/rtgupdates.html
(http://www.cisco.com/en/US/tech/tk543/tk544/technologies_tech_note09186a0080094612.shtml)
03-15-2013 12:51 PM
Thank you both for explaning this "default" behaviour for control traffic. It's also sound logic, because control traffic should be priority treated before any other "production" traffic. I can assume that BPDU's also falls into the category "control traffic"?
03-15-2013 07:41 PM
Disclaimer
The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.
Liability Disclaimer
In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.
Posting
I'm unsure such L2 frames do too.
03-18-2013 02:36 AM
routers don't care of L2 control plane PDU's (among which you have BPDU's) as they don't process them (nor route them).
So no particular mechanism to make sure they are not dropped.
On the other hand swithes care of them and they make sure they are correctly sent to the CPU to be processed. The way they do is that as soon as the hardware plane see frames destined to those well known MAC addresses it 'punts' them to the CPU right away bypassying any other forwarding or traffic handling logic.
Riccardo
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