How does FIFO queueing handles network traffic

[richardgosen]

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?

[rsimoni]

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

[Joseph W. Doherty]

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.

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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.

Posting

IP Routing

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)

[richardgosen]

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"?

[Joseph W. Doherty]

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.

[rsimoni]

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