swiches,routers, software and hardware queues.

[sarahr202]

Hi everybody.

Does a cisco cat switch also use software queues like  a cisco router at ingress and egress ports Or  a Cisco router only uses software queues at ingress and egress int ?

thanks and have a great day.              

[Joseph W. Doherty · Accepted solution]

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 believe switches generally only use hardware supported queuing.

[sarahr202]

thanks Joseph.

I just want to confirm one more thing if you dont't mind.

Regardless if  we are trusting dscp or Cos at ingress port/int of a router or a switch, dscp values are mapped to ingress queues.  At egress port/int , dsc values are mapped to egress queues.  So if we are trusting COS at a ingress port, switch will use cos to dscp table and compute the dscp value which will decides which queue the packet should be placed at the ingress port.  Same for router as well.  Am i correct?

thanks.

[dominic.caron · Accepted solution]

I dont think you can trust the COS in router. (those without a switching module)

[dominic.caron · Accepted solution]

I believe it's a fuzzy line.

In most router cases, memory is shared. The queuing process is responsible for managing the RX and TX ring. (those are populated by pointers to memory locations(buffer) where packets are). Moving packets would be a waste of ressources.

Hardware or software is related to who doing the switching. In lowmid-end router, the interface processor will send a interrup to the cpu so it can do the routing and L2 forwarding job.  In bigger router like the 12000 and the 6500, ASIC will do that job without the help of the cpu.

[sarahr202]

  Hi there.

A quote from my book:

The Cisco 3550 uses a relatively simple classification scheme, assuming you consider only what

happens once the forwarding decision has been made. Cisco 3550 switches make most internal

QoS decisions based on an

internal DSCP setting. The internal DSCP has been determined once

the frame is forwarded. So, once a frame has been assigned an internal DSCP and an egress

interface, the following logic determines into which of the four interface output queues the frame

is placed:

The per-interface CoS-to-queue map determines the queue for a frame based on the assigned

The frame’s internal DSCP is compared to a global DSCP-to-CoS map to determine a CoS

value.

The per-interface CoS-to-queue map determines the queue for a frame based on the assigned

CoS.

1)My questions do all switches map cos to queues at egress ports like 3550 switch?

2) If we configure 3550 port f1/1 as layer 3 port. Will egress queues at f1/1  are still mapped to cos values ?

3) what about ingress queues at a port of a switch?  Do switches map cos to ingress queues at a port ?

thanks and have a great day.

[Joseph W. Doherty · Accepted solution]

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

#1 No, different Cisco switches support different QoS features, including whether they can use L2 CoS and/or L3 ToS.

#2 I believe most switches don't really concern themselves with whether a port is configured as a switchport or routed port.  However, only tagged frames can provide L2 CoS.  (NB: some switches set an internal CoS, not to be confused with a L2 frame's CoS.)

#3 Depends on the switch.  Also on many switches, ingress queues usually have less features as generally there's less queuing there.