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I’m the chairman of the wiring committee for a hundred-unit condominium, and not a Cisco expert. We have a trio of Cisco Catalyst 3550 switches, connected to an old Cisco 1417 router, connected to a DSL connection which we realize we need to upgrade. (See Bandwidth sharing via police exceed-action drop on Cisco Catalyst 3550s: anything less draconian? for more background.) It looks to me like the bandwidth field in our ATM0 configuration is wrong about our DSL connection’s speed, which is less than 7900 both up and down:

 interface ATM0
  bandwidth 7900
  no ip address
  no atm ilmi-keepalive
  dsl operating-mode auto
 !
 interface ATM0.1 point-to-point
  bandwidth 7900
  bridge-group 1
  bridge-group 1 subscriber-trunk
  pvc 0/35 
   vbr-nrt 384 384 1
   encapsulation aal5snap

Should I care about this? Behavior when users were hogging bandwidth, before I enabled QoS, was annoying, with everyone getting equally huge ping times, which made the shared internet connection pretty much unusable. I was hoping that giving the router a more accurate idea of how much bandwidth we actually had, might make behavior under excess load somewhat less annoying. But my limited acquaintance with the Cisco documentation failed to turn up a description of what the field actually does, though I suspect it influences the report of the proportion of bandwidth used.

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1 Answer 1

up vote 2 down vote accepted

The bandwidth statement is informative, and has only a small effect.

IIRC, it's only used for dynamic routing protocols that care about the link bandwidth, and to display a correct usage when doing a show interface.

c1721#sh int atm0
ATM0 is up, line protocol is up 
  Hardware is DSLSAR (with Alcatel ADSL Module)
  Description: --- adsl link
  MTU 4470 bytes, sub MTU 4470, BW 800 Kbit/sec, DLY 640 usec, 
     reliability 255/255, txload 1/255, rxload 1/255

Here, line speed is equal to the bandwidth, so there is no need to specify it.

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