[c-nsp] 2950 Questions

[Andre Beck]

On Thu, Mar 24, 2005 at 01:12:01AM +1000, Virgil wrote:
> On 24/3/05 12:33 AM, "Justin M. Streiner" <streiner at cluebyfour.org> wrote:
> 
> > The switch doesn't know what SIP is, and it doesn't care - it just
> > forwards packets :-)
> 
> Being pedantic and all, but a switch (layer 2 device) forwards *frames*.
> Routers (layer 3 device) forward packets.

Then again, the 2950 has "mls qos" capabilities. I'm not currently
up to date on which subset exactly, but that could well mean:

* The switch could access 802.1Q/D frame priority information (aka
  Ethernet CoS) and take it into consideration in queuing decisions
* The "switch" could access the TOS field of IPv4 packet carrying
  frames to get alternate CoS information (either pure IP precedence
  or even DSCP)
* The "switch" could classify IPv4 packet bearing frames on a subset
  of extended IP access list semantics to further control QoS decisions
* The "switch" could even *manipulate* the TOS field of said IPv4
  bearing frames, enforcing some policy or trust scheme. For instance,
  a 29xx/355x with "mls qos" (default is "no mls qos") but nothing else
  configured will *zero* the DSCP.

So yes, it's basically a frame bouncing device, but it has bells and
whistles that can take IP into account, primarily for QoS reasons.
With the protocol of interest beeing SIP, having a look on that stuff
might not be the worst of ideas. As long as "mls qos" is not engaged,
it will stay just a bridge^Wswitch, though.

Andre.
-- 
                  The _S_anta _C_laus _O_peration
  or "how to turn a complete illusion into a neverending money source"

-> Andre Beck    +++ ABP-RIPE +++    IBH Prof. Dr. Horn GmbH, Dresden <-