From: Plank, Jason (JPlank@concordefs.com)
Date: Sat Jun 17 2006 - 12:33:06 ART
This has always been confusing to me as well. I read somewhere once that the
FRSwitch will send a FECN in the direction of the data that is being sent
and that the router would then send a BECN to router that originated the
data (so to speak - the other side of the frame). I've been "informed" that
that isn't correct though so I don't know.
-------------------
J. Marshall Plank
Network Engineer
101 Bellevue Parkway
Wilmington, DE 19809
E-mail: JPlank@concordefs.com
Phone: 302-793-5913
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of Arun
Arumuganainar
Sent: Saturday, June 17, 2006 11:08 AM
To: Scott Morris; 'Petr Lapukhov'; 'Ken'
Cc: 'Cisco certification'
Subject: Re: What does FECN do?
Hi Scott,
Frame Relay traffic is unidirectional. i.e FR Switch does not know who
sender was . DLCI provides information only about the destination.
I don't think frame-relay switch can actually set the BECN. It can only be
set in the PVC end-points .
Thanks and Regards
Arun
----- Original Message -----
From: "Scott Morris" <swm@emanon.com>
To: "'Petr Lapukhov'" <petr@internetworkexpert.com>; "'Ken'"
<hpnkpn103@yahoo.co.jp>
Cc: "'Cisco certification'" <ccielab@groupstudy.com>
Sent: Saturday, June 17, 2006 6:57 PM
Subject: RE: What does FECN do?
> While very true, this gives an example of us being concerned with our
> endpoints only. (Which very well may be the design of the network these
> days!)
>
> The history of FECN and BECN was revolving around a true end-to-end
> frame-relay network, and a Frame Switch in the middle would actually
> generate both types of ECNs (each a separate bit within the frame relay
> frame format).
>
> The FECN would follow with the traffic telling receiving stations (upper
> layer protocols, if informed) to expect some delays in incoming traffic
due
> to congestion. The BECN would go on the back path telling the receiver to
> slow down in transmission. If, of course, it were paying attention to
> those! Cisco's default behavior is to ignore these notifications. Go
> figure.
>
> In today's frame-relay networks though, where SPs often re-encapsulate the
> frames into IP-IP tunnels, or MPLS clouds or whatever their choice is, the
> functionality is a bit warped.
>
>
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/frame.htm#wp1020620
>
> There are capabilities for ECN marking within MPLS clouds (MPLS-VPN), but
I
> don't have any data suggesting which service providers do or do not
actually
> support this.... Bottom line, in real life you should know your network's
> capabilities. In the lab, you should do whatever the lab asks for!
>
> Ahhhhh..... Evolution. ;)
>
>
> Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713, JNCIE
> #153, CISSP, et al.
> CCSI/JNCI
> IPExpert CCIE Program Manager
> IPExpert Sr. Technical Instructor
> smorris@ipexpert.com
> http://www.ipexpert.com
>
>
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Petr
> Lapukhov
> Sent: Saturday, June 17, 2006 8:27 AM
> To: Ken
> Cc: Cisco certification
> Subject: Re: What does FECN do?
>
> Ken,
>
> Most of the time FECN is an informative bit, that may be used simply for
> statistical purposes.
>
> On the over hand, BECN frames actually signal source to slow down, but
that
> would work okay only if data exchange is bidirectional.
>
> Now, for a good example of FECN usefulness consider "frame-relay
> fecn-adapt" command wihin map-class.
> (Or it's equivalent "shape fecn-adapt" with MQC).
>
> Imagine that you have unidirectional stream of packets (e.g. video feed)
> that overloads your FR network. You will have a lot of FECN bit set
packets
> in direction from sender to receivers, but not a single packet backwards,
so
> no BECN frames will arrive to sender.
>
> Here you may use fecn-adapt, that enables reflection of FECN bit in
special
> FR frames back to sender with BECN bit set. In this way, a FECN signalling
> is converted to BECNs, that actually signal sender to slow down sending
> rate.
>
>
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos
> _r/qrfcmd9.htm#wp1103558
>
> HTH
>
> --
> Petr Lapukhov, CCIE #16379
> petr@internetworkexpert.com
>
> Internetwork Expert, Inc.
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