From: Brian Dennis (bdennis@internetworkexpert.com)
Date: Sat Jun 17 2006 - 13:36:31 ART
It really depends on the upper layer protocols in regards to Cisco's support and there has been support for FECN and BECN for protocols that understood the concept of an ECN (Explicit Congestion Notification). The IOS has supported DECnet and OSI for a very long time. Also SNA using direct encapsulation LLC2 encapsulation supports BECN.
Brian Dennis, CCIE #2210 (R&S/ISP-Dial/Security)
bdennis@internetworkexpert.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987
Direct: 775-745-6404 (Outside the US and Canada)
________________________________
From: nobody@groupstudy.com on behalf of Scott Morris
Sent: Sat 6/17/2006 9:27 AM
To: Petr Lapukhov; 'Ken'
Cc: 'Cisco certification'
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.comInternetwork Expert, Inc. http://www.InternetworkExpert.com Toll Free: 877-224-8987 Outside US: 775-826-4344 24/7 Support: http://forum.internetworkexpert.com Live Chat: http://www.internetworkexpert.com/chat/
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