RE: What does FECN do?

From: Scott Morris (swm@emanon.com)
Date: Mon Jun 19 2006 - 00:03:50 ART

Does that cover ALL words in Texas? ;)

-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Faryar Zabihi (fzabihi)
Sent: Sunday, June 18, 2006 12:56 PM
To: Scott Morris; Brian Dennis; Petr Lapukhov; Ken
Cc: Cisco certification
Subject: RE: What does FECN do?

Them's fightin words!!!
At least it is in TX

 Faryar

-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Scott Morris
Sent: Sunday, June 18, 2006 9:26 AM
To: 'Brian Dennis'; 'Petr Lapukhov'; 'Ken'
Cc: 'Cisco certification'
Subject: RE: What does FECN do?

Wow... I'm sorry, I didn't know these were still current lab topics.

I'll jump right up and test that out. And while I'm at it, I think I'll
worry about Appletalk and IPX as well in case they come back. What about
LLC2 or RSRB? I hear token ring is on the rebound.

DecNet and CLNS have built in mechanisms for ECN and are much more closely
related with Layer 2 framing. IP on the other hand has had minor mechanisms
in the past, but more enhancements lately than for "a very long time".
Hence the terminology of passing to "upper layer protocols".

So... For the record, if we look at EVERY possibility that you ever may run
across, then my statement may at times be incorrect. If we have some
semblance of paying attention to our lab discussions around here, which
revolve in the IP world, then I think you'll find the statement to make
perfect sense.

And the DocCD writers have had no comment either way on that part.
Since you know more than they do, you may want to point that out so then at
least we can be sure everyone is as politically correct...

 Scott

-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Brian Dennis
Sent: Sunday, June 18, 2006 2:36 AM
To: swm@emanon.com; Petr Lapukhov; Ken
Cc: Cisco certification
Subject: RE: What does FECN do?

Scott,
Can you post an example configuration of how to configure DECnet support for
FECN since you say it's not the default? The reason I ask is because the
DocCD states that "no configuration is required" which normally means
"default".
 
<DocCD>
Cisco's Implementation of Frame Relay
 
The Frame Relay software provides the following capabilities:

* Transmission of congestion information from Frame Relay to DECnet Phase IV
and CLNS. This mechanism promotes Forward Explicit Congestion Notification
(FECN) bits from the Frame Relay layer to upper-layer protocols after
checking for the FECN bit on the incoming DLCI. Use this Frame Relay
congestion information to adjust the sending rates of end hosts.
FECN-bit promotion is enabled by default on any interface using Frame Relay
encapsulation. No configuration is required.
</DocCD>
 
Also if this is against Cisco's long-standing idea as you say someone better
let the people who write the DocCD know ;-)
 
Thanks!
 
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: Scott Morris [mailto:swm@emanon.com]
Sent: Sat 6/17/2006 1:22 PM
To: Brian Dennis; Petr Lapukhov; 'Ken'
Cc: 'Cisco certification'
Subject: RE: What does FECN do?

However, unless specifically configured on Cisco devices, received BECN's or
FECNs have been routinely ignored regardless of the upper layer protocol
support. Cisco is not alone in this long-standing idea though.
 
Just because you CAN do it, doesn't mean you will. ;)
 
scott

________________________________

From: Brian Dennis [mailto:bdennis@internetworkexpert.com]
Sent: Saturday, June 17, 2006 12:37 PM
To: Scott Morris; Petr Lapukhov; Ken
Cc: Cisco certification
Subject: RE: What does FECN do?

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#wp102
0620

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

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