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”. 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. 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)
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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
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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)
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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
Internetwork 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/
Category: CCIE Study
