From: Scott Morris (swm@emanon.com)
Date: Wed May 18 2005 - 09:34:57 GMT-3
When looking through the command reference, both "frame-relay" and
"interface" configurations can be used with the keyword. Which really leads
us to the "same" encapsulation type in the end. Both direct encapsulation
and dlsw-lite encapsulation show the command "pass-thru" available to it.
Interestingly enough this all boils down to the sessions themselves.
When looking to implement DLSW you have the end-to-end SNA session
occurring, but we look at whether the LLC2 session is also end-to-end or
not. By default, TCP and DLSW-Lite encapsulations implement local
acknowledgement of LLC2 sessions. The whole original design of DLSW (RFC
1795 and RFC 2166) was using the important concept of limiting the LLC2
session (and timeouts and other problems) to the local segments to assure
reachability.
Also, per the RFC, TCP was referred to as the only encapsulation supported.
Hence the "+" and the room for mis-documented oddities. FST and Direct
encapsulations do not use local acknowledgements. The "pass-thru" statement
dictates the performance of the LLC2 session.
Direct and Lite are both what we would see as "Layer 2" encapsulations
meaning a single common network link separating the two end-points (which
could also be an extended set of fully bridged interfaces). FST and TCP are
both multihop encapsulations.
When we see in the docs that frame-relay can support both the Lite or Direct
encapsulations, this really leads us to the frame-map stuff. Frame-relay
map dlsw implies that you are using direct encapsulation (a point-to-point
connection). Direct is a more efficient encapsulation due to the reduced
header size and the ability to fast-switch ("efficient" only refers to
packets per second, not overall processing which may vary based on your
network) The "pass-thru" option on the dlsw remote-peer is what really
makes the connection Direct. All LLC2 traffic is end-to-end.
If we don't use the "pass-thru" option, then we are running DLSW-Lite and
locally acknowledging stuff.
They really are essentially the same thing just a variant of the
acknowledgement. You'll find some documentation that references Direct as
"Direct/Passthru" and Lite as "Direct/LACK" for Local Acknowledgement.
It adds to the confusion of things!
In real life, we may do this, and implement many other commands to extend
the timing of the LLC2 sessions and timeouts (and other things like "dlsw
llc2 nornr") to tweak how these are handled. In the lab, we really don't
care about this stuff!
My personal suggestion is to not really sweat this stuff unless you are
looking to implement it in real life. Check out the number of points on
your exam for DLSw and you will likely find that within the limited 8 hours
that you have, DLSW is not considered to be a core topic and therefore won't
take a huge implementation to make it work!
If you're interested in some features and availability, check out:
http://www.cisco.com/en/US/tech/tk331/tk336/technologies_design_guide09186a0
08011a67f.shtml
http://www.cisco.com/en/US/partner/tech/tk331/tk336/technologies_design_guid
e09186a0080237a5e.shtml#wp16430
HTH,
Scott
PS. As for the workbook, that actually came out of my course book. But
that section has been revised and reworded after I went back and read
through it!
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
simon hart
Sent: Wednesday, May 18, 2005 2:36 AM
To: Bob Nelson; ccielab@groupstudy.com
Subject: RE: DLSW and passthru
Bob,
As I understand it, using the pass-thru statement determines whether the
transport mechanism to the remote peer on Frame Relay will be either Direct
or DLSW lite.
This is peculiar to Frame Relay.
If you were conducting direct encapsulation on say HDLC, then the pass-thru
statement would be meaningless - because HDLC does not support DLSW-lite -
therefore everything is passed forward anyway.
Direct Encapsulation
1. dlsw remote-peer 0 frame-relay interface serial 0/0 513 pass-thru 2.
dlsw frame map dlsw 513 broadcast
Now these two statements together ensure that the traffic is sent with no
local acknowledgements - hence pass through.
DLSW Lite
1. dlsw remote-peer 0 frame-relay interface serial 0/0 513 2. dlsw frame
map llc2 513 broadcast
Now there will be local acknowledgement, hence DLSW lite.
The only 'coding' difference between the two methods is the addition of the
pass-thru statement. By adding this you are forcing the router to forward
on llc2 to the distant end. And with each method you need to tell frame
relay how to encapsulate. If you use the wrong encapsulation method it will
not work.
Lastly - I would suggest that the CCIE bootcamp manual is incorrect.
HTH
Simon
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of Bob
Nelson
Sent: 18 May 2005 02:38
To: ccielab@groupstudy.com
Subject: DLSW and passthru
All:
I have searched the archives on this site and still do not have a good
handle on this issue. Can someone clarify this?
What I have researched:
1. TCP and LLC2 encapsulation have local acknowledgement and reliable
transport 2. FST and Direct do not have either local acknowledgement or
reliable transport.
Sources: Solie - CCIE Practical Studies Vol 1 page 898 - Cisco
DLSw+ Design and Implementation Guide - pages 12-14
Statements in documentation that confuse me:
Cisco IOS Bridging and IBM Networking Guide page BC-291 (under the
Direct configuration heading)
dlsw remote-peer 0 frame-relay interface serial 0/1 pass-thru
"Specifying the pass-thru option configures the router so that the
traffic will not be locally acknowledged
(DLSw+ normally locally acknowledges traffic to keep traffic on the
WAN to a minimum)"
Question 1 - Why do we need the pass-thru option here since by default
Direct encapsulation does not
perform local acknowledgement. (see #2 above)
Cisco DLSw+ Design and Implementation Guide - pages 12-14 (Advanced
Configuration Section)
dlsw remote-peer 0 frame-relay interface serial 01 33 pass-thru
int s1
frame-relay map dlsw 33
"In this example, data-link connection identifier (DLCI) 33 on
serial interface 1 will be used to transport DLSw+ traffic.
Specifying pass-thru implies that the traffic is not locally
acknowledged. Leaving pass-thru off will cause the
traffic to be locally acknowledged, which means it is transported in
LLC2 to ensure reliable delivery."
Question 2 - Again, why specify pass-thru when Direct encapsulation does
not do local acknowledgment anyway?
Additionally, how can leaving pass-through off magically make this
configuration transport packets using LLC2
without changing the frame-relay map statement from dlsw to frame-relay
map llc2 33?
A CCIE bootcamp student manual
dlsw remote-peer 0 frame-relay interface 0/1 201 pass-thru
"The pass-thru option can be added to other remote-peer methods as
well.
This will prevent the local acknowledgements and keepalives from
going across the WAN"
Question 3 - My understanding of the pass-thru is that this will not perform
local acknowledgement and will pass both the acknowledgements and keepalives
end-to-end. Is this what pass-thru actually does or does it prevent local
acknowledgements? Additionally, why do we need this if according to Cisco,
Direct encapsulation does not support local acknowledgements and keepalives
are end-to-end.
Thanks,
Bob
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