From: Arun Arumuganainar (aarumuga@hotmail.com)
Date: Mon Dec 12 2005 - 12:12:48 GMT-3
Hi Paresh ,
I think to understand this feature we need some good understanding of
Integrated ISIS !!!
Let me explain :
Default Behavior in ISIS
~~~~~~~~~~~~~~~~~~
ISIS says that a L1L2 router should set the attached-bit in its L1 LSP when
that router thinks it is attached to the L2 backbone. Currently IOS uses a
simple algorithm to determine if it is attached. This algorithm says if a
L1L2 router during an SPF computation sees other area addresses, then it
thinks it is attached to the L2 backbone and hence it should set att bit.
Problem with Default Behavior .
~~~~~~~~~~~~~~~~~~~~~~
Often this is good enough. However, in some network designs it can happen
that L1L2 routers in two different areas get separated from the L2 backbone.
Because they see each others area, they still think they are attached to the
L2 backbone. This might cause L1-only routers to send traffic for the
backbone to the wrong L1L2 routers.
Cisco Solution
~~~~~~~~~~~
To solve this problem , IOS gives network administrators better control
over, when a L1L2 router should decide,it is attached to the L2 backbone.
Here Admistrator has the option of setting the area that should be visible
in order to set the ATT bit .
Note : Irrespective of IP or CLNS, area address in ISIS is represented by
CLNS address format ( We use " net " under "config-router" prompt for
specifying area ).
Hence route-map for "set-attached-bit " should match area address( an CLNS
address ) that has to visible in order to set the att bit .This command is
applicable to both CLNS and IP routing .
By the way , command reference is right . Route map will infact refer to
CLNS routing table for the presence or absence of matching criteria . FYI :
To view clns route use "SHOW CLNS ROUTE" on the router in which ISIS is
configured .
Hope this helps .
Thanks and Regards
Arun
----- Original Message -----
From: "Paresh Khatri" <Paresh.Khatri@aapt.com.au>
To: "Arun Arumuganainar" <aarumuga@hotmail.com>; "Andrew Lissitz (alissitz)"
<alissitz@cisco.com>; <comserv@groupstudy.com>; <ccielab@groupstudy.com>
Sent: Monday, December 12, 2005 4:09 PM
Subject: RE: ISIS - set-attached-bit and set-overload-bit
Actually Arun, I still think my initial post was correct...the command
reference for this feature indicates that the route-map can only be used for
matching on CLNS routes, and not IP routes.
Here's a quote from it:
"The route map can specify one or more CLNS routes. If at least one of the
match address route-map clauses matches a route in the L2 CLNS routing
table, and if all other requirements for setting the attached-bit are met,
the L1L2 router will continue to set the attached-bit in its L1 LSP. If the
requirements are not met or no match address route-map clauses match a route
in the L2 CLNS routing table, the attached-bit will not be set."
Regards,
Paresh.
-----Original Message-----
From: Arun Arumuganainar [mailto:aarumuga@hotmail.com]
Sent: Mon 12/12/2005 9:16 PM
To: Andrew Lissitz (alissitz); Paresh Khatri; comserv@groupstudy.com;
ccielab@groupstudy.com
Cc:
Subject: Re: ISIS - set-attached-bit and set-overload-bit
Hi Andrew ,
Paresh is almost correct .We do not have full flexibility for setting the
ATT bit .
But times are changing and features gets added up !!! Check this out !!!
This is one of the feature that are used to optimize L1-Routing . After
introduction of this feature , network admins can influence ATT bit setting
using "ser-attached-bit Route-map " command .
FYI : You should be using 12.2(4)T or Later ..
Pls. refer to the CCO Link
http://www.cisco.com/en/US/products/ps6599/products_white_paper09186a00804fa
7a0.shtml
or In case you have access Cisco Bug tracking tool . You can also check out
Bug ID : CSCdp64489 .
Pls. Note : Even after this feature we can arbitarly set att bit in IOS .
Your L1/L2 router still need to have visibility to atleast one area other
than its own !!!
Thanks and Regards
Arun
----- Original Message -----
From: "Andrew Lissitz (alissitz)" <alissitz@cisco.com>
To: "Paresh Khatri" <Paresh.Khatri@aapt.com.au>; <comserv@groupstudy.com>;
<ccielab@groupstudy.com>
Sent: Thursday, December 08, 2005 10:56 AM
Subject: RE: ISIS - set-attached-bit and set-overload-bit
> Paresh you rock! I am going to try this now, thanks
>
>
> -----Original Message-----
> From: Paresh Khatri [mailto:Paresh.Khatri@aapt.com.au]
> Sent: Thursday, December 08, 2005 12:23 AM
> To: Andrew Lissitz (alissitz); comserv@groupstudy.com;
ccielab@groupstudy.com
> Subject: RE: ISIS - set-attached-bit and set-overload-bit
>
> Hi Andrew,
>
> I don't believe that you can use the 'set-attached-bit' command when using
ISIS for routing IP; while the ATT bit is also used by IP, you can only set
it via this command when routing CLNS via ISIS.
>
> As for the second command, one possible way a question could require you
to use this is: configure your interior routers so that they do not
advertise themselves as available for transit routing until BGP has
converged on these routers. Or: configure your interior routers so that
they do advertise themselves as available for transit routing until 2
minutes after the IGP routing process has started.
>
> Here's my little summary of how you can use the set-overload-bit command:
>
> set-overload-bit
> ' immediately and unconditionally sets the overload bit
>
> set-overload-bit on-startup <seconds>
> ' sets the overload bit on startup for the specified number of seconds
(from 5 to 86400)
>
> set-overload-bit on-startup <seconds> suppress interlevel
> ' sets the overload bit on startup for the specified number of seconds
(from 5 to 86400). Suppresses advertisement of any inter-level routes while
the overload bit is set.
>
> set-overload-bit on-startup <seconds> suppress external
> ' sets the overload bit on startup for the specified number of seconds
(from 5 to 86400). Suppresses advertisement of any redistributed routes
while the overload bit is set.
>
> set-overload-bit on-startup <seconds> suppress interlevel external
> ' sets the overload bit on startup for the specified number of seconds
(from 5 to 86400). Suppresses advertisement of any inter-level or external
routes while the overload bit is set.
>
> set-overload-bit on-startup wait-for-bgp
> ' sets the overload bit on startup. It's cleared on receiving a signal
from BGP or after 10 minutes, whichever comes first.
>
> set-overload-bit on-startup wait-for-bgp suppress interlevel
> ' sets the overload bit on startup. It's cleared on receiving a signal
from BGP or after 10 minutes, whichever comes first. Suppresses
advertisement of any inter-level routes while the overload bit is set.
>
> set-overload-bit on-startup wait-for-bgp suppress external
> ' sets the overload bit on startup. It's cleared on receiving a signal
from BGP or after 10 minutes, whichever comes first. Suppresses
advertisement of any redistributed routes while the overload bit is set.
>
> set-overload-bit on-startup wait-for-bgp suppress interlevel external
> ' sets the overload bit on startup. It's cleared on receiving a signal
from BGP or after 10 minutes, whichever comes first. Suppresses
advertisement of any inter-level or external routes while the overload bit
is set.
>
> set-overload-bit suppress interlevel
> ' immediately and unconditionally sets the overload bit. Suppresses
advertisement of any inter-level routes while the overload bit is set.
>
> set-overload-bit suppress external
> ' immediately and unconditionally sets the overload bit. Suppresses
advertisement of any external routes while the overload bit is set.
>
> set-overload-bit suppress interlevel external
> ' immediately and unconditionally sets the overload bit. Suppresses
advertisement of any inter-level or redistributed routes while the overload
bit is set.
>
> HTH,
> Paresh.
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
Andrew Lissitz (alissitz)
> Sent: Thursday, 08 December 2005 03:01 PM
> To: comserv@groupstudy.com; ccielab@groupstudy.com
> Subject: ISIS - set-attached-bit and set-overload-bit
>
>
> Hello group,
>
> I am trying to think of a scenario / question that may cause me to think
of this feature.
>
> pe1(config)#router isis
> pe1(config-router)#?
> set-attached-bit Conditionally advertise us as attached to L2
> set-overload-bit Signal other routers not to use us in SPF
> pe1(config-router)#
>
> My lab:
>
> R1 <---> R2 (My lab follows the 'KISS' design guide)
>
> Has anyone used these features and or can think of a question / scenario
that would cause me to look at this feature for the solution. I am really
trying to think of how a question / scenario could be written.
> Kindest regards group,
>
> Andrew
>
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