From: Andrew Lissitz \(alissitz\) (alissitz@cisco.com)
Date: Tue Dec 13 2005 - 03:05:23 GMT-3
Ok, I got this working once I put a different L2 area on an adjacent
router. Thanks Arun for the tip, previously my lab did not have any
other L2 areas and thus ISIS did not create the default router as
expected. Once I added a different ISIS area, I now see the default
route generated.
A follow up question: How do I determine which router is the DIS router
on a multi-access interface? In my case I was using serial multipoint
interfaces and realized that I needed the hub to be the DIS. Once I
configured this on the hub router, I had complete routing tables on all
remote routers.
Thanks Arun and Group!
________________________________
From: Andrew Lissitz (alissitz)
Sent: Monday, December 12, 2005 12:21 PM
To: 'Arun Arumuganainar'; Paresh Khatri; comserv@groupstudy.com;
ccielab@groupstudy.com
Subject: RE: ISIS - set-attached-bit and set-overload-bit
My L2 router did not have a L2 neighbor in another area. Perhaps this
is why I could not lab this? More answers in line ... in line -->
<Andrew>
________________________________
From: Arun Arumuganainar [mailto:aarumuga@hotmail.com]
Sent: Monday, December 12, 2005 12:08 PM
To: Andrew Lissitz (alissitz); Paresh Khatri; comserv@groupstudy.com;
ccielab@groupstudy.com
Subject: Re: ISIS - set-attached-bit and set-overload-bit
Hi Andrew ,
My comments inline .
Thanks and Regards
Arun
----- Original Message -----
From: "Andrew Lissitz (alissitz)" <alissitz@cisco.com
<mailto:alissitz@cisco.com> >
To: "Arun Arumuganainar" <aarumuga@hotmail.com
<mailto:aarumuga@hotmail.com> >; "Paresh Khatri"
<Paresh.Khatri@aapt.com.au <mailto:Paresh.Khatri@aapt.com.au> >;
<comserv@groupstudy.com <mailto:comserv@groupstudy.com> >;
<ccielab@groupstudy.com <mailto:ccielab@groupstudy.com> >
Sent: Monday, December 12, 2005 8:52 PM
Subject: RE: ISIS - set-attached-bit and set-overload-bit
Hey Guys,
Arun ... Nemeste, you rock man!
I tried to use this in order to create default route on L1 routers. I
do not have configs with me, but the L1 routers saw routes from all
routers but no default route was in the routing table.
[Arun]
Could you explain me bit more . What do you mean create a default-route
on L1 routers ???
<Andrew> - I was setting the attached bit on the L2 router and hoping to
see a default route on the L1 routers
Actually there two methods for creating default route .
Method 1 : setting the ATT bit
This is done by default and no configuration is needed . But only thing
that is needed is to have L2 Neighbor that belongs to different area .
By the way can you me provide your topology details in case your L1/L2
router is not able to inject default route to L1 routers .
<Andrew> - I had one router as core and two remotes in L1 only.
Method 2 : default-information-originate command .
This will work independent of your topology .But you should remember
default-information will originate only L2-default . For generating
L1-default you need special configuration ( route-map needs to be
configured ) !!! Skeleton configuration is given below .
******************************************
!
route-map level1-default permit 10
set level level-1
!
!
Router isis
default-information originate route-map level1-default
!
******************************************
Note : If both methods are used to generate the default , Method 2 will
be preferred over Method 1 .
<Andrew> - Thanks for the configs!
[Arun]
Here is what my thoughts are for this:
You are using ISIS for IGP, you are asked to configure peer stub router
as L1 only. You are not allowed to use static default or default
information commands. Can this method work for this remote router to
receive a default route? This seems like a good method right?
[Arun]
"set-attached-bit" does not alter basic characteristic of L1-default
routing . It only adds more constraints to it To generate L1-Default by
setting ATT bit , what you need, is the visibility of atleast one L2
router that belongs to different area
***Hence the conclusion is ,this method will work only if your topology
permits***.
[Arun ]
Perhaps this is good practice for the core router? In the lab there
will likely be some redistribution point between multiple routing
domains. This would be good, any comments on this idea?
[Arun]
One Caution on Multiprotocol scenarios.
Just be careful about this in Multi-protocol scenario . EIGRP is a real
villain here !!! Whenever you configure auto summary( default in EIGRP )
, then EIGRP generates a static default route to Null0 . The main point
of contention is that EIGRP default has got an admin distance very much
lower that any other routing protocol . So even though default route is
generated in ISIS it may not get installed Such scenarios more
configurations are need to correct it . Some of techniques you can
employ are as follows .
1) Manually increase the admin distance for EIGRP .
2) Redistribute L2 routes in to L1 . By this way you will inject more
specific route in to L1 routing . Hence Black holing may not happen .
<Andrew> - EIGRP auto-summary ... I can not think of a time to use
this... Did you mean using the configuring a EIGRP summary on an
interface? Also, if EIGRP has a default in the routing table, and the
ISIS default does not get installed, it will still advertise it out to
ISIS neighbors?
[Arun ]
-----Original Message-----
From: Arun Arumuganainar [mailto:aarumuga@hotmail.com]
Sent: Monday, December 12, 2005 10:13 AM
To: Paresh Khatri; Andrew Lissitz (alissitz); comserv@groupstudy.com
<mailto:comserv@groupstudy.com> ;
ccielab@groupstudy.com <mailto:ccielab@groupstudy.com>
Subject: Re: ISIS - set-attached-bit and set-overload-bit
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
<mailto:Paresh.Khatri@aapt.com.au> >
To: "Arun Arumuganainar" <aarumuga@hotmail.com
<mailto:aarumuga@hotmail.com> >; "Andrew Lissitz
(alissitz)"
<alissitz@cisco.com <mailto:alissitz@cisco.com> >;
<comserv@groupstudy.com <mailto:comserv@groupstudy.com> >;
<ccielab@groupstudy.com <mailto: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
<mailto:comserv@groupstudy.com> ;
ccielab@groupstudy.com <mailto: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_paper09186a008
<http://www.cisco.com/en/US/products/ps6599/products_white_paper09186a00
8>
04fa
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
<mailto:alissitz@cisco.com> >
To: "Paresh Khatri" <Paresh.Khatri@aapt.com.au
<mailto:Paresh.Khatri@aapt.com.au> >;
<comserv@groupstudy.com <mailto:comserv@groupstudy.com> >;
<ccielab@groupstudy.com <mailto: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
<mailto:comserv@groupstudy.com> ;
ccielab@groupstudy.com <mailto: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>
[mailto:nobody@groupstudy.com]On Behalf Of
Andrew Lissitz (alissitz)
> Sent: Thursday, 08 December 2005 03:01 PM
> To: comserv@groupstudy.com <mailto:comserv@groupstudy.com> ;
ccielab@groupstudy.com <mailto: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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