From: Brian McGahan (bmcgahan@internetworkexpert.com)
Date: Sun Aug 08 2004 - 13:12:35 GMT-3
John,
What you just posted confirms what Scott said:
"A Level 1/Level 2 router that is attached to another area will set the
"attached bit" in its Level 1 LSP; all the Level 1 ISs in an area will
get a copy of this LSP and know where to forward packets to destinations
outside the area"
The reason Scott is mentioning this is because this is different
than originating a default route with the default-originate statement.
When an L1/L2 router is attached to both a L1 domain and a L2 domain, it
sets the ATT (attached) bit in its LSP advertisements. When routers in
the L1 domain see the LSP with the ATT bit set, they know that the L1/L2
router is candidate to be defaulted to. If there are multiple L1/L2
routers that are setting the ATT bit, the lowest cost to the L1/L2 will
dictate which default gets installed.
This is different than a situation when you want to originate a
default via default-originate, which is typically used in L2. As L2
routers do not listen to the ATT bit, default routing must be configured
either with the default-originate statement, or via redistribution of a
static default into the domain. This is fundamentally different than a
default route being installed via the ATT bit, as an LSA for 0.0.0.0 is
actually generated when using the latter methods.
HTH,
Brian McGahan, CCIE #8593
bmcgahan@internetworkexpert.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
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> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
Of
> john matijevic
> Sent: Sunday, August 08, 2004 10:01 AM
> To: 'Scott Morris'; k_kaloianov@eircom.net; ccielab@groupstudy.com
> Subject: RE: isis and ATT bit=1
>
> Hello Scott,
> The default route is installed per Cisco doc.
> "A Level 1/Level 2 router that is attached to another area will set
the
> "attached bit" in its Level 1 LSP; all the Level 1 ISs in an area will
> get a copy of this LSP and know where to forward packets to
destinations
> outside the area. If the routers are running Integrated IS-IS, a
default
> IP route will automatically be installed in the Level 1 routers
pointing
> toward the nearest Level 1/Level 2 router that originally set the
> attached bit in its Level 1 LSP. A Level 1/Level 2 router that is not
> attached to another area can also detect that a Level 2-only neighbor
is
> attached to another area and set the "attached bit" on behalf of this
> Level 2-only neighbor."
> I think the point of the question was if that using the command
default
> information originate was necessary to install a default route. And if
> the default route being installed was normal behavior. I am just
trying
> to help the individual. Unlike other companies, I don't focus on
> attacking other individuals. I focus on solutions. I don't have to
take
> the exam. It is Kaloyan who has to take the exam. So it is important
> that he understands why the default route appears in the routing
table.
> Here is the link for more information:
>
http://www.cisco.com/en/US/tech/tk365/tk381/technologies_white_paper0918
> 6a00800a3e6f.shtml
> Team, if anyone else has any trouble understanding this, please let me
> know.
> :) Again im not here to debate, I don't like to see a lot of threads,
im
> here to help.
> Sincerely,
> John Matijevic, CCIE #13254, MCSE, CNE, CCEA
> Network Consultant
> Hablo Espanol
> 305-321-6232
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
Of
> Scott Morris
> Sent: Sunday, August 08, 2004 10:42 AM
> To: 'john matijevic'; k_kaloianov@eircom.net; ccielab@groupstudy.com
> Subject: RE: isis and ATT bit=1
>
> Actually, the default route is not "sent" at all. The ATT bit
indicates
> connection to an outside area. As soon as L1 routers see routes with
> the
> ATT bit set, they will create a default route pointing to the router
> that
> sent the ATT-bit routes.
>
> HTH,
>
>
> Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713,
> CISSP,
> JNCIP, et al.
> IPExpert CCIE Program Manager
> IPExpert Sr. Technical Instructor
> swm@emanon.com/smorris@ipexpert.net
> http://www.ipexpert.net
>
>
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
Of
> john
> matijevic
> Sent: Sunday, August 08, 2004 9:54 AM
> To: k_kaloianov@eircom.net; ccielab@groupstudy.com
> Subject: RE: isis and ATT bit=1
>
> Hello Kaloyan,
> What you are seeing is normal behavior with IS-IS, basically it treats
> each
> level-1 router as a stub area. It will send a default route to a
> level-1 area. IF you have a level-1-2 domain you can configure route
> leaking
> so that the individual routes will show up with the default. For more
> information on that see the following link:
>
http://www.cisco.com/en/US/tech/tk365/tk381/technologies_tech_note09186a
> 0080093f39.shtml
> :)
>
> Sincerely,
> John Matijevic, CCIE #13254, MCSE, CNE, CCEA Network Consultant Hablo
> Espanol
> 305-321-6232
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
Of
> k_kaloianov@eircom.net
> Sent: Sunday, August 08, 2004 7:06 AM
> To: ccielab@groupstudy.com
> Subject: isis and ATT bit=1
>
> Hi Group,
>
> just a dumb question about isis and IOS ver 12.2, I'm just having two
> areas
> and one router in one area and two in the other one, the two routers
are
> L1
> and L1/L2 and the other one is L2 or L2/L1, I just want to know
whether
> we
> have to use clns router isis command on L2/L1 iterface connecting /w
L1
> router or alternatively default-information originate and default
static
> route to null interface in order to propagate default route to L1
> router?
> The behaviour that I saw was that even without any of the above
> configuration there was a default route created in L1 routing table?
> Thanks
> in advance:)
>
> Reg,
>
> Kaloyan-:)
>
>
This archive was generated by hypermail 2.1.4 : Fri Sep 03 2004 - 07:02:35 GMT-3