RE: EIGRP IP Default-Network - Confusion Reigns !!!

Nope. I have never seen this documented.

Regards,

Tyson Scott - CCIE #13513 R&S, Security, and SP

Managing Partner / Sr. Instructor - IPexpert, Inc.

Mailto: <mailto:tscott_at_ipexpert.com> tscott_at_ipexpert.com

From: Ravi Singh [mailto:way2ccie_at_googlemail.com]
Sent: Sunday, December 05, 2010 4:27 PM
To: Tyson Scott
Cc: Cisco certification
Subject: Re: EIGRP IP Default-Network - Confusion Reigns !!!

Hi Tyson,

If I have understood you correctly, shouldn't the reserved field in the
sniffer output shown before , set to 0x02 ?

 Do you know of a write-up or any document which tells about the usage of
the this reserved field for the purpose of marking Candidate defaults ?

Regards,
Ravi

On Sun, Dec 5, 2010 at 3:29 AM, Tyson Scott <tscott_at_ipexpert.com> wrote:

Interesting. Hmm...

I guess I had to do a sniffer trace but for the internal candidate default
the Reserved 2 bytes are used. If the reserved field is set to 0x02 then it
is a candidate default. The reserved field is not used for the EX or EX
candidate nor a regular internal route.

Regards,

Tyson Scott - CCIE #13513 R&S, Security, and SP

Managing Partner / Sr. Instructor - IPexpert, Inc.

Mailto: tscott_at_ipexpert.com

From: Ravi Singh [mailto:way2ccie_at_googlemail.com]
Sent: Saturday, December 04, 2010 6:01 PM

To: Tyson Scott
Cc: Cisco certification
Subject: Re: EIGRP IP Default-Network - Confusion Reigns !!!

Allow me to explain in a little detail what the confusion is .Here is the
simple setup .

(1.1.1.1-Lo0) R1 (F0/0) <--->(F0/0) R2 .

Here is the config.
R1
!
interface Loopback0
 ip address 1.1.1.1 255.0.0.0
!
interface FastEthernet0/0
 ip address 10.1.12.1 255.255.255.0
!
router eigrp 100
 network 1.0.0.0
 network 10.0.0.0
 no auto-summary
!
ip default-network 1.0.0.0

On R2
interface FastEthernet0/0
 ip address 10.1.12.2 255.255.255.0
!
router eigrp 100
 network 10.0.0.0
 no auto-summary
!

Since I can't attach files on GS , here is a screenshot of the displayed
packet and the Internal TLV as it shows on wireshark

http://img440.imageshack.us/img440/7054/internal.png

Note that there is no flag field in the TLV, still the route was marked as a
candidate default on R2.

R2#sh ip route
Gateway of last resort is 10.1.12.1 to network 1.0.0.0

     1.0.0.0/8 is subnetted, 1 subnets
D* 1.0.0.0 [90/409600] via 10.1.12.1, 00:08:15, FastEthernet0/0
     10.0.0.0/24 is subnetted, 1 subnets
C 10.1.12.0 is directly connected, FastEthernet0/0

If I change the way the default network is advertised i.e using redistribute
command, the flag field is present in the external TLV and everything is as
would have been expected. Here is the wireshark output

http://img3.imageshack.us/img3/2126/external0.png

Hope I am not missing something silly here.

Regards,
Ravi

On Sat, Dec 4, 2010 at 9:50 PM, Tyson Scott <tscott_at_ipexpert.com> wrote:

the flag is not just for external. It is for two things. To mark as
external or candidate default. 0x01 would say external 0x02 would say
candidate default. 0x03 would be external and candidate default.

Regards,

Tyson Scott - CCIE #13513 R&S, Security, and SP

Managing Partner / Sr. Instructor - IPexpert, Inc.

Mailto: tscott_at_ipexpert.com

From: Ravi Singh [mailto:way2ccie_at_googlemail.com]
Sent: Saturday, December 04, 2010 3:51 PM

To: Tyson Scott
Cc: Cisco certification
Subject: Re: EIGRP IP Default-Network - Confusion Reigns !!!

Thanks Again Tyson . My inputs on this --

1) Jeff Doyle's Routing TCP/IP - Vol 1 also says what Cisco's documentation
says. But I have a pretty old version of that book so it might be an error
in the book. Therefore, I give question 1 the benefit of doubt and would
agree with you that a candidate default can be an external or internal
route.

2 ) The flag bit is only a part of EIGRP external TLVs so question 2 still
exists - What field or bit in an EIGRP internal route tells the receiving
router to mark the route as candidate default ?

Regards,
Ravi

On Sat, Dec 4, 2010 at 8:23 PM, Tyson Scott <tscott_at_ipexpert.com> wrote:

The post shows that the default-network may be internal or external. It
depends on how it is used. In the article it shows the two ways you use it
and if you look at the output you will see that one way it shows as an EX
route and the other way it shows as an internal, which I thought was your
first question

So both are correct to an extent. The Cisco post should read that it may be
an external route

The flag bit being set to 0x02 will mark the route as a candidate default

http://www.iphelp.ru/faq/21/ch08lev1sec1.html

Regards,

Tyson Scott - CCIE #13513 R&S, Security, and SP

Managing Partner / Sr. Instructor - IPexpert, Inc.

Mailto: tscott_at_ipexpert.com

From: Ravi Singh [mailto:way2ccie_at_googlemail.com]
Sent: Saturday, December 04, 2010 2:29 PM
To: Tyson Scott
Cc: Cisco certification
Subject: Re: EIGRP IP Default-Network - Confusion Reigns !!!

Hi Tyson,

That post does not actually answer any of the questions, to be
frank.However, I really appreciate your response to this query. Thanks a lot
!!

Regards,
Ravi

On Sat, Dec 4, 2010 at 6:56 PM, Tyson Scott <tscott_at_ipexpert.com> wrote:

Ravi,

Take a look at this blog post,
http://blog.ipexpert.com/2010/03/29/eigrp-ip-default-network-command/, it
should help to answer your questions.

This may also be use for you for other information.
http://www.ipexpert.com/Cisco/CCIE/Library

Regards,
 
Tyson Scott - CCIE #13513 R&S, Security, and SP
Managing Partner / Sr. Instructor - IPexpert, Inc.
Mailto: tscott_at_ipexpert.com

-----Original Message-----
From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of Ravi
Singh
Sent: Friday, December 03, 2010 11:16 PM
To: Cisco certification
Subject: EIGRP IP Default-Network - Confusion Reigns !!!

Hello All,

We have had numerous discussions on the use of the IP Default-network
command in EIGRP, but I would like to ask a couple of questions which I
couldn't find answers to anywhere on the web.

1) Cisco's documentation and quite a few books say that with EIGRP, the
network which has been configured as the default-network is advertised and
flagged as an external route. Here is a link to the Cisco Doc -

http://www.cisco.com/en/US/docs/ios/iproute_pi/configuration/guide/iri_ip_pr
<http://www.cisco.com/en/US/docs/ios/iproute_pi/configuration/guide/iri_ip_p
r%0Aot_indep_ps6441_TSD_Products_Configuration_Guide_Chapter.html#wp1056283>

ot_indep_ps6441_TSD_Products_Configuration_Guide_Chapter.html#wp1056283

This does not seem correct to me . If you read Narbik's puzzles about this
command a couple of weeks back or simply connect two routers back to back
running EIGRP, you would find that the network being advertised as a
candidate-default is being sent and learned as an EIGRP internal route.
Please note that the network is being advertised using the network statement
so i am not talking about routes being genuinely advertised as external
routes using redistribute commands. So the question is- what am I missing
here ?

2) Considering the above is incorrect and the candidate default route can be
an internal route, what bit in the update packet tells the receiving router
that the route is a candidate default. An EIGRP update having an external
TLV contains the Flag field which tells the receiving router whether the
route is supposed to be a candidate default. Doing a packet dissection using
wireshark , I don't seem to find anything in an Internal EIGRP update that
would make the router decide whether to insert the route as a candidate
default or not.

Awaiting your valuable responses !!

Regards,
Ravi

Blogs and organic groups at http://www.ccie.net
Received on Sun Dec 05 2010 - 17:06:45 ART