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
> ot_indep_ps6441_TSD_Products_Configuration_Guide_Chapter.html#wp1056283<http://www.cisco.com/en/US/docs/ios/iproute_pi/configuration/guide/iri_ip_pr%0Aot_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
>
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Received on Sat Dec 04 2010 - 23:00:30 ART