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
> 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 Sun Dec 05 2010 - 21:26:37 ART