From: Peter Van Oene (pvo@xxxxxxxxxxxx)
Date: Fri Apr 27 2001 - 15:18:13 GMT-3
Just once more for emphasis, I would highly recommend that classless is enabled
on all routers at all times. It has no negative consequences.
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On 4/27/2001 at 1:59 PM radha rani wrote:
>This is the best explanation I have seen so far. I guess it makes sense.
>
>In our production network, we are running ospf only. The default is
>configured as a static route. From what I have repeatedly observed, the
>router DOES NOT forward packets using the default route until the command
>ip
>classless is added. From the explanation below, it seems that the router
>behaves classlessly with OSPF but if the default comes from another
>process
>(static here), then classfull behaviour applies and thus we need to add
>the
>ip classless command specifically to override that behaviour.
>
>
>
>>From: "Peter Van Oene" <pvo@usermail.com>
>>Reply-To: "Peter Van Oene" <pvo@usermail.com>
>>To: ccielab@groupstudy.com
>>Subject: IP Classless
>>Date: Fri, 27 Apr 2001 11:56:35 -0400
>>
>>I'm not sure if John is on this list, but the following is a post he
>>submitted to the NP list at groupstudy. It thoroughly and in my opinion
>>accurately describes the behavior of ip classless when link state IP
>>routing protocols are in use.
>>
>>Peter
>>
>>
>>*********** BEGIN FORWARDED MESSAGE ***********
>>
>>On 4/24/2001 at 3:12 PM John Neiberger <john.neiberger@efirstbank.com>
>>wrote:
>>
>> >I have received results from a TAC case about this familiar issue. For
>> >those coming late to the picnic, the issue was that the presence of OSPF
>> >or IS-IS overrides 'no ip classless' in the router configuration and
>> >would force the router to behave classlessly. The TAC engineer
>> >consulted with some Development Engineers and here is what they said:
>> >
>> >
>> >
>> >Yes, we do this if the default (or the supernet) route
>> > is supplied by OSPF or ISIS (I guess EIGRP should be there,
>> > but it is not) and the part of the code that requested
>> > the RT lookup didn't not specify to ignore the default
>> > route if there's no specific subnet (which is the case
>> > for the locally originated and transit packets).
>> > The assumption is that it is safe to use a default/supernet
>> > route installed by a classless protocol.
>> >
>> >So what ever you and me have seen in our testing is correct behaviour.
>> >
>> >
>> >
>> >So, our original guesses were correct. The router assumes that if
>> >we're running OSPF or IS-IS then we want classless routing even if we
>> >didn't specify 'ip classless' in the config. However, an important
>> >point is that this applies only if the supernet was installed by the
>> >classless protocol. If OSPF or IS-IS is running on a router but the
>> >supernet was installed by another process, then classful routing would
>> >still apply without the addition of 'ip classless' to the config.
>> >
>> >I've also discovered that if you add 'ip clueless' to the config, the
>> >following occurs:
>> >
>> >First, ip classless is overridden but only in cases where no ip
>> >classless was manually configured previously and was not the default
>> >setting, unless the router has not had any previous configuration and is
>> >running at least 12.1(5F)T12. This does not apply for any 12.0 images
>> >except 12.0(6)S but does apply to any 11.2 image after 11.2(26c)P;
>> >
>> >Second, the gateway of last resort might be chosen by the RT lookup
>> >process if the GOLR was set by a classless routing protocol with an
>> >administrative distance lower than that of any other classless or
>> >classful routing protocol on the router, except in the case of BGP or
>> >EGP in which case the administrative distance must be at least equal to
>> >that of the routing protocol which previously installed the GOLR, if
>> >already present;
>> >
>> >Thirdly, if the lowest-weighted routing protocol is OSPF and the GOLR
>> >is advertised to neighbor as an E2 route, then the neighbor router may
>> >choose to use that route unless another neighbor has advertised the same
>> >supernet route as an E1 route. In which case--but especially when
>> >utilizing IP over Avian Carriers (with QoS)--the RT lookup will choose
>> >the Type 1 External route unless EIGRP is running on this router as
>> >well. In that case, the GOLR will be set via EIGRP because Cisco
>> >prefers EIGRP to OSPF and we should all use that anyway because, don't
>> >ya know, OSPF is harder to configure and requires way too much thought
>> >to begin with. IS-IS is just out of the question. However, if a router
>> >learns a supernet route via OSPF and IS-IS *and* EIGRP then you will be
>> >severely punished. Flogging is generally suggested. As an alternative,
>> >only run EIGRP and leave "ip clueless" configured.
>> >
>> >Any other configuration will provide ambiguous results.
>> >
>> >HTH,
>> >John
>> >FAQ, list archives, and subscription info:
>> >http://www.groupstudy.com/list/cisco.html
>> >Report misconduct and Nondisclosure violations to abuse@groupstudy.com
>>
>>*********** END FORWARDED MESSAGE ***********
>>**Please read:http://www.groupstudy.com/list/posting.html
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