From: Peter Van Oene (pvo@xxxxxxxxxxxx)
Date: Fri Apr 27 2001 - 12:56:35 GMT-3
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 descri
bes the behavior of ip classless when link state IP routing protocols are in us
e.
Peter
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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
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