From: Scott M Vermillion (scott_ccie_list@it-ag.com)
Date: Mon Dec 01 2008 - 15:15:43 ARST
Ah, well, I just waded through the first five pages from the results of a
google search for "manipulate eigrp update" and came up empty handed.
I think the challenge here would be that the EIGRP packets/updates
themselves don't actually traverse the network. Every update, regardless of
the hop count TLV value, has an IP TTL of 2. You'd have to completely
emulate an EIGRP router so that you could neighbor up before any updates
would be exchanged. And given that EIGRP is proprietary...
At the end of the day, an EIGRP route in excess of 200 hops sounds like just
plain bad network design/engineering to me! Can you imagine trying to keep
a handle on that query scope!? So it's kind of an academic point one way or
the other, IMHO...
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of paul
cosgrove
Sent: Monday, December 01, 2008 2:32 AM
To: Scott M Vermillion
Cc: Timothy Chin; Oleg Konovalov; Cisco certification
Subject: Re: EIGRP Maximum-hop
Hi Scott,
I was thinking of generating a packet as if it had passed over that number
of hops, rather than using an actual network.
You can get tools to do that kind of thing for most protocols and there is
indeed one for eigrp:-
http://www.hackingciscoexposed.com/tools/eigrp-tools.tar.gz
Paul.
On Mon, Dec 1, 2008 at 12:12 AM, Scott M Vermillion <
scott_ccie_list@it-ag.com> wrote:
> Hi again Paul!
>
> Hop count is actually carried in the EIGRP "IP Internal Routes TLV" and
the
> "IP External Routes TLV" (Doyle Vol I is a great reference for this and
> similar such topics). A router advertises a directly connected network
> with
> a hop count of zero and it's incremented from there by subsequent routers
> learning of that network. When a router receiving an update increments
hop
> count and the resulting value exceeds the locally configured max-hop
value,
> it will be marked unreachable with a delay of 0xFFFFFF. I don't believe
> that any update is sent on regarding that route with a delay of 0xFFFFFF -
> I
> believe this to be a completely local affair on a router-by-router basis.
> But I could be wrong on that last count. Semantics at that point anyway.
>
> So I'd think that you'd either need a really big EIGRP network to test
what
> the truly maximum implemented value might be or you'd need a means to
> manipulate the "Hop Count" field in the TLV. I'm not aware that this can
be
> manipulated in a route-map or any such thing...
>
> Cheers,
>
> Scott
>
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> paul
> cosgrove
> Sent: Sunday, November 30, 2008 2:29 PM
> To: Timothy Chin
> Cc: Oleg Konovalov; Cisco certification
> Subject: Re: EIGRP Maximum-hop
>
> p.s. I should have said we need to set maximum hops to 255 and generate a
> packet as if it has passed over >224 hops. IPv6 hop limit, like TTL,
count
> down not up.
>
>
> On Sun, Nov 30, 2008 at 9:15 PM, paul cosgrove
> <paul.cosgrove@gmail.com>wrote:
>
> > Hi Timothy,
> >
> > Thanks for the link. The paragraph which explains the 224 limit is
> > incorrect in that the Transport Control Field does not exist in IP or
> IPv6
> > headers. The IPX TC header field functions like a hop count, and IPX
> > packets with a TC of 16 are dropped, hence the need for a workaround if
> > packets need to be sent further. The TTL in IP, or hop limit in IPv6
> both
> > allow 255 hops so there is no need to do that.
> > The text looks to have been incorrectly copied from the old
> documentation.
> > The IPX workaround is explained in the old EIGRP paper at
> >
> http://ccrg.soe.ucsc.edu/<
> http://ccrg.soe.ucsc.edu/publications/interop94.pd
> f <http://ccrg.soe.ucsc.edu/publications/interop94.pdf>>
> >
> publications/interop94.pdf<
> http://ccrg.soe.ucsc.edu/publications/interop94.p
> df>
> > and all the references I've seen to 224 appear to have been
> derived/copied
> > from that text. I've left a comment on the web page that it needs
> updating.
> >
> > Since the documentation about the max hop limit is conflicting, some of
> it
> > must be incorrect. My guess is that the limit is indeed 255, but the
> only
> > way we will know for sure is to generate an eigrp packet with the hop
> limit
> > set above 224 and see if other routers consider the advertisement as
> valid.
> >
> > Paul.
> >
> >
> > On Sun, Nov 30, 2008 at 7:05 PM, Timothy Chin <Tim@1c-solutions.com
> >wrote:
> >
> >> Hi,
> >>
> >>
> >>
> >> I know there has been conflicting information on this for awhile. I
> don't
> >> know why but the 12.4 command reference for IPv4 doesn't provide this
> >> information but the IPv6 reference does specify the maximum network
> width:
> >>
> >>
> >>
> >>
> >>
>
>
http://www.cisco.com/en/US/docs/ios/ipv6/configuration/guide/ip6-eigrp_ps644
>
1_TSD_Products_Configuration_Guide_Chapter.html<http://www.cisco.com/en/US/d
ocs/ios/ipv6/configuration/guide/ip6-eigrp_ps6441_TSD_Products_Configuration
_Guide_Chapter.html>
> >>
> >>
> >>
> >> I don't see an update on the 12.4 IPv4 documentation regarding this but
> >> going by previous releases I figure it would still be the same. From
> what
> I
> >> know a maximum hop count of 255 can be configured but traffic would
only
> >> traverse 224.
> >>
> >>
> >> ------------------------------
> >>
> >> *From:* paul cosgrove [mailto:paul.cosgrove@gmail.com]
> >> *Sent:* Sunday, November 30, 2008 12:56 PM
> >> *To:* Timothy Chin
> >> *Cc:* Oleg Konovalov; Cisco certification
> >> *Subject:* Re: EIGRP Maximum-hop
> >>
> >>
> >>
> >> Hi Timothy,
> >>
> >> Just wondering if you have verified that?
> >>
> >> I know there is conflicting information about this point but the 12.4
> >> command reference says the limit is 255.
> >>
> >>
>
>
http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_eig2.html#
>
wp1011619<http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_
eig2.html#wp1011619>
> >>
> >> Also the field is indeed 8 bits long in the packets (see figure 4-7).
> >> http://oreilly.com/catalog/iprouting/chapter/ch04.html
> >>
> >> The following paper mentions the limit of 224, but only as a workaround
> >> used on IPX networks to overcome limitations of IPX.
> >> http://ccrg.soe.ucsc.edu/publications/interop94.pdf
> >>
> >> Paul.
> >>
> >> On Sun, Nov 30, 2008 at 12:49 PM, Timothy Chin <Tim@1c-solutions.com>
> >> wrote:
> >>
> >> The maximum definable hop count is 255 but 224 hops is the true limit.
> >>
> >>
> >>
> >> -----Original Message-----
> >> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> >> Oleg Konovalov
> >> Sent: Sunday, November 30, 2008 5:05 AM
> >> To: Cisco certification
> >> Subject: EIGRP Maximum-hop
> >>
> >> Hi all, I have question about EIGRP maximum hops, based on
> >>
> http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080
> >>
> 094cb<
> http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a00
> 80094cb>
> >> 7.shtml we can set up maximum 220 hops, based on some CCNP materials it
> >> is
> >> 224. I test it on my router and 255 hops was OK.
> >>
> >> Routing Protocol is "eigrp 100"
> >> Outgoing update filter list for all interfaces is not set
> >> Incoming update filter list for all interfaces is not set
> >> Default networks flagged in outgoing updates
> >> Default networks accepted from incoming updates
> >> EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
> >> EIGRP maximum hopcount 100
> >> EIGRP maximum metric variance 1
> >> Redistributing: eigrp 100
> >> EIGRP NSF-aware route hold timer is 240s
> >> Automatic network summarization is in effect
> >> Maximum path: 4
> >> Routing for Networks:
> >> Routing Information Sources:
> >> Gateway Distance Last Update
> >> Distance: internal 90 external 170
> >>
> >>
> >> Then I have changed metric maximum-hops
> >>
> >> Router(config)#router eigrp 100
> >> Router(config-router)#metric maximum-hops 255
> >> Router(config-router)#end
> >> Router#sh ip protocols
> >> Routing Protocol is "eigrp 100"
> >> Outgoing update filter list for all interfaces is not set
> >> Incoming update filter list for all interfaces is not set
> >> Default networks flagged in outgoing updates
> >> Default networks accepted from incoming updates
> >> EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
> >> EIGRP maximum hopcount 255
> >> EIGRP maximum metric variance 1
> >> Redistributing: eigrp 100
> >> EIGRP NSF-aware route hold timer is 240s
> >> Automatic network summarization is in effect
> >> Maximum path: 4
> >> Routing for Networks:
> >> Routing Information Sources:
> >> Gateway Distance Last Update
> >> Distance: internal 90 external 170
> >>
> >>
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