Go CCIE Go!! - A Cisco Certified Internetwork Expert

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 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/ > publications/interop94.pdf > 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 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 >> >> >> >> 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 >> >> 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 >> 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 >> 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 >> >> >> Blogs and organic groups at http://www.ccie.net >> >> _______________________________________________________________________ >> Subscription information may be found at: >> http://www.groupstudy.com/list/CCIELab.html >> >> >> Blogs and organic groups at http://www.ccie.net >> >> _______________________________________________________________________ >> Subscription information may be found at: >> http://www.groupstudy.com/list/CCIELab.html
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