EIGRP hop limitation

Answered Question
May 23rd, 2010
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Hi all.


Following is the excerpt from this link


http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1ceigrp.pdf


Increased network width—With IP RIP, the largest possible width of your network is 15 hops.
When EIGRP is enabled, the largest possible width is 224 hops. Because the EIGRP metric is
large enough to support thousands of hops, the only barrier to expanding the network is the
transport layer hop counter. Cisco works around this problem by incrementing the transport
control field only when an IP packet has traversed 15 routers and the next hop to the destination

was learned by way of EIGRP. When a RIP route is being used as the next hop to the destination,
the transport control field is incremented as usual.


I connected 4 routers in line.



R1------------------R2--------------------R3---------------------------R4


I advertised a loopback from R1. Now on R4 i started wireshark. I was expecting to see under EIGRP header, the hop field to be "1", but it was "2" why ?


Can someone explain me why the hop field is not "1" as defined in the above paragraph ?

Correct Answer by Giuseppe Larosa about 7 years 6 days ago

Hello Jonn,

I had assumed that you were running RIP between R1 and R2.


if all routers are running EIGRP in the same domain that route is an internal route, however IP next-hop may be equal or different from originator router.


forget about that hop count updated every 15 hops is no sense



Hope to help

Giuseppe

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Giuseppe Larosa Sun, 05/23/2010 - 12:35
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Hello Jonn,


>> Because the EIGRP metric is

large enough to support thousands of hops, the only barrier to expanding the network is the

transport layer hop counter.


as far as I know IPv4 header TTL is one byte in size, so speaking of a way to go behind 255 router hops is simply meaningless.


this document is related to 12.0 that is still used only on backbone devices with 12.0S train.


>> I advertised a loopback from R1. Now on R4 i started wireshark. I was expecting to see under EIGRP header, the hop field to be "1", but it was "2" why ?


R2 is the first EIGRP router to inject an external route in EIGRP domain so next-hop is R2 a next-hop has to be reachable and known by all devices in EIGRP domain. This is simply consistency.


>> Cisco works around this problem by incrementing the transport

control field only when an IP packet has traversed 15 routers and the next hop to the destination

was learned by way of EIGRP.


this may be referring to the Hop Count field in IP internal routes TLV and in IP external routes, but again there is no sense in this, even if an EIGRP packet could travel 15*255 router hops standard IP packets for the destination advertised in the EIGRP update cannot do the same (TTL is one byte).


Hope to help

Giuseppe

Jonn cos Mon, 05/24/2010 - 01:33
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Dear Sir,


I think i am still missing something. I advertised loopback from R1 via network command so its an internal route isnt it ?


Secondly, when cisco is saying that it increments the hop count only in transport layer after traversing 15 hops, so where is this hop count ? i have checked it via wireshark and Hop count in eigrp header is incremented normally !!


Where as should i look ? i know it doesnt make sense as per your saying but i just wanted to know that if in IOS 12.4 this approach was removed ?

Correct Answer
Giuseppe Larosa Mon, 05/24/2010 - 11:53
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Hello Jonn,

I had assumed that you were running RIP between R1 and R2.


if all routers are running EIGRP in the same domain that route is an internal route, however IP next-hop may be equal or different from originator router.


forget about that hop count updated every 15 hops is no sense



Hope to help

Giuseppe

Jonn cos Mon, 05/24/2010 - 21:49
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Dear sir,


thanks alot for confirming. I think cisco doc is mistyped or referred to some past feature.

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