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question in the command # sh ip eigrp to all-links

Unanswered Question
Nov 5th, 2011
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i have a auestion in the command #sh ip eigrp to all


i enclosed a figure s0 as to explain my question


==========================


now when i go to wan1 and type sh ip eigrp to all

===================================================

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,

r - reply Status, s - sia Status

P 10.11.1.0/24, 1 successors, FD is 2172416

via 10.1.1.2 (2172416/28160), Serial0/0/0.1

! lines omitted for brevity; no other lines of output pertain to 10.11.1.0/24.

WAN1#show ip eigrp topology all-links

IP-EIGRP Topology Table for AS(1)/ID(10.9.1.1)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,

r - reply Status, s - sia Status

P 10.11.1.0/24, 1 successors, FD is 2172416, serno 45

via 10.1.1.2 (2172416/28160), Serial0/0/0.1

via 10.9.1.2 (2174976/2172416), FastEthernet0/0

! lines omitted for brevity; no other lines of output pertain to 10.11.1.0/24.

=====================================================


why there is no  path to the route 10.11.1.0/24 through B2????


i read from the book

(

the path to 10..11.1.0/24 is not listed  through Router B2 (10.1.1.6) at all, because B2’s current successor route

for 10.11.1.0/24 is through WAN1. EIGRP Split Horizon rules tell B2 to not advertise

10.11.1.0/24 to WAN1.

)



what does the split horiozon do here???????"


should i disable it??



or it is a

Precaution to prevent the loops???



thanks alot

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Giuseppe Larosa Sat, 11/05/2011 - 06:56
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Hello Ahmed,

the show ip eigrp topology all-links shows for each prefix all possible paths including those that are not eligible as feasible successors, as a result of this you see also a sub-optimal path that does not satisfy the feasible condition if you omit the all-links option it should not be seen.

These are EIGRP specific concepts that are not shared with RIP.


the output of the command tells us how many successors = best path and how many feasible successors = alternate best paths are present in topology for a given prefix.


split horizon is a distance vector concept and the idea is that to avoid to advertise an IP prefix out the interface on which it is learned.

Split horizon concept is present also in RIPv1 and RIPv2.


you don't need to disable it in EIGRP except some special cases and yes it is part of loop prevention in distance vector protocols. EIGRP DUAL protocol has its own mechanisms to provide fast loop free convergence but it is best practice to keep split horizon enabled



edit:

the feasible condition requires that the advertised distance from neighbor is less then current best metric (named feasible distance) to the IP prefix, that is only a router that is nearest to the IP prefix can be considered a feasible alternate path to it.

Some colleagues are confused by the fact that it is not a less equal then but strictly less then

in your case should the best path fail, the router should put the route in active state and should send out a query for it asking to EIGRP neighbors help in finding an alternate path. This is very specific of EIGRP because the local node involves the other ones in the computation. Even OSPF and ISIS do not do this.

It is called a distributed approach.



Hope to help

Giuseppe

Ahmed M Alzaeem Sat, 11/05/2011 - 07:21
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thanks Giuseppe Larosa , but u did not answer my question which is ,


why there is no  path to the route 10.11.1.0/24 through B2????



waiting ur reply



thanks so much

Giuseppe Larosa Sat, 11/05/2011 - 08:47
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Hello Ahmed,

may you post


show ip eigrp neig


show ip eigrp interface



taken from WAN1 router and B2 routers?


my impression was that there is no direct link betweeen them, but you may have a FR cloud in the middle so we could expect an EIGRP neighborship between them


Hope to help

Giuseppe

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