cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
1244
Views
10
Helpful
7
Replies

EIGRP - All routes not showing up in show ip eigrp topology all-links

dk3874
Beginner
Beginner

Diagram.PNGEverything is working as expected except for one thing. When I do a show ip eigrp topology all-links on R2, specifically looking for a route to 66.66.66.66/32, I am only seeing two routes. One thru R4 and one thru R5, but I'm not seeing a route thru R1.

 

When I do a show ip eigrp events on R2, I can see poison reverse is preventing the advertisement for 66.66.66.66/32 from R1, but I do not understand why. Making this more difficult to understand is that R2 is advertising 66.66.66.66/32 to R4 and R4 is advertising 66.66.66.66/32 to R2 and both these routes are showing up in both routers from the other. Poison reverse is not being implemented between R2 or R4, so I don't understand why it is being implemented between R1 and R2. 

 

Also R2 is advertising 66.66.66.66/32 to R1 and R1 is advertising 66.66.66.66/32 to R3, but R3 is also advertising 66.66.66.66/32 to R1 and yet poison reverse is not occurring at all between R1 and R3 for route 66.66.66.66/32.

 

Why is poison reverse affecting R1's advertisement to R2 for 66.66.66.66/32, but it's not affecting R2 / R3 for route 66.66.66.66/32 and it's not affecting R1 / R3 for 66.66.66.66/32?

 

Note that the last octet for each neighbor is the same as the hostname number (e.g. for R5, all ip's configured end with .5; for R2, all ip's end with .2)

 

 

 

R1#sh ip eigrp topology all-links | sec 66.66
P 66.66.66.66/32, 1 successors, FD is 156416, serno 94
via 10.10.10.2 (156416/130816), FastEthernet0/0
via 11.11.11.3 (158720/156160), FastEthernet1/0
R1#

 

 

Why does R2 only show two routes instead of three. Why is poison reverse in effect here for 66.66.66.66/32, but not between R2/R4 or R1/R3? For R2/R4 and R1/R3, 66.66.66.66/32 is being advertised out the same interface which learned of the route and split horizon is not turned off. Why is this?

 

R2#sh ip eigrp topology all-links | sec 66.66    
P 66.66.66.66/32, 1 successors, FD is 130816, serno 16
via 13.13.13.5 (130816/128256), GigabitEthernet2/0
via 12.12.12.4 (158720/156160), FastEthernet0/0
R2#

 

R3#sh ip eigrp topology all-links | sec 66.66
P 66.66.66.66/32, 1 successors, FD is 156160, serno 7
via 14.14.14.5 (156160/128256), FastEthernet1/0
via 11.11.11.1 (158976/156416), FastEthernet0/0
R3#

 

R4#sh ip eigrp topology all-links | sec 66.66
P 66.66.66.66/32, 1 successors, FD is 156160, serno 5
via 15.15.15.5 (156160/128256), FastEthernet0/0
via 12.12.12.2 (156416/130816), FastEthernet1/0
R4#

 

 

1 Accepted Solution

Accepted Solutions

Martin L
VIP Advisor VIP Advisor
VIP Advisor

 

I think the issue here depends on 3 factors: split horizon (SH), routing by rumor, and Feasibility condition (FC).   

Routing by rumor is one of characteristics of Distance Vector prot.  like EIGRP and RIP.   Definition goes like "Router learns routes from its neighboring routers' perspective and then advertise the routes from its own perspective." So, it is like I make my best routing decision based on the best info I got from you.  Feasibility condition and SH go together sometimes depending on topology.  Making things worst, sometimes output depends on the order in which router/eigrp came up. 

 

You have 2 triangles (or sort of 2); making R1 on left dependable on what R2 and R5 reporting.  R1 best route is via R2, so SH tells R1 not to advertise that back to R2. In other words, SH depends on current successor route when making decision about networks.  You will not see that route in All-links topology on R2. 

 

how did u determine that Router is using split horizon?

Re: R4,  I will try to explain more later; 

any possibility of getting gns3 file ? i think just single gns3 2.x project file will include all info including configs. 

 

Regards, ML
**Please Rate All Helpful Responses **

View solution in original post

7 Replies 7

Martin L
VIP Advisor VIP Advisor
VIP Advisor

 

I think the issue here depends on 3 factors: split horizon (SH), routing by rumor, and Feasibility condition (FC).   

Routing by rumor is one of characteristics of Distance Vector prot.  like EIGRP and RIP.   Definition goes like "Router learns routes from its neighboring routers' perspective and then advertise the routes from its own perspective." So, it is like I make my best routing decision based on the best info I got from you.  Feasibility condition and SH go together sometimes depending on topology.  Making things worst, sometimes output depends on the order in which router/eigrp came up. 

 

You have 2 triangles (or sort of 2); making R1 on left dependable on what R2 and R5 reporting.  R1 best route is via R2, so SH tells R1 not to advertise that back to R2. In other words, SH depends on current successor route when making decision about networks.  You will not see that route in All-links topology on R2. 

 

how did u determine that Router is using split horizon?

Re: R4,  I will try to explain more later; 

any possibility of getting gns3 file ? i think just single gns3 2.x project file will include all info including configs. 

 

Regards, ML
**Please Rate All Helpful Responses **

Although, I am not 100% certain, will check my notes, doesn't eigrp router advertise only best route?