CBT Nuggets ROUTE - Lab 1 (EIGRP)

A little confused about something on the first CBT Nuggets lab covering EIGRP config.

I have three routers connected, as shown here: Whoops! - TinyGrab

I have EIGRP (AS90) running on all three routers. All recognize each other:

BB#sh run | s eigrp
router eigrp 90
network 10.1.24.0 0.0.0.3
network 10.1.34.0 0.0.0.3
network 192.168.1.0
no auto-summary

R2#sh run | s eigrp
router eigrp 90
network 10.1.2.0 0.0.0.255
network 10.1.24.0 0.0.0.3
no auto-summary

R3#sh run | s eigrp
router eigrp 90
network 10.1.2.0 0.0.0.255
network 10.1.34.0 0.0.0.3
no auto-summary

One of the questions asks for a static route on BB to point at Null0 - this is why I am also advertising "network 192.168.1.0 0.0.0.255" on the BB router.

When I do a "show ip EIGRP topology" from R2 or R3, I expected to see routes to 192.168.1.0/24, advertised by BB, and the "other" router. So on R2, I would expect to see two routes to 192.168.1.0 - one through BB (successor), and one through R3 (feasible successor).

Oddly enough, when I do "show ip EIGRP top", I get the following:
R2#sh ip eigrp top
IP-EIGRP Topology Table for AS(90)/ID(10.1.24.2)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status

P 10.1.2.0/24, 1 successors, FD is 28160
via Connected, FastEthernet1/0
P 10.1.24.0/30, 1 successors, FD is 20512000
via Connected, Serial0/0
P 10.1.34.0/30, 1 successors, FD is 10514432
via 10.1.2.3 (10514432/10511872), FastEthernet1/0
via 10.1.24.1 (21024000/10511872), Serial0/0
P 192.168.1.0/24, 1 successors, FD is 10514432
via 10.1.2.3 (10514432/10511872), FastEthernet1/0
via 10.1.24.1 (20512000/256), Serial0/0

R3#sh ip eigrp top
IP-EIGRP Topology Table for AS(90)/ID(10.1.34.2)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status

P 10.1.2.0/24, 1 successors, FD is 28160
via Connected, FastEthernet1/0
P 10.1.24.0/30, 1 successors, FD is 20514560
via 10.1.2.2 (20514560/20512000), FastEthernet1/0
via 10.1.34.1 (21024000/20512000), Serial0/0
P 10.1.34.0/30, 1 successors, FD is 10511872
via Connected, Serial0/0
P 192.168.1.0/24, 1 successors, FD is 10511872
via 10.1.34.1 (10511872/256), Serial0/0

So my question is this: Why doesn't R3 recognize R2 as a feasible successor to the 192.168.1.0 network?

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Comments

Netstudent

Split Horizon...

bermovick

Pretty sure EIGRP doesn't have to deal with split horizon; isn't that one of the advantages over distance-vector protocols?

At a guess, I would say it has to do with the cost of the R2-BB link putting the total cost to 192.168.1.0 through R2 high enough that it's not eligible to be a feasible successor.

spartangtr

bermovick wrote: »

Pretty sure EIGRP doesn't have to deal with split horizon; isn't that one of the advantages over distance-vector protocols?

Sure it does. Even though EIGRP is hybrid protocol isn't it technically still considered distance vector? In any case, EIGRP still uses split horizon.

DevilWAH

is this is not just advertised the distance through R2 is higher than FD of successor so not added in to the topology. my last post on my blog (see bellow) i posted last night explains this.

CCNP ROUTE (Part 6 EIGRP Terminology in Diagrams)

peanutnoggin

bermovick wrote: »

At a guess, I would say it has to do with the cost of the R2-BB link putting the total cost to 192.168.1.0 through R2 high enough that it's not eligible to be a feasible successor.

I would agree with this! I think if you were to calculate the K values, the 128 link would probably not be low enough to qualify as a feasible successor. Just my thoughts... HTH.

-Peanut

peanutnoggin

I'm going to have to rescind my statement above. I labbed up the config you have in GNS3 and I came to the same results. It has something to do with the bandwidth (I don't know what). When I set the bandwidth to 256 on both links, each link only had a successor for the static route. Each link discovered the static via its serial interface. That's weird and I unfortunately don't know enough about EIGRP to answer this question... I am however very curious as to why its doing what its doing. HTH.

-Peanut

DevilWAH

try a

#show ip eigrp topology all-links

on my 4506 show ip eigrp topology only shows the sucessors.

fesiable sucessors are not included.

DevilWAH

from a #show ip eigrp topology all-links i get this where the vlan50 link is a back up route.

P 156.295.112.0/21, 1 successors, FD is 2816, serno 160
        via Connected, Vlan1
        via 172.31.0.0 (3072/2816), Vlan50

but if i only do #show ip eigrp

P 156.295.112.0/21, 1 successors, FD is 2816
        via Connected, Vlan1

mzinz

DevilWAH wrote: »
from a #show ip eigrp topology all-links i get this where the vlan50 link is a back up route.
P 156.295.112.0/21, 1 successors, FD is 2816, serno 160
        via Connected, Vlan1
        via 172.31.0.0 (3072/2816), Vlan50
but if i only do #show ip eigrp
P 156.295.112.0/21, 1 successors, FD is 2816
        via Connected, Vlan1

Hmm... Interesting. So what is the difference between 'sh ip eigrp top' and 'sh ip eigrp top all-links'? I wonder if the 'all-links' command will show ALL routes, even ones not considered a feasible successor?

It is possible that the cost is too high, although I find that very odd considering that both serial links have the same bandwidth set. Seems like one end should not have different results than the other.

mzinz

So, yes, 'sh ip eigrp top all-links' shows active routes that are not considered feasible successors - ie: routes where the RD is higher than the FD through the successor.

I'm going to open up GNS3 again and look up all of the different costs and see what's causing that.... The cost should be identical through either path.

DevilWAH

i was looking on my production network and see the same thing. in fact the advertised cost should be identical to the fesibable distance. assuming all delays and bandwiths are identical. do a show ip eigrp topology all links and compare all metrics.

mzinz

DevilWAH wrote: »

i was looking on my production network and see the same thing. in fact the advertised cost should be identical to the fesibable distance. assuming all delays and bandwiths are identical. do a show ip eigrp topology all links and compare all metrics.

Interesting... even when I do 'sh ip eigrp top all' on R2... it still does not list routes through R3.

R3#sh ip eigrp top all
IP-EIGRP Topology Table for AS(90)/ID(10.1.34.2)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status

P 10.1.2.0/24, 1 successors, FD is 28160, serno 1
via Connected, FastEthernet1/0
P 10.1.24.0/30, 1 successors, FD is 20514560, serno 2
via 10.1.2.2 (20514560/20512000), FastEthernet1/0
via 10.1.34.1 (21024000/20512000), Serial0/0
P 10.1.34.0/30, 1 successors, FD is 10511872, serno 3
via Connected, Serial0/0
P 192.168.1.0/24, 1 successors, FD is 10511872, serno 14
via 10.1.34.1 (10511872/256), Serial0/0
P 172.30.2.0/24, 1 successors, FD is 10639872, serno 7
via 10.1.34.1 (10639872/128256), Serial0/0
P 172.30.3.0/24, 1 successors, FD is 10639872, serno 8
via 10.1.34.1 (10639872/128256), Serial0/0
P 172.30.0.0/24, 1 successors, FD is 10639872, serno 5
via 10.1.34.1 (10639872/128256), Serial0/0
P 172.30.1.0/24, 1 successors, FD is 10639872, serno 6
via 10.1.34.1 (10639872/128256), Serial0/0
P 172.30.6.0/24, 1 successors, FD is 10639872, serno 11
via 10.1.34.1 (10639872/128256), Serial0/0
P 172.30.7.0/24, 1 successors, FD is 10639872, serno 12
via 10.1.34.1 (10639872/128256), Serial0/0
P 172.30.4.0/24, 1 successors, FD is 10639872, serno 9
via 10.1.34.1 (10639872/128256), Serial0/0
P 172.30.5.0/24, 1 successors, FD is 10639872, serno 10
via 10.1.34.1 (10639872/128256), Serial0/0
P 172.30.8.0/24, 1 successors, FD is 10639872, serno 13
via 10.1.34.1 (10639872/128256), Serial0/0

mzinz

Ok.

So the more I play with this, the more it makes sense.

I for some reason kept thinking that the connection on the two serial links was the same, but the BB->R2 link is 128k, and the BB->R3 link is 256k.

Since BB->R3 is so much faster, R2 naturally chooses to go through R3 for its path to BB.

"For a route to be a feasible successor the advertised distance cannot be more than the feasbile distance of the current sucessor route."

R2:
P 172.30.8.0/24, 1 successors, FD is 10642432
via 10.1.2.3 (10642432/10639872), FastEthernet1/0
via 10.1.24.1 (20640000/128256), Serial0/0

From R2's point of view:
"I have two options to get to 172.30.8.0/24"

Option 1 "My total distance to destination is 10642432. This is via 10.1.2.3, my neighbor, who's total distance is 10639872" ("This means that the distance from me to 10.1.2.3 is 10642432 minus 10639872, which is 2560").

Option 2 "My total distance to destination is 20640000. This is via 10.1.24.1, my neighbor, who's total distance is 128256" ("This means that the distance from me to 10.1.24.1 is 20640000 minus 128256, which is 20511744").

Using 10.1.24.1 as a feasible successor is legit because the distance from him to the destination (128256) is less than the total distance from me to the destination via my primary route (10642432).

R3 will not consider R2 a reasonable backup because his advertised distance (20640000) is more than the feasible distance on his current route (10642432).

mzinz

PS: Is there an easy way to find the cost between 2 links? Or do I need to always calculate FD minus RD?

DevilWAH

#show int (to get the delay)

256*(10^7/banwidth in kbs + delay/10)

mzinz

DevilWAH wrote: »

#show int (to get the delay)

256*(10^7/banwidth in kbs + delay/10)

Hehe thanks

Sorry for the ramblings... just kinda thinking out loud and typing it helped.

DevilWAH

mzinz wrote: »

Hehe thanks

Sorry for the ramblings... just kinda thinking out loud and typing it helped.

no its great

working through threads like this only help with understanding.

Why do you think I started a blog. Writing it all out long hand and explaining it to myself, really helps with getting it to sink in..

Who cares if any one else reads it. Like you say it helps to think out loud, and thinking out load as though you are explaing it to other is a good way to insure you yourself understand it.

And to the person who left feedback, I really don't care that no one cares. Thats not the point.