Administrative Distance Vs Longest Prefix Match

please setup your lab similar to previous lab

add this line to R2


router eigrp 1
 passive-interface Serial0/0
 network 10.1.1.0 0.0.0.255
 network 172.16.2.0 0.0.0.3
 no auto-summary

ip route 192.168.0.0 255.255.0.0 172.16.2.1

now R2 have two routes to reach 192.168.1.1

via EIGRP with AD = 90

via static route with AD = 1

R2#sh ip route

...

D    192.168.1.0/24 [90/2323456] via 10.1.1.1, 00:05:57, FastEthernet0/0

S    192.168.0.0/16 [1/0] via 172.16.2.1

guess.. which path will the router use to reach 192.168.1.1 ? some of you may though that it'll be static because it has AD = 1, let's test..

R2#traceroute 192.168.1.1

..

  1 10.1.1.1 4 msec 4 msec 4 msec

  2 172.16.1.1 12 msec 12 msec * 

unfortunately it choose to use route learn via EIGRP.. thru R3. This is because it prefer longest prefix match compare to AD

destination : 192.168.1.1

EIGRP route = 192.168.1.0/24

Static route = 192.168.0.0/16

EIGRP has the longest match..

static floating route - gns3

using static route when dynamic route not available

primary link between network 192.168.1.0/24 and 10.1.1.0/24 are thru R3, learn each others via EIGRP . R2 is a backup link in case the primary link is not available

*this is not a complete routing solution, this lab only looking from R1 point of view to reach 10.1.1.0/24

R1

interface Loopback1

ip address 192.168.1.1 255.255.255.0

!

interface Serial0/0

ip address 172.16.2.1 255.255.255.252

clock rate 56000

!

interface Serial0/1

ip address 172.16.1.1 255.255.255.252

clock rate 56000

!

router eigrp 1

network 10.0.0.0

network 172.16.1.0 0.0.0.3

network 192.168.1.0

no auto-summary

!

ip route 10.1.1.0 255.255.255.0 172.16.2.2 190

!

R2

interface FastEthernet0/0

ip address 10.1.1.2 255.255.255.0

duplex auto

speed auto

!

interface Serial0/0

ip address 172.16.2.2 255.255.255.252

clock rate 56000

!

ip route 192.168.0.0 255.255.0.0 172.16.2.1

R3

interface FastEthernet0/0

ip address 10.1.1.1 255.255.255.0

duplex auto

speed auto

!

interface Serial0/0

ip address 172.16.1.2 255.255.255.252

clock rate 56000

!

router eigrp 1

network 10.1.1.0 0.0.0.255

network 172.16.1.0 0.0.0.3

no auto-summary

-----------------------------------------------------

we've configure two route for R1 to reach 10.1.1.0

router eigrp 1

network 10.0.0.0

..

and

ip route 10.1.1.0 255.255.255.0 172.16.2.2 190

only one route with the lowest AD installed into the routing table

in this case, route from EIGRP (AD=90), static route(AD=190 as we defined)

R1#sh ip route

..

10.0.0.0/24 is subnetted, 1 subnets

D 10.1.1.0 [90/2195456] via 172.16.1.2, 00:05:22, Serial0/1

C 192.168.1.0/24 is directly connected, Loopback1

this what we call floating static route, by default AD for static route is 1, because we want this route to backup primary link w/ EIGRP in case it's not available.. we made it's AD higher than EIGRP AD

shutdown primary link to see the effect..


R1(config)#int ser 0/1
R1(config-if)#shut

R1#sh ip route

..

     10.0.0.0/24 is subnetted, 1 subnets

S       10.1.1.0 [190/0] via 172.16.2.2

new route come into place.. via static with AD 190, thru 172.16.2.2 which is R2

next we'll discuss about Administrative Distance Vs Longest Prefix Match

a few useful show IP command..

sh ip route


R1#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route


Gateway of last resort is not set


     172.16.0.0/30 is subnetted, 2 subnets
C       172.16.1.0 is directly connected, Serial0/1
C       172.16.2.0 is directly connected, Serial0/0
     10.0.0.0/24 is subnetted, 1 subnets
D       10.1.1.0 [90/2195456] via 172.16.1.2, 00:02:01, Serial0/1
C    192.168.1.0/24 is directly connected, Loopback1

• type of network ; static, connected
• or how the router learn it ; EIGRP, OSPF.from which interface it learned

let's be more specific

show ip route [address]

R1#sh ip rou 10.1.1.1

Routing entry for 10.1.1.0/24

  Known via "eigrp 1", distance 90, metric 2195456, type internal

  Redistributing via eigrp 1

  Last update from 172.16.1.2 on Serial0/1, 00:07:16 ago

  Routing Descriptor Blocks:

  * 172.16.1.2, from 172.16.1.2, 00:07:16 ago, via Serial0/1

      Route metric is 2195456, traffic share count is 1

      Total delay is 21000 microseconds, minimum bandwidth is 1544 Kbit

      Reliability 255/255, minimum MTU 1500 bytes

      Loading 1/255, Hops 1

• how it's learned . ex; EIGRP, static or connected..
• how long since the route learned, this can be useful when you want to check how long link down, up time.. etc,
• the bandwidth information are reflected by the value configured on the interface .. what ever value you put, it's gonna show the same in here

show ip route connected

show ip route static

• issue this two cmd if you want to filter or be more specific on the type of route

show ip route summary

R1#sh ip rou sum

IP routing table name is Default-IP-Routing-Table(0)

IP routing table maximum-paths is 16

Route Source    Networks    Subnets     Overhead    Memory (bytes)

connected       1           2           216         408

static               0           0           0           0

eigrp 1            0           1           72          136

internal            2                                   2312

Total               3           3           288         2856

Removing Queue Size 0

• this will summarize all you information base on Net, Subnet, Overhead(other memory used) and Memory

EIGRP route summarization, default gateway, variance and passive-interface

this lab will cover route summarization, default gateway using network-default command, variance and EIGRP passive-interface concept

R1

interface Loopback11
 ip address 172.16.1.1 255.255.255.0
!
interface Loopback12
 ip address 172.16.2.1 255.255.255.0
!
interface Loopback13
 ip address 172.16.3.1 255.255.255.0
!
interface Loopback14
 ip address 172.16.4.1 255.255.255.0
!
interface Loopback15
 ip address 172.16.5.1 255.255.255.0

interface Serial0/0

 ip address 30.30.12.1 255.255.255.252

 clock rate 56000

interface Serial0/1

 ip address 30.30.13.1 255.255.255.252

 clock rate 56000

router eigrp 1

 network 30.0.0.0

 network 172.16.0.0

 no auto-summary

R2

interface FastEthernet0/0

 ip address 30.30.23.1 255.255.255.0

 duplex auto

 speed auto

!

interface Serial0/0

 ip address 30.30.12.2 255.255.255.252

 clock rate 2000000

router eigrp 1

 network 30.0.0.0

 no auto-summary

R3

interface FastEthernet0/0

 ip address 30.30.23.2 255.255.255.0

 duplex auto

 speed auto

!

interface Serial0/0

 ip address 30.30.13.2 255.255.255.252

 clock rate 2000000

router eigrp 1

 network 30.0.0.0

 no auto-summary

Route Summarization 

show ip route on R2 and R3 will show all the route for prefixes 172.16.x.x, what if you have thousands of prefixes? your routing table gonna be a mess. to reduce this, we can summarize it

R2>sh ip rou

..

     172.16.0.0/24 is subnetted, 5 subnets

D       172.16.4.0 [90/2297856] via 30.30.12.1, 00:42:12, Serial0/0

D       172.16.5.0 [90/2297856] via 30.30.12.1, 00:42:12, Serial0/0

D       172.16.1.0 [90/2297856] via 30.30.12.1, 00:42:12, Serial0/0

D       172.16.2.0 [90/2297856] via 30.30.12.1, 00:42:12, Serial0/0

D       172.16.3.0 [90/2297856] via 30.30.12.1, 00:42:12, Serial0/0

..

so we have 172.16.1.0 - 172.16.5.0, the best range to contain all the above prefixes is 172.16.0.0/21 or mask 255.255.248.0.. 172.16.0.0 - 172.16.7.255

summarization is applied on the interface, lets apply this R1 Ser 0/0

interface Serial0/0

 ip address 30.30.12.1 255.255.255.252

 ip summary-address eigrp 1 172.16.0.0 255.255.248.0 5

 clock rate 56000

end

check R2 routing table

R2>sh ip rou

....

     172.16.0.0/16 is variably subnetted, 6 subnets, 2 masks

D       172.16.4.0/24 [90/2323456] via 30.30.23.2, 00:02:51, FastEthernet0/0

D       172.16.5.0/24 [90/2323456] via 30.30.23.2, 00:02:51, FastEthernet0/0

D       172.16.0.0/21 [90/2297856] via 30.30.12.1, 00:02:51, Serial0/0

D       172.16.1.0/24 [90/2323456] via 30.30.23.2, 00:02:51, FastEthernet0/0

D       172.16.2.0/24 [90/2323456] via 30.30.23.2, 00:02:51, FastEthernet0/0

D       172.16.3.0/24 [90/2323456] via 30.30.23.2, 00:02:51, FastEthernet0/0

you see that (in yellow), route to 172.16.x.x summarize into /21 subnet.. but why there's still storing the un-summarize (green)? notice that it is learned via 30.30.12.2 (R3).. and we didn't summarize route from R1 - R3.

*one more thing we need to remember, above Administrative Distance, the highest priority always the smallest prefix.. even it's learned from RIP, it's still the preferred route compare to EIGRP, OSPF... 

ok, now let's summarize those network on R1 Ser0/1 (connected to R3)

ip summary-address eigrp 1 172.16.0.0 255.255.248.0 5

check routing table on both R2 and R3


R2>sh ip rou
...
     172.16.0.0/21 is subnetted, 1 subnets
D       172.16.0.0 [90/2297856] via 30.30.12.1, 00:00:41, Serial0/0

now looks good :)

Default Route

let's assume 192.168.99.0/24 is gateway to outside network, and it's connected to R1

create a static route to this gateway on R1 and point it to NULL 0

ip route 192.168.99.0 255.255.255.0 Null0

and advertise it in EIGRP 1

router eigrp 1

 network 30.0.0.0

 network 172.16.0.0

 network 192.168.99.0

 no auto-summary

and make it default gateway for R2 and R3

ip default-network 192.168.99.0

show routing table on R2 and R3

R3>sh ip rou

Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP

       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 

       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

       E1 - OSPF external type 1, E2 - OSPF external type 2

       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

       ia - IS-IS inter area, * - candidate default, U - per-user static route

       o - ODR, P - periodic downloaded static route

Gateway of last resort is 30.30.13.1 to network 192.168.99.0

     172.16.0.0/21 is subnetted, 1 subnets

D       172.16.0.0 [90/2297856] via 30.30.13.1, 00:15:21, Serial0/0

D*   192.168.99.0/24 [90/2169856] via 30.30.13.1, 1d06h, Serial0/0

     30.0.0.0/8 is variably subnetted ....

yes.. that's correct :)

un-equal load balance using Variance

show R1 routing table

R1#sh ip rou

...

D       30.30.23.0/24 [90/2195456] via 30.30.13.2, 02:05:56, Serial0/1

                      [90/2195456] via 30.30.12.2, 02:05:56, Serial0/0

1

there's two route with same metric from R1 to network 30.30.23.0/24.. let's change this to simulate un-equal load balance

up the bandwith on R1 Ser0/1 to make the preferred path is via R3

interface Serial0/1

 bandwidth 256

 ip address 30.30.13.1 255.255.255.252

 ip summary-address eigrp 1 172.16.0.0 255.255.248.0 5

 clock rate 56000

end

and make R1 Ser0/0 to have half of Ser0/1 bandwidth

.. 

 bandwidth 256

..

show routing table again now and you should only see on path

R1#sh ip rou

..

D       30.30.23.0/24 [90/2195456] via 30.30.12.2, 00:01:47, Serial0/0

now check metric to 30.30.23.0/24 on EIGRP topology table

R1#sh ip ei top

..

P 30.30.23.0/24, 1 successors, FD is 2195456

        via 30.30.13.2 (10537472/281600), Serial0/1

        via 30.30.12.2 (20537600/281600), Serial0/0

now you see the thru R3 has the lower metric.. and value is almost two times

now apply this line

router eigrp 1

 variance 2

 network 30.0.0.0

 network 172.16.0.0

..

this line actually saying that, for every backup path.. where the primary link has twice metric value compare to it, will be load balanced

show routing table should see both path again now.. with un-equal load balance

R1#sh ip route

..

D       30.30.23.0/24 [90/10537472] via 30.30.13.2, 00:06:07, Serial0/1

                      [90/20537600] via 30.30.12.2, 00:06:07, Serial0/0

see... that's cool isn't it :)

passive-interface

check your EIGRP configuration on R1

router eigrp 1

 variance 2

 network 30.0.0.0

 network 172.16.0.0

 network 192.168.99.0

 no auto-summary

the line in yellow saying that

-send hello packet to all this network

-advertise all this network

actually it is a security hole to have all interface to be able to send hello packet, any time bad guy can plug in to it and form a neighbor and they can manipulate your network .. 

the best practice is to make all the interface passive (not send hello but still being advertised) and only open for the interface that connected to your other EIGRP router

do this on R1

router eigrp 1

 variance 2

 passive-interface default

 network 30.0.0.0

now you will loose all neighbor... now open again for R1 Ser0/0 and Ser0/1

router eigrp 1

 no passive-interface Serial0/0

 no passive-interface Serial0/1

do the same for R2 and R3, now your router a lot more safer :) happy routing !

EIGRP filtering route - GNS3

R1

router eigrp 1
 network 10.0.0.0
 network 30.0.0.0
 no auto-summary

R2

router eigrp 1

 network 30.30.12.2 0.0.0.0

 network 172.16.0.0

 no auto-summary

R3

router eigrp 1

 network 30.30.13.2 0.0.0.0

 network 192.168.0.0 0.0.255.255

 no auto-summary

*we didn't advertise direct link between R2 and R3, just let it be that way for now

make sure show ip route on R1, R2 and R3 can show all the network advertised by all the 3 routers.

Distribute List + Access list

we'll start with filtering certain route that EIGRP receive on particular interface 

let's stop 192.168.1.0 - 192.168.5.0 from being learned on Serial 0/1 on R1, R2 also should not see this since R2 learn it via R1

show current IP route

R1#sh ip rou

...

D    192.168.4.0/24 [90/2297856] via 30.30.13.2, 00:12:16, Serial0/1

D    192.168.5.0/24 [90/2297856] via 30.30.13.2, 00:12:16, Serial0/1

..

D    192.168.1.0/24 [90/2297856] via 30.30.13.2, 00:12:17, Serial0/1

D    192.168.2.0/24 [90/2297856] via 30.30.13.2, 00:12:17, Serial0/1

D    192.168.3.0/24 [90/2297856] via 30.30.13.2, 00:12:17, Serial0/1

...

R2#sh ip rou

D    192.168.4.0/24 [90/2809856] via 30.30.12.1, 00:12:57, Serial0/0 notice that R2 learned via R1 interface

D    192.168.5.0/24 [90/2809856] via 30.30.12.1, 00:12:57, Serial0/0

..

D    192.168.1.0/24 [90/2809856] via 30.30.12.1, 00:12:58, Serial0/0

D    192.168.2.0/24 [90/2809856] via 30.30.12.1, 00:12:58, Serial0/0

D    192.168.3.0/24 [90/2809856] via 30.30.12.1, 00:12:58, Serial0/0

..

first we need to create access list to define the what IP to be filtered on R1 Ser 0/1 (another way to define it is by using ip prefix-list, will show you below )

access-list 10 deny   192.168.0.0 0.0.255.255

access-list 10 permit any

then apply it on EIGRP 1, in mean 'incoming'

router eigrp 1

 network 10.0.0.0

 network 30.0.0.0

 distribute-list 10 in Serial0/1 

 no auto-summary

now try again show ip route on R1 and R2, all the route above should be gone :)

now let's try filter on out interface, mean stop the route from go out on particular interface

let's filter 172.16.0.0 on R2 from received by R3, will stop this route at R1 ser 0/1

first, make sure show ip route on R3 have this IPs

R3>sh ip rou

..

     172.16.0.0/24 is subnetted, 5 subnets

D       172.16.4.0 [90/2809856] via 30.30.13.1, 00:43:31, Serial0/0

D       172.16.5.0 [90/2809856] via 30.30.13.1, 00:43:31, Serial0/0

D       172.16.1.0 [90/2809856] via 30.30.13.1, 00:43:31, Serial0/0

D       172.16.2.0 [90/2809856] via 30.30.13.1, 00:43:31, Serial0/0

D       172.16.3.0 [90/2809856] via 30.30.13.1, 00:43:31, Serial0/0

..

create an access-list (another way to define it is by using ip prefix-list, will show you below )

access-list 20 deny   172.16.0.0 0.0.255.255

access-list 20 permit any

apply it on EIGRP 1 

..

 distribute-list 20 out Serial0/1

..

now show ip route on R3 see 172.16.0.0 anymore, but R1 still can see it because it only stop it from going out on ser0/1, but it still learn it from Ser0/0 :)

distribute-list + prefix-list + gateway 

by right R2 and R3 should be able to see route to 10.1.x.x on R1

R2#sh ip rou

..

D       10.1.3.0 [90/2297856] via 30.30.12.1, 00:01:46, Serial0/0

D       10.1.2.0 [90/2297856] via 30.30.12.1, 00:01:46, Serial0/0

D       10.1.1.0 [90/2297856] via 30.30.12.1, 00:02:15, Serial0/0

D       10.1.5.0 [90/2297856] via 30.30.12.1, 00:01:46, Serial0/0

D       10.1.4.0 [90/2297856] via 30.30.12.1, 00:01:46, Serial0/0

..

R3#sh ip rou

..

D       10.1.3.0 [90/2297856] via 30.30.12.1, 00:01:46, Serial0/0

D       10.1.2.0 [90/2297856] via 30.30.12.1, 00:01:46, Serial0/0

D       10.1.1.0 [90/2297856] via 30.30.12.1, 00:02:15, Serial0/0

D       10.1.5.0 [90/2297856] via 30.30.12.1, 00:01:46, Serial0/0

D       10.1.4.0 [90/2297856] via 30.30.12.1, 00:01:46, Serial0/0

..

what we'll do is, to make R2 only can see 10.1.1.0 and 10.1.2.0, and R3 only can see 10.1.3.0 and 10.1.4.0 on his routing table

create the prefix list for the interesting traffic

ip prefix-list PREFIX2 seq 5 permit 10.1.1.0/24 le 32

ip prefix-list PREFIX2 seq 10 permit 10.1.2.0/24 le 32

ip prefix-list PREFIX2 seq 15 deny 0.0.0.0/8 le 32

create prefix list to allow only specific gateway to advertise route to this router

ip prefix-list NEG2 seq 5 permit 30.30.12.1/32

ip prefix-list NEG2 seq 10 deny 0.0.0.0/0 le 32

apply it on EIGRP 1

 distribute-list prefix PREFIX2 gateway NEG2 in

now show route, you should only see the allowed prefixes 

R2#sh ip rou

..

     10.0.0.0/24 is subnetted, 2 subnets

D       10.1.2.0 [90/2297856] via 30.30.12.1, 00:20:06, Serial0/0

D       10.1.1.0 [90/2297856] via 30.30.12.1, 00:20:36, Serial0/0

..

now, do the same on R3 to allow only specific route from R1 into his routing table :)

EIGRP metric

prepare the config as below 

R1

interface Loopback0

 ip address 10.10.10.1 255.255.255.255

!

interface Loopback1

 ip address 10.239.1.1 255.255.255.0

!

interface Loopback2

 ip address 10.239.2.1 255.255.255.0

!

interface Loopback3

 ip address 10.239.3.1 255.255.255.0

!

interface Loopback4

 ip address 10.239.4.1 255.255.255.0

!

interface Loopback5

 ip address 10.239.5.1 255.255.255.0

!

interface Loopback6

 ip address 10.239.6.1 255.255.255.0

!

interface Loopback7

 ip address 10.239.7.1 255.255.255.0

!

interface Loopback8

 ip address 10.239.8.1 255.255.255.0

!

interface Loopback9

 ip address 10.239.9.1 255.255.255.0

!

interface Serial0/0

 bandwidth 128

 ip address 10.1.12.1 255.255.255.252

 clock rate 128000

!

interface Serial0/1

 bandwidth 128

 ip address 10.1.13.1 255.255.255.252

 clock rate 128000

!

router eigrp 1

 network 10.1.0.0 0.0.255.255

 network 10.10.10.1 0.0.0.0

 network 10.239.0.0 0.0.255.255

 no auto-summary

R2

interface Loopback0

 ip address 10.10.10.2 255.255.255.255

!

interface FastEthernet0/0

 ip address 10.151.1.1 255.255.255.0

 duplex auto

 speed auto

!

interface Serial0/0

 bandwidth 128

 ip address 10.1.12.2 255.255.255.252

!

router eigrp 1

 network 10.1.12.2 0.0.0.0

 network 10.10.10.2 0.0.0.0

 network 10.151.1.0 0.0.0.255

 no auto-summary

R3

interface Loopback0

 ip address 10.10.10.3 255.255.255.255

!

interface FastEthernet0/0

 ip address 10.151.1.3 255.255.255.0

 duplex auto

 speed auto

!

interface Serial0/0

 bandwidth 128

 ip address 10.1.13.2 255.255.255.0

!

interface FastEthernet0/1

 ip address 10.232.1.3 255.255.255.0

 duplex auto

 speed auto

!

router eigrp 1

 network 10.1.13.2 0.0.0.0

 network 10.10.10.3 0.0.0.0

 network 10.151.1.0 0.0.0.255

 network 10.232.1.0 0.0.0.255

 no auto-summary

----------------------------------------------------------------------

let's get started.. ROUTING TABLE !!

start with 'sh ip route' on R1 make sure you can see all route below

     10.0.0.0/8 is variably subnetted, 17 subnets, 3 masks

C       10.1.13.0/30 is directly connected, Serial0/1

D       10.1.13.0/24 [90/21026560] via 10.1.12.2, 07:02:08, Serial0/0

C       10.1.12.0/30 is directly connected, Serial0/0

D       10.10.10.2/32 [90/20640000] via 10.1.12.2, 00:00:52, Serial0/0

D       10.10.10.3/32 [90/20640000] via 10.1.13.2, 00:00:52, Serial0/1

C       10.10.10.1/32 is directly connected, Loopback0

D       10.151.1.0/24 [90/20514560] via 10.1.12.2, 00:00:52, Serial0/0

                      [90/20514560] via 10.1.13.2, 00:00:52, Serial0/1

C       10.239.5.0/24 is directly connected, Loopback5

C       10.239.4.0/24 is directly connected, Loopback4

C       10.239.7.0/24 is directly connected, Loopback7

C       10.239.6.0/24 is directly connected, Loopback6

D       10.232.1.0/24 [90/20514560] via 10.1.13.2, 00:00:53, Serial0/1

C       10.239.1.0/24 is directly connected, Loopback1

C       10.239.3.0/24 is directly connected, Loopback3

C       10.239.2.0/24 is directly connected, Loopback2

C       10.239.9.0/24 is directly connected, Loopback9

C       10.239.8.0/24 is directly connected, Loopback8

basically, from the diagram, you can see that from R1 there are two routes to all the network that connected to R2 and R3, but from the routing table.. only network 10.151.1.0 have two routes

actually only one and the best route installed into Routing Table, that explain why there's only one route for every network 


..EXCEPT 10.151.1.0.. ok, that's is because this network has a same metric value, so routing table install both.. so, which route R1 will pick to go to that network.. well.. (I haven't figured it out yet, ha2. why don't u figured it out)

EIGRP TOPOLOGY TABLE


there's another table storing the routing information.. SHOW IP EIGRP TOPO, remember. Only EIGRP has this !! EIGRP store all the available routing path inside this table



....
P 10.1.12.0/30, 1 successors, FD is 20512000
        via Connected, Serial0/0
P 10.10.10.2/32, 1 successors, FD is 20640000
        via 10.1.12.2 (20640000/128256), Serial0/0
        via 10.1.13.2 (20642560/156160), Serial0/1
P 10.10.10.3/32, 1 successors, FD is 20640000, U
        via 10.1.13.2 (20640000/128256), Serial0/1
        via 10.1.12.2 (20642560/156160), Serial0/0
P 10.10.10.1/32, 1 successors, FD is 128256
        via Connected, Loopback0
P 10.151.1.0/24, 2 successors, FD is 20514560, U
        via 10.1.13.2 (20514560/28160), Serial0/1
        via 10.1.12.2 (20514560/28160), Serial0/0
P 10.239.5.0/24, 1 successors, FD is ...


notice that if a network have two routes(blue), only the smallest metric(or cost) make it to the routing table(green).. unless all the route has the same value. there's a term for this

• successor - the best route that goes into the routing table (red)
• feasible successor - all the backup route only reside inside the EIGRP topology table (lite red)

• feasible distance (FD) - overall cost ; cost between R1 and network 10.151.1.0 (orange)
• advertise distance (AD) - cost from neighbor to destination network ; cost between R2 and 10.151.1.0 (lite orange)

let's adjust some the metric, there's a few method to do this

let's start with OFFSET-LIST to modify metric of routes that learned thru particular interface, what we gonna do is, add metric value of 10000 on R1 for every route learned via serial 0/1

R1(config)#router ei 1

R1(config-router)#offset-list 22 in 10000 serial 0/1

by right, now only one route to 10.151.1.0 installed into ROUTING TABLE because one of the interface configured to have additional metric 10000, SHOW IP ROUTE

C       10.10.10.1/32 is directly connected, Loopback0

D       10.151.1.0/24 [90/20514560] via 10.1.12.2, 00:00:35, Serial0/0

C       10.239.5.0/24 is directly connected, Loopback5

SHOW IP TOPO to confirm the value now is different, compare the metric value for every route learn via serial 0/1 (compare with previous EIGRP topology table on top of the page) all have extra 10000

P 10.10.10.2/32, 1 successors, FD is 20640000

        via 10.1.12.2 (20640000/128256), Serial0/0

        via 10.1.13.2 (20652560/166160), Serial0/1

P 10.10.10.3/32, 1 successors, FD is 20640000

        via 10.1.12.2 (20642560/156160), Serial0/0

        via 10.1.13.2 (20650000/138256), Serial0/1

P 10.10.10.1/32, 1 successors, FD is 128256

        via Connected, Loopback0

P 10.151.1.0/24, 1 successors, FD is 20514560

        via 10.1.12.2 (20514560/28160), Serial0/0

        via 10.1.13.2 (20524560/38160), Serial0/1

but remember, this only apply one way only, for example in this config, only R1 see the different.. there's no affect on route learned by R2 and R3. but you still can modify the outgoing traffic just replace IN with OUT in the command