In this example we have taken a setup which is more or less
a simulation from cisco example but I have done a little modification to it.
We have 4 routers and 1 ether router/switch. The router R1
is connected to R2 && R2 connects to R3 and R4 over different subnets. R3 f0/0
shares 4.4.4.1 /24 subnet with R4 f0/1, 4.4.4.2 and interface SVI vlan 3 with
IP 4.4.4.3 /24.
The connection style is given below:
R1(f0/0 {.1})<->R2(f0/0 {.2})= 1.1.1.0/24
R2(f0/1
{.1})<->R3(f0/1
{.2})=2.2.2.0 /24
R2(f1/0
{.1})<->R4(f0/0
{.2})=3.3.3.0 /24
R3(f0/0
{.1})<->R5(f1/1
{.3}{SVI})=4.4.4.0 /24
R4(f0/1
{.2})<->R5(f1/2
{.3}{SVI})=4.4.4.0 /24
Objective:
To learn the 4.4.4.3 sub-network IP from R5 and making the R2-R3-R5 as preferred
route and R2-R4-R5 as backup one.
R1 set up:
interface FastEthernet0/0
ip address 1.1.1.1
255.255.255.252
duplex auto
speed auto
!
interface FastEthernet0/1
ip address
192.168.1.254 255.255.255.0
duplex auto
speed auto
router eigrp 100
network 1.1.1.0
0.0.0.3
network
192.168.1.0
auto-summary
R2 Set up:
interface FastEthernet0/0
ip address 1.1.1.2
255.255.255.252
duplex auto
speed auto
!
interface FastEthernet0/1
ip address 2.2.2.1
255.255.255.252
duplex auto
speed auto
interface Vlan2
ip address 3.3.3.1
255.255.255.252
!
router eigrp 100
network 1.1.1.0
0.0.0.3
network 2.2.2.0
0.0.0.3
network 3.3.3.0
0.0.0.3
auto-summary
Above in R2 I had only one routed port and other one was switch
port so I chose to do something usually different and made one SVI 2 0f 3.3.3.1
putting the vlan 2 on f1/0 of R2 which then will connect router R4 port f0/0 IP
3.3.3.2 as above.
R3 Set up:
interface FastEthernet0/0
ip address 4.4.4.1
255.255.255.0
duplex auto
speed auto
!
interface FastEthernet0/1
ip address 2.2.2.2
255.255.255.252
duplex auto
speed auto
router eigrp 100
network 2.2.2.0
0.0.0.3
network 4.4.4.0
0.0.0.255
no auto-summary
R4 Set up:
interface FastEthernet0/0
ip address 3.3.3.2
255.255.255.252
duplex auto
speed auto
!
interface FastEthernet0/1
ip address 4.4.4.2
255.255.255.0
delay 18
//This delay was used to manipulate the path by delay. change.
duplex auto
speed auto
router eigrp 100
offset-list 99 out
300 FastEthernet0/0
network 3.3.3.0
0.0.0.255
network 4.4.4.0
0.0.0.255
no auto-summary
!
ip http server
ip classless
!
!
access-list 99 permit 4.4.4.0 0.0.0.255
R5 Set up:
interface FastEthernet1/1
switchport access
vlan 3
no ip address
speed 100
!
interface FastEthernet1/2
switchport access
vlan 3
no ip address
!
interface Vlan3
ip address 4.4.4.3
255.255.255.0
!
router eigrp 100
network 4.4.4.0
0.0.0.255
auto-summary
Identified solution to make R2-R3-R5 as preferred route:
1. Check the FD/AD of
the route to network 4.4.4.0 with sh ip eigrp topology command
R2#sh ip eigrp topology
IP-EIGRP Topology Table for AS(100)/ID(3.3.3.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R
- Reply,
r - reply
Status, s - sia Status
P 1.0.0.0/8, 1 successors, FD is 28160
via Summary
(28160/0), Null0
P 1.1.1.0/30, 1 successors, FD is 28160
via
Connected, FastEthernet0/0
P 2.0.0.0/8, 1 successors, FD is 28160
via Summary
(28160/0), Null0
P 2.2.2.0/30, 1 successors, FD is 28160
via
Connected, FastEthernet0/1
P 3.0.0.0/8, 1 successors, FD is 2816
via Summary
(2816/0), Null0
P 3.3.3.0/30, 1 successors, FD is 2816
via Connected,
Vlan2
P 4.0.0.0/8, 1 successors, FD is 30464
via 3.3.3.2
(30464/30208), Vlan2 //**This
goes in routing table to reach net of 4.4.4.0 /24- from R4
via 2.2.2.2
(30720/28160), FastEthernet0/1
P 192.168.1.0/24, 1 successors, FD is 30720
via 1.1.1.1
(30720/28160), FastEthernet0/0
One way is to change one of the metric used in route
selection of 5; Bandwidth, load, delay, reliability and MTU.
For ex. if we change the delay of interface f0/1 of R4
the metric will become poor the traffic over R3 will be preferred. Go on to the interface and change the metric
to any(in tens of microsec) like delay 20.
This will spoil R4 to be the best route to reach network
4.4.4.0 and R3 thus will be preferred.
Note: One often should consider while changing Bandwidth as it
will have a huge impact on route calculation and might ruin all possibility of
either becoming backup route OR no route at all.
Second way, is through manipulation of sum up of all
metrics using offset list.
If we go back to set up of R4 you will observed there
I've used access-list:
access-list 99 permit 4.4.4.0 0.0.0.255
This states that a standard ACL will permit all route
with source 4.4.4.0.
Now this ACL is to be called in eigrp subcommand of R4,
router eigrp 100
offset-list 99 out
300 FastEthernet0/0
The above states that ACL 99 passes out with a modified
composite metric and added with value
300. So earlier the FD/AD of R2 was
4.0.0.0/8, 1 successors, FD is 30464
via 3.3.3.2
(30464/30208), Vlan2
via 2.2.2.2 (30720/28160),
FastEthernet0/1
Now it is,
4.0.0.0/8, 1 successors, FD is 30464
via 3.3.3.2
(30464/30208), Vlan2
via 2.2.2.2
(30720/28160), FastEthernet0/1
oops!!!!! It
didn't change.
Wow, I just happened to have found out the reason. The
ACL above has 4.4.4.0 as source but my R4 is summarizing the whole route to
class A boundary because we all know EIGRP's romance with summarization. So all you need is to
add the command no auto summary and the problem is solved or the other way change
the ACL with 4.0.0.0 255.0.0.0
cmd. Either way you achieve the same result.
Output now on R2 after disabling auto-summarization is below:
P 4.4.4.0/24, 1 successors, FD is 30564
via 2.2.2.2
(30720/28160), FastEthernet0/1 //**best
path via R3
via 3.3.3.2
(30764/30508), Vlan2
//** path via R4 as backup now
Now path via R3 become the best path.
This way you can have some fun with change in metric or applying offset list to tell that I will spoil this route. Remember MTU as metric is a container and not used in route calc for best path.
Share, If you have something new or to disagree.
Cheers!!
1 comment:
I do not deal with route manipulation on a daily basis. I needed to do some today and come up with a plan. This is exactly what I wanted without having to read a wordy Cisco press book. Thank you Anupam.
-Dev.
Post a Comment