01-06-2012 07:51 AM - edited 03-07-2019 04:12 AM
Greetings!
I'm practicing some routing techniques related to the Implementing Cisco IP Routing class and exam. I'm trying to manufacture a routing loop in order to correct it with route maps and prefix lists, etc. I can't seem to get the routing loop to appear the way I thought it would, and I would like a hand.
Please consider the topology attached to this post.
The lab is intended to feature mutual redistribution at two points, creating a routing loop. When I redistribute EIGRP into OSPF at R1, for example, R2 gets the routes expected in it's routing table. R2's next hop is R1 to reach the 192.168.253/254.0 networks (this is expected for now). When I try to redistribute EIGRP into OSPF at R2 there seems to be no effect. R1 maintains its routing table - it's as if the redistribution commands at R2 never happened. If I redistribute in the opposite order (R2 before R1) I have the opposite scenario.
Can someone please tell me what I'm doing wrong? I have the lab sitting in front of me, so I'll be happy to provide any other details required.
Thanks for your time.
Eric
Solved! Go to Solution.
01-06-2012 11:49 AM
One of the redistribution rules is that only routes from the routing table are considered for redistribution, i.e. only the best routes. In this way R1 learns the routes to 192.168.253 & 254 from R2. Because they are redistributed routes, they are flagged as External type 2 which is the default value.
When R1 subsequently considers the routes to redistribute, for 192.168.253 & 254 it only has the OSPF routes because those are currently in the routing table.
Another default behavior for OSPF is to only redistribute Internal routes.
Because the routes to 192.168.253 & 254 are of the type E2, they are not considered for redistribution.
The EIGRP routes are overruled by OSPF because of their lower AD so they cannot be used. (not in table)
You will get different results when you change the redistribute as follows:
redistribute ospf 1 metric 1 1 255 1 300 match external 2
http://www.cisco.com/en/US/partner/docs/ios/12_2t/ip_route/command/reference/p2ftind2.html#wp1041381
regards,
Leo
01-06-2012 08:31 AM
This is a classic problem related to administrative distance.
After redistibuting on R1, R2 learns a route via OSPF.
R2 is however also having a route via EIGRP and this one is preferred because of its lower AD 90 vs 110.
We will need routing tables and configs in order to help you out but fiddling this one out for yourself is of course an excellent learning experience. Have fun!
regards,
Leo
01-06-2012 08:50 AM
Yes, but the .253/.254 are being redistributed into EIGRP. Distributing EIGRP into OSPF should make the OSPF path look better, am I not correct?
Configs and routing tables are as follows. Thanks again for your help.
R1:
Building configuration...
Current configuration : 943 bytes
!
version 12.2
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname ro1
!
!
ip subnet-zero
!
!
no ip domain-lookup
!
!
!
!
interface FastEthernet0/0
ip address 10.2.7.1 255.255.255.0
duplex auto
speed auto
!
interface Serial0/0
ip address 192.168.2.1 255.255.255.0
no fair-queue
!
interface FastEthernet0/1
no ip address
shutdown
duplex auto
speed auto
!
interface Serial0/1
ip address 10.3.7.1 255.255.255.0
!
router eigrp 5
redistribute ospf 1 metric 1 1 255 1 300
network 10.3.7.0 0.0.0.255
network 192.168.2.0
no auto-summary
eigrp log-neighbor-changes
!
router ospf 1
router-id 1.1.1.1
log-adjacency-changes
redistribute eigrp 5 subnets
network 10.2.7.0 0.0.0.255 area 0
!
ip classless
ip http server
ip pim bidir-enable
!
!
alias exec blank copy flash:blank-config.txt start
!
line con 0
exec-timeout 0 0
logging synchronous
line aux 0
line vty 0 4
login
R1's routing table:
Gateway of last resort is not set
10.0.0.0/24 is subnetted, 2 subnets
C 10.3.7.0 is directly connected, Serial0/1
C 10.2.7.0 is directly connected, FastEthernet0/0
O E2 192.168.254.0/24 [110/20] via 10.2.7.2, 00:07:54, FastEthernet0/0
D 192.168.1.0/24 [90/2681856] via 192.168.2.2, 00:12:33, Serial0/0
O E2 192.168.253.0/24 [110/20] via 10.2.7.2, 00:07:54, FastEthernet0/0
C 192.168.2.0/24 is directly connected, Serial0/0
R2:
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname ro2
!
boot-start-marker
boot-end-marker
!
!
no aaa new-model
!
resource policy
!
ip subnet-zero
!
!
ip cef
no ip domain lookup
no ip dhcp use vrf connected
!
!
interface FastEthernet0/0
ip address 10.2.7.2 255.255.255.0
duplex auto
speed auto
!
R2's routing table:
Gateway of last resort is not set
10.0.0.0/24 is subnetted, 2 subnets
C 10.3.7.0 is directly connected, Serial2/0
C 10.2.7.0 is directly connected, FastEthernet0/0
D EX 192.168.254.0/24 [170/2560512256] via 192.168.1.2, 00:10:20, Serial3/0
[170/2560512256] via 10.3.7.1, 00:10:20, Serial2/0
C 192.168.1.0/24 is directly connected, Serial3/0
D EX 192.168.253.0/24 [170/2560512256] via 192.168.1.2, 00:10:20, Serial3/0
[170/2560512256] via 10.3.7.1, 00:10:20, Serial2/0
D 192.168.2.0/24 [90/2681856] via 10.3.7.1, 00:14:54, Serial2/0
R3:
version 12.1
no service single-slot-reload-enable
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname ro4
!
!
!
!
!
!
ip subnet-zero
no ip domain-lookup
!
!
!
!
interface Ethernet0
ip address 192.168.253.1 255.255.255.0
no keepalive
!
interface Ethernet1
ip address 192.168.254.1 255.255.255.0
no keepalive
!
interface Serial0
ip address 192.168.2.2 255.255.255.0
no fair-queue
clockrate 56000
!
interface Serial1
ip address 192.168.1.2 255.255.255.0
clockrate 56000
!
router eigrp 5
redistribute connected metric 1 1 255 1 300
network 192.168.1.0
network 192.168.2.0
no auto-summary
no eigrp log-neighbor-changes
!
ip classless
ip http server
!
!
line con 0
exec-timeout 0 0
logging synchronous
line aux 0
line vty 0 4
!
end
R3's routing table:
Gateway of last resort is not set
10.0.0.0/24 is subnetted, 2 subnets
D 10.3.7.0 [90/2681856] via 192.168.2.1, 00:15:37, Serial0
[90/2681856] via 192.168.1.1, 00:15:37, Serial1
D EX 10.2.7.0 [170/2560512256] via 192.168.2.1, 00:15:37, Serial0
[170/2560512256] via 192.168.1.1, 00:15:37, Serial1
C 192.168.254.0/24 is directly connected, Ethernet1
C 192.168.1.0/24 is directly connected, Serial1
C 192.168.253.0/24 is directly connected, Ethernet0
C 192.168.2.0/24 is directly connected, Serial0
01-06-2012 11:49 AM
One of the redistribution rules is that only routes from the routing table are considered for redistribution, i.e. only the best routes. In this way R1 learns the routes to 192.168.253 & 254 from R2. Because they are redistributed routes, they are flagged as External type 2 which is the default value.
When R1 subsequently considers the routes to redistribute, for 192.168.253 & 254 it only has the OSPF routes because those are currently in the routing table.
Another default behavior for OSPF is to only redistribute Internal routes.
Because the routes to 192.168.253 & 254 are of the type E2, they are not considered for redistribution.
The EIGRP routes are overruled by OSPF because of their lower AD so they cannot be used. (not in table)
You will get different results when you change the redistribute as follows:
redistribute ospf 1 metric 1 1 255 1 300 match external 2
http://www.cisco.com/en/US/partner/docs/ios/12_2t/ip_route/command/reference/p2ftind2.html#wp1041381
regards,
Leo
01-06-2012 12:16 PM
Yes, thank you for reminding me of things I should know or remember if I'm going to be taking the exam next week. This is absolutely correct, but my problem was that of metrics. In the lab I was trying to recreate, I missed the part where the redistributed routes were seeded in such a way that the routing protocols were stepping on each other. I tweaked my configs and now I've got a real nasty loop going on. Now the fun starts!
Thanks for your time sir!
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