While there are several thoughts in market about how good Design skills can be gained - Where some people have opinion that Design can be only learned from Experience while other may talk about Study as Tool or perhaps availability of Case Study can fasten the process.
On the flip side some Expert Level Certification like CCIE usually doesn't focus on Design side of protocol but rather feature set. For example the Scenario I shown above - It's very easy to trick a CCIE with even this small OSPF setup by asking if they see any design issues with this design. And Most likely the answer is going to be - "No"
Now lets take a look at Design issue with even this small setup:
Lets say R1's Loopback wants to Talk to R5's Loopback. Let's see what Traceroute has to say about path being followed:
R3#traceroute 5.5.5.5 source loopback0
Type escape sequence to abort.
Tracing the route to 5.5.5.5
1 175.13.0.1 40 msec 100 msec 48 msec
2 150.15.0.5 160 msec 48 msec 52 msec
Looks good so far as we are talking the shortest path to reach the destination.
Lets review R1's routing table which is next hop in this case:
R1#sh ip route 5.5.5.5
Routing entry for 5.5.5.5/32
Known via "ospf 1", distance 110, metric 2, type intra area
Last update from 150.15.0.5 on FastEthernet1/0, 00:01:04 ago
Routing Descriptor Blocks:
* 150.15.0.5, from 5.5.5.5, 00:01:04 ago, via FastEthernet1/0
Route metric is 2, traffic share count is 1
Still being CCIE and looking at the output above, you might not have observed design issue here.
Let's bring the link down between R1-R5:
R5(config)#int f0/0
R5(config-if)#shutdown
R5(config-if)#exit
R5(config)#
*Mar 1 00:39:10.651: %OSPF-5-ADJCHG: Process 1, Nbr 1.1.1.1 on FastEthernet0/0 from FULL to DOWN, Neighbor Down: Interface down or detached
And now let's run the Traceroute again to see if we are are following the shortest path as shown in diagram below:
R3#traceroute 5.5.5.5 source loopback0
Type escape sequence to abort.
Tracing the route to 5.5.5.5
1 175.13.0.1 44 msec 100 msec 12 msec
2 150.16.0.6 52 msec 48 msec 36 msec
3 150.26.0.2 72 msec 56 msec 76 msec
4 150.25.0.5 156 msec 240 msec 116 msec
Dahhhh.....
So we aren't following the shortest path. But rather a far longer/sub-optimal path to reach the destination prefix as shown in diagram below:
So next question that comes in mind is - Why is this happening ? Since rather asking - How do we fix it ? is not the harder part usually for CCIEs :)
Now returning back to first HOW ? part. Let's return back to basics of OSPF route preference which says basically - Prefer O over IA O over E1 over E2 over N1 over N2 routes.
So when Packet comes to R1, it sees 5.5.5.5 as Intra Area route from Area 1 and Inter Area route from Area 0 Prospective.
And per rule - Intra Area route should be preferred over Inter Area route.
Now How do we fix it ?
Hmmm..... Actually Depends on which IOS and Box you are are running.
In newer generation of IOS like IOS-XR, they solved the problem using RFC-5185.
But our old friend IOS still doesn't allow that implementation. So we got to fix it other way which of course involves Downtime if you got to make those changes in production environment.
Let's first revert the configuration to be on same stage we were on earlier:
R5(config)#int f0/0
R5(config-if)#no shutdown
R5(config-if)#exit
R5(config)#
*Mar 1 00:40:37.735: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up
*Mar 1 00:40:38.735: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up
R5(config)#
*Mar 1 00:40:41.159: %OSPF-5-ADJCHG: Process 1, Nbr 1.1.1.1 on FastEthernet0/0 from LOADING to FULL, Loading Done
R3#traceroute 5.5.5.5 source loopback 0
Type escape sequence to abort.
Tracing the route to 5.5.5.5
1 175.13.0.1 52 msec 40 msec 68 msec
2 150.15.0.5 32 msec 64 msec 100 msec
Now Let's apply the fix:
R1(config)#default interface f0/0
Building configuration...
Interface FastEthernet0/0 set to default configuration
R2(config)#default interface f0/0
Building configuration...
Interface FastEthernet0/0 set to default configuration
R1(config)#int f0/0
R1(config-if)#no shutdown
R1(config-if)#exit
R1(config)#int f0/0.12
R1(config-subif)#encapsulation dot1Q 12
R1(config-subif)#ip add 175.12.0.1 255.255.255.0
R1(config-subif)#exit
R1(config)#int f0/0.112
R1(config-subif)#encapsulation dot1Q 112
R1(config-subif)#ip add 150.12.0.1 255.255.255.0
R1(config-subif)#exit
R1(config)#router ospf 1
R1(config-router)#network 150.12.0.1 0.0.0.0 area 1
R1(config-router)#exit
R2(config)#int f0/0
R2(config-if)#no shutdown
R2(config-if)#exit
R2(config)#int f0/0.12
R2(config-subif)#encapsulation dot1Q 12
R2(config-subif)#ip add 175.12.0.2 255.255.255.0
R2(config-subif)#exit
R2(config)#int f0/0.112
R2(config-subif)#encapsulation dot1Q 112
R2(config-subif)#ip add 150.12.0.2 255.255.255.0
R2(config-subif)#exit
R2(config)#router ospf 1
R2(config-router)#network 150.12.0.2 0.0.0.0 area 1
R2(config-router)#exit
R1#sh ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 1 FULL/BDR 00:00:36 175.12.0.2 FastEthernet0/0.12
3.3.3.3 1 FULL/BDR 00:00:33 175.13.0.3 FastEthernet0/1
2.2.2.2 1 FULL/BDR 00:00:33 150.12.0.2 FastEthernet0/0.112
6.6.6.6 1 FULL/BDR 00:00:34 150.16.0.6 FastEthernet2/0
5.5.5.5 1 FULL/BDR 00:00:33 150.15.0.5 FastEthernet1/0
Let's perform the testing one final time to see if this fixes the design issue:
R5(config)#int f0/0
R5(config-if)#shutdown
R5(config-if)#exit
R3#traceroute 5.5.5.5 source loopback 0
Type escape sequence to abort.
Tracing the route to 5.5.5.5
1 175.13.0.1 40 msec 40 msec 48 msec
2 150.12.0.2 108 msec 1424 msec 88 msec
3 150.25.0.5 216 msec 96 msec 144 msec
So we are good now :)
Below are the initial configuration in case you want to test this out by your own:
Further Readings:
http://cciethebeginning.wordpress.com/2013/04/10/ospf-external-e1-e2-n1-n2-who-is-the-winner/
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Initials
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
R1
++
!
en
!
conf t
!
no cdp run
!
no ip do lo
!
ho R1
!
line con 0
logg syn
no exec-time
exit
!
logging buff
!
logging buff 7
!
logging buff 5000
!
int lo0
ip add 1.1.1.1 255.255.255.255
exit
!
int f0/0
ip add 175.12.0.1 255.255.255.0
no sh
exit
!
int f0/1
ip add 175.13.0.1 255.255.255.0
no sh
exit
!
int f1/0
ip add 150.15.0.1 255.255.255.0
no sh
exit
!
int f2/0
ip add 150.16.0.1 255.255.255.0
no sh
exit
!
router ospf 1
net 1.1.1.1 0.0.0.0 a 0
net 175.12.0.1 0.0.0.0 a 0
net 175.13.0.1 0.0.0.0 a 0
net 150.15.0.1 0.0.0.0 a 1
net 150.16.0.1 0.0.0.0 a 1
exit
!
end
!
#################################
R2
++
!
en
!
conf t
!
no cdp run
!
no ip do lo
!
ho R2
!
line con 0
logg syn
no exec-time
exit
!
logging buff
!
logging buff 7
!
logging buff 5000
!
int lo0
ip add 2.2.2.2 255.255.255.255
exit
!
int f0/0
ip add 175.12.0.2 255.255.255.0
no sh
exit
!
int f0/1
ip add 175.24.0.2 255.255.255.0
no sh
exit
!
int f1/0
ip add 150.26.0.2 255.255.255.0
no sh
exit
!
int f2/0
ip add 150.25.0.2 255.255.255.0
no sh
exit
!
router ospf 1
net 2.2.2.2 0.0.0.0 a 0
net 175.12.0.2 0.0.0.0 a 0
net 175.24.0.2 0.0.0.0 a 0
net 150.26.0.2 0.0.0.0 a 1
net 150.25.0.2 0.0.0.0 a 1
exit
!
end
!
#################################
R3
++
!
en
!
conf t
!
no cdp run
!
no ip do lo
!
ho R3
!
line con 0
logg syn
no exec-time
exit
!
logging buff
!
logging buff 7
!
logging buff 5000
!
int lo0
ip add 3.3.3.3 255.255.255.255
exit
!
int f0/0
ip add 175.13.0.3 255.255.255.0
no sh
exit
!
router ospf 1
net 3.3.3.3 0.0.0.0 a 0
net 175.13.0.3 0.0.0.0 a 0
exit
!
end
!
#################################
R4
++
!
en
!
conf t
!
no cdp run
!
no ip do lo
!
ho R4
!
line con 0
logg syn
no exec-time
exit
!
logging buff
!
logging buff 7
!
logging buff 5000
!
int lo0
ip add 4.4.4.4 255.255.255.255
exit
!
int f0/0
ip add 175.24.0.4 255.255.255.0
no sh
exit
!
router ospf 1
net 4.4.4.4 0.0.0.0 a 0
net 175.24.0.4 0.0.0.0 a 0
exit
!
end
!
#################################
R5
++
!
en
!
conf t
!
no cdp run
!
no ip do lo
!
ho R5
!
line con 0
logg syn
no exec-time
exit
!
logging buff
!
logging buff 7
!
logging buff 5000
!
int lo0
ip add 5.5.5.5 255.255.255.255
exit
!
int f0/0
ip add 150.15.0.5 255.255.255.0
no sh
exit
!
int f0/1
ip add 150.25.0.5 255.255.255.0
no sh
exit
!
router ospf 1
net 5.5.5.5 0.0.0.0 a 1
net 150.15.0.5 0.0.0.0 a 1
net 150.25.0.5 0.0.0.0 a 1
exit
!
end
!
#################################
R6
++
!
en
!
conf t
!
no cdp run
!
no ip do lo
!
ho R6
!
line con 0
logg syn
no exec-time
exit
!
logging buff
!
logging buff 7
!
logging buff 5000
!
int lo0
ip add 6.6.6.6 255.255.255.255
exit
!
int f0/0
ip add 150.26.0.6 255.255.255.0
no sh
exit
!
int f0/1
ip add 150.16.0.6 255.255.255.0
no sh
exit
!
router ospf 1
net 6.6.6.6 0.0.0.0 a 1
net 150.26.0.6 0.0.0.0 a 1
net 150.16.0.6 0.0.0.0 a 1
exit
!
end
!
HTH...
Deepak Arora
Deepak Arora
Evil CCIE
