While browsing blogs, when I should be labbing, I came across INE’s OSPF Traffic Engineering challenge. I normally ignore these because I happen to see them when they usually already expired. Well this one somehow managed to pull me in. Petr has a very valid point, most often real network challenges are fixed with the first, quick and easy solution (or future problem). I know if this was a scenario in our network, static routes would definitely be the prime candidate.
My summarized version of the answer to Petr’s challenge is to use multiple ‘logical’ interfaces or in other words multiple subinterfaces, using only loopbacks for addressing or ip unnumbered loopback, since configuring additional ip addresses were not permitted. After the subinterfaces were configured all what was left to do is then manipulating the ospf cost on the R4-R1 link to 3 and set the maximum ospf paths. The net result: 6 paths to a subnet on R1, 3 going via R1, 2 paths via R3 and 1 path via R5:
Routing entry for 100.100.100.0/24
Known via “ospf 1″, distance 110, metric 4, type intra area
Last update from 3.3.3.3 on Serial1/0.1, 00:00:01 ago
Routing Descriptor Blocks:
* 5.5.5.5, from 1.1.1.1, 00:00:01 ago, via Serial1/3
Route metric is 4, traffic share count is 1
3.3.3.3, from 1.1.1.1, 00:00:01 ago, via Serial1/0.1
Route metric is 4, traffic share count is 1
3.3.3.3, from 1.1.1.1, 00:00:01 ago, via Serial1/0.2
Route metric is 4, traffic share count is 1
1.1.1.1, from 1.1.1.1, 00:00:01 ago, via Serial1/1.2
Route metric is 4, traffic share count is 1
1.1.1.1, from 1.1.1.1, 00:00:01 ago, via Serial1/1.1
Route metric is 4, traffic share count is 1
1.1.1.1, from 1.1.1.1, 00:00:01 ago, via Serial1/1.3
Route metric is 4, traffic share count is 1
The topology from URL above:
I fumbled a quick dynamips config together.
autostart=false
####################
# http://21500.net #
####################[localhost:7200]
[[3725]]
image = /dyn/images/C3725-adv-ent-mz.124-23.BIN
ram = 160
mmap = true
idlepc = 0×60a8141c[[Router R1]]
model = 3725
console = 2001
S1/0 = R2 S1/0
S1/1 = R4 S1/1
S1/2 = R5 S1/2
F0/0 = LAN 1[[Router R2]]
model = 3725
console = 2002
S1/1 = R3 S1/1[[Router R3]]
model = 3725
console = 2003
S1/0 = R4 S1/0[[Router R4]]
model = 3725
console = 2004
S1/3 = R5 S1/3[[Router R5]]
model = 3725
console = 2005
Completed config files if you want to run this minilab and perhaps find some more ways to solve the riddle.
R1.txt
R2,txt
R3.txt
R4.txt
R5.txt
Update: And what do you know, the solution gets the prize 
