David Hedleys Tuesday Tech Talk Bad Design

David Hedley’s
Tuesday Tech Talks
Bad Design
Let’s have some fun!
© 2018 David M. Hedley All Rights Reserved.

David Hedley's Tuesday Tech Talks – Bad Design
• Specific brands and models are for illustration purposes only.
• They do not imply any endorsement by the vendor, in any way.
• This talk does not represent the business process of any employer or
client, past or present, this is based on my own work and study.
• I only include equipment that I have experience with.
• At the time of writing, I have not received any compensation, or
inducement from any vendor.

• Definition
• Bad Design
• We can use terms like sub-optimal, to try and soften the blow, but what we’re
talking about is where a design requires some extraordinary workarounds to
function efficiently.

• Purpose: We often inherit designs, so how to we fix them.
• Question: What makes the most sense!

• Triple Constraints
• Time
• Quality or Scope
• Budget

• Our example
OSPF 1
AREA 5
Eigrp 150

• Our Mission
• R5 needs to be able to access a network hung off of R1
• We want to used the highest bandwidth
• But, we want to have redundancy in case the R2-R3 ethernet link goes down.
• We can’t use static routes.

• What are we going to do?
• 1. Redistribute OSPF in to EIGRP and visa versa.
• What options will we use?
• Do we need to anything special going from EIGRP to OSPF?
• E1 or E2?
• From OSPF to EIGRP
• R4: Redistribute ospf 1 metric 1544 100000 255 1 1500
• R3: Redistribute osp1 metric 100000 10 255 1 1500
• What do these redistribute statements accomplish?
• Do I need to manipulate the metric when I redistribute eigrp in to OSPF

• Does this accomplish all our goals?
• R5 needs to be able to access a network hung off of R1 -- Yes
• We want to used the highest bandwidth -- No
• But, we want to have redundancy in case the R2-R3 ethernet link goes down. -
-Yes
• We can’t use static routes –Yes

• Why NO!
• EIGRP external routes for an Administrative Distance (AD) of 170, and OSPF an AD of 110,
so on R4, the packets will take the T-1.
• Which is why this is a bad design!

• Other options?
• 2. Can we move the EIGRP OSPF Boundary to R2?
• What would that take?
• Add EIGRP 150 to R2
• Add the redistribute commands to redistribute OSPF to EIGRP and visa versa
• Add the appropriate network commands.
• On R3 and R4, update the EIGRP network statements to include the formerly OSPF interfaces
(or configure the interfaces for EIGRP)
• On R3 and R4, remove the OSPF process, and redistribute commands.

• We want to used the highest bandwidth -- Yes
-Yes
• We can’t use static routes –Yes

• What if R2 is another company or division so we can’t move the
boundary?
• What’s our next guess?

• 3. Add a direct link from R5 to R3
• If close enough, add a copper link between R5 and R3.
• If all three are in different closets, can I still run copper, using the structured
cabling, remember that the additional air gaps, will need to be factored in to
the distance calculation.
• If I have fiber between the closets, can I use fiber, again, the additional air
gaps will need to be factored in to the distance calculation.
• This link should be in the EIGRP domain.

• What if the distance is too far, or we lack single mode fiber, etc.?
• 4. R4 distance eigrp 90 100
• What does this do?
• Redistibuted OSPF routes from R3 now have a better AD than the OSPF routes. R4 will
install the EIGRP routes in to the routing table, so we shouldn’t even have anything to
redistribute in to EIGRP on R4.
• What does it do it OSPF on R4?
• It should try and redistribute these routes in to OSPF causing loops.
• So we still have a bad design.

• 4a. Take 1 + 4, and add tags
• From OSPF to EIGRP
• R4: Redistribute ospf 1 metric 1544 100000 255 1 1500 tag 66
• R3: Redistribute osp1 metric 100000 10 255 1 1500 tag 66
• R3 & R4. Redistribute eigrp 150 metric-type 1 route-map FIXME
• R3 & R4
• Route-map FIXME deny 10
• Match tag 66
• Route-map FIXME permit 20
• NB: Could do more to tailor the metrics so Routes from R4 would be feasible successors.

• What if I could use a static route?
• Assume we can summarize.
• 5. RE: ip route 10.10.0.0 255.255.0.0 192.168.255.2 (where the summary is
10.10.0.0/16 and 192.168.255.2 is R3’s ethernet link to R4)

-No
• We can’t use static routes

• 5a. Static route plus tracking
• R4:
• Ip sla 1
• Icmp-echo 10.10.1.255 source-interface gigabitethernet 0/0/2 (where 10.10.1.255 is the
loopback for R2 and gi0/0/2 is R4’s link to R3)
• Frequency 10
• Ip sla schedule 1 start-time now life forever
• Track 5 ip sla 1 reachability
• Ip route 10.10.0.0 255.255.0.0 192.168.255.2 track 5

-Yes
• We can’t use static routes
• It’s still a bad design!

• Why this exercise?
• I see really bad designs where we have to do some heroics in software to
make up for bad physical and logical designs.
• Keep the what if’s to a minimum.

• Homework
• Do you have any designs like this?
• If so, start making plans to fix them!

• Thanks for watching!
• You can subscribe to my YouTube Channel
https://www.youtube.com/channel/UCZ3pcIh5Zmbp3rdjhfR7BOg
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hedley-541985/
• You can suggest topics in the comments!

David Hedleys Tuesday Tech Talk Bad Design

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