CCIE Network Design FAQ: Redundancy

CCIE Network Design FAQ: Redundancy

Q1. Why is it important to consider link capacities when designing redundancy?

Answer: The backup link should be able to handle the full traffic load normally placed on the primary link.

Q2. Why is designing redundancy in the core easier than at other layers?

Answer: Suboptimal routing should be easier to deal with because the devices in the network core should have full routing information.

Q3. If all the core routers are in one building, what is a natural way to provide redundancy?

Answer: Connect them with multiple high speed LANs.

Q4. How many links on a ring core can fail before at least one section of the core is isolated?

Answer: Two.
ccie-network-design-faq-redundancy
Figure: A Ring Core

Q5. Do ring designs provide consistent hop count through the core network when a link fails?

Answer: No—the hop count can increase dramatically when a single link fails.

Q6. What ring technologies provide redundancy at Layer 2?

Answer: FDDI and SONET.

Q7. Do redundant ring technologies provide redundancy against failed devices?

Answer: No.

Q8. Given a full mesh core with 25 routers, how many paths would there be through the network?

Answer: 300

Q9. What method does a Cisco router use to differentiate between routes from two different routing protocols?

Answer: Administrative distance.

Q10. What is the first, and most important fac tor, used in deciding which route to use for a particular destination?

Answer: Prefix length. The longest specific match is used.

Q11. What mechanism in OSPF needs to be considered when it is being configured on a partial mesh network?

Answer: Designated router election.

Q12. What are the possible techniques you can use in OSPF partial mesh network designs to get around this problem?

Answer: Using point-to-point subinterfaces, using the router priority to predetermine that only the hub router becomes DR, using OSPF network type point-tomultipoint, or configuring the network as a non-broadcast OSPF network type and manually configuring the neighbors.

Q13. When dual homing a distribution layer or access layer router, what major problem should you be careful of?

Answer: Transiting traffic across the router that should be passed through the next higher layer in the network and increasing the size of the routing table in the next higher layer of the network.

Q14. When interconnecting the distribution or access layer router to provide redundancy, what issues should you be careful of?

Answer: Transiting traffic across the router that should be passed through the next higher layer in the network, increasing the size of the routing table in the next higher layer of the network, and that the path between the routers could be preferred over the normal (correct) path through the next higher layer.

Q15. What are the two main goals you must be careful to address when building redundancy into a network?

Answer: Redundant paths should only be used when the main path is down, unless they are engineered specifically for load sharing. Traffic shouldn’t be allowed, under any network conditions, to pass through links that aren’t designed to handle the full load of the primary link.

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