Resolution to the Challenge – Part 2 with Virgilio Spaziani
Network Design Methodology In Action:
Challenge Resolution

In my first blog of this series, “A Design Challenge”, I proposed a network design challenge whose goal was to apply the solution methodology described on the “Now What?” blog. Let’s analyze the question and find the solution that best fits according to that methodology.

 

 

 

List all of the explicit and implicit business requirements in this challenge

Below are the business requirements to resolve the challenge. In any case, it’s very important to know them as most of the time they will drive you in the right direction when you find more “right” answers than you have to select.

1. Route First Service Provider wants to be leader in his business.

2. They are interested in video, voice, cloud computing, collaboration, and classic data services.

3. They need to efficiently use network resources to be more aggressive from a cost point of view.

4. There is the idea to divide customers using different profiles: bronze and gold at least.

 

List all of the explicit and implicit technical requirements in this challenge

1. Migrate from L2 overlay VPN (ATM, Frame Relay, etc.) to MPLS L3/L2 VPN

2. Solution scalability: Double the number of aggregation rings and aggregation routers

3. Network topology: It’s really important to have a clear idea about quantity of routers  and topology

Core Zone: 10 routers, no requirements to scale the core routers.

Aggregation Zone: The scalability requirement asks to double aggregation rings and number of aggregation routers per ring. There are currently 5 aggregation zones, 10 routers each – a total of 50 routers. To scale to 10 aggregation zones with 20 routers each would be a total of 200 aggregation routers. The dimension of the ring will be huge –  a 22-router ring - and might not be the solution we would naturally choose, but it’s not about the solution we prefer. Instead, we have to understand and apply requirements to choose between the proposed options. So we can assume that to scale to 10 aggregation rings, we will have two rings connected to the same core routers pair.

Peripheral zone: There are currently 30 peripheral zones (peripheral zones will be built on core router pairs, too) with 3 routers each – 90 routers total. To scale to 55 peripheral zones (explanation: core routers will not double) with 3 routers each would be a total of 165 peripheral routers. The current design consists of 10 core routers + 50 aggregation routers + 90 peripheral routers = 150 routers, and to scale to 10 core routers + 200 aggregation routers +165 peripheral routers = 375. So your chosen option should be able to grow 2.5 times.

4. Implicit requirement: The challenge asks you to migrate to MPLS, efficiently using the bandwidth, which brings us straight to having a MPLS TE support requirement.

 

List all of the explicit and implicit constraints in this challenge.

1. Route First wants to avoid switching OSPF with other routing protocols, unless it’s unavoidable.

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Figure 1: Current topology

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Figure 2: Proposed topology

 

Compare and contrast all options side-by-side against the requirements and constraints

To resolve the challenge we can make a summary of all of the requirements and all the proposed solutions using a table like the one below.

L2 IS-ISL1/L2 IS-ISSingle area OSPFMulti area OSPFMulti area, multi process OSPFOSPF virtual-linkEIGRP
1. Leader in SP industryfully meets requirementsfully meets requirementsfully meets requirementsfully meets requirementsfully meets requirementsfully meets requirementsfully meets requirements
2. Video, voice, cloud, collaboration and data supportfully meets requirementsfully meets requirementsfully meets requirementsfully meets requirementsfully meets requirementsfully meets requirementsfully meets requirements
3. Efficient use of network resources ==> MPLS TE supportfully meets requirementspartially meets requirementsfully meets requirementspartially meets requirementspartially meets requirementspartially meets requirementsdoes not meet requirements
4. Customer profilingfully meets requirementsfully meets requirementsfully meets requirementsfully meets requirementsfully meets requirementsfully meets requirementsfully meets requirements
5. Migrate to MPLS L3/L2 VPNfully meets requirementsfully meets requirementsfully meets requirementsfully meets requirementsfully meets requirementsfully meets requirementsfully meets requirements
6. 2.5 times growthfully meets requirementsfully meets requirementsdoes not meet requirementsdoes not meet requirementsfully meets requirementspartially meets requirementsdoes not meet requirements
7. Prefer OSPFdoes not meet requirementsdoes not meet requirementsfully meets requirementsfully meets requirementsfully meets requirementsfully meets requirementsdoes not meet requirements

 

Justify the option chosen (why correct) and the options not chosen (why incorrect)

Some requirements, such as (1) Leader in the SP industry, (2) Video, voice, cloud, collaboration and data support, (4) Customer profiling, and (5) Migrate to MPLS L3/L2 VPN do not depend on the IGP, so all boxes are marked as “Fully meets requirements”. Now analyzing all options:

 

L2 IS-IS

3. MPLS TE support: CSPF extension is required for both IS-IS and OSPF.

6. 2.5 times growth: IS-IS can scale to over 1,000 routers in one single area due to its more efficient use of LSP packet.

7. Prefer OSPF: Route First prefers to use OSPF unless it’s not an option.

 

L1/L2 IS-IS

3. MPLS TE support: CSPF extension is required for both IS-IS and OSPF. It can be complex to use MPLS TE on a multi-level environment, but still possible using inter-area tunnels.

6. 2.5 times growth: IS-IS can scale to over 1,000 routers in one single area due to its more efficient use of LSP packet.

7. Prefer OSPF: Route First prefers to use OSPF unless it’s not an option.

 

Single area OSPF

3. MPLS TE support: CSPF extension is required for both IS-IS and OSPF.

6. 2.5 times growth: As a best practice, and given that the 165 peripheral routers are low-end, the maximum number of routers in one single area is around 50, but of course the exact number depends on the topology and the number of adjacencies more than from the number of routers itself. Anyway, in this case we could have 375 routers in one single area, which is so largely above 50.

7. Prefer OSPF: Route First prefers to use OSPF.

 

Multi area OSPF

3. MPLS TE support: CSPF extension is required for both IS-IS and OSPF. It can be complex to use MPLS TE on a multi-level environment, but still possible using inter-area tunnels.

6. 2.5 times growth: As a best practice, and given the 165 peripheral routers are low-end, the maximum number of routers in one single area is around 50, of course the exact number depends on the topology, on the number of adjacencies more than from the number of routers itself. Anyway, in this case we could have 55 routers in one single area (2 core routers +20 aggregation routers + (11*3) 33 peripheral routers), and typically you don’t want to be so close to the 50 routers limit.

7. Prefer OSPF: Route First prefers to use OSPF.

 

Multi area, multi process OSPF

3. MPLS TE support: CSPF extension is required for both IS-IS and OSPF. It can be complex to use MPLS TE on a multi-process environment, but still possible using tunnel stitching solutions.

6. 2.5 times growth: With this solution, the maximum number of routers in one area will be 22 for aggregation zone routers.

7. Prefer OSPF: Route First prefers to use OSPF.

 

Multi area OSPF with virtual-link

3. MPLS TE support: CSPF extension is required for both IS-IS and OSPF. It can be complex to use MPLS TE on a multi-level environment, but still possible using inter-area tunnels.

6. 2.5 times growth: With this solution, the maximum number of routers in one area will be 22 for aggregation zones. However, this would incur a maze of  100 virtual-links, potentially creating instability in the network and limiting growth.

7. Prefer OSPF: Route First prefers to use OSPF.

 

EIGRP

3. MPLS TE support: EIGRP doesn’t support MPLS TE.

6. 2.5 times growth: There is not a specific maximum number of routers for an EIGRP network, but the network diameter for the query process can go over 20 routers for the 32 loopback PE addresses, and could cause the stuck in active (SIA) effect.

7. Prefer OSPF: Route First prefers to use OSPF unless it’s not an option.

 

In the end, the 2 finalists were the multi area, multi process OSPF and the OSPF virtual-link options. From these 2 options, the one that mostly meets the requirements is the multi area, multi process OSPF. This is an example of how to approach the CCDE Practical exam items. A solution that may seem “unusual” in this case was the one that best meet customer requirements. So the conclusion is that a right design in absolute doesn’t exist, but there are designs that meet requirements and designs that don’t. Therefore, the deep understanding of the requirements is the key for a right option selection.

 

Virgilio Spaziani is CCDE #20140003 and triple CCIE #35471 (R&S, SP, and Security). He’s a network designer and a Cisco official instructor based in Switzerland. He loves to solve complex network requirements using easy network designs, to teach complex technologies using easy examples.

 

About the Author

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Virgilio Spaziani is CCDE #20140003 and triple CCIE #35471 (R&S, SP, and Security). He’s a network designer and a Cisco official instructor based in Switzerland. He loves to solve complex network requirements using easy network designs, to teach complex technologies using easy examples.

 

 

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