Tag: Design

Review/Perspective: Troubleshooting Methodologies

1.3.b Apply troubleshooting methodologies

  • 1.3.b [i] Diagnose the root cause of networking issue [analyze symptoms, identify and describe root cause]
  • 1.3.b [ii] Design and implement valid solutions according to constraints
  • 1.3.b [iii] Verify and monitor resolution


Diagnose the root cause of networking issue

There are two basic approaches to troubleshooting:

  • Climb the Stack approach – begin at layer 1 and work your way up until you find a problem.  You can do it the other way around and start at layer 7 and work your way down until you find something wrong as well.
  • Divide and Conquer method – usually faster at finding the problem because you start at the layer that’s having the issue.  You then either go up or down the OSI stack to find the issue based on the observed problems in the problem layer.

Design and implement valid solutions according to constraints and Verify and monitor resolution

Before coming up with a plan for a network it’s important to understand the design requirements, this includes organizational and technical goals, and organizational and technical restraints.

Organizational constraints include budget, personnel, policy, and schedule.

Technical constraints include parameters that may limit the solution such as legacy applications or protocols that exist that must be supported may limit the design.  other examples include, existing wiring does not support new technology, and bandwidth doesn’t support new applications.

PPDIOO is the Cisco method of implementing new network designs based on organizational goals and provides a change process to apply to an existing network.

Prepare – this phase establishes business requirements and develops a network strategy, propose a high level conceptual architecture to support the strategy.

Plan – this phase identifies network requirements based on goals, facilities and user needs.  This phase also characterizes sites, analyzes the network, performs gap analysis against best practice architectures and looks at the operational environment.  Project plan is developed to manage the tasks and responsible parties to do the design and implementation.  The project plan aligns with the scope, cost, and resources defined in the original business requirements (Prepare phase)

Design – The network design is developed based on the technical and business requirements from the previous two phases.  This is a comprehensive and detailed design that meets business and technical requirements.  Design includes plans for redundancy and high availability.

Implement – equipment is installed and configured according to detailed design specifications.  Devices can be new or replace current infrastructure in this phase to support the project plans requirements based on business needs.  Each step in the implementation phase should be documented including plans for rollback in case of failure and any additional reference information as needed.

Operate – this phase maintains the networks day to day operations.  Network management applications to monitor the networks health are used to gauge how well the implemented design is managing the real production traffic planned in the project.  Any problems,  traps, or network events must be documented for review.

Optimize – this involves proactive network management by identifying issues seen on the existing network and modifying the network design to appropriately correct those issues and improve overall network performance.  Ultimately leading to the network life cycle where this entire process starts over again for each of the changes required for the network.

OSPF: Design

OSPF design requires groups of links to be collected in a contiguous area.

Routers that connect to two or more areas are referred to as Area Border Routers (ABR)

All areas are supposed to connect to Area 0, so ABRs should have at least one interface in Area 0.

It is possible to have routers in between areas that AREN’T area 0, Cisco does not consider these routers as ABRs its simply referred to as an internal router.

Autonomous System Boundary Routers (ASBR) inject external routes into OSPF.

OSPF routers keep an independant and seperate LSDB for each area it is connected too.  When computing best path to a network, SPF is run on each LSDB independantly where the results are combined and injected into the routing table.

Using areas provide the following benefits:

  • Smaller per area LSDBs require less memory
  • Faster SPF computations on smaller LSDBs
  • Link failures in a specific area only require partial LSDB updates.
  • routes can be summarized and filtered at ABRs and ASBRs as needed to reduce the number of active routes in the cores Area 0 routing table.

Transit Networks – are a network over which two or more OSPF routers are neighbors and elected a DR so traffic can transit from one to the other.  The exception to this rule is point to point connections between two routers.

Stub Networks – A subnet on which a router has not formed any neighbor relationships.