Redundancy

1
Redundancy

2
Single point of failure

• Hierarchical design produces many single points
  of failure
• Redundancy provides alternate paths, but may
  undermine stability
• Fig 3-1 Retana

3
Issues and strategies of redundancy

• Redundant paths should be used only when the
  normal path is broken
• Load sharing must be carefully engineered to
  prevent instability
• Traffic shouldnt pass through devices and links
  not designed for through traffic

4
Redundant core design

• Reduce hop count
• Increase the number of failures the core can
  withstand

5
Ring core design

• Two paths for each pair of routers
• Diameter is N/2
• Losing a link increases the diameter
• Losing any two links isolates a piece of network
• (Analysis)
• Reduced number of available paths
• Number of hops increases dramatically on a sing
  link failure --gt session timeout

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Ring core

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Redundant fiber ring technology

• SONET a pair of fiber optic links of
  OC-48(2.4G), 2nd fiber is used as backup
• FDDI two rings passing data in opposite
  directions. If fiber fails, the ring will wrap
  using dual attached nodes, healing the break
• Redundancy in layer 2

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Full mesh core design

• Large number of alternate paths
• Two hop path to any destination
• N hops maximum in the worst case
• Highest redundancy
• (Analysis)
• Smallest hop count
• Maximum redundancy
• Too many alternate paths --gt larger convergence
  time, difficult to engineer traffic

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Full mesh core

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Partial mesh core design

• Good compromise in hop count, redundancy and
  number of paths
• No more than three hops for Fig. 3-6
• If any single link fails, maximum number of hops
  will increase to four
• Provides full connectivity with three links down
  as long as no single router loses both of its
  connections
• Some routing protocols dont handle multipoint
  partial mesh

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Partial mesh core

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OSPF on NBMA networks

• PVCs on Frame relay, ATM has non-broadcast
  multiple access(NBMA)

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Solutions

• Problem non-broadcast, since a packet on a
  subnet might not be seen by all routers connected
  to it, making it hard to choose a DR(designated
  router)
• Solution
• 1) Configure priorities to select Router A as DR
• 2) Treat all as point-to-point link --gt subnet id
  space wasted
• 3) make all the links in the multipoint network
  to have same netid --gt should add a lot of host
  routes to each hosts routing table

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Distribution redundancy

• Dual homing
• Backup links to other distribution layer
• Consider unexpected traffic patterns

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Dual homing to the core

• Doubles the number of available paths to core
  routers, slowing network convergence

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Dual homing to the core

• Double avail. paths --gt Advertise only over one
  link unless that link becomes unusable
• Distribution routers may be drawn into core role,
  passing through traffic -gt prevent D from
  advertising routes learned from C back to B

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Redundant links to other distribution layer
devices

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Drawbacks

• Double the cores routing table size
• Possible use of redundant paths for core traffic
• Prefer redundant links to passing core
• Routing info will leak from A and spread to other
  distribution layer routers, causing instability

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Access redundancy

• Dual homed --gt size of routing table in the core
  increases prevent advertising redundant links

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Access redundancy

• Provide redundant links in access layer saves
  one link and reduces number of available paths
  --gt should provide enough bandwidth to handle
  traffic from both remote sites to core

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Access redundancy
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Redundancy through same distribution layer branch

• Load sharing and redundancy

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Redundancy through same distribution layer branch

• Filter in router G, H prevents advertising
  networks not below them in hierarchy such as D, F
  to C and E
• Or use dial-up

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Connections to common services

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Case whats the best route?

• How to determine best routes from a mix of
  different routing metrics?
• Ex) RIP uses hop count, EIGRP uses bandwidth and
  delay
• Longest match gt adminstrative distance gt metric
• Each routing protocol has a default adminstrative
  distance (lower prefered)
• connected 0 static 1, OSPF 110, RIP 120
• floating static routes a static route with high
  adminstrative distance, 200 or more used for
  backing up or conditionally advertised (BGP)

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Case Redundancy at layer 2 using switches

• (Using spanning tree avoiding loops at layer 2,
  block ports)

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Dial-up backup with a single router

• Configure ISDN link as a backup interface