IS-IS Design Options

  1. How to prevent IS-IS router from being a transit router when waiting for LDP or BGP to converge?
    1. Enable “overload-bit” feature
  2. When do we need to enable wide metric?
    1. when we want to use IS-IS for traffic engineering, or L2 to L1 route leaking
  3. Does IS-IS has backbone area like OSPF?
    1. All IS-IS routers belong to only one area, no area is considered the backbone area. The backbone of IS-IS consists of contiguous link of L2 routers.
  4. How to improve IS-IS convergence time?
    1. Improve link or node failure detection. Use BFD, reduce hello timer
    2. reduce LSP generation delay using “lsp-interval” and “lsp-gen-interval”
      1. “lsp-gen-interval” control the delay between a topology change and a lsp generation
      2. “lsp-interval” control the number of LSP advertisement per second
    3. reduce SPF generation delay using “spf-interval”
    4.  enable prefix priority for BGP next-hop (loopback prefixes)
  5. What kind of design is best for NMBA network?
    1. IS-IS does not support NMBA network. The preference is to use point-to-point interface at hub and spoke
    2. The second option is to fully meshed the NMBA nodes, and treat the network as a broadcast network
  6. How to reduce flooding on NBMA network with many connections between nodes running in IS-IS point-to-point mode?
    1. use mesh-group. A router does not advertise LSPs received from a interface to other interfaces in the same mesh group.
  7. how to prevent back-hole traffic when summarization is deployed?
    1. make sure there is a direct link between the L1/L2 routers that have summarization configured. in the event of a link/node failure, the link can be used to transmit the packets to the final destination
  8. how to ensure optimum routing out of a L1 area?
    1. use “route leaking” at the L1/L2 routers

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