Core: Transport technologies

Transport resiliency, diversity, and migration

Miro Kosut

Miro Kosut

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Physical-layer protection and path diversity decide whether higher-layer convergence ever gets tested - and legacy transports must be migrated cleanly.

Physical resiliency

1+1 / 1:1 protection, optical rings with protection switching, or mesh; dual entrance and diverse fibre paths. The trap is SRLG / fate-sharing: two links that look redundant but share a conduit, duct, or upstream are a single failure (see Factors affecting convergence and Network hierarchy and topologies).

Where to absorb failure

Decide which layer protects: fast optical / L1 protection (sub-50 ms, OTN/ring style) vs letting L3 reconverge (IGP + BFD + LFA/TI-LFA, see Fast convergence techniques). Don't blindly double-protect - pick the layer that meets the recovery target at the lowest complexity/cost.

Migration (incl. legacy)

Migrate Frame Relay / legacy TDM / ATM to Ethernet / IP / MPLS or SD-WAN. Use ships-in-the-night (run old and new together) or overlay-then-cut, minimise change windows, and always keep a rollback plan (see Reference models and migration considerations).

What would change this (mid-scenario twist)

  • Sub-50 ms recovery required on a span -> optical/L1 protection rather than L3 alone.
  • Legacy circuit being retired -> staged migration with parallel run + rollback.

Validation checks

  • Is "redundancy" genuinely non-fate-sharing end to end?
  • Is protection applied at one appropriate layer (not redundantly fighting itself)?

IPv6 / dual-stack note

Transport-agnostic; verify dual-stack survives the migration on the new transport.

Spaced repetition

Two physically separate-looking links still form a single point of failure when they [...].

Two physically separate-looking links still form a single point of failure when they share an SRLG (same conduit/duct/fibre/upstream) - fate-sharing.

The design question "protect at L1 or let L3 reconverge?" is decided by [...].

The design question "protect at L1 or let L3 reconverge?" is decided by the recovery-time target vs complexity/cost - don't double-protect.

Running the old and new transports simultaneously during a migration is [...].

Running the old and new transports simultaneously during a migration is ships-in-the-night.

Sources

  • Cisco Press, Optimal Routing Design; The Art of Network Architecture.

domain: Core · blueprint-ref: Core 1.3 Frame Relay (migration) / transport resiliency · type: design-decision · status: complete · tags: [core, core/transport, tradeoff/convergence, tradeoff/cost]

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