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Course Intermediate

Switching and Routing

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01

Network Devices

The effect of the repeater, hub, and switch on collision and broadcast domains, the MAC address table's learning and forwarding decision, segmentation with VLANs, loop prevention, and link aggregation.

  1. 01 Repeater, Hub, and Switch Three device classes are measured on the same forty frames: the repeater and the hub set up a single collision domain and produce 280 deliveries and 34 collisions, a table-less switch zeroes the collisions but keeps 240 unnecessary copies, and a switch with a table brings deliveries down to 40.
  2. 02 MAC Address Tables A table's age is a two-way trade-off: on forty frames, an entry with unlimited lifetime gives 110 deliveries and 3 black holes, a two-frame lifetime zeroes the black holes but raises deliveries to 278.
  3. 03 VLANs Segmentation splits the broadcast domain but not the table row: on forty frames, flooding delivered drops from 280 to 51 and to 8, row count stays at 16, and the cost is a black hole of between 23 and 32.
  4. 04 Spanning Tree Protocol In an unblocked ring, a single broadcast frame produces 142 copies by round twelve and does not stop; on the tree it ends at 7 copies. When the tree is rebuilt with stale tables, 20 packets fall into a black hole; when only two switches have refreshed their tables, 10 packets enter a loop.
  5. 05 Link Aggregation Per-flow distribution splits no flow and breaks no order but spreads port load between 0 and 19; per-frame distribution equalizes load and splits 7 of 8 flows. When a port drops, modulus remapping moves 7 of 8 flows, in-place remapping only 3.

02

Routing

From longest prefix match to the routing table, from the criterion for choosing between static and dynamic routing to the convergence of the distance vector and link state families, from policy-based routing to table splitting, label switching, and overlay networks.

  1. 01 Routing Decision The routing decision is made by looking not at what the message says but at the device's own table; when the same table is read with two different rules, it diverges on 12 of the 256 addresses, and because the table is a copy of a shared reality, at the moment of the break 26 of the 40 packets reach their destination and 14 fall into a black hole.
  2. 02 Static and Dynamic Routing The static table never hears about the break, and the black-hole count stays at 14 across six rounds; in the dynamic regime where the news spreads, the loss ends in two rounds, but when only two nodes hear the news, the black hole drops to zero while 6 packets enter a loop and hops climb from 61 to 94.
  3. 03 Distance Vector Protocols A neighbor is heard from only as a sequence of distances; from a cold start, the correct table is reached in four rounds, and reached packets per round go 0, 9, 20, 32, 40. The one trade-off needed to learn bad news gives rise to counting to infinity: when a destination becomes unreachable, all sixty of the sixty packets in a twelve-round window enter a loop.
  4. 04 Link State Protocols A neighbor is sent not a decision but raw link information; every node builds its own link-state database and computes the shortest path itself. The correct table is reached in three rounds, and it is ahead of distance vector in every round: 9, 20, 32, 40 against 0, 9, 20, 32.
  5. 05 Border Gateway Protocol At an autonomous system border, the decision looks not at the shortest path but at policy: a transit ban still delivers all forty of the forty packets but raises total hops from 83 to 92; when a border advertisement is withdrawn, 14 packets fall into a black hole; and a single node's route leak brings hops back down to 83 while routing 7 packets off the contracted path.
  6. 06 Routing Instances When a single device keeps more than one routing table, the same destination goes to two separate next hops: three instances carry the same forty packets in 83, 99, and 91 hops, 18 of the 56 rows diverge, and when the instances are collapsed into a single table, 14 packets fall into a black hole.
  7. 07 MPLS and Label Switching When the decision is taken again at every node, forty packets produce 83 full-table lookups; when the decision is taken once at the edge and carried by a label, the count drops to 40 full lookups and 43 label lookups, a label-built path brings the chord's load down from 14 packets to 0, and rows kept climb from 56 to 110.
  8. 08 Overlay Networks A logical topology built with tunnels keeps its own table and never sees the table beneath it: when the underlay breaks, not a single row of the overlay's table changes, but the underlay's hops climb from 188 to 211; when the underlay's tables go stale, 21 packets die even though the overlay's table is correct; and the overlay tables' partial information puts 12 packets into a loop with not a single wrong decision in the underlay.

03

Topology Design

Layered enterprise designs, spine-leaf for east-west traffic, gateway redundancy and failover, and an address and naming plan that scales as the network grows.

  1. 01 Layered Design The number of layers determines hop count and convergence rounds together: on the same nine access switches, a flat chain spends 121 hops, a two-layer design 80, and a three-layer design 138, and the link-state family converges in 7, 1, and 3 rounds respectively.
  2. 02 Spine–Leaf Design In a design where every leaf connects to every spine, the leaf-to-leaf hop count stays constant at 2, independent of spine count, leaf pair, and break; the cost of that constancy is table rows, and with four spines the single-path row count climbs from 90 to 156 and the multi-path row count from 90 to 468.
  3. 03 Redundancy and Failover The failover window in gateway redundancy is measured: when the upstream link breaks and nobody has heard, all forty flows fall into a black hole one hop later; when the gateway fails over, the black hole clears but the peer link carries forty crossings; when two gateways each believe themselves active, all forty flows enter a loop and the peer link carries eighty crossings.
  4. 04 Address and Naming Plan On the same topology, a summarizable plan holds the table at 73 rows as the subnet count climbs from nine to thirty-six, while a plan distributed by creation order climbs from 90 rows to 335 on the same path; a single moved network leaves a summary announcing an address it no longer owns, and two packets fall into a black hole.

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