Core: Layer 3 Control Plane
In one line Structured layering bounds failure domains, scales the control plane, and localizes policy - it is the foundation every other L3 decision sits on. Layers and their purpose * Access - host/edge connectivity and the policy edge (QoS marking, NAC, first-hop security). * Distribution - aggregation and the routing/policy boundary:
Core: Layer 3 Control Plane
In one line The addressing plan enables (or prevents) summarization, and PBR/NAT are deliberate exceptions to normal routing that carry real costs. Policy-based routing (PBR) Overrides destination-based forwarding by source, application, or markings. Use sparingly - it bypasses normal routing and TE, complicates troubleshooting, and can hide from
Core: Layer 3 Control Plane
The interior routing protocol choice. Usually decided by multivendor needs, scale, SR/TE direction, and operational familiarity - rarely by raw performance, since all three converge fast when tuned. Requirement / business driver A stable, scalable interior routing foundation that fits the customer's vendor mix, growth, and operations team - and
Core: Layer 3 Control Plane
Requirement / business driver Use BGP where the requirement is policy control, scale beyond an IGP, internet multihoming, or a DC/WAN fabric - not as a general interior protocol. Where BGP earns its place * Internet edge / multihoming - provider-independent policy and failover. * DC underlay - eBGP per-rack ASN (simple, scalable) and
Core: Layer 3 Control Plane
Requirement / business driver Shrink routing tables and bound how far a failure propagates, while not blackholing or badly de-optimizing traffic. Benefits Smaller RIB/FIB; fewer SPF/flooding events (topology hiding) so faster, more stable convergence; a single flap behind a summary doesn't ripple outward. Costs / risks * Suboptimal
Core: Layer 3 Control Plane
Requirement / business driver Exchange routes between protocols or domains (mergers, migrations, multi-protocol cores) without creating loops or suboptimal routing. The hazard Mutual redistribution at multiple points is the classic trap: a route redistributed from protocol A into B can be learned back into A as if external, creating a
Core: Layer 3 Control Plane
In one line Protect adjacency formation and route integrity so the control plane can't be spoofed, hijacked, or DoSed. Techniques (apply at every trust boundary) * Neighbor authentication - OSPF/IS-IS/EIGRP key-chains; BGP TCP-AO (legacy MD5). * Passive interfaces - never form adjacencies toward hosts/access edges. * Control-
Core: Layer 3 Control Plane
Requirement / business driver Minimize traffic loss on failure to meet application SLAs (voice/video, storage, trading). The convergence budget - attack each phase Detect -> propagate -> compute (SPF/DUAL) -> update FIB. You shrink total loss by improving every phase, not just one. * Detection: carrier-delay/debounce tuning, link-down
Core: Layer 3 Control Plane
In one line The things that slow or break convergence beyond raw timers - so a design that looks fast on paper can still lose traffic. The factors * Recursion - a route whose next hop must itself be resolved (e.g. BGP next-hop via the IGP) adds a dependency; next-hop
Core: Layer 3 Control Plane
In one line Steer traffic by manipulating routing metrics/attributes - the cheapest TE lever, before you reach for tunnels (SR-TE / RSVP-TE). The levers * IGP - cost/metric tuning: OSPF reference-bandwidth and per-link cost, IS-IS wide metrics, EIGRP delay/K-values; used to set primary vs backup
Core: Layer 3 Control Plane
Requirement / business driver Deliver one-to-many (or many-to-many) efficiently with the right tree type and a resilient rendezvous-point (RP) design. PIM modes ModeTree / RPBest forNote PIM-SSMSource-specific, no RPKnown sources (IPTV, market data)Simplest; needs IGMPv3 / MLDv2 PIM-SMShared tree via RP, then switch to
Core: Layer 3 Control Plane
Requirement / business driver IPv6 is driven by public-IPv4 scarcity, regulatory or customer mandates, reaching IPv6-only services, or future-proofing. The hard design question is rarely "how does IPv6 work" - it's where to start and how to coexist with IPv4 without breaking applications. The phased