CEF — FIB, Adjacency & Why Process Switching Died

CEF is the default forwarding engine: no per-packet CPU work. Two precomputed structures: FIB — optimised copy of the RIB, longest-prefix entries, best path per prefix. Adjacency table — the rewritten L2 header per next-hop (MAC + outgoing interface), built from ARP/ND. Flow: RIB → FIB → Adjacency. Both update event-driven on routing or ARP […]

Routing Protocol Authentication & Passive Interfaces

If routing updates are unauthenticated, anyone on-path can inject a better route. Per-interface (IGP) or per-session (BGP) authentication fixes it — and a mismatch simply means no adjacency, which is also how you spot a partially-broken design. ! EIGRP key chain AUTH key 1 key-string cisco123 interface Gi0/0 ip authentication mode eigrp 100 md5 ip […]

Prefix-Lists & the Route Filtering Stack

Route filtering needs to match both address and mask length — ACLs don’t. Prefix-lists do: ip prefix-list PL seq 20 permit 10.0.0.0/8 ge 16 le 24 Matches any 10/8 subnet with mask between /16 and /24. Exact match = no ge/le. 0.0.0.0/0 le 32 = everything. Top-down, sequential numbers, implicit deny at the bottom. The […]

Summarization vs Aggregation (Null0)

Collapsing many specific routes into one supernet. Same math, two exam words: summarization inside an AS (OSPF/EIGRP/RIP), aggregation between ASes (BGP). The summary route points at Null0 — a local loop-prevention anchor so specific routes don’t recurse into a blackhole loop. Benefits: smaller tables, less update traffic, faster convergence, hidden internal detail. BGP extras: summary-only […]

Route Redistribution — And Its Loop Traps

Redistribution feeds routes learned by one protocol into another at a router running both. Two facts define the behaviour: the redistributed route gets the receiving protocol’s AD, and the metric must be supplied if the target can’t invent one. router ospf 1 redistribute static subnets ! subnets flag or classful-only (classic trap) router eigrp 100 […]

Inter-VLAN Routing — SVI & Router-on-a-Stick

Two ways to route between VLANs. SVI on a Layer 3 switch ip routing interface vlan 10 ip address 10.10.10.1 255.255.255.0 One SVI per routed VLAN; routing happens in hardware. Note: a L2 switch SVI is management-only (SSH, SNMP) plus ip default-gateway for the switch itself — it never routes user traffic. Router-on-a-stick interface g0/0.10 […]

VRF — Several Routers in One

VRF gives one router multiple independent routing contexts — separate RIB, separate CEF table, separate routing processes. Overlapping IPs across tenants are fine; a failure in one VRF doesn’t touch the others. vrf forwarding on an interface removes its IP — re-add the address afterwards. Each VRF runs its own OSPF/EIGRP/BGP: router ospf 10 vrf […]

Static Routes — Default, Floating, Recursive

Manual routes with ip route <net> <mask> <next-hop|interface> — four flavours worth knowing: Default route 0.0.0.0/0 — the gateway of last resort, used only when nothing more specific matches. Floating static: same prefix with a higher AD (e.g. … 192.168.1.2 5) — sleeps until the primary route vanishes. Backup via AD, nothing else. Recursive: next-hop […]

Best Path Selection: LPM, AD, Metric, ECMP

How a Cisco router picks one best path — one fixed chain: Longest Prefix Match → Administrative Distance → Metric → ECMP → FIB LPM beats everything. To 10.10.10.5, a /24 wins over a /16 no matter what AD or metric says. AD chooses between protocols at equal prefix length: connected 0, static 1, eBGP […]