Tagged: Routing
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FortiGate BGP Cookbook Part 1: Dual-ISP Edge Recipes
Six FortiGate BGP recipes for a dual-homed internet edge: stop being a transit AS, accept only a default route, steer traffic in both directions, advertise a prefix with no connected route, originate a default, and authenticate the session with TCP-AO.
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FortiGate BGP Cookbook Part 2: Verify and Debug
The commands I run after every FortiGate BGP change: read the session, compare received and advertised routes before and after policy, soft-clear safely, triage a session that will not establish, and run BGP debug without leaving it on.
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Proving the Path Part 3: Policy Route and Routing
The pinned session predates the fault. Before blaming SD-WAN or routing for what's happening now, I need to know what a brand-new flow to the same destination would actually do today, and that's a different question from what the config says on paper.
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Same Job, Different Shell Part 4: Routing Tables
ip route, route -n, route print, Get-NetRoute: reading a routing table is the same question on both platforms, why is this packet going that way, but Windows answers it with a metric that Linux's route table doesn't expose the same way at all.
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BGP Deep Dive Part 1: The Application That Runs Between Autonomous Systems
The first post in a deep dive into BGP. Why EGP failed, the two napkins that became RFC 1105, and the distinction that took a bad Halabi read and a good Doyle read to finally land: an IGP finds the best path inside a network you own, BGP negotiates policy between networks you don't.
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BGP Deep Dive Part 10: Multiprotocol BGP, or How One Session Learned to Carry Everything
RFC 4760 turned BGP from an IPv4-unicast-only protocol into a generic reachability-advertisement transport. Here's how AFI/SAFI, MP_REACH_NLRI, and one session carrying multiple address families actually works.
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BGP Deep Dive Part 11: Convergence, Stability, and Scale
Route flap dampening, graceful restart, BFD, and ADD-PATH: four mechanisms that shape how fast, how stable, and how scalable a BGP deployment actually is.
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BGP Deep Dive Part 12: Security, or Why BGP Trusts You Until Proven Otherwise
BGP was built on trust between operators, not cryptographic proof. Real hijack incidents, RPKI/ROV, BGPsec, and MANRS: what actually protects the internet's routing today.
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BGP Deep Dive Part 2: The Session, From TCP SYN to Established
Part 2 of the BGP Deep Dive: TCP port 179 and why BGP needed a transport layer at all, the full six-state finite state machine from Idle to Established, what the OPEN message actually negotiates, and the hold timer, keepalive, and session-security mechanics underneath it.
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BGP Deep Dive Part 3: The UPDATE Message and the Attribute Catalog That Is BGP's Actual Policy Engine
Part 3 of the BGP Deep Dive: the UPDATE message's real structure, RFC 4271's four categories of path attribute, and a walk through ORIGIN, AS_PATH, NEXT_HOP, MED, LOCAL_PREF, ATOMIC_AGGREGATE, and AGGREGATOR, including the classic unreachable-next-hop gotcha.
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BGP Deep Dive Part 4: The Best Path Algorithm, Step by Step
Part 4 of the BGP Deep Dive: the full best path selection algorithm that consumes every attribute from Part 3, walked step by step from Cisco-proprietary Weight down to the final router-ID tiebreak, including why hot-potato routing is baked into step 8, not a separate design choice.
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BGP Deep Dive Part 5: eBGP and iBGP Are Not the Same Protocol Wearing One Name
Part 5 of the BGP Deep Dive: why AS_PATH loop prevention does nothing inside an AS, the iBGP split-horizon rule that actually prevents internal loops, and why that one rule is the direct, mechanical reason full-mesh iBGP exists at all.
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BGP Deep Dive Part 6: Route Reflectors and Confederations, Two Ways to Break the Full Mesh
Part 6 of the BGP Deep Dive: RFC 4456 route reflection and RFC 5065 confederations, the two sanctioned ways to relax iBGP's full-mesh requirement from Part 5 without giving up its loop safety, plus the path-hiding tradeoff route reflection quietly introduces.
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BGP Deep Dive Part 7: Communities, BGP's General-Purpose Signaling Channel
Part 7 of the BGP Deep Dive: standard communities from RFC 1997, extended and large communities that fixed their real limitations, the three well-known values every implementation understands, and why the whole mechanism only works because two networks agree on what a number means.
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BGP Deep Dive Part 8: Policy in Practice, Prefix Filters, AS-Path Filters, and RFC 7454
Part 8 of the BGP Deep Dive: the actual tools operators use to apply policy, prefix filtering, AS-path filtering, the route-map pattern every vendor implements some version of, and RFC 7454's real operational recommendations for filtering, max-prefix limits, and session hardening.
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BGP Deep Dive Part 9: Traffic Engineering, or Why Outbound Is Easy and Inbound Never Fully Is
Part 9 of the BGP Deep Dive: multihoming and traffic engineering using nothing but attributes this series has already covered. Outbound TE is a solved problem with LOCAL_PREF. Inbound TE never fully is, and this post explains why prepending, MED, and communities are all, honestly, best-effort.
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OSPF on Linux Part 1: A Brief History of Routing on Linux
The first post in a companion series to OSPF Deep Dive: how Linux went from a kernel that could route packets but not speak a routing protocol, through GNU Zebra and Quagga, to FRRouting. History first, because the config in Part 3 makes more sense once you know why it looks the way it does.
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OSPF on Linux Part 2: Why FRRouting Is the Defacto Choice
The zebra/protocol-daemon/vtysh architecture from the inside, FRR against its one real remaining competitor, BIRD, and why FRR's Cisco-shaped CLI is a genuine advantage for this specific series rather than a cosmetic one.
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OSPF on Linux Part 3: Installing and Configuring OSPF with FRR
Getting FRR onto a Linux box for real: the package, the users it creates without asking, the daemon on/off switch in /etc/frr/daemons, and the first router ospf block in vtysh. Verified against a real install, not written from memory.
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OSPF on Linux Part 4: Wiring LAB-LNX into the OSPF Deep Dive Lab
Adding a fourth router to CORE-CSR, BRANCH-FGT, and EDGE-MX: a Linux box running the FRR config from Part 3, walked from Down to Full against the neighbor state machine from OSPF Deep Dive Part 1, with an LSDB parity check across all four platforms.
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OSPF on Linux Part 5: Troubleshooting OSPF on Linux
The closing post: OSPF Deep Dive Part 11's three-vendor show-command table extended to four with FRR's vtysh, then the failure modes that only exist on Linux, ip_forward, a firewall dropping protocol 89, multicast group membership, the wrong network namespace, veth MTU, and tcpdump on the wire.
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OSPF Deep Dive Part 1: Neighbor States, DR/BDR Election, and Network Types
The first post in a full OSPF series covering Cisco, FortiOS, and Junos: the hello protocol, the neighbor state machine, why DR/BDR election exists, and the network types that decide whether an election happens at all.
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OSPF Deep Dive Part 10: A Vendor-Neutral Troubleshooting Methodology
A structured way to diagnose OSPF problems that works on any vendor, because it's built on the neighbor state machine from Part 1, not on any platform's specific commands: what state is it stuck at, and what does that state rule in or out.
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OSPF Deep Dive Part 11: Show Commands and the LSDB, Side by Side
The same neighbor table, interface state, and LSDB, read through three different command sets: Cisco IOS/IOS-XE, FortiOS, and Junos, mapped side by side against the CORE-CSR/BRANCH-FGT/EDGE-MX lab from this series.
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OSPF Deep Dive Part 12: A Multi-Vendor Outage, Start to Finish
A closing walkthrough on the CORE-CSR/BRANCH-FGT/EDGE-MX lab: a complaint with no down interfaces and no failed adjacencies anywhere, a red herring that turns out to be real but irrelevant, and a root cause that traces straight back to Part 3's reference-bandwidth warning.
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OSPF Deep Dive Part 2: LSA Types and the Area Type Hierarchy
What actually travels in the LSDB: the seven LSA types that matter in practice, and how stub, totally stubby, NSSA, and totally NSSA areas each filter a different subset of them.
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OSPF Deep Dive Part 3: The SPF Algorithm, Cost, and Route Selection
How a full LSDB actually becomes a routing table: Dijkstra's algorithm, the reference-bandwidth cost trap every vendor shares, and why intra-area, inter-area, and external routes are preferred in that exact order regardless of cost.
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OSPF Deep Dive Part 4: Virtual Links, Summarization, and Redistribution
Fixing the lab's missing backbone connection with a virtual link, then summarizing at the ABR and redistributing a static route at the NSSA's ASBR, with working syntax on Cisco, FortiOS, and Junos for each.
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OSPF Deep Dive Part 5: OSPFv3 and Running OSPF Over IPv6
OSPFv3 isn't OSPFv2 with bigger addresses. It separates topology from addressing, drops built-in authentication in favor of IPsec, and runs over link-local addresses by default, plus the per-vendor syntax to bring it up on Cisco, FortiOS, and Junos.
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OSPF Deep Dive Part 6: Authentication and Security Hardening
What an unauthenticated OSPF segment actually exposes, why plaintext and MD5 authentication are both weaker than they look, and how to configure HMAC-SHA key chains on Cisco, FortiOS, and Junos.
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OSPF Deep Dive Part 7: Cisco IOS/IOS-XE Implementation and Gotchas
CORE-CSR's actual configuration: process ID scope, the network-statement vs interface-based config split, the passive-interface-default trap, and why raising reference-bandwidth only ever warns once, locally, at configuration time.
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OSPF Deep Dive Part 8: FortiOS Implementation and Gotchas
BRANCH-FGT's configuration: why FortiOS has no passive-interface-default equivalent, how OSPF scopes to a VDOM, the router-id selection rule that actually differs from Cisco's, and how OSPF interacts with SD-WAN policy routes.
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OSPF Deep Dive Part 9: Junos Implementation and Gotchas
EDGE-MX's configuration: why Junos never adds an interface to OSPF by accident, the policy-based redistribution model, and the one default that catches Cisco and FortiOS engineers off guard, OSPF outranking eBGP by default.
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Contrail SD-WAN Deep Dive Part 2: E-Hub, P-Hub, and the Anatomy of a Hub-and-Spoke Overlay
Contrail's hub-and-spoke model ran on two hub roles — the customer-owned enterprise hub and the multi-tenant provider hub. Routing, VRF segmentation, and traffic-flow mechanics of both, and why the model got heavy at scale.
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Juniper Session Smart SD-WAN Deep Dive Part 2: Secure Vector Routing — Tenants, Services, and Session Metadata
How Secure Vector Routing actually forwards a session: the tenant/service data model, the metadata the ingress router stamps on the first packet, and why that's enough to route symmetrically without a tunnel.
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One Box, Many Firewalls: A Practical Guide to FortiGate VDOMs
Virtual domains let a single FortiGate behave as several independent firewalls — separate routing tables, policies, and administrative boundaries on shared hardware. When that's the right tool, when a VRF is the better one, and how inter-VDOM routing actually moves a packet between them.
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BGP Route Dampening Part 1: The Flapping Problem, Exponential Decay, and Cisco Configuration
A deep dive into how BGP route dampening works: the 1990s internet instability that created it, the exponential decay algorithm behind it, every Cisco parameter explained, and a full configuration and verification reference.
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BGP Route Dampening Part 2: RFC 7454, BFD, and Where Dampening Still Belongs
Why the IETF now discourages global BGP route dampening, how Bidirectional Forwarding Detection interacts with it, what RFC 7454 actually says, and the specific modern scenarios where dampening remains the right answer.
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A Day in the Life of a Packet on a 50G FortiGate, Part 3: Routing, Policy Routes, and SDWAN Service Rules
The packet has a session entry and now needs to know where to go. FortiOS resolves that in a strict order: policy routes, then SDWAN service rules, then the FIB. Each layer has its own logic, its own match criteria, and its own diagnostic surface.
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Comparing Route Tables Between Two Sources: A Small Python Tool for Audits and Migrations
A self-contained Python utility that takes two Excel route lists, normalises every prefix through ipaddress, finds exact matches and overlaps, preserves invalid entries for audit, and writes a colour-coded Excel report plus CSVs. Includes install guide and full source.
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NSE4 Part 9: Routing & SDWAN
Part 9 of the NSE4 study series — static and policy routing, distance vs priority, RPF, OSPF and BGP basics, and how SDWAN turns a pile of WAN links into a single steered zone with performance SLAs.