TTL Expired

JNCIA Reference Guide

Fri, 03 Jul 2026 00:00:00 +0000

Last Updated: 3/17/26

Note to Test Takers: This document is a Summary Reference, not a replacement for a comprehensive course and hands on experience in a lab. I recommend the CBT Nuggets course because Knox is great at explaining networking concepts with the right amount of enthusiasm.

Table of Contents

Networking Fundamentals

The OSI Model
- Where it all starts and I’m not sure if its really covered in the JNCIA exam. This is an industry standard crafted back in the 1970s.
- The model has seven layers from physical up through application.
- The really important bit is the breakdown of the lower layers as you will see L1, L2, L3, L4 all over the place.
- These are logical separations of the networking stack
  - L1 - Physical layer - Think cabling and electrical or photonic pulses on said cabling
  - L2 - Data-link layer - Think frames, ethernet, and mac addresses
  - L3 - Network layer - Packets, IPs, and routing
  - L4 - Transport layer - Segments, TCP/UDP, and SYN+ACKs
Function of routers and switches
- Routers use L3 information to forward packets between networks
- Switches use L2 info to forward packets on the lan
Ethernet networks
- Major concept here is Mac addresses
  - Physical address made up of 48 bits and displayed using hexadecimal format
  - Broadcast address is ffff.ffff.ffff
- Uses mac addresses to forward ethernet frame
- Ethernet header + trailing checksum

JNCIS-SP IS-IS (Best IGP)

Tue, 30 Jun 2026 00:00:00 +0000

IS-IS

IS-IS (Intermediate System to Intermediate System) is a link-state routing protocol commonly used in service provider networks and hey, you’re studying for the JNCIS-SP, so you’re in luck. Like OSPF, it uses the Dijkstra SPF algorithm to compute shortest paths, but it was designed to handle all sorts of traffic. So even though it was designed with CLNP in mind, it can carry things like IPv4, IPv6, and label information for mpls and srv6.

JNCIS-SP Quick Reference

Tue, 30 Jun 2026 00:00:00 +0000

OSPF

OSPF Packet Types

Type	Name	Purpose
1	Hello	Neighbor discovery, DR/BDR election, keepalive
2	DBD	LSDB table of contents exchange
3	LSR	Request specific missing LSAs
4	LSU	Delivers actual LSAs
5	LSAck	Confirms LSU receipt

LSA Types

Type	Name	Originated By	Scope	Blocked By
1	Router	Every router	Area	—
2	Network	DR	Area	—
3	Summary	ABR	Domain	Totally Stubby, Totally NSSA
4	ASBR Summary	ABR	Domain	Totally Stubby, Totally NSSA, NSSA, Stub
5	AS-External	ASBR	Domain	All stub and NSSA areas
7	NSSA External	ASBR (NSSA)	NSSA only	— (translated to Type 5 at ABR)

Area Types

Area	Type 3	Type 4	Type 5	Type 7	Default Route
Standard	Yes	Yes	Yes	No	No
Backbone	Yes	Yes	Yes	No	No
Stub	Yes	No	No	No	Yes (auto)
Totally Stubby	No	No	No	No	Yes (auto)
NSSA	Yes	No	No	Yes	No (configurable)
Totally NSSA	No	No	No	Yes	Yes (auto)

Adjacency States

Down → Init → 2-Way → ExStart → Exchange → Loading → Full

Broadcom Switch Chipset Families

Wed, 29 Apr 2026 00:00:00 +0000

Broadcom Switch Chipset Families

Broadcom dominates merchant silicon for data center and carrier switching. Their three main ASIC families, Tomahawk, Trident, and Jericho, each make different tradeoffs between bandwidth, feature depth, and buffer size. Most Arista, Cisco Nexus, and Juniper QFX/PTX platforms run one of these under the hood.

1. Tomahawk Series

Design philosophy: Maximum port density and throughput at the cost of feature depth. These chips use cut-through forwarding, carry shallow on-chip buffers (~50–100 MB), and support little to no L3 routing table depth. The trade-off is intentional; at spine and AI fabric scale, you want wire-rate forwarding with predictable low latency, not a large TCAM.

Laid off and learning AI tools

Wed, 29 Apr 2026 00:00:00 +0000

I was laid off from Block at the end of February and the reason provided by Jack was AI efficiencies were reshaping how work is done. Bummer. After sitting for a day or so with the existential dread of an AI dominated future, I decided that instead of sitting still and letting that future catch up to me, I needed to grab the bull by the horns and steer where my career is heading.

Optical Transceiver Reference

Wed, 29 Apr 2026 00:00:00 +0000

Form Factor	Speed	Lanes (Elec)	Modulation	Connector Types	Approx. Intro
SFP+	10 Gbps	1 x 10G	NRZ	LC Duplex, RJ45	2006
SFP28	25 Gbps	1 x 25G	NRZ	LC Duplex	2014
SFP56	50 Gbps	1 x 50G	PAM4	LC Duplex	2019
SFP-DD	100 Gbps	2 x 50G	PAM4	LC Duplex	2019/20
QSFP+	40 Gbps	4 x 10G	NRZ	MPO-12, LC	2012
QSFP28	100 Gbps	4 x 25G	NRZ	MPO-12, LC	2014
QSFP56	200 Gbps	4 x 50G	PAM4	MPO-12, LC	2019
QSFP-DD	400G / 800G	8 x 50/100G	PAM4	MPO-16, LC, CS	2017/21
OSFP	400G / 800G	8 x 50/100G	PAM4	MPO-12/16, LC	2019
OSFP1600	1.6 Tbps	8 x 200G	PAM4	MPO, Dual LC	2024/25

JNCIS-SP OSPF Concepts

Wed, 22 Apr 2026 00:00:00 +0000

OSPF (Open Shortest Path First)

OSPF is a link-state interior gateway protocol (IGP). Each router floods Link-State Advertisements (LSAs) describing its interfaces and neighbors. Every router builds an identical Link-State Database (LSDB) and runs the Dijkstra SPF algorithm to compute the shortest path tree. OSPF runs directly over IP (protocol 89) and uses multicast for efficiency.

Default route preferences in Junos:

OSPF Internal routes: 10
OSPF AS External routes: 150

Terms

LSDB (Link-State Database) - The topological database. Within a single area, all routers must have an identical LSDB.
SPF (Shortest Path First) - The Dijkstra algorithm each router runs against the LSDB to compute best paths.
Router ID (RID) - A 32-bit identifier unique to each OSPF router. Junos selects the RID in this order: explicitly configured → highest active loopback IP → highest physical interface IP. Best practice is to configure it explicitly.
ABR (Area Border Router) - A router with interfaces in multiple OSPF areas. Generates Type 3 (Summary) LSAs between areas.
ASBR (AS Boundary Router) - A router that redistributes routes from outside OSPF into the OSPF domain. Generates Type 5 LSAs.
Backbone Router - Any router with at least one interface in Area 0.
Internal Router - All interfaces are in the same single area.

Remember, best practice to explicitly configure the router-id. See below:

Trying out this whole blogging thing.

Sun, 19 Apr 2026 00:00:00 +0000

Well I’ve tried it before and the site languished in the forgotten corners of the internet after I lost interest. The domain lapsed. Its now an orphaned Wordpress instance participating in some botnet. I’ll eventually put the droplet out of its misery but thats for another day.

The deployment toil for blogs has dropped considerably since the last time I gave it a go. I’m writing this in Obsidian which is backed by a local git repo. When I’m finished I’ll run a commit and push right here in Obsidian. The push sends it to a Github repo thats tied to Cloudflare Pages. After merge, the Cloudflare plugin runs the deployment to Pages. And we’re done. I could automate it more using the auto commit/push functionality in the Obsidian git plugin but we’ll get to that when I’m more confident in my ability to write coherently.

JNCIS-SP - IP Tunnels

Wed, 15 Apr 2026 00:00:00 +0000

IP Tunnels

Tunnels encapsulate one protocol inside another, creating a virtual point-to-point link across a network that wouldn’t otherwise carry that traffic. The encapsulating network is called the underlay; the encapsulated traffic and the logical topology it creates is called the overlay. Both GRE and IP-IP are stateless — they hold no session state and provide no encryption or reliability guarantees.

Common use cases:

Carry IPv6 traffic across an IPv4-only core (6in4)
Carry IPv4 traffic across an IPv6-only core (4in6)
Extend IGP adjacencies across a WAN that doesn’t support multicast
Tunnel MPLS across a non-MPLS network
Bridge Layer 2 domains across a routed network

Tunnel Concepts

Underlay vs Overlay:

JNCIS-SP High Availability

Wed, 15 Apr 2026 00:00:00 +0000

High Availability

Junos provides a layered HA architecture. Link aggregation handles physical link redundancy. Graceful Restart, GRES, and NSR handle control plane failures at increasing levels of sophistication. BFD accelerates failure detection for all routing protocols. Understanding which technology does what — and what each one requires — is the core exam objective for this topic.

Link Aggregation Groups (LAG / LACP)

LAG bundles multiple physical interfaces into a single logical ae (aggregated Ethernet) interface, providing both redundancy and increased bandwidth. IEEE standard 802.3ad — not to be confused with 802.1ad (Q-in-Q).

JNCIS-SP IPv6 Concepts

Wed, 15 Apr 2026 00:00:00 +0000

IPv6

IPv6 was designed to solve IPv4 address exhaustion while simplifying the protocol. The header is fixed-length and streamlined, broadcast is eliminated in favor of multicast, and address configuration can be fully automatic. For service providers, the most exam-relevant areas are address types, NDP, autoconfiguration, and how routing protocols (OSPF, IS-IS) extend to support IPv6.

IPv4 vs IPv6 Key Differences

Feature	IPv4	IPv6
Address size	32 bits	128 bits
Header size	Variable (20–60 bytes)	Fixed 40 bytes
Header checksum	Yes	No (relies on L4)
Fragmentation	Routers and source	Source only
Broadcast	Yes	No — replaced by multicast
Address resolution	ARP	NDP (ICMPv6)
Autoconfiguration	DHCP only	SLAAC + DHCPv6
IPsec	Optional	Built into extension header framework

IPv6 Header

The base IPv6 header is always exactly 40 bytes. It is simpler than IPv4 — no checksum, no options field, and no fragmentation fields (those are handled by extension headers when needed).

JNCIS-SP Layer 2 Bridging, VLANs, and STP

Wed, 15 Apr 2026 00:00:00 +0000

Layer 2 Bridging and VLANs

Service provider networks often need to deliver Layer 2 connectivity between geographically separated customer sites. Junos implements this using bridge domains, which define the L2 forwarding boundaries, and 802.1ad (Q-in-Q) to tunnel customer VLAN spaces across the provider network without overlap.

Terms

Bridge Domain — a Layer 2 forwarding domain. Like a VLAN. It defines which interfaces share the same broadcast domain and MAC table.
EVC (Ethernet Virtual Connection) — the L2 service sold by a SP to a customer. It defines the endpoints of a Layer 2 circuit.
C-Tag (Customer Tag) — the inner 802.1q tag. Any VLAN 1–4094 from the customer’s space.
S-Tag (Service Tag) — the outer 802.1ad tag. Assigned by the SP to identify the customer. Encapsulates all of that customer’s C-Tags.
PBN (Provider Bridge Network) — the entire SP Layer 2 fabric.
PEB (Provider Edge Bridge) — the SP edge device. Pushes/pops S-Tags on customer-facing ports.
S-VLAN Bridge — an interior SP device that only examines and switches based on the S-Tag.
Customer ports — PEB ports facing the customer. S-Tags are applied or removed here.
Network ports — interior SP ports that carry double-tagged frames without modification.
IRB (Integrated Routing and Bridging) — a logical interface that gives a bridge domain an IP address, enabling the router to act as the default gateway for hosts in that domain.

802.1q

The standard VLAN tagging protocol. Inserts a 4-byte tag into the Ethernet frame.

JNCIS-SP MPLS Concepts

Wed, 15 Apr 2026 00:00:00 +0000

MPLS (Multiprotocol Label Switching)

MPLS is a forwarding mechanism that uses short, fixed-length labels to make packet-forwarding decisions instead of performing a full IP lookup at every hop. Labels are applied at the ingress of an MPLS domain and stripped at the egress, with each transit router performing only a label swap — making forwarding fast and enabling traffic engineering, VPNs, and QoS capabilities.

Terms

LSR (Label Switching Router) - Any router participating in MPLS forwarding. Performs label actions push, swap, or pop.
LSP (Label Switched Path) - The unidirectional path a labeled packet takes from ingress to egress LSR.
FEC (Forwarding Equivalence Class) - A group of packets that receive identical forwarding treatment and are assigned the same label at ingress. The ingress router decides the FEC assignment; downstream routers just label-switch.
Ingress LSR - The first router in an LSP. Classifies traffic into FECs and pushes labels.
Egress LSR - The last router in an LSP. Removes the label and forwards the original packet.
Transit LSR (P router) - An interior provider router. Swaps labels and forwards without examining the inner IP header.
PE (Provider Edge) - ISP router at the edge of the MPLS domain that interfaces with customer equipment. Performs label push/pop for customer traffic.
CE (Customer Edge) - Customer device that connects to the PE. Not aware of MPLS.
LIB (Label Information Base) - The full table of all label bindings a router has received. Not all entries are actively used for forwarding.
LFIB (Label Forwarding Information Base) - The active subset of the LIB used for actual forwarding decisions. This is what the data plane uses.
TED (Traffic Engineering Database) - Populated by IGP TE extensions; stores link-state info (bandwidth, admin groups) used by CSPF to calculate constrained paths.
MBB (Make-before-Break) - Default Junos behavior where the new LSP is fully signaled and verified before traffic is switched over from the old path.

Label Operations

Operation	Description
Push	Add a new label to the top of the label stack. Done by the ingress LSR.
Swap	Replace the top label with a new one. Done by transit LSRs.
Pop	Remove the top label from the stack. Done by the egress LSR or the penultimate hop.

MPLS Label Structure

Each MPLS label is a 32-bit field inserted between the Layer 2 and Layer 3 headers (sometimes called a “shim header”). Multiple labels can be stacked.

JNCIS-SP Protocol-Independent Routing

Wed, 15 Apr 2026 00:00:00 +0000

Protocol-Independent Routing

Protocol-independent routing features work regardless of which dynamic routing protocol is running. This covers how Junos selects routes when multiple sources compete, how to define static and summary routes, how to filter unwanted prefixes, and how to carve the routing table into separate instances for policy-based forwarding and VPNs.

Route Preferences

When multiple protocols learn a route to the same destination, Junos uses preference (administrative distance) to pick the winner. Lower value wins.

About

Mon, 01 Jan 0001 00:00:00 +0000

about

TTL Expired

JNCIA Reference Guide

Networking Fundamentals

The OSI Model

Function of routers and switches

Ethernet networks

JNCIS-SP IS-IS (Best IGP)

IS-IS

JNCIS-SP Quick Reference

OSPF

OSPF Packet Types

LSA Types

Area Types

Adjacency States

Broadcom Switch Chipset Families

Broadcom Switch Chipset Families

1. Tomahawk Series

Laid off and learning AI tools

Optical Transceiver Reference

JNCIS-SP OSPF Concepts

OSPF (Open Shortest Path First)

Terms

Trying out this whole blogging thing.

JNCIS-SP - IP Tunnels

IP Tunnels

Tunnel Concepts

JNCIS-SP High Availability

High Availability

Link Aggregation Groups (LAG / LACP)

JNCIS-SP IPv6 Concepts

IPv6

IPv4 vs IPv6 Key Differences

IPv6 Header

JNCIS-SP Layer 2 Bridging, VLANs, and STP

Layer 2 Bridging and VLANs

Terms

802.1q

JNCIS-SP MPLS Concepts

MPLS (Multiprotocol Label Switching)

Terms

Label Operations

MPLS Label Structure

JNCIS-SP Protocol-Independent Routing

Protocol-Independent Routing

Route Preferences

About