Authority and Format Identifier (always 49 for private)
49
Area ID
Area identifier (variable length, 1-13 bytes)
0001
System ID
Unique router ID (6 bytes, often derived from loopback IP)
1921.6800.1001 (from 192.168.1.1)
SEL
NSAP Selector (always 00 for NET)
00
Full NET
AFI.AreaID.SystemID.SEL
49.0001.1921.6800.1001.00
IS-IS Levels
Level
Function
Comparable To
Level 1 (L1)
Intra-area routing — knows all routes within own area
OSPF intra-area (within single area)
Level 2 (L2)
Inter-area routing — knows backbone and routes between areas
OSPF backbone (Area 0)
Level 1-2 (L1/L2)
Both: participates in L1 and L2 — area border router
OSPF ABR (Area Border Router)
L1 Default Route
L1/L2 router sets attached bit → L1 routers use default to reach L2
OSPF: ABR advertises default into stub area
DIS (Designated Intermediate System)
Feature
IS-IS DIS
OSPF DR/BDR
Election
Highest priority → highest MAC address (no BDR)
Highest priority → highest Router ID (DR + BDR)
Preemption
Yes — higher priority router takes over immediately
No — DR/BDR stable until failure
Backup
No BDR — all routers form adjacency with all others
BDR exists as backup
Pseudonode LSP
DIS creates pseudonode LSP representing the LAN
DR creates Network LSA (Type 2)
Hello Timer
DIS: 3.3s (1/3 of normal 10s) — faster failure detection
DR: same timer as other routers (10s)
TLV (Type-Length-Value)
TLV
Type
Carries
TLV 2
IS Neighbors
Adjacent IS-IS neighbors and metrics
TLV 22
Extended IS Reachability
Wide metrics (24-bit), sub-TLVs for TE
TLV 128
IP Internal Reachability
Internal IP prefixes (narrow metrics)
TLV 135
Extended IP Reachability
IP prefixes with wide metrics + sub-TLVs (standard)
TLV 236
IPv6 IP Reachability
IPv6 prefixes — multi-topology support
TLV 242
Router Capability
Segment Routing, SR-MPLS SID information
IS-IS vs OSPF
Feature
IS-IS
OSPF
Layer
Layer 2 (CLNP — directly on data link)
Layer 3 (IP protocol 89)
Extensibility
TLV-based — add new features via new TLVs (very easy)
New LSA types needed — harder to extend
Area Design
2 levels (L1 intra-area, L2 backbone) — simple
Multiple area types (stub, NSSA, totally stub) — complex
Scalability
Better for very large networks (ISP scale)
Good for enterprise (hundreds of routers)
IPv6 Support
Multi-topology or new TLVs — same process handles IPv4+IPv6
OSPFv3 — separate process for IPv6
Segment Routing
Native TLV support — preferred for SR deployments
Supported but IS-IS preferred by vendors
Adoption
ISPs, hyperscalers, data centers
Enterprise, campus, smaller SP networks
IS-IS Design Best Practices
Practice
Detail
L2-Only Backbone
Core/backbone routers as L2-only → simplifies LSDB, faster convergence
L1/L2 at Border
Area border routers as L1/L2 → connect L1 areas to L2 backbone
Wide Metrics
Always use wide metrics (metric-style wide) — narrow metrics max 63 per link, too limiting
Mesh Groups
Use mesh groups on fully-meshed links to reduce LSP flooding overhead
BFD
Enable BFD for fast failure detection (50ms) instead of relying on hello timers
Authentication
Use HMAC-MD5 or key-chain authentication to prevent unauthorized routers
Overload Bit
Set overload bit during maintenance → traffic routes around this router
ทิ้งท้าย: IS-IS = The ISP’s Choice for Scalable Routing
IS-IS Protocol NET: 49.AreaID.SystemID.00 — Layer 2 protocol, doesn’t depend on IP Levels: L1 (intra-area), L2 (backbone/inter-area), L1/L2 (border) — simpler than OSPF area types DIS: elected by priority/MAC, preemptive (no BDR), creates pseudonode LSP, 3.3s hello TLV: extensible format — TLV 22 (IS neighbors), 135 (IPv4), 236 (IPv6), 242 (Segment Routing) vs OSPF: L2 based (not IP), TLV extensible (easy), simpler areas, better at scale, preferred for SR Design: L2-only backbone, L1/L2 at border, wide metrics, BFD, authentication, overload bit for maintenance Key: IS-IS is the routing protocol of choice for ISPs and hyperscalers — simpler, more extensible, scales better
