isp-backbone-course/modules/01-isis.md

Module 1: The Underlay — IS-IS

Course: ISP Backbone Lab Course Next: Module 2: MPLS

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Network Diagram

IS-IS Level 2 domain — all P and PE routers with NET addresses and link subnets

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Why IS-IS and Not OSPF?

Every major ISP on the planet runs IS-IS as their IGP (Interior Gateway Protocol). Here's why:

1. IS-IS runs on Layer 2 — it doesn't need IP to function. OSPF runs on top of IP. This means IS-IS is more resilient; if your IP config is broken, IS-IS still forms adjacencies.
2. Protocol-agnostic — IS-IS carried IPv4, IPv6, and MPLS labels long before OSPF could. It was designed to carry any protocol (it originally carried CLNS).
3. Scales better — IS-IS uses a flat TLV (Type-Length-Value) structure, making it trivially extensible. Adding Segment Routing support to IS-IS was easy. Adding it to OSPF required new LSA types and was messy.
4. Faster convergence — IS-IS partial route calculations (PRC) are more efficient than OSPF's.
5. Convention — When everyone uses IS-IS, interop is easier. It's the industry standard for SP networks.

IS-IS Key Concepts

Levels:

• Level 1 (L1) = Intra-area routing (like OSPF's intra-area routes)
• Level 2 (L2) = Inter-area / backbone routing (like OSPF Area 0)
• Level 1-2 = A router that participates in both (most PE routers)

For our ISP: Everything runs Level 2 only. This is standard practice in SP networks. We're one big backbone — no need for L1 areas. Keeps it simple and fast.

NET (Network Entity Title): This is IS-IS's address format. It looks weird but it's simple:

49.0001.0000.0000.0001.00
│    │    │              │
│    │    └──────────────┘── System ID (unique per router, often based on loopback IP)
│    └── Area ID
└── AFI (49 = private, always use this in labs)

Metric: IS-IS uses a flat metric (default: 10 on every link). We'll use wide metrics (mandatory for Segment Routing) and set costs based on link speed to influence traffic paths.

Lab 1 Config: IS-IS on the Core

Goal: Full IS-IS adjacency across all P and PE routers. Every router can ping every other router's loopback.

Addressing Plan:

Router	Loopback0	IS-IS NET	Role
P1	10.0.0.1/32	49.0001.0000.0000.0001.00	Core P
P2	10.0.0.2/32	49.0001.0000.0000.0002.00	Core P
P3	10.0.0.3/32	49.0001.0000.0000.0003.00	Core P
P4	10.0.0.4/32	49.0001.0000.0000.0004.00	Core P
P-CORE	10.0.0.5/32	49.0001.0000.0000.0005.00	Core P / RR
PE-EDGE1	10.0.0.11/32	49.0001.0000.0000.0011.00	PE (AS65000 border)
PE-EDGE2	10.0.0.12/32	49.0001.0000.0000.0012.00	PE (AS65000 cust)
PE-EDGE3	10.0.0.13/32	49.0001.0000.0000.0013.00	PE (AS65100 border)
PE-EDGE4	10.0.0.14/32	49.0001.0000.0000.0014.00	PE (AS65100 cust)

Link Addressing (point-to-point, /30s):

Link	Subnet	Router A IP	Router B IP
P1 — PE-EDGE1	10.1.1.0/30	.1	.2
P1 — P-CORE	10.1.1.4/30	.5	.6
P1 — P2	10.1.1.8/30	.9	.10
P2 — PE-EDGE2	10.1.1.12/30	.13	.14
P2 — P-CORE	10.1.1.16/30	.17	.18
P3 — PE-EDGE3	10.1.1.20/30	.21	.22
P3 — P-CORE	10.1.1.24/30	.25	.26
P3 — P4	10.1.1.28/30	.29	.30
P4 — PE-EDGE4	10.1.1.32/30	.33	.34
P4 — P-CORE	10.1.1.36/30	.37	.38
PE-EDGE1 — IXP	172.16.0.0/24	.1	—
PE-EDGE3 — IXP	172.16.0.0/24	.3	—

Sample Config — P1 (IOS-XE / IOSv):

hostname P1
!
interface Loopback0
 ip address 10.0.0.1 255.255.255.255
 ip router isis YOURSP
!
interface GigabitEthernet0/1
 description TO PE-EDGE1
 ip address 10.1.1.1 255.255.255.252
 ip router isis YOURSP
 isis network point-to-point
 isis metric 10
 no shutdown
!
interface GigabitEthernet0/2
 description TO P-CORE
 ip address 10.1.1.5 255.255.255.252
 ip router isis YOURSP
 isis network point-to-point
 isis metric 10
 no shutdown
!
interface GigabitEthernet0/3
 description TO P2
 ip address 10.1.1.9 255.255.255.252
 ip router isis YOURSP
 isis network point-to-point
 isis metric 10
 no shutdown
!
router isis YOURSP
 net 49.0001.0000.0000.0001.00
 is-type level-2-only
 metric-style wide
 log-adjacency-changes
 passive-interface Loopback0

Verification Commands

show isis neighbors          ! Are adjacencies UP?
show isis database detail    ! What LSPs do we have?
show ip route isis           ! Are all loopbacks in the table?
ping 10.0.0.5 source 10.0.0.1  ! Can P1 reach P-CORE?
show isis topology           ! Visual of the IS-IS graph

Understanding Check

Before moving on, you should be able to answer:

1. Why does the ISP use Level 2 only?
2. What's the System ID in the NET, and why do we derive it from the loopback?
3. Why isis network point-to-point on every link?
4. What happens if you forget metric-style wide? (Hint: Segment Routing won't work)

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Next Module: Module 2: MPLS — Labeling the Backbone →