Comparison
OSPFv2 versus IS-IS
The same shortest-path idea, two different machines. The differences that bite are election, adjacency, metrics and how an area actually bounds flooding.
Side-by-side walkthrough
One shared step index advances both scenarios. Share the URL — ?step= is part of the link.
OSPFv2
Three routers on one Ethernet segment, OSPF just enabled. Every neighbor table is empty. Nothing has been elected, and nothing has been flooded. R1 · 1.1.1.1: Neighbor state Down. R2 · 2.2.2.2: Neighbor state Down. Broadcast segment. R3 · 3.3.3.3: Neighbor state Down.
- Link
- Blocking
- Packet in flight
- Discarded
- Emphasis
Text equivalent of this diagram
| Element | Kind | State |
|---|---|---|
| R1 · 1.1.1.1 | router | Neighbor state: Down |
| R2 · 2.2.2.2 | router | Neighbor state: Down |
| Broadcast segment | switch | — |
| R3 · 3.3.3.3 | router | Neighbor state: Down |
| R1 · 1.1.1.1 — Broadcast segment | link | up · pri 1 |
| R2 · 2.2.2.2 — Broadcast segment | link | up · pri 1 |
| R3 · 3.3.3.3 — Broadcast segment | link | up · pri 1 |
Three routers on one Ethernet segment, OSPF just enabled. Every neighbor table is empty. Nothing has been elected, and nothing has been flooded.
What changed
- R1 · 1.1.1.1: Neighbor state → Down
- R2 · 2.2.2.2: Neighbor state → Down
- R3 · 3.3.3.3: Neighbor state → Down
- No DR yet
IS-IS
R3 is DIS. Its pseudonode LSP is in every database. OSPF instincts say a newcomer will sit quietly. Those instincts are about to be wrong. R1 · pri 64. R3 · DIS · pri 80: Role DIS. LAN.
- Link
- Blocking
- Packet in flight
- Discarded
- Emphasis
Text equivalent of this diagram
| Element | Kind | State |
|---|---|---|
| R1 · pri 64 | router | — |
| R3 · DIS · pri 80 | router | Role: DIS |
| LAN | switch | — |
| R1 · pri 64 — LAN | link | up · L1 |
| R3 · DIS · pri 80 — LAN | link | up · L1 |
R3 is DIS. Its pseudonode LSP is in every database. OSPF instincts say a newcomer will sit quietly. Those instincts are about to be wrong.
What changed
- Emphasis: DIS
- R3 · DIS · pri 80: Role → DIS
- Hidden: R4 · pri 127
Side by side
| Mechanism | OSPFv2 | IS-IS |
|---|---|---|
| What floods | LSAs, typed 1–5 (and 7, 9–11). Flooding scope is per type: some stay in the area, Type-5 leaves it. LS typeRFC 2328 · INTERNET STANDARD · April 1998 | LSPs, one (or more fragments) per IS per level. A TLV inside the LSP carries the prefix; the PDU is not typed the way an LSA is. Multiple LSPs and SNPsRFC 1195 · PROPOSED STANDARD · December 1990 |
| LAN election | DR and BDR. A higher-priority latecomer does not preempt. DROTHERs are adjacent only to DR and BDR. Electing the Designated RouterRFC 2328 · INTERNET STANDARD · April 1998 | DIS. There is no Backup DIS. A higher-priority arrival takes over immediately. Adjacencies are still to every IS on the LAN; the DIS originates the pseudonode LSP. LANs, Designated Routers, and PseudonodesRFC 1195 · PROPOSED STANDARD · December 1990 |
| Adjacency | Hello matching (mask, timers, area, E-bit) then DBD exchange through ExStart / Exchange / Loading / Full. MTU is in the DBD, not the Hello. Neighbor statesRFC 2328 · INTERNET STANDARD · April 1998 | LAN Hello three-way by seeing yourself in the neighbour TLV. Point-to-point uses RFC 5303 handshake with an extended circuit ID. HandshakingRFC 5303 · PROPOSED STANDARD · October 2008 |
| Areas | Area 0 is the backbone. Every ABR must attach to it (or use a virtual link). Stub and NSSA change which LSA types may enter. The backbone of the Autonomous SystemRFC 2328 · INTERNET STANDARD · April 1998 | Level-1 is the area, Level-2 is the backbone. An L1L2 IS is the ABR analogue. L1 routers use a default toward attached L1L2 ISes. Overview of the ISO IS-IS ProtocolRFC 1195 · PROPOSED STANDARD · December 1990 |
| Metric | 16-bit interface cost, typically reference-bandwidth / interface-bandwidth. Default reference is 100 Mb/s, so every faster link costs 1 unless the reference is raised. Calculating the shortest-path tree for an areaRFC 2328 · INTERNET STANDARD · April 1998 | Narrow metric is 6 bits (max 63) in the original neighbour TLV. Wide metric (RFC 5305) is 24 bits in TLV 22. Running both is how two ISes silently disagree about a path. The Extended IS Reachability TLVRFC 5305 · PROPOSED STANDARD · October 2008 |
| Transport | IP protocol 89. Hellos go to 224.0.0.5 / 224.0.0.6. The Router ID is an IPv4 address that need not be configured on an interface. Routing protocol packetsRFC 2328 · INTERNET STANDARD · April 1998 | CLNS, not IP. Hellos use 01:80:C2:00:00:14 / :15. The IS is addressed by a NET (area + System ID + NSEL), which is why a missing NET is a missing process, not a missing address. Addressing Routers in IS-IS PacketsRFC 1195 · PROPOSED STANDARD · December 1990 |
| SPF | Dijkstra over the area’s Router- and Network-LSAs, then Type-3, then Type-5. RFC 2328 section 16.1. Calculating the shortest-path tree for an areaRFC 2328 · INTERNET STANDARD · April 1998 | The same algorithm over the LSP database for that level. Preference between L1, L2 and leaked prefixes is a separate order, not a different SPF. Order of Preference of Routes / Dijkstra ComputationRFC 1195 · PROPOSED STANDARD · December 1990 |
What does not carry over
Carry OSPF’s DR instincts onto an IS-IS LAN and the DIS “flapping” you see is the protocol working: IS-IS preempts, OSPF does not. Carry IS-IS’s “just raise the priority” onto OSPF and nothing happens until the current DR dies.
Both compute a shortest-path tree. The atlas uses the same SPF stepper for both. What differs is the database you feed it, and the election that decided who originated the LAN’s node.
IS-IS areas are levels, not OSPF area IDs with a reserved zero. Numbering an IS-IS area 0.0.0.0 because OSPF wanted a backbone is a configuration that happens to parse and then does not mean what the author thought.