Terminology
OSPF | ISIS |
Host | End System (ES) |
Router | Intermediate System (IS) |
Link | Circuit |
Packet | Protocol Data Unit (PDU) |
Designated router (DR) | Designated IS (DIS) |
Backup DR (BDR) | N/A (no BDIS is used) |
Link-state advertisement (LSA) | Link-state PDU (LSP) |
Hello packet | IIH PDU |
Database Description (DBD) | Complete Sequence Number PDU (CSNP) |
LS update | LSP (ISIS runs over layer-2) |
LS acknowledgement | Partial Sequence Number PDU (PSNP) |
Area | Subdomain (area) |
Non-backbone area | Level-1 area |
Backbone area | Level-2 area |
Area Border Router (ABR) | L1L2 router |
Virtual link | Virtual link (not used though) |
AS Boundary Router (ASBR) | any IS |
Router ID | System ID |
Link-state ID | N/A |
Advertising router ID | N/A |
Packets
OSPF: 5 types of basic packets | ISIS: 3 types of basic packets |
1. Hello | 1. Hello (3 types L1 LAN, L2 LAN, Point-to-point) |
2. DBD | 2. Link state packet (L1, L2) |
3. LS Request | 3. Sequence number packet (CSNP, PSNP) |
4. LS Update | |
5. Link State Ack |
LSAs
OSPF LSAs | ISIS LSPs • Up to 256 LSPs per IS | |||
Type | LSA | |||
1 | Router | TLV | Purpose | |
2 | Network | 2 | Neighbor announcement | |
3 | Summary Network | 10 | Authentication | |
4 | Summary ASBR | 22 | Extended neighbor info(TE) | |
5 | External | 128 | Internal IP Routing info | |
6 | Group Membership | 129 | NLPID announcement (IP) | |
7 | NSSA | 130 | External IP Routing info | |
8 | External Attributes | 132 | IP Interface addresses | |
9-11 | Opaque | 135 | Wide scale metrics | |
Adjacency Establishment
OSPF | ISIS |
LSDB synchronization is performed before a neighbor is reported in the router-LSA | Adjacency is reported once two-way connectivity has been ensured |
On point-to-point links adjacencies are established between every pair of neighbors that can see each other | Point-to-point links are treated the same way as in OSPF |
On LAN segments adjacencies are established with the DR and BDR | On LAN segments, adjacencies are established with the DIS (no BDIS is elected) |
MTU mismatch is detected | MTU mismatch is detected |
OSPF | ISIS |
Links and areas
OSPF | ISIS |
In OSPF link can be only in one area, and routers must agree on area ID | In ISIS, if routers do not agree on area ID, they form L2 adjacency |
Area borders cross routers in OSPF | Area borders cross links in ISIS |
In ISIS, link can be associated with a L1 and a L2 area simultaneously |
