OSPF General information/spec

1. Area
for scalability
the size of an area is measured with number of routers and the size of the link-state database

2. Process-id
Local significant, gak harus sama tiap ruter.

3. Router-id




2. OSPF metric
bandwidth
auto-cost reference-bandwidth


3. OSPF packet types:
Hello
Database Description (DD)
Link-state request
Link-state update: used interchangebly with term LSA
Link-state acknowledgment

Each packet serves a specific purpose.
Hello: Used to establish adjacency and maintain adjacency with other OSPF router
DBD: descriptor / teaser of the sending router's link-state database and is used by receiving routers to check against the local link-state database
LSR: Receiving routers can then request more information about any entry in the DBD by sending a Link-state Request (LSR)
LSU: Link-state update used to reply LSR; to announce new information (routing updates)
LSAck: the router sends a link-state acknowledgement (LSAck) to confirm receipt of the LSU

4. Dissecting hello packet
Type, RouterID, AreaID, Network mask, Hello Interval, Router priority, Designated Router(DR), Backup designated router(BDR), List of neighbors.

5. Process of OSPF router

Processes (before sending/flood its link-states to other routers):
1. Determine if there are any neighbor on those links
Receiving an OSPF hello packet on an interface confirms for a router that there is another OSPF router on this link.
2. Establish adjacency
Before 2 routers can form an OSPF neighbor adjacency, they must agree on three values: hello interval, dead interval, and network type.
3. Keeps sending hello packet to check apakah neighbor masih hidup / mati?
4. Floods the link-state information about the "down" neighbor out all OSPF enabled interface.



6. OSPF Hello intervals and dead interval
OSPF hello interval indicate how often OSPF router transmit its hello packets.
10 seconds : multi-access network and point-to-point networ
30 seconds: NBMA(non-broadcast multiaccess) frame-relay, X.25, ATM

OSPF dead interval: 4 kali hello interval

7. Network type
- Point-to-point
- Broadcast multiaccess
- Nonbroadcast multiaccess (NBMA)
- Point-to-multipoint
- Virtual links

8. OSPF interface priority
Digunakan pada proses pemilihan DR dan BDR
DR Router: ruter dengan interface priority yang lebih tinggi
BDR: next tertinggi

Default nilai ospf interface priority = 1



8. Link-state database

If the Dead interval expires before the routers receive a Hello packet, OSPF will remove that neighbor from its link-state database.


10. OSPF algorithm

Any time a router receives new information about the topology (addition, deletion, or modification of a link), the router must rerun the SPF algorithm, create a new SPF tree, and update the routing table.

11. Tools show-show:
show ip protocols
show ip ospf
show ip ospf interface
show ip ospf neighbors:
- Untuk melihat siapa saja yang telah menjadi neighbor / tetangga
- Untuk melihat dead time counter

12. DR/BDR election

Bagaimana ruter DR atau ruter BDR terpilih? Dibawah ini kriteria yang dipakai dalam pemilihan ruter DR/BDR pada jaringan multiaccess(Ethernet):
1. DR: Router with the highest OSPF interface priority.
2. BDR: Router with the second highest OSPF interface priority.
3. If OSPF interface priorities are equal, the highest router ID is used to break the tie.


show ip protocols:

Adjacency troubleshooting:
Two routers may not form an OSPF adjacency if:
The subnet masks do not match, causing the routers to be on separate networks.
OSPF Hello or Dead Timers do not match.
OSPF Network Types do not match.
There is a missing or incorrect OSPF network command.