notes: OSPF Overview & Peering

- Uses own transport protocol: 89
- Supports equal-cost load balancing for more efficient use. (The amount is not an OSPF limitation instead this set by the vendor depending on their hardware platform. For most IOS versions this is limited to either 6 or 8 paths).
- Supports the use of route tagging for the tracking of external routes.
- OSPF packets are exchanged only between neighbors on a network. They are never routed beyond the network on which they originated.
- OSPF multicast packets use a TTL of 1.

- OSPF sees secondary networks as stub networks and therefore will not send hellos on them. Consequently, no adjacencies can be established on secondary networks.

- Classless with variable: Length subnet mask (VLSM) support.
- Uses multicasts (224.0.0.5–all shortest path first [SPF] routers; 224.0.0.6: Designated Router [DR]/ Backup Designated Router [BDR]) for hellos and updates.
- Plain text and Message Digest Algorithm 5 (MD5) authentication available. Null authentication is the default.
- Dijkstra’s algorithm is used to produce a shortest-path tree for each destination. Link-state advertisements are used to build a database of the topology.

OSPF Peering

in order to form a neighbor relationship the following must be satisfied.

- timers must match
- mtu size must match
- area must match
- stub flag must match (e-bit)
- network must match ( except for point-to-point and virtual links)
note: changing hello will automatically adjust dead time.

Peering considerations (troubleshooting)

-  show ip ospf neigbhor
-  show ip ospf interface
- debug ip ospf adjacencies

if you notice Extart & peer keeps on dropping too many

DBD retransmission
- most likely mtu mistmatch.
- usually in router to switch connection
- switch higher mtu (for tagging)
3560 - "system mtu routing"
3550 - no option
router - "ip ospf mtu ignore"