Friday, July 25, 2008

OSPF, some useful info

OSPF is a Link-Stat routing protocol, which relies on the Djikestra algorithm to calculate all of its tables (routing table ... etc), here are some of its key specifications:

* There is a property called Adjacency, which is the relationship between two or more routers that its formation is a condition to begin exchange routing information, there are variables must match for that adjacency to form, and other variables must be unique:
-- Must match:
- Area number
- Hello and dead timers
- Mtu (the maximum transmission unit)
. ip ospf mtu-ignore - in case we want to ignore this variable
- Compatible network types
- Stub flags
- Authentication

-- Must be unique
- IP address
- OSPF router-id

OSPF network types:
-- Types
- BC (broadcast network)
- Non-BC (non-broadcast network)
- P2P (point-to-point network)
- P2MultiP (point-to-multi-point network)
- P2MultiP Non-BC (point-to-multi-point non-broadcast)

Network types affects the following:
- Unicast/ Multicast
- DR/BDR election
- Next-hop processing


Broadcast network type:
- Default on broadcast medias - Ethernet/Token Ring
- Sends hellos as multicast -
- Performs DR/BDR election

DR/BDR election:
- Used to minimize LSA replications
. Like BGP router-reflector
- Election based on priority and router-id
. 0-255
. Higher is better
. 0=never prompt the election
- Router-id - higher wins
. Highest loopback/interface IP
. Can be statically set
. higher is better

Non-broadcast network
- Default on multi-point NBMA medias
. Frame relay / ATM
- Sends hellos as unicast
. static "neighbor" command is necessary
- Performs DR/BDR election

- Non a default option
- Sends hellos as multicast
- No DR/BDR election happens
- Modifies next-hop processing
- Best option for NBMA

Point-to-multipoint non-broadcast
- Not a default option
- Sends hellos as unicast
. Static "neighbor" statement
- No DR/BDR election
- Modifies next-hop processing

OSPF Convergence Timers
- Convergence based on hello and dead timers.
- Different timers for different network types
- Changing hello time automatically adjusts dead time
. ip ospf hello-interval
. ip ospf dead-interval
- New feature: sub-second convergence

Changing the network type will cause adjacency to form but no routing information will be exchanged.
Broadcast -------
|--> will adjacent and will exchange routing information
Non-broadcast ---
|------> will adjacent but will not exchange routing information
Point-to-Point ----------------
Point-to-Multipoint |--> Will adjacent and will exchange routing information
Point-to-Multipoint Non-Broadcast ----

Kind regards,

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