OSPF: Open Shortest Path
First Protocol (OSPFv2)
OSPF is an interior gateway
protocol used for routing between routers belonging to a single Autonomous
System. OSPF uses link-state technology in which routers send each other
information about the direct connections and links which they have to other
routers. Each OSPF router maintains an identical database describing the
Autonomous System's topology. From this database, a routing table is calculated
by constructing a shortest- path tree. OSPF recalculates routes quickly in the
face of topological changes, utilizing a minimum of routing protocol traffic.
OSPF provides support for equal-cost multi-path. An area routing capability is
provided, enabling an additional level of routing protection and a reduction in
routing protocol traffic. In addition, all OSPF routing protocol exchanges are
authenticated.
OSPF has been designed
expressly for the TCP/IP internet environment, including explicit support for
CIDR and the tagging of externally-derived routing information. OSPF also
provides for the authentication of routing updates, and utilizes IP multicast
when sending/receiving the updates.
OSPF routes IP packets
based solely on the destination IP address found in the IP packet header. IP
packets are routed "as is" - they are not encapsulated in any further
protocol headers as they transit the Autonomous System.
OSPF allows sets of
networks to be grouped together. Such a grouping is called an area. The
topology of an area is hidden from the rest of the Autonomous System. This
information hiding enables a significant reduction in routing traffic. Also,
routing within the area is determined only by the area's own topology, lending
the area protection from bad routing data.
OSPF enables the
flexible configuration of IP subnets. Each route distributed by OSPF has a
destination and mask. Two different subnets of the same IP network number may
have different sizes (i.e., different masks). This is commonly referred to as
variable length subnetting. A packet is routed to the best (i.e., longest or
most specific) match.
Protocol Structure - OSPF (Open Shortest Path First version 2)
8
|
16
|
32 bit
|
Version No.
|
Packet Type
|
Packet length
|
Router ID
|
||
Area ID
|
||
Checksum
|
AuType
|
|
Authentication (64 bits)
- Version number - Protocol version number (currently 2).
- Packet type - Valid types are as follows:
- 1 : Hello
- 2 : Database Description
- 3 : Link State Request
- 4 : Link State Update
- 5 : Link State Acknowledgment.
- Packet length - The length of the protocol packet in
bytes. This length includes the standard OSPF header.
- Router ID - The router ID of the packet's source. In
OSPF, the source and destination of a routing protocol packet are the two
ends of an (potential) adjacency.
- Area ID - identifying the area that this packet belongs
to. All OSPF packets are associated with a single area. Most travel a
single hop only.
- Checksum - The standard IP checksum of the entire
contents of the packet, starting with the OSPF packet header but excluding
the 64-bit authentication field.
- AuType - Identifies the authentication scheme to be
used for the packet.
Authentication - A 64-bit
field for use by the authentication scheme
