OSPF routing protocol

Some facts

• Supports variable-length subnet masking (VLSM, CIDR)
• Reduces the routing overhead using areas
• Doesn't perform route autosummarization by default
• Supports manual route summarization
• OSPF allows only equal cost load balancing
• It is Interior Gateway Protocol(IGP)
• OSPF uses the SPF algorithm, conceived by a mathematician named Dijkstra.
• It supports IP and IPv6
• The area-id can be an integer between 0 and 4294967295
• Link layer
• Uses Dijkstra SPF algorithm
• Unlimited hop count
• OSPF is supposed to be designed in a hierarchical fashion, which basically means that you can separate the larger internetwork into smaller internetworks called areas. The following are reasons for creating OSPF in a hierarchical design:

1. To decrease routing overhead
2. To speed up convergence
3. To confine network instability to single areas of the network

• Uses areas, area 0 is a backbone area
• By default support 4 equal routes to one destination in routing table. It can also be configured with:

Router(config-router)#maximum-paths number

• Multicast addresses 224.0.0.5 (all SPF/link state routers, also known as AllSPFRouters) and 224.0.0.6 (all Designated Routers, AllDRouters) are reserved for OSPF (RFC 2328) and FF02::5, FF02::6 for IPv6
• The router types are attributes of an OSPF process. A given physical router may have one or more OSPF processes. OSPF defines the following router types:

1. Area border router (ABR) - Routers that connect other areas to the backbone area within an AS.ABR is a router that connects one or more OSPF areas to the main backbone network. It is considered a member of all areas it is connected to. An ABR keeps multiple copies of the link-state database in memory, one for each area to which that router is connected.


2. Autonomous system border router (ASBR) - An ASBR is a router that is connected to more than one AS and that exchanges routing information with routers in other ASs. ASBRs typically also run a non-IGP routing protocol (e.g., BGP), or use static routes, or both. An ASBR is used to distribute routes received from other ASs throughout its own AS.


3. Internal router (IR) - An IR is a router that has only OSPF neighbor relationships with routers in the same area.


4. Backbone router (BR)

• An area border router is always a backbone router, but a backbone router is not necessarily an area border router

Becoming neighbors

• To become neighbors:

1. The same area ID
2. Authentication – the same password
3. The same hello and dead intervals
4. Subnet mask and subnet

• Hello packets every 10 seconds, are addressed to 224.0.0.5
• Dead interval is 4 * Hello intervals - 40 seconds. “Down” in neighbor table

Neighbor states:

1. Down
2. Init
3. 2 way
4. Full

DR & BDR

• DR and BDR are elected on broadcast and non-broadcast multi-access networks
• To elect DR priority on interface is used(show ip ospf interface), if priority is identical, router ID is used
• Router with the highest OSPF priority setting becomes the DR.
• A priority setting of 0 means that the router does not participate in the election and can never become the DR or BDR.
• The range of priority values that allow a router to be a candidate are 1 through 255.

Router ID (RID) is:

• router-id rid OSPF subcommand OR
• The highest numeric IP address among the up/up loopback interfaces OR
• The highest IP address of logical interface configured on the router

Path cost

• Calculate routes based on Bandwidth
• OSPF router collects link-state information to construct the entire network topology of so-called “areas” from which it computes the shortest path tree for each route using a method based on Dijkstra's algorithm
• The cost of the entire path is the sum of the costs of the outgoing interfaces along the path
• Cisco uses a simple equation of 108/bandwidth(in bits)
• OSPF has an administrative distance of 110
• * In contrast to the Routing Information Protocol (RIP) or the Border Gateway Protocol (BGP), OSPF does not use TCP or UDP but uses IP directly, via IP protocol 89. OSPF handles its own error detection and correction, therefore negating the need for TCP or UDP functions
• The OSPF Protocol can operate securely between routers, optionally using a clear-text password or using MD5 to authenticate peers before forming adjacencies and before accepting link-state advertisements (LSA)
• As a link state routing protocol, OSPF establishes and maintains neighbour relationships in order to exchange routing updates with other routers. The neighbour relationship table is called an adjacency database in OSPF. Provided that OSPF is configured correctly, OSPF forms neighbour relationships only with the routers directly connected to it. The routers that it forms a neighbour relationship with must be in the same area as the interface with which it is using to form a neighbor relationship. An interface can only belong to a single area
• An OSPF network is divided into areas, which have 32-bit area identifiers commonly, but not always, written in the dotted decimal format of an IP address. Area identifiers are not IP addresses and may duplicate, without conflict, any IP address. While most OSPF implementations will right-justify an area number written in other than dotted decimal format (e.g., area 1), it is wise always to use dotted decimal formats. Most implementations would expand area 1 to the area identifier 0.0.0.1, but some have been known to expand it as 1.0.0.0. These are logical groupings of routers whose information may be summarized towards the rest of the network
• There are 4 types of areas: backbone, stub area, totally stubby area, not-so-stubby area
• Each router has a router identifier, customarily written in the dotted decimal format (e.g.: 1.2.3.4) of an IP address

Configuration

router ospf 1(zero is unacceptable)
Router(config-router)# network 192.168.10.0 0.0.0.255 area 0

Show commands

show ip ospf - display details of all OSPF routing processes enabled on a router.

show ip ospf interface - display interface-specific OSPF information.

show ip ospf neighbor - display all OSPF neighbors.

show ip ospf database - display all different OSPF route types that are currently known by the router.