Introduction
The Open Shortest Path First (OSPF) protocol, defined in RFC 2328 , is an Interior Gateway Protocol used to distribute routing information within a single Autonomous System. This paper examines how OSPF works and how it can be used to design and build today's large and complicated networks.
Background Information
OSPF protocol was developed due to a need in the internet community to introduce a high functionality non-proprietary Internal Gateway Protocol (IGP) for the TCP/IP protocol family. The discussion of creating a common interoperable IGP for the Internet started in 1988 and did not get formalized until 1991. At that time the OSPF Working Group requested that OSPF be considered for advancement to Draft Internet Standard.
The OSPF protocol is based on link-state technology which is a departure from the Bellman-Ford vector based algorithms used in traditional Internet routing protocols such as RIP. OSPF has introduced new concepts such as authentication of routing updates, Variable Length Subnet Masks (VLSM), route summarization, etc.
OSPF versus RIP
The rapid growth and expansion of today's networks has pushed RIP to its limits. RIP has certain limitations that could cause problems in large networks:
RIP has a limit of 15 hops. A RIP network that spans more than 15 hops (15 routers) is considered unreachable.
RIP cannot handle Variable Length Subnet Masks (VLSM). Given the shortage of IP addresses and the flexibility VLSM gives in the efficient assignment of IP addresses, this is considered a major flaw.
Periodic broadcasts of the full routing table will consume a large amount of bandwidth. This is a major problem with large networks especially on slow links and WAN clouds.
RIP converges slower than OSPF. In large networks convergence gets to be in the order of minutes. RIP routers will go through a period of a hold-down and garbage collection and will slowly time-out information that has not been received recently. This is inappropriate in large environments and could cause routing inconsistencies.
RIP has no concept of network delays and link costs. Routing decisions are based on hop counts. The path with the lowest hop count to the destination is always preferred even if the longer path has a better aggregate link bandwidth and slower delays.
RIP networks are flat networks. There is no concept of areas or boundaries. With the introduction of classless routing and the intelligent use of aggregation and summarization, RIP networks seem to have fallen behind.
Some enhancements were introduced in a new version of RIP called RIP2. RIP2 addresses the issues of VLSM, authentication, and multicast routing updates. RIP2 is not a big improvement over RIP (now called RIP 1) because it still has the limitations of hop counts and slow convergence which are essential in todays large networks.
OSPF, on the other hand, addresses most of the issues presented above:
With OSPF, there is no limitation on the hop count.
The intelligent use of VLSM is very useful in IP address allocation.
OSPF uses IP multicast to send link-state updates. This ensures less processing on routers that are not listening to OSPF packets. Also, updates are only sent in case routing changes occur instead of periodically. This ensures a better use of bandwidth.
OSPF has better convergence than RIP. This is because routing changes are propagated instantaneously and not periodically.
OSPF allows for better load balancing.
OSPF allows for a logical definition of networks where routers can be divided into areas. This will limit the explosion of link state updates over the whole network. This also provides a mechanism for aggregating routes and cutting down on the unnecessary propagation of subnet information.
OSPF allows for routing authentication by using different methods of password authentication.
OSPF allows for the transfer and tagging of external routes injected into an Autonomous System. This keeps track of external routes injected by exterior protocols such as BGP.
This of course would lead to more complexity in configuring and troubleshooting OSPF networks. Administrators that are used to the simplicity of RIP will be challenged with the amount of new information they have to learn in order to keep up with OSPF networks. Also, this will introduce more overhead in memory allocation and CPU utilization. Some of the routers running RIP might have to be upgraded in order to handle the overhead caused by OSPF.
What Do We Mean by Link-States?
OSPF is a link-state protocol. We could think of a link as being an interface on the router. The state of the link is a description of that interface and of its relationship to its neighboring routers. A description of the interface would include, for example, the IP address of the interface, the mask, the type of network it is connected to, the routers connected to that network and so on. The collection of all these link-states would form a link-state database.
Link-State Algorithm
OSPF uses a link-state algorithm in order to build and calculate the shortest path to all known destinations. The algorithm by itself is quite complicated. The following is a very high level, simplified way of looking at the various steps of the algorithm:
Upon initialization or due to any change in routing information, a router will generate a link-state advertisement. This advertisement will represent the collection of all link-states on that router.
All routers will exchange link-states by means of flooding. Each router that receives a link-state update should store a copy in its link-state database and then propagate the update to other routers.
After the database of each router is completed, the router will calculate a Shortest Path Tree to all destinations. The router uses the Dijkstra algorithm to calculate the shortest path tree. The destinations, the associated cost and the next hop to reach those destinations will form the IP routing table.
In case no changes in the OSPF network occur, such as cost of a link or a network being added or deleted, OSPF should be very quiet. Any changes that occur are communicated via link-state packets, and the Dijkstra algorithm is recalculated to find the shortest path.
Shortest Path Algorithm
The shortest path is calculated using the Dijkstra algorithm. The algorithm places each router at the root of a tree and calculates the shortest path to each destination based on the cumulative cost required to reach that destination. Each router will have its own view of the topology even though all the routers will build a shortest path tree using the same link-state database. The following sections indicate what is involved in building a shortest path tree.
OSPF Cost
The cost (also called metric) of an interface in OSPF is an indication of the overhead required to send packets across a certain interface. The cost of an interface is inversely proportional to the bandwidth of that interface. A higher bandwidth indicates a lower cost. There is more overhead (higher cost) and time delays involved in crossing a 56k serial line than crossing a 10M ethernet line. The formula used to calculate the cost is:
cost= 10000 0000/bandwith in bps
For example, it will cost 10 EXP8/10 EXP7 = 10 to cross a 10M Ethernet line and will cost 10 EXP8/1544000 = 64 to cross a T1 line.
By default, the cost of an interface is calculated based on the bandwidth; you can force the cost of an interface with the ip ospf cost
Enabling OSPF on the Router
Enabling OSPF on the router involves the following two steps in config mode:
Enabling an OSPF process using the router ospf
Assigning areas to the interfaces using the network
The OSPF process-id is a numeric value local to the router. It does not have to match process-ids on other routers. It is possible to run multiple OSPF processes on the same router, but is not recommended as it creates multiple database instances that add extra overhead to the router.
The network command is a way of assigning an interface to a certain area. The mask is used as a shortcut and it helps putting a list of interfaces in the same area with one line configuration line. The mask contains wild card bits where 0 is a match and 1 is a "do not care" bit, e.g. 0.0.255.255 indicates a match in the first two bytes of the network number.
OSPF Authentication
It is possible to authenticate the OSPF packets such that routers can participate in routing domains based on predefined passwords. By default, a router uses a Null authentication which means that routing exchanges over a network are not authenticated. Two other authentication methods exist: Simple password authentication and Message Digest authentication (MD-5).
Simple Password Authentication
Simple password authentication allows a password (key) to be configured per area. Routers in the same area that want to participate in the routing domain will have to be configured with the same key. The drawback of this method is that it is vulnerable to passive attacks. Anybody with a link analyzer could easily get the password off the wire. To enable password authentication use the following commands:
ip ospf authentication-key key (this goes under the specific interface)
area area-id authentication (this goes under "router ospf
The area-id is the area number we want the interface to be in. The area-id can be an integer between 0 and 4294967295 or can take a form similar to an IP address A.B.C.D.
Message Digest Authentication
Message Digest authentication is a cryptographic authentication. A key (password) and key-id are configured on each router. The router uses an algorithm based on the OSPF packet, the key, and the key-id to generate a "message digest" that gets appended to the packet. Unlike the simple authentication, the key is not exchanged over the wire. A non-decreasing sequence number is also included in each OSPF packet to protect against replay attacks.
This method also allows for uninterrupted transitions between keys. This is helpful for administrators who wish to change the OSPF password without disrupting communication. If an interface is configured with a new key, the router will send multiple copies of the same packet, each authenticated by different keys. The router will stop sending duplicate packets once it detects that all of its neighbors have adopted the new key. Following are the commands used for message digest authentication:
ip ospf message-digest-key keyid md5 key (used under the interface)
area area-id authentication message-digest (used under "router ospf
OSPF and Route Summarization
Summarizing is the consolidation of multiple routes into one single advertisement. This is normally done at the boundaries of Area Border Routers (ABRs). Although summarization could be configured between any two areas, it is better to summarize in the direction of the backbone. This way the backbone receives all the aggregate addresses and in turn will injects them, already summarized, into other areas. There are two types of summarization:
Inter-area route summarization
External route summarization
Inter-Area Route Summarization
Inter-area route summarization is done on ABRs and it applies to routes from within the AS. It does not apply to external routes injected into OSPF via redistribution. In order to take advantage of summarization, network numbers in areas should be assigned in a contiguous way to be able to lump these addresses into one range. To specify an address range, perform the following task in router configuration mode:
area area-id range address mask
Where the "area-id" is the area containing networks to be summarized. The "address" and "mask" will specify the range of addresses to be summarized in one range.
External Route Summarization
External route summarization is specific to external routes that are injected into OSPF via redistribution. Also, make sure that external ranges that are being summarized are contiguous. Summarization overlapping ranges from two different routers could cause packets to be sent to the wrong destination. Summarization is done via the following router ospf subcommand:
summary-address ip-address mask
This command is effective only on ASBRs doing redistribution into OSPF
Summary
The OSPF protocol defined in RFC 1583, provides a high functionality open protocol that allows multiple vendor networks to communicate using the TCP/IP protocol family. Some of the benefits of OSPF are, fast convergence, VLSM, authentication, hierarchical segmentation, route summarization, and aggregation which are needed to handle large and complicated networks.
Ref : http://www.cisco.com/warp/public/104/1.html
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