Routers that act as gateways (redistribution) between OSPF and other routing protocols (IGRP, EIGRP, IS-IS, RIP, BGP, Static) or other instances of the OSPF routing process are called autonomous system boundary router (ASBR). Any router can be an ABR or an ASBR.
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The notation for these routes is O IA in the IP routing table. Routes that originate from other routing protocols (or different OSPF processes) and that are injected into OSPF via redistribution are called external routes.
These routes are represented by O E2 or O E1 in the IP routing table. Multiple routes to the same destination are preferred in this order: intra-area, inter-area, external E1, external E2. External types E1 and E2 are explained later.
Redistribute routes into OSPF from other routing protocols or from static causes these routes to become OSPF external routes. To redistribute routes into OSPF, use this command in router configuration mode:
Distribute-list out works on the ASBR to filter redistributed routes into other protocols. Distribute-list in works on any router to prevent routes from the routing table, but it does not prevent link-state packets from propagation; downstream routers would still have the routes.
Split Horizon: The split horizon is a method employed by distance vector protocols to prevent network routing loops. The underlying premise is straightforward: never send routing information back in the same direction it came from. It is necessary to have a split-horizon because distance vector protocols like Routing Information Protocol (RIP) are prone to routing loops, which occur when a data packet is caught in an unending loop and routed through the same routers over and over again. Split horizon is frequently used in protocols to avoid loops. Different strategies are used to prevent packet looping in other protocols, such as Open Shortest Path First.
BGP peers are two routers that have established a link for exchanging BGP information. Such BGP peers provide routing information via TCP-based BGP sessions, which are dependable, connection-oriented, and error-free protocols.
Enhanced Interior Gateway Routing Protocol (EIGRP): If two routers in the same area exist, EIGRP is used to share information between them. It's also a complicated protocol, but it's simple to set up and use in both small and big networks. It's also a hybrid protocol, as it combines elements of both distance vector and link-state routing protocols.
A routing protocol is a set of rules that specify how routers identify and forward packets along a network path. Routing protocols are grouped into two distinct categories: interior gateway protocols and exterior gateway protocols.
Routing algorithms are software programs that implement different routing protocols. They work by assigning a cost number to each link; the cost number is calculated using various network metrics. Every router tries to forward the data packet to the next best link with the lowest cost.
File Transfer Protocol. FTP is a client-server protocol, with which a client requests a file and the server supplies it. FTP runs over TCP/IP -- a suite of communications protocols -- and requires a command channel and a data channel to communicate and exchange files, respectively. Clients request files through the command channel and receive access to download, edit and copy the file, among other actions, through the data channel.
Dynamic routing protocols, as their name suggests, are used to dynamically exchange routing information between routers. Their implementation allows network topologies to dynamically adjust to changing network conditions, and to ensure that efficient and redundant routing continues in spite of any changes. In addition, they are invaluable to the management, administration, and configuration of networks as relatively low administrative overhead is required to configure highly complex routing scenarios. Compared to statically configuring routing in topology, the implementation of dynamic routing protocols vastly improves the scalability of networks.
The first dynamic routing protocol was the Exterior Gateway Protocol (EGP), introduced in 1982 by Eric C. Rosen. Since then, many more advanced protocols have been developed and have matured over the years, protocols that we will cover extensively in this series of articles.
Dynamic routing protocols were designed to address the previously mentioned shortcomings of static routing such as the need for human involvement to route traffic around failures, the human mistakes made when typing route information, and the scaling limit of the few routes one person can track in a text file. These benefits come at the expense of requiring significant computing power in the routers, and the need for training network administrators who specialize in taming routing algorithms.
The dynamic routing protocols fall into one of two categories: interior gateway protocols (IGPs) and exterior gateway protocols (EGPs). Generally speaking, an interior gateway protocol operates within a particular Autonomous System (AS), while an exterior gateway protocol operates between ASes. An autonomous system is a set of routers under a common administration with common routing policies.
Different routing protocols use different ways to calculate metrics. The metric calculated by one routing protocol cannot be compared to the metric calculated by another. Different routing protocols might not choose the same best path because of how they calculate their metrics.
Some protocols use just a single variable to calculate metrics, while others may use a combination of several. For instance, RIPv2, which is a distance-vector routing protocol, uses as a metric the hop count, which is how many routers exist between the router and the remote network.
In this case, the router uses administrative distance (AD) to determine which is the best path. The AD can be seen as the trustiness of the source of information, and a lower value is a better value. It ranges from 0 and 255, with 0 being for the connected routes and 255 for unreachable routes. Each vendor has values assigned to the routing protocols, which might or might not be identical to the values assigned by other vendors.
Although it is preferable to use a single routing protocol in the network, this is not always possible. Reasons may include mergers of entities or routing devices that do not support a given routing protocol, or the implementation of different protocols for different purposes. For example, OSPF may be used within an AS, and BGP may be used between ASes.
Route redistribution occurs when the routes learned through one protocol are advertised in another routing protocol on one or more routers. To perform route redistribution, the router must run both routing protocols, and the routes must be in the routing table.
As you can see, every routing protocol has its own positive and negative and situations where they work best. Because of this, many organizations run multiple routing protocols on the same network and use route redistribution techniques to play them alongside each other. 2ff7e9595c
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