Static routing tables are most efficient when there are a limited number of routers. If the network has only one(1) router, the correct configuration is to use a static default route. Use dynamic routing protocols when the routing environment of a network is changeable or complex. when there are multiple routers that can reach the same destinations.
Thus a Routing Protocol is a technique for defining routes and for disseminating routes.
Routing Protocols
Routing and routing protocols are not the same thing. All Linux systems make routing decisions, but very few systems run routing protocols.
Routing Protocols Functions
- They select the "best" route to a destination
- They communicate that route to other routers on the network
Type of Routing Protocols
Interior Protocols are used inside of a routing domain
- RIP - routing information protocol
- RIPv2 - routing information protocol version 2
- OSPF - open shortest path first
Exterior Protocols are used to exchange routing information between routing domain
- BGP - border gateway protocol
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Border Gateway Protocol
BGP is the only exterior routing protocol in widespread use. Supports policy-based routing, which allows you to define organizational or political reasons for choosing a route.
BGP is a path-vector protocol. The path vector provides an end-to-end list of every routing domain along the route, which allows you to decide whether or not you trust the advertisements that comes from those domains.
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Routing Information Protocol
RIP, is a widely used interior routing protocol. It is included as part of the operating system with many Linux and Unix systems, as well as the Windows NT/2000 RRAS package.
RIP defines the "best" route as being the lowest-cost route, which is the one with the lowest routing metrics. RIP calls each router a "hop" and the metric a "hop count". The best route to a destination is the one that passes through the fewest "hops". This technique for determining the best route is called a distance-vector algorithm.
Things to know ?
- RIP issues update packets every 30 seconds
- standard time-out is 180 seconds
- If a router stops issuing updates for 180 seconds, the other Routers on the network assume that it is DEAD and delete any routes that go through that router
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RIP version 2
RIPv2 enhances the original RIP packet with the addition of a network mask field and a "next hop address" field. The network mask is the bit mask that is used to determine the destination network.
The "next hop address" field provides the address of the gateway. In RIP, only the destination address and the metric are provided. The gateway is always assumed to be the router that sends out the update. The next hop address field specifically provides the address of the gateway, which allows the system that sends the update to be different from the gateway that will handle the route.
Thus, RIPv2-capable systems can provide updates for routers that don't run RIPv2. If the next hop address is 0.0.0.0, the router that sends the update is assumed to be the gateway for the route.
Other Information regading RIPv2
- RIPv2 is completely compatible with RIP, coz the packet format is unchanged
- RIPv2 uses multicasting instead of broadcasting to reduce the load on systems that don't want RIPv2 updates
- RIPv2 provides an authentication scheme that prevents routing updates from a misconfigured host from accidentally being accepted as valid
- RIPv2 and RIP routers can coexist on a single network without a problem
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Open Shortest Path First Protocol
OSPF is very different from RIP. A router running RIP sends information about the entire network to its neighbors. The router running OSPF floods information about its neighbors to the entire network. Flooding means that the router sends the update out of every network port, and every router that receives the update also sends it out of every port except the one it receives it on.
OSPF is called a link-state protocol because it creates a graph of the state of all of the links in the network and set itself as root. The graph is built using the Dijkstra Shortest Path First algorithm. Clearly, building a link-state graph for a large network every time a route changes creates a lot of overhead for the router.
For this reason, OSPF divides the routing domain up into smaller, more manageable pieces. The entire routing domain is called autonomous system and the pieces are called areas.
A special area, called the backbone area, is defined to interconnect all the areas in the autonomous system. Routers within an area only have knowledge of their area and therefore only create a graph of the systems in that area.
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