1. Distance Vector Protocols
Distance vector protocols such as Routing Information Protocol (RIP), Interior Gateway Routing Protocol (IGRP), Internetwork Packet Exchange (IPX) RIP, IPX Service Advertisement Protocol (SAP), and Routing Table Maintenance Protocol (RTMP), broadcast their complete routing table periodically, regardless of whether the routing table has changed. This periodic advertisement varies from every 10 seconds for RTMP to every 90 seconds for IGRP. When the network is stable, distance vector protocols behave well but waste of bandwidth because of the periodic sending of routing table updates, even when no change has occurred. When a failure occurs in the network, distance vector protocols do not add excessive load to the network, but they take a long time to reconverge to an alternate path or to flush a bad path from the network.
Distance Vector Routing protocols are dynamic. Routers that use distance vector routing share information, or a routing map, with other routers on the network. As changes to the network occur, the router with the change propagates the new routing information across the entire network.
In routing based on distance-vector algorithms, routers periodically pass copies of their entire routing table to routers that are their immediate neighbors. Each recipient of this information adds a distance vector (it’s own distance value) to the routing table before it forwards it on to its neighbors. This process continues in an omni directional manner among connected routers. Eventually each router on the network learns about all the others and is able to develop a cumulative network “map.” Each router then knows how to reach any other router, and any other network connected to the router.
Distance vector routing provides a tremendous advantage over static routing. Routers are able to discover the state of the network, and to propagate changes as they occur. The most common, and most ubiquitous of distance vector routing protocols is the Routing Information Protocol, or RIP.
However, there are also some disadvantages to distance vector routing that preclude its use on the <Client> network:
· Because distance vector routing protocols periodically transmit the entire routing table to all immediate neighbors, they can add significant traffic. This is particularly problematic on a wireless network with limited bandwidth.
· Distance vector protocols are notoriously slow to converge, or adapt to network topology changes. After a change to the network, and before all the routers have converged, there is the probability of routing errors and lost data.
2. Link-State Protocols
Link-state routing protocols, such as Open Shortest Path First (OSPF), Intermediate System-to-Intermediate System (IS-IS), and NetWare Link Services Protocol (NLSP), were designed to address the limitations of distance vector routing protocols (slow convergence and unnecessary bandwidth usage). Link-state protocols are more complex than distance vector protocols, and running them adds to the router's overhead. The additional overhead (in the form of memory utilization and bandwidth consumption when link-state protocols first start up) constrains the number of neighbors that a router can support and the number of neighbors that can be in an area. When the network is stable, link-state protocols minimize bandwidth usage by sending updates only when a change occurs. A hello mechanism ascertains reachability of neighbors. When a failure occurs in the network, link-state protocols flood Link-State Advertisements (LSAs) throughout an area. LSAs cause every router within the failed area to recalculate routes. The fact that LSAs need to be flooded throughout the area in failure mode and the fact that all routers recalculate routing tables constrain the number of neighbors that can be in an area.
Link state routing protocols, like distance vector protocols, are dynamic. They propagate route information across networks. However, they have a number of advantages over distance vector protocols.
One of the major advantages of link-state routing is that they calculate the best route for data based on cost rather than distance. The algorithms used to determine cost vary from protocol to protocol, but it is generally based on a link’s bandwidth. Thus, the router that the data packet takes to get to its destination is optimized.
Additionally, link state protocols do not transmit their entire topology database across the network on a periodic basis. Once the network has converged, protocol traffic is limited to changes in specific links (link state advertisement packets) and keep-alive or “hello” packets.
Finally, convergence times for link state protocols are generally much shorter than for distance vector protocols. A network based on link-state routing will recognize and adapt to failures and changes much more quickly.
There are a few disadvantages to link state routing protocols that must be considered. They are generally much more complex than either static routes or distance-vector routing. This translates into higher implementation costs, higher CPU utilization, and greater memory requirements.
3. Advanced Distance Vector Protocol
Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance vector protocol that has some of the properties of link-state protocols. Enhanced IGRP addresses the limitations of conventional distance vector routing protocols (slow convergence and high bandwidth consumption in a steady state network
4 Path Vector Protocols
There is really only one Path Vector routing protocol and it is Border Gateway Protocol version 4 (BGP-4). This is the primary routing protocol used on the Internet to share routing updates between Autonomous Systems (AS). An Autonomous System is a network under a single administrative and technical control. ASs are typically defined by the boundaries of a single company or organizational entity. BGP-4 is typically used between Internet Service Providers (ISPs) and between companies and the multiple ISPs they use for upstream Internet connectivity. BGP-4 routers operate in either External BGP (EBGP) or Internal BGP (IBGP) configurations depending on whether the connectivity is between ASs or within ASs respectively. Since <Client> currently default routes toward their Internet points of presence there is little reason for <Client> to use this protocol. Regardless, BGP-4 would not be used within the corporate network and only in the future would it be used in a limited capacity at the Internet edges of the <Client> intranet.
0 comments:
Post a Comment