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Network Security Management Lecture sharing portion for Network Security Management, Sir Moizuddin


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Old 10-05-2010, 10:55 PM
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Read Distance vector versus link-state

Overview

"Distance Vector" and "Link State" are terms used to describe routing protocols which are used by routers to forward packets between networks. The purpose of any routing protocol is to dynamically communicate information about all network paths used to reach a destination and to select the from those paths, the best path to reach a destination network. The terms distance vector and link state are used to group routing protocols into two broad categories based on whether the routing protocol selects the best routing path based on a distance metric (the distance) and an interface (the vector), or selects the best routing path by calculating the state of each link in a path and finding the path that has the lowest total metric to reach the destination.
DISTANCE VECTOR

Distance
Distance is the cost of reaching a destination, usually based on the number of hosts the path passes through, or the total of all the administrative metrics assigned to the links in the path.
Vector
From the standpoint of routing protocols, the vector is the interface traffic will be forwarded out in order to reach an given destination network along a route or path selected by the routing protocol as the best path to the destination network.
Distance vector protocols use a distance calculation plus an outgoing network interface (a vector) to choose the best path to a destination network. The network protocol (IPX, SPX, IP, Appletalk, DECnet etc.) will forward data using the best paths selected.
Common distance vector routing protocols include:

  • Appletalk RTMP
  • IPX RIP
  • IP RIP
  • IGRP
Advantages of Distance Vector Protocols

Well Supported
Protocols such as RIP have been around a long time and most, if not all devices that perform routing will understand RIP.
LINK STATE

Link State protocols track the status and connection type of each link and produces a calculated metric based on these and other factors, including some set by the network administrator. Link state protocols know whether a link is up or down and how fast it is and calculates a cost to 'get there'. Since routers run routing protocols to figure out how to get to a destination, you can think of the 'link states' as being the status of the interfaces on the router. Link State protocols will take a path which has more hops, but that uses a faster medium over a path using a slower medium with fewer hops.
Because of their awareness of media types and other factors, link state protocols require more processing power (more circuit logic in the case of ASICs) and memory. Distance vector algorithms being simpler require simpler hardware.
A Comparison: Link State vs. Distance Vector

See Fig. 1-1 below. If all routers were running a Distance Vector protocol, the path or 'route' chosen would be from A B directly over the ISDN serial link, even though that link is about 10 times slower than the indirect route from A C D B.
A Link State protocol would choose the A C D B path because it's using a faster medium (100 Mb ethernet). In this example, it would be better to run a Link State routing protocol, but if all the links in the network are the same speed, then a Distance Vector protocol is better.


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There are two major classes of routing protocol: distance vector and link-state. It's easy to remember which protocols belong to either class, but comprehending their differences between the two classes takes a bit more effort.
Distance vector routing is so named because it involves two factors: the distance, or metric, of a destination, and the vector, or direction to take to get there. Routing information is only exchanged between directly connected neighbors. This means a router knows from which neighbor a route was learned, but it does not know where that neighbor learned the route; a router can't see beyond its own neighbors. This aspect of distance vector routing is sometimes referred to as "routing by rumor." Measures like split horizon and poison reverse are employed to avoid routing loops.
Link-state routing, in contrast, requires that all routers know about the paths reachable by all other routers in the network. Link-state information is flooded throughout the link-state domain (an area in OSPF or IS-IS) to ensure all routers posess a synchronized copy of the area's link-state database. From this common database, each router constructs its own relative shortest-path tree, with itself as the root, for all known routes.
Consider the following topology.
Name:  lab_topology.png
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Both distance vector and link-state routing protocols are suitable for deployment on this network, but each will go about propagating routes in a different manner.
Distance Vector

If we were to run a distance vector routing protocol like RIP or EIGRP on this topology, here's how R1 would see the network, assuming each link has a metric of 1 (locally connected routes have been omitted):
Name:  R1_DV.png
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Notice that although R1 has connectivity to all subnets, it has no knowledge of the network's structure beyond its own links. R4 has even less insight:
Name:  R4_DV.png
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Because they do not require routers to maintain the state of all links in the network, distance vector protocols typically consume less overhead at the expense of limited visibility. Because routers have only a limited view of the network, tools like split horizon and poision reverse are needed to prevent routing loops.
Link-State

Now, let's look at the same topology running a link-state routing protocol (in a single area). Because each router records the state of all links in the area, each router can construct a shortest-path tree from itself to all known destinations. Here's what R1's tree would look like:
Name:  R1_LS.png
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R4 has constructed its own shortest-path tree, different from that of R1:
Name:  R4_LS.png
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Although maintaining link-state information for the entire area typically requires more overhead than does processing advertisements only from direct neighbors, but provide more robust operation and scalability.
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