IP subnet routing
Chapter 3. Routing Protocols
3.2 Routing Algorithms
3.2.2 Distance Vector Routing
The principle behind distance vector routing is very simple. Each router in an internetwork maintains the distance from itself to every known destination in a distance vector table. Distance vector tables consist of a series of destinations (vectors) and costs (distances) to reach them and define the lowest costs to destinations at the time of transmission.
The distances in the tables are computed from information provided by neighbor routers. Each router transmits its own distance vector table across the shared network. The sequence of operations for doing this is as follows:
Each router is configured with an identifier and a cost for each of its network links. The cost is normally fixed at 1, reflecting a single hop, but can reflect some other measurement taken for the link such as the traffic, speed, etc.
Each router initializes with a distance vector table containing zero for itself, one for directly attached networks, and infinity for every other destination.
Each router periodically (typically every 30 seconds) transmits its distance vector table to each of its neighbors. It can also transmit the table when a link first comes up or when the table changes.
Each router saves the most recent table it receives from each neighbor and uses the information to calculate its own distance vector table.
The total cost to each destination is calculated by adding the cost reported to it in a neighbor's distance vector table to the cost of the link to that neighbor.
The distance vector table (the routing table) for the router is then created by taking the lowest cost calculated for each destination.
Figure 71 on page 100 shows the distance vector tables for three routers within a simple internetwork.
Router R5
N6 Router R4
N5 Router R3
Router R1
Router R2
N1
N2
N4
Router R2 Distance Vector
Table Net
Next
Hop Metric
N1 R1 2
N2 = 1
N3 = 1
N4 R3 2
N5 R3 3
N6 R3 4
Router R3 Distance Vector
Table Net
Next
Hop Metric
N1 R2 3
N2 R2 2
N3 = 1
N4 = 1
N5 R4 2
N6 R4 3
Router R4 Distance Vector
Table Net
Next
Hop Metric
N1 R3 4
N2 R3 3
N3 R3 2
N4 = 1
N5 = 1
N6 R5 2
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Figure 71. Distance Vector - Routing Table Calculation
The distance vector algorithm produces a stable routing table after a period directly related to the number of routers across the network. This period is referred to as the convergence time and represents the time it takes for distance vector
information to traverse the network. In a large internetwork, this time may become too long to be useful.
Routing tables are recalculated if a changed distance vector table is received from a neighbor, or if the state of a link to a neighbor changes. If a network link goes down, the distance vector tables that have been received over it are discarded and the routing table is recalculated.
The chief advantage of distance vector is that it is very easy to implement. There are also the following significant disadvantages:
The instability caused by old routes persisting in an internetwork
The long convergence time on large internetworks
The limit to the size of an internetwork imposed by maximum hop counts
The fact that distance vector tables are always transmitted even if their contents have not changed
Enhancements to the basic algorithm have evolved to overcome the first two of these problems. They are described in the following subsections.
3.2.2.1 The Count to Infinity Problem
The basic distance vector algorithm will always allow a router to correctly calculate its distance vector table.
Using the example shown in Figure 72 you can see one of the problems of distance vector protocols known as counting to infinity.
Counting to infinity occurs when a network becomes unreachable, but erroneous routes to that network persist because of the time for the distance vector tables to converge.
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A
(1)
B
C
D
(10)
Target Network (1)
(1)
(1)
(10)
(n) = Network Cost
Figure 72. Counting to Infinity - Example Network
The example network shows four routers interconnected by five network links. The networks all have a cost of 1 except for that from C to D, which has a cost of 10.
Each of the routers A, B, C and D has routes to all networks. If we show only the routes to the target network, we will see they are as follows:
For D : Directly connected network. Metric 1.
For B : Route via D. Metric 2.
For C : Route via B. Metric 3.
For A : Route via B. Metric 3.
If the link from B to D fails, then all routes will be adjusted in time to use the link from C to D. However, the convergence time for this can be considerable.
Distance vector tables begin to change when B notices that the route to D has become unavailable. Figure 73 shows how the routes to the target network will change, assuming all routers send distance vector table updates at the same time.
D:
B:
C:
A:
Direct Unreachable B
B
1
3 3
Direct C A C
1 4 4 4
Direct C A C
1 5 5 5
Direct C A C
1 6 6 6
Direct C A C
1 11 11 11
Direct C D C
1 12 11 12 ....
....
Time
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Figure 73. Counting to Infinity
The problem can be seen clearly. B is able to remove the failed route immediately because it times out the link. Other routers, however, have tables that contain references to this route for many update periods after the link has failed.
1. Initially A and C have a route to D via B.
2. Link from D to B fails.
3. A and C then send updates based on the route to D via B even after the link has failed.
4. B then believes it has a route to D via either A or C. But, in reality it does not have such a route, as the routes are vestiges of the previous route via B, which has failed.
5. A and C then see that the route via B has failed, but believe a route exists via one another.
Slowly the distance vector tables converge, but not until the metrics have counted up, in theory, to infinity. To avoid this happening, practical implementations of distance vector have a low value for infinity; for example, RIP uses a maximum metric of 16.
The manner in which the metrics increment to infinity gives rise to the term
counting to infinity. It occurs because A and C are engaged in an extended period of mutual deception, each claiming to be able to get to the target network D via one another.
3.2.2.2 Split Horizon
Counting to infinity can easily be prevented if a route to a destination is never reported back in the distance vector table that is sent to the neighbor from which the route was learned. Split horizon is the term used for this technique.
The incorporation of split horizon would modify the sequence of distance vector table changes to that shown in Figure 74 on page 103. The tables can be seen to converge considerably faster than before (see Figure 73).
D:
B:
C:
A:
Direct Unreachable B
B
1
3 3
Direct Unreachable A
C
1
4 4
Direct Unreachable D
Unreachable 1
11 Direct C D C
1 12 11 12
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Time
Figure 74. Split Horizon
3.2.2.3 Split Horizon with Poison Reverse
Poison reverse is an enhancement to split horizon, whereby routes learned from a neighbor router are reported back to it but with a metric of infinity (that is, network unreachable).
The use of poison reverse is safer than split horizon alone because it breaks erroneous looping routes immediately.
If two routers receive routes pointing at each other, and they are advertised with a metric of infinity, the routes will be eliminated immediately as unreachable. If the routes are not advertised in this way, they must be eliminated by the timeout that results from a route not being reported by a neighbor router for several periods (for example, six periods for RIP).
Poison reverse does have one disadvantage. It significantly increases the size of distance vector tables that must be exchanged between neighbor routers because all routes are included in the distance vector tables. While this is generally not a problem on LANs, it can cause real problems on point-to-point connections in large internetworks.
3.2.2.4 Triggered Updates
Split horizon with poison reverse will break routing loops involving two routers. It is still possible, however, for there to be routing loops involving three or more routers.
For example, A may believe it has a route through B, B through C, and C through A. This loop can only be eliminated by the timeout that results from counting to infinity.
Triggered updates are designed to reduce the convergence time for routing tables, and hence reduce the period during which such erroneous loops are present in an internetwork.
When a router changes the cost for a route in its distance vector table, it must send the modified table immediately to neighbor routers. This simple mechanism
ensures that topology changes in a network are propagated quickly, rather than at a rate dependent on normal periodic updates.