IPV6: Distance Vector Routing Protocol - Revision history

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Route Invalidation Timers

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Common Characteristics

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Created page with "Distance Vector Routing Protocols Most routing protocols fall into one of two classes: distance vector or link state. The basics of distance vector routing protocols are exami..."

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Distance Vector Routing Protocols
Most routing protocols fall into one of two classes: distance vector or link
state. The basics of distance vector routing protocols are examined here;
the next section covers link state routing protocols. Most distance vector
algorithms are based on the work done of R. E. Bellman, [3] L. R. Ford,
and D. R. Fulkerson, [4] and for this reason occasionally are referred to as
Bellman-Ford or Ford-Fulkerson algorithms. A notable exception is
EIGRP, which is based on an algorithm developed by J. J. Garcia Luna
Aceves.
[3]
R. E. Bellman. Dynamic Programming. Princeton, New Jersey: Princeton University
Press; 1957.
[4]
L. R. Ford Jr. and D. R. Fulkerson. Flows in Networks. Princeton, New Jersey:
Princeton University Press; 1962.
The name distance vector is derived from the fact that routes are
advertised as vectors of (distance, direction), where distance is defined in
terms of a metric and direction is defined in terms of the next-hop router.
For example, "Destination A is a distance of five hops away, in the
direction of next-hop Router X." As that statement implies, each router
learns routes from its neighboring routers' perspectives and then
advertises the routes from its own perspective. Because each router
depends on its neighbors for information, which the neighbors in turn
might have learned from their neighbors, and so on, distance vector
routing is sometimes facetiously referred to as "routing by rumor."
Distance vector routing protocols include the following:
Routing Information Protocol (RIP) for IP
Xerox Networking System's XNS RIP
Novell's IPX RIPThe Cisco Systems Internet Gateway Routing Protocol (IGRP) and
Enhanced Internet Gateway Routing Protocol (EIGRP)
DEC's DNA Phase IV
AppleTalk's Routing Table Maintenance Protocol (RTMP)
Common Characteristics
A typical distance vector routing protocol uses a routing algorithm in
which routers periodically send routing updates to all neighbors by
broadcasting their entire route tables. [5]
[5]
A notable exception to this convention is the Cisco Enhanced IGRP. EIGRP is a
distance vector protocol, but its updates are not periodic, are not broadcasted, and do not
contain the full route table. EIGRP is covered in Chapter 7, "Enhanced Interior Gateway
Routing Protocol (EIGRP)."
The preceding statement contains a lot of information. Following sections
consider it in more detail.
Periodic Updates
Periodic updates means that at the end of a certain time period, updates
will be transmitted. This period typically ranges from 10 seconds for
AppleTalk's RTMP to 90 seconds for the Cisco IGRP. At issue here is the
fact that if updates are sent too frequently, congestion and router CPU
overloading might occur; if updates are sent too infrequently,
convergence time might be unacceptably high.
Neighbors
In the context of routers, neighbors means routers sharing a commondata link or some higher-level logical adjacency. A distance vector routing
protocol sends its updates to neighboring routers [6] and depends on them
to pass the update information along to their neighbors. For this reason,
distance vector routing is said to use hop-by-hop updates.
[6]
This statement is not entirely true. Hosts also can listen to routing updates in some
implementations; but all that is important for this discussion is how routers work.
Broadcast Updates
When a router first becomes active on a network, how does it find other
routers and how does it announce its own presence? Several methods
are available. The simplest is to send the updates to the broadcast
address (in the case of IP, 255.255.255.255). Neighboring routers
speaking the same routing protocol will hear the broadcasts and take
appropriate action. Hosts and other devices uninterested in the routing
updates will simply drop the packets.
Full Routing Table Updates
Most distance vector routing protocols take the very simple approach of
telling their neighbors everything they know by broadcasting their entire
route table, with some exceptions that are covered in following sections.
Neighbors receiving these updates glean the information they need and
discard everything else.
Routing by Rumor
Figure 4-3 shows a distance vector algorithm in action. In this example,
the metric is hop count. At time t 0 , Routers A through D have just become
active. Looking at the route tables across the top row, at t 0 the only
information any of the four routers has is its own directly connected
networks. The tables identify these networks and indicate that they aredirectly connected by having no next-hop router and by having a hop
count of 0. Each of the four routers will broadcast this information on all
links.
Figure 4-3. Distance vector protocols converge hop-by-
hop.
[View full size image]
At time t 1 , the first updates have been received and processed by the
routers. Look at Router A's table at t 1 . Router B's update to Router A said
that Router B can reach networks 10.1.2.0 and 10.1.3.0, both zero hops
away. If the networks are zero hops from B, they must be one hop from
A. Router A incremented the hop count by one and then examined its
route table. It already recognized 10.1.2.0, and the hop count (zero) was
less than the hop count B advertised, (one), so A disregarded that
information.
Network 10.1.3.0 was new information, however, so A entered this in theNetwork 10.1.3.0 was new information, however, so A entered this in the
route table. The source address of the update packet was Router B's
interface (10.1.2.2) so that information is entered along with the
calculated hop count.
Notice that the other routers performed similar operations at the same
time t 1 . Router C, for instance, disregarded the information about
10.1.3.0 from B and 10.1.4.0 from C but entered information about
10.1.2.0, reachable via B's interface address 10.1.3.1, and 10.1.5.0,
reachable via C's interface 10.1.4.2. Both networks were calculated as
one hop away.
At time t 2 , the update period has again expired and another set of
updates has been broadcast. Router B sent its latest table; Router A
again incremented B's advertised hop counts by one and compared. The
information about 10.1.2.0 is again discarded for the same reason as
before. 10.1.3.0 is already known, and the hop count hasn't changed, so
that information is also discarded. 10.1.4.0 is new information and is
entered into the route table.
The network is converged at time t 3 . Every router recognizes every
network, the address of the next-hop router for every network, and the
distance in hops to every network.
It is time for an analogy. You are wandering in the Sangre de Cristo
mountains of northern New Mexicoa wonderful place to wander if you
aren't lost. But you are lost. You come upon a fork in the trail and a sign
pointing west, reading "Taos, 15 miles." You have no choice but to trust
the sign. You have no clue what the terrain is like over that 15 miles; you
don't know whether there is a better route or even whether the sign is
correct. Someone could have turned it around, in which case you will
travel deeper into the forest instead of to safety!
Distance vector algorithms provide road signs to networks. [7] They
provide the direction and the distance, but no details about what lies
along the route. And like the sign at the fork in the trail, they are
vulnerable to accidental or intentional misdirection. Following are some of
the difficulties and refinements associated with distance vectoralgorithms.
[7]
The road sign analogy is commonly used. You can find a good presentation in Radia
Perlman's Interconnections, pp. 205210.
Route Invalidation Timers
Now that the network in Figure 4-3 is fully converged, how will it handle
reconvergence when some part of the topology changes? If network
10.1.5.0 goes down, the answer is simple enoughRouter D, in its next
scheduled update, flags the network as unreachable and passes the
information along.
But what if, instead of 10.1.5.0 going down, Router D fails? Routers A, B,
and C still have entries in their route tables about 10.1.5.0; the
information is no longer valid, but there's no router to inform them of this
fact. They will unknowingly forward packets to an unreachable
destinationa black hole has opened in the network.
This problem is handled by setting a route invalidation timer for each
entry in the route table. For example, when Router C first hears about
10.1.5.0 and enters the information into its route table, C sets a timer for
that route. At every regularly scheduled update from Router D, C
discards the update's already-known information about 10.1.5.0 as
described in "Routing by Rumor." But as C does so, it resets the timer on
that route.
If Router D goes down, C will no longer hear updates about 10.1.5.0. The
timer will expire, C will flag the route as unreachable and will pass the
information along in the next update.
Typical periods for route timeouts range from three to six update periods.
A router would not want to invalidate a route after a single update has
been missed, because this event might be the result of a corrupted or lost
packet or some sort of network delay. At the same time, if the period is
too long, reconvergence will be excessively slow.Split Horizon
According to the distance vector algorithm as it has been described so
far, at every update period each router broadcasts its entire route table to
every neighbor. But is this really necessary? Every network known by
Router A in Figure 4-3, with a hop count higher than zero, has been
learned from Router B. Common sense suggests that for Router A to
broadcast the networks it has learned from Router B back to Router B is
a waste of resources. Obviously, B already "knows" about those
networks.
A route pointing back to the router from which packets were received is
called a reverse route. Split horizon is a technique for preventing reverse
routes between two routers.
Besides not wasting resources, there is a more important reason for not
sending reachability information back to the router from which the
information was learned. The most important function of a dynamic
routing protocol is to detect and compensate for topology changesif the
best path to a network becomes unreachable, the protocol must look for
a next-best path.
Look yet again at the converged network of Figure 4-3 and suppose that
network 10.1.5.0 goes down. Router D will detect the failure, flag the
network as unreachable, and pass the information along to Router C at
the next update interval. However, before D's update timer triggers an
update, something unexpected happens. C's update arrives, claiming
that it can reach 10.1.5.0, one hop away! Remember the road sign
analogy? Router D has no way of recognizing that C is not advertising a
legitimate next-best path. It will increment the hop count and make an
entry into its route table indicating that 10.1.5.0 is reachable via Router
C's interface 10.1.4.1, just two hops away.
Now a packet with a destination address of 10.1.5.3 arrives at Router C,
which consults its route table and forwards the packet to D. Router D
consults its route table and forwards the packet to C, C forwards it back
to D, ad infinitum. A routing loop has occurred.Implementing split horizon prevents the possibility of such a routing loop.
There are two categories of split horizon: simple split horizon and split
horizon with poisoned reverse.
The rule for simple split horizon is, when sending updates out a particular
interface, do not include networks that were learned from updates
received on that interface.
The routers in Figure 4-4 implement simple split horizon. Router C sends
an update to Router D for networks 10.1.1.0, 10.1.2.0, and 10.1.3.0.
Networks 10.1.4.0 and 10.1.5.0 are not included because they were
learned from Router D. Likewise, updates to Router B include 10.1.4.0
and 10.1.5.0 with no mention of 10.1.1.0, 10.1.2.0, and 10.1.3.0.
Figure 4-4. Simple split horizon does not advertise routes
back to the neighbors from whom the routes were learned.
[View full size image]
Simple split horizon works by suppressing information. Split horizon with
poisoned reverse is a modification that provides more positive
information.
The rule for split horizon with poisoned reverse is, when sending updates
out a particular interface, designate any networks that were learned from
updates received on that interface as unreachable.In the scenario of Figure 4-4, Router C would in fact advertise 10.1.4.0
and 10.1.5.0 to Router D, but the network would be marked as
unreachable. Figure 4-5 shows what the route tables from C to B and D
would look like. Notice that a route is marked as unreachable by setting
the metric to infinity; in other words, the network is an infinite distance
away. Coverage of a routing protocol's concept of infinity continues in the
next section.
Figure 4-5. Split horizon with poisoned reverse advertises
reverse routes but with an unreachable (infinite) metric.
[View full size image]
Split horizon with poisoned reverse is considered safer and stronger than
simple split horizona sort of "bad news is better than no news at all"
approach. For example, suppose that Router B in Figure 4-5 receives
corrupted information, causing it to proceed as if subnet 10.1.1.0 is
reachable via Router C. Simple split horizon would do nothing to correct
this misperception, whereas a poisoned reverse update from Router C
would immediately stop the potential loop. For this reason, most modern
distance vector implementations use split horizon with poisoned reverse.
The trade-off is that routing update packets are larger, which might
exacerbate any congestion problems on a link.
Counting to InfinityCounting to Infinity
Split horizon will break loops between neighbors, but it will not stop loops
in a network such as the one in Figure 4-6. Again, 10.1.5.0 has failed.
Router D sends the appropriate updates to its neighbors, Router C (the
dashed arrows), and Router B (the solid arrows). Router B marks the
route via D as unreachable, but Router A is advertising a next-best path
to 10.1.5.0, which is three hops away. B posts that route in its route table.
Figure 4-6. Split horizon will not prevent routing loops
here.
B now informs D that it has an alternative route to 10.1.5.0. D posts this
information and updates C, saying that it has a four-hop route to the
network. C tells A that 10.1.5.0 is five hops away. A tells B that the
network is now six hops away.
"Ah," Router B thinks, "Router A's path to 10.1.5.0 has increased in
length. Nonetheless, it's the only route I've got, so I'll use it!"length. Nonetheless, it's the only route I've got, so I'll use it!"
B changes the hop count to seven, updates D, and around it goes again.
This situation is the counting-to-infinity problem because the hop count to
10.1.5.0 will continue to increase to infinity. All routers are implementing
split horizon, but it doesn't help.
The way to alleviate the effects of counting to infinity is to define infinity.
Most distance vector protocols define infinity to be 16 hops. As the
updates continue to loop among the routers in Figure 4-6, the hop count
to 10.1.5.0 in all routers will eventually increment to 16. At that time, the
network will be considered unreachable.
This method is also how routers advertise a network as unreachable.
Whether it is a poisoned reverse route, a network that has failed, or a
network beyond the maximum network diameter of 15 hops, a router will
recognize any 16-hop route as unreachable.
Setting a maximum hop count of 15 helps solve the counting-to-infinity
problem, but convergence will still be very slow. Given an update period
of 30 seconds, a network could take up to 7.5 minutes to reconverge and
is susceptible to routing errors during this time. Triggered updates can be
used to reduce this convergence time.
Triggered Updates
Triggered updates, also known as flash updates, are very simple: If a
metric changes for better or for worse, a router will immediately send out
an update without waiting for its update timer to expire. Reconvergence
will occur far more quickly than if every router had to wait for regularly
scheduled updates, and the problem of counting to infinity is greatly
reduced, although not completely eliminated. Regular updates might still
occur along with triggered updates. Thus a router might receive bad
information about a route from a not-yet-reconverged router after having
received correct information from a triggered update. Such a situation
shows that confusion and routing errors might still occur while a networkis reconverging, but triggered updates will help to iron things out more
quickly.
A further refinement is to include in the update only the networks that
actually triggered it, rather than the entire route table. This technique
reduces the processing time and the impact on network bandwidth.
Holddown Timers
Triggered updates add responsiveness to a reconverging network.
Holddown timers introduce a certain amount of skepticism to reduce the
acceptance of bad routing information.
If the distance to a destination increases (for example, the hop count
increases from two to four), the router sets a holddown timer for that
route. Until the timer expires, the router will not accept any new updates
for the route.
Obviously, a trade-off is involved here. The likelihood of bad routing
information getting into a table is reduced but at the expense of the
reconvergence time. Like other timers, holddown timers must be set with
care. If the holddown period is too short, it will be ineffective, and if it is
too long, normal routing will be adversely affected.
Asynchronous Updates
Figure 4-7 shows a group of routers connected to an Ethernet backbone.
The routers should not broadcast their updates at the same time; if they
do, the update packets will collide. Yet this situation is exactly what can
happen when several routers share a broadcast network. System delays
related to the processing of updates in the routers tend to cause the
update timers to become synchronized. As a few routers become
synchronized, collisions will begin to occur, further contributing to system
delays, and eventually all routers sharing the broadcast network mightbecome synchronized.
Figure 4-7. If update timers become synchronized,
collisions might occur.
Asynchronous updates might be maintained by one of two methods:
Each router's update timer is independent of the routing process and
is, therefore, not affected by processing loads on the router.
A small random time, or timing jitter, is added to each update period
as an offset.
If routers implement the method of rigid, system-independent timers, all
routers sharing a broadcast network must be brought online in a random
fashion. Rebooting the entire group of routers simultaneouslysuch as
might happen during a widespread power outage, for examplecould
result in all the timers attempting to update at the same time.
Adding randomness to the update period is effective if the variable is
large enough in proportion to the number of routers sharing the broadcast
network. Sally Floyd and Van Jacobson [8] have calculated that a too-
small randomization will be overcome by a large enough network of
routers, and that to be effective the update timer should range up to 50
percent of the median update period.[8]
S. Floyd and V. Jacobson. "The Synchronisation of Periodic Routing Messages." ACM
Sigcomm '93 Symposium, September 1993.

==Pranala Menarik==

* [[IPv6: Advanced Routing]]

@@ Line 22: / Line 22: @@
 A notable exception to this convention is the Cisco Enhanced IGRP. EIGRP is a distance vector protocol, but its updates are not periodic, are not broadcasted, and do not contain the full route table. EIGRP is covered in Chapter 7, "Enhanced Interior Gateway Routing Protocol (EIGRP)."
-The preceding statement contains a lot of information. Following sections
+The preceding statement contains a lot of information. Following sections consider it in more detail.
 consider it in more detail.
-Periodic Updates
+===Periodic Updates===
 Periodic updates means that at the end of a certain time period, updates
-will be transmitted. This period typically ranges from 10 seconds for
+Periodic updates means that at the end of a certain time period, updates will be transmitted. This period typically ranges from 10 seconds for
-AppleTalk's RTMP to 90 seconds for the Cisco IGRP. At issue here is the
+AppleTalk's RTMP to 90 seconds for the Cisco IGRP. At issue here is the fact that if updates are sent too frequently, congestion and router CPU overloading might occur; if updates are sent too infrequently, convergence time might be unacceptably high.
 fact that if updates are sent too frequently, congestion and router CPU
-overloading might occur; if updates are sent too infrequently,
+===Neighbors===
 convergence time might be unacceptably high.
-Neighbors
+In the context of routers, neighbors means routers sharing a commondata link or some higher-level logical adjacency. A distance vector routing protocol sends its updates to neighboring routers [6] and depends on them to pass the update information along to their neighbors. For this reason, distance vector routing is said to use hop-by-hop updates. [6]
 In the context of routers, neighbors means routers sharing a commondata link or some higher-level logical adjacency. A distance vector routing
-protocol sends its updates to neighboring routers [6] and depends on them
+This statement is not entirely true. Hosts also can listen to routing updates in some implementations; but all that is important for this discussion is how routers work.
 to pass the update information along to their neighbors. For this reason,
-distance vector routing is said to use hop-by-hop updates.
+===Broadcast Updates===
 [6]
-This statement is not entirely true. Hosts also can listen to routing updates in some
+When a router first becomes active on a network, how does it find other routers and how does it announce its own presence? Several methods are available. The simplest is to send the updates to the broadcast address (in the case of IP, 255.255.255.255). Neighboring routers speaking the same routing protocol will hear the broadcasts and take appropriate action. Hosts and other devices uninterested in the routing updates will simply drop the packets.
 implementations; but all that is important for this discussion is how routers work.
-Broadcast Updates
+===Full Routing Table Updates===
 When a router first becomes active on a network, how does it find other
 Most distance vector routing protocols take the very simple approach of
 telling their neighbors everything they know by broadcasting their entire
@@ Line 53: / Line 46: @@
 Neighbors receiving these updates glean the information they need and
 discard everything else.
 Routing by Rumor
-Figure 4-3 shows a distance vector algorithm in action. In this example,
+===Routing by Rumor===
 the metric is hop count. At time t 0 , Routers A through D have just become
-active. Looking at the route tables across the top row, at t 0 the only
+Figure 4-3 shows a distance vector algorithm in action. In this example, the metric is hop count. At time t 0 , Routers A through D have just become active. Looking at the route tables across the top row, at t 0 the only information any of the four routers has is its own directly connected networks. The tables identify these networks and indicate that they aredirectly connected by having no next-hop router and by having a hop count of 0. Each of the four routers will broadcast this information on all links.
 information any of the four routers has is its own directly connected
-networks. The tables identify these networks and indicate that they aredirectly connected by having no next-hop router and by having a hop
+Figure 4-3. Distance vector protocols converge hop-by-hop.
 [View full size image]
 At time t 1 , the first updates have been received and processed by the
-routers. Look at Router A's table at t 1 . Router B's update to Router A said
+At time t 1 , the first updates have been received and processed by the routers. Look at Router A's table at t 1 . Router B's update to Router A said that Router B can reach networks 10.1.2.0 and 10.1.3.0, both zero hops away. If the networks are zero hops from B, they must be one hop from A. Router A incremented the hop count by one and then examined its route table. It already recognized 10.1.2.0, and the hop count (zero) was less than the hop count B advertised, (one), so A disregarded that information.
 that Router B can reach networks 10.1.2.0 and 10.1.3.0, both zero hops
 Network 10.1.3.0 was new information, however, so A entered this in theNetwork 10.1.3.0 was new information, however, so A entered this in the
-route table. The source address of the update packet was Router B's
+route table. The source address of the update packet was Router B's interface (10.1.2.2) so that information is entered along with the calculated hop count.
 interface (10.1.2.2) so that information is entered along with the
-calculated hop count.
+Notice that the other routers performed similar operations at the same time t 1 . Router C, for instance, disregarded the information about 10.1.3.0 from B and 10.1.4.0 from C but entered information about 10.1.2.0, reachable via B's interface address 10.1.3.1, and 10.1.5.0, reachable via C's interface 10.1.4.2. Both networks were calculated as one hop away.
 Notice that the other routers performed similar operations at the same
-time t 1 . Router C, for instance, disregarded the information about
+At time t 2 , the update period has again expired and another set of updates has been broadcast. Router B sent its latest table; Router A again incremented B's advertised hop counts by one and compared. The information about 10.1.2.0 is again discarded for the same reason as before. 10.1.3.0 is already known, and the hop count hasn't changed, so that information is also discarded. 10.1.4.0 is new information and is entered into the route table.
 .1.3.0 from B and 10.1.4.0 from C but entered information about
-.1.2.0, reachable via B's interface address 10.1.3.1, and 10.1.5.0,
+The network is converged at time t 3 . Every router recognizes every network, the address of the next-hop router for every network, and the distance in hops to every network.
 reachable via C's interface 10.1.4.2. Both networks were calculated as
-one hop away.
+It is time for an analogy. You are wandering in the Sangre de Cristo mountains of northern New Mexicoa wonderful place to wander if you aren't lost. But you are lost. You come upon a fork in the trail and a sign pointing west, reading "Taos, 15 miles." You have no choice but to trust the sign. You have no clue what the terrain is like over that 15 miles; you don't know whether there is a better route or even whether the sign is correct. Someone could have turned it around, in which case you will travel deeper into the forest instead of to safety!
 At time t 2 , the update period has again expired and another set of
-updates has been broadcast. Router B sent its latest table; Router A
+Distance vector algorithms provide road signs to networks. [7] They provide the direction and the distance, but no details about what lies along the route. And like the sign at the fork in the trail, they are vulnerable to accidental or intentional misdirection. Following are some of the difficulties and refinements associated with distance vectoralgorithms. [7]
 again incremented B's advertised hop counts by one and compared. The
-information about 10.1.2.0 is again discarded for the same reason as
+The road sign analogy is commonly used. You can find a good presentation in Radia Perlman's Interconnections, pp. 205210.
 before. 10.1.3.0 is already known, and the hop count hasn't changed, so
-that information is also discarded. 10.1.4.0 is new information and is
+===Route Invalidation Timers===
 Now that the network in Figure 4-3 is fully converged, how will it handle
 reconvergence when some part of the topology changes? If network
@@ Line 293: / Line 256: @@
 S. Floyd and V. Jacobson. "The Synchronisation of Periodic Routing Messages." ACM
 Sigcomm '93 Symposium, September 1993.
 ==Pranala Menarik==
 * [[IPv6: Advanced Routing]]

@@ Line 1: / Line 1: @@
 Distance Vector Routing Protocols
 Most routing protocols fall into one of two classes: distance vector or link
-state. The basics of distance vector routing protocols are examined here;
+Most routing protocols fall into one of two classes: distance vector or link state. The basics of distance vector routing protocols are examined here; the next section covers link state routing protocols. Most distance vector algorithms are based on the work done of R. E. Bellman, [3] L. R. Ford, and D. R. Fulkerson, [4] and for this reason occasionally are referred to as Bellman-Ford or Ford-Fulkerson algorithms. A notable exception is EIGRP, which is based on an algorithm developed by J. J. Garcia Luna Aceves. [3] R. E. Bellman. Dynamic Programming. Princeton, New Jersey: Princeton University Press; 1957. [4] L. R. Ford Jr. and D. R. Fulkerson. Flows in Networks. Princeton, New Jersey: Princeton University Press; 1962.
 the next section covers link state routing protocols. Most distance vector
-algorithms are based on the work done of R. E. Bellman, [3] L. R. Ford,
+The name distance vector is derived from the fact that routes are advertised as vectors of (distance, direction), where distance is defined in terms of a metric and direction is defined in terms of the next-hop router. For example, "Destination A is a distance of five hops away, in the direction of next-hop Router X." As that statement implies, each router learns routes from its neighboring routers' perspectives and then
-and D. R. Fulkerson, [4] and for this reason occasionally are referred to as
+advertises the routes from its own perspective. Because each router depends on its neighbors for information, which the neighbors in turn
-Bellman-Ford or Ford-Fulkerson algorithms. A notable exception is
+might have learned from their neighbors, and so on, distance vector routing is sometimes facetiously referred to as "routing by rumor."
 Distance vector routing protocols include the following:
 Routing Information Protocol (RIP) for IP
-Xerox Networking System's XNS RIP
+ Routing Information Protocol (RIP) for IP
-Novell's IPX RIPThe Cisco Systems Internet Gateway Routing Protocol (IGRP) and
+ Xerox Networking System's XNS RIP
-Enhanced Internet Gateway Routing Protocol (EIGRP)
+ Novell's IPX RIPThe Cisco Systems Internet Gateway Routing Protocol (IGRP) and
-DEC's DNA Phase IV
+ Enhanced Internet Gateway Routing Protocol (EIGRP)
-AppleTalk's Routing Table Maintenance Protocol (RTMP)
+ DEC's DNA Phase IV
-Common Characteristics
+ AppleTalk's Routing Table Maintenance Protocol (RTMP)
 A typical distance vector routing protocol uses a routing algorithm in
-which routers periodically send routing updates to all neighbors by
+==Common Characteristics==
 broadcasting their entire route tables. [5]
-[5]
+A typical distance vector routing protocol uses a routing algorithm in which routers periodically send routing updates to all neighbors by
-A notable exception to this convention is the Cisco Enhanced IGRP. EIGRP is a
+broadcasting their entire route tables. [5][5]
 distance vector protocol, but its updates are not periodic, are not broadcasted, and do not
-contain the full route table. EIGRP is covered in Chapter 7, "Enhanced Interior Gateway
+A notable exception to this convention is the Cisco Enhanced IGRP. EIGRP is a distance vector protocol, but its updates are not periodic, are not broadcasted, and do not contain the full route table. EIGRP is covered in Chapter 7, "Enhanced Interior Gateway Routing Protocol (EIGRP)."
 Routing Protocol (EIGRP)."
 The preceding statement contains a lot of information. Following sections
 consider it in more detail.