Download Routing protocols

ROUTING PROTOCOLS
PART I
ET4187/ET5187 Advanced Telecommunication Network
Autonomous systems
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An autonomous system (AS) is defined as a logical
portion of a larger IP network
An AS normally consists of an internetwork within an
organization
It is administered by a single management authority.
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Interior Gateway Protocols (IGPs): Interior Gateway Protocols allow routers to
exchange information within an AS. Examples Open Short Path First (OSPF)
and Routing Information Protocol (RIP)
Exterior Gateway Protocols (EGPs): Exterior Gateway Protocols allow the
exchange of summary information between autonomous systems. Example:
Border Gateway Protocol (BGP).
Types of IP routing and IP routing algorithms
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Routing algorithms build and maintain the IP routing table on a
device
There are two primary methods used to build the routing table:
 Static routing
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Static routing uses preprogrammed definitions representing paths
through the network.
Dynamic routing
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It allow routers to automatically discover and maintain awareness of
the paths through the network.
This automatic discovery can use a number of currently available
dynamic routing protocols.
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Distance vector protocols
Link state protocols
Path vector protocols
Hybrid protocols
Static routing
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Static routing is manually performed by the network administrator
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Cons
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In small and simple networks with minimal redundancy, this process is relatively simple to
administer
Static routes require a considerable amount of coordination and maintenance in non-trivial
network environments.
Static routes cannot dynamically adapt to the current operational state of the network
Pros in some circumstances
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To manually define a default route. This route is used to forward traffic when the routing
table does not contain a more specific route to the destination.
To define a route that is not automatically advertised within a network.
When utilization or line tariffs make it undesirable to send routing advertisement traffic
through lower-capacity WAN connections.
When complex routing policies are required. For example, static routes can be used to
guarantee that traffic destined for a specific host traverses a designated network path.
To provide a more secure network environment. The administrator is aware of all
subnetworks defined in the environment. The administrator specifically authorizes all
communication permitted between these subnetworks.
To provide more efficient resource utilization
Distance vector routing
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Each router in the internetwork maintains the distance or cost
from itself to every known destination
Paths associated with a smaller cost value are more
attractive to use than paths associated with a larger value
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The path represented by the smallest cost becomes the preferred
path to reach the destination
This information is maintained in a distance vector table
The table is periodically advertised to each neighboring
router
Each router processes these advertisements to determine the
best paths through the network
Example: RIP
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Pros
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Easy to implement and debug
Very useful in small networks with limited redundancy
Cons
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During an adverse condition, the length of time for every device in the network
to produce an accurate routing table is called the convergence time.
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In large, complex internetworks using distance vector algorithms, this time can be
excessive.
While the routing tables are converging, networks are susceptible to inconsistent
routing behavior.
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To reduce convergence time, a limit is often placed on the maximum number of
hops contained in a single route.
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This can cause routing loops or other types of unstable packet forwarding
Valid paths exceeding this limit are not usable in distance vector networks
Distance vector routing tables are periodically transmitted to neighboring
devices.
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They are sent even if no changes have been made to the contents of the table.
This can cause noticeable periods of increased utilization in reduced capacity
environments
Link state routing
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It use the principle of a link state to determine
network topology.
A link state is the description of an interface on a
router (for example, IP address, subnet mask, type
of network) and its relationship to neighboring
routers.
The collection of these link states forms a link state
database.
Example: OSPF
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Process:
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Each router identifies all other routing devices on the directly
connected networks.
Each router advertises a list of all directly connected network
links and the associated cost of each link. This is performed
through the exchange of link state advertisements (LSAs) with
other routers in the network.
Using these advertisements, each router creates a database
detailing the current network topology. The topology database in
each router is identical.
Each router uses the information in the topology database to
compute the most desirable routes to each destination network.
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This information is used to update the IP routing table
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Shortest-Path First (SPF) algorithm
 It
is used to process the information in the topology
database.
 It provides a tree-representation of the network.
 The device running the SPF algorithm is the root of the
tree.
 The output of the algorithm is the list of shortest-paths
to each destination network.
Path vector routing
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The path vector routing algorithm is similar to the
distance vector algorithm
 Each
border router advertises the destinations it can
reach to its neighboring router

Networks are advertised as destination addresses
and path descriptions to reach those destinations.
 In
distance vector: networks are advertised in terms of
a destination and the distance to that destination
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A route is defined as a pairing between a destination and
the attributes of the path to that destination
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The name, path vector routing, where the routers receive a vector
that contains paths to a set of destinations.
The path, expressed in terms of the domains (or
confederations) traversed so far, is carried in a special path
attribute that records the sequence of routing domains
through which the reachability information has passed.
The path represented by the smallest number of domains
becomes the preferred path to reach the destination.
Example: BGP
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Pros
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The computational complexity is smaller than that of the link state protocol.
The path vector computation consists of evaluating a newly arrived route and
comparing it with the existing one, while conventional link state computation
requires execution of an SPF algorithm.
Path vector routing does not require all routing domains to have homogeneous
policies for route selection; route selection policies used by one routing domain
are not necessarily known to other routing domains.
Only the domains whose routes are affected by the changes have to recompute.
Suppression of routing loops is implemented through the path attribute,
Route computation precedes routing information dissemination.
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Therefore, only routing information associated with the routes selected by a domain is
distributed to adjacent domains.
Path vector routing has the ability to selectively hide information.
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Cons
 Topology
changes only result in the recomputation of
routes affected by these changes, which is more
efficient than complete recomputation. However,
because of the inclusion of full path information with
each distance vector, the effect of a topology change
can propagate farther than in traditional distance
vector algorithms.
 Unless the network topology is fully meshed or is able
to appear so, routing loops can become an issue.
Hybrid routing
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These protocols attempt to combine the positive attributes of both
distance vector and link state protocols.
Like distance vector, hybrid protocols use metrics to assign a
preference to a route. However, the metrics are more accurate than
conventional distance vector protocols.
Like link state algorithms, routing updates in hybrid protocols are
event driven rather than periodic.
Networks using hybrid protocols tend to converge more quickly than
networks using distance vector protocols.
Finally, these protocols potentially reduce the costs of link state
updates and distance vector advertisements.
Although open hybrid protocols exist, this category is almost
exclusively associated with the proprietary EIGRP algorithm.

EIGRP was developed by Cisco Systems, Inc.
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Routing Information Protocol (RIP)
RIP packet types
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Request packets
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It queries neighboring RIP devices to obtain their distance vector table.
The request indicates if the neighbor should return either a specific subset or the
entire contents of the table.
Response packets
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A response packet is sent by a device to advertise the information maintained in
its local distance vector table.
The table is sent during the following situations:
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The table is automatically sent every 30 seconds.
The table is sent as a response to a request packet generated by another RIP node.
If triggered updates are supported, the table is sent when there is a change to the
local distance vector table.
When a response packet is received by a device, the information contained
in the update is compared against the local distance vector table.
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If the update contains a lower cost route to a destination, the table is updated to
reflect the new path.
RIP packet format
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RIP packets are transmitted using UDP datagrams.
RIP sends and receives datagrams using UDP port 520.
RIP datagrams have a maximum size of 512 octets.
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Updates larger than this size must be advertised in multiple
datagrams.
In LAN environments, RIP datagrams are sent using the MAC
all-stations broadcast address and an IP network broadcast
address.
In point-to-point or non-broadcast environments, datagrams
are specifically addressed to the destination device.
A 512 byte packet size allows a maximum of 25 routing
entries to be included in a single RIP advertisement.
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RIP modes of operation
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RIP hosts have two modes of operation:
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Active mode
Devices operating in active mode advertise their distance vector
table and also receive routing updates from neighboring RIP hosts.
 Routing devices are typically configured to operate in active
mode.
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Passive (or silent) mode
Devices operating in this mode simply receive routing updates
from neighboring RIP devices.
 They do not advertise their distance vector table.
 End stations are typically configured to operate in passive mode.
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Calculating distance vectors
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The distance vector table describes each destination
network.
The entries in this table contain the following
information:
The destination network (vector) described by this entry in
the table.
 The associated cost (distance) of the most attractive path to
reach this destination.
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It provides the ability to differentiate between multiple paths to a
destination.
The IP address of the next-hop device used to reach the
destination network.
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RIP distance vector algorithm
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At router initialization, each device contains a distance vector table listing each
directly attached networks and configured cost.
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Typically, each network is assigned a cost of 1.
This represents a single hop through the network.
The total number of hops in a route is equal to the total cost of the route.
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Each router periodically (typically every 30 seconds) transmits its distance vector
table to each of its neighbors.
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The router can also transmit the table when a topology change occurs.
Each router uses this information to update its local distance vector table:
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Cost can be changed to reflect other measurements such as utilization, speed, or reliability.
The total cost to each destination is calculated by adding the cost reported in a neighbor's
distance vector table to the cost of the link to that neighbor.
The path with the least cost is stored in the distance vector table.
All updates automatically supersede the previous information in the distance vector table.
This allows RIP to maintain the integrity of the routes in the routing table.
The IP routing table is updated to reflect the least-cost path to each destination.
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RIP routing table update example
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converged
Count to infinity problem
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A converged internetwork
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For simplicity, assume that the announcements sent
by Router 1 on Network 1 and Router 2 on Network
3 are not included.
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Now assume that the link from Router 2 to Network 3 fails
and is sensed by Router 2.
As shown in Figure, Router 2 changes the hop count for the
route to Network 3 to indicate that it is unreachable, an
infinite distance away.
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For RIP for IP, infinity is 16
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However, before Router 2 can advertise the new hop count to Network 3 in
a scheduled announcement, it receives an announcement from Router 1.
The Router 1 announcement contains a route to Network 3 which is two hops
away.
Because two hops away is a better route than 16 hops, Router 2 updates its
routing table entry for Network 3, changing it from 16 hops to three hops
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When Router 2 announces its new routes, Router 1 notes that
Network 3 is available three hops away through Router 2.
Because the route to Network 3 on Router 1 was originally learned
from Router 2, Router 1 updates its route to Network 3 to four hops.
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When Router 1 announces its new routes, Router 2 notes that Network 3 is available
four hops away through Router 1.
Because the route to Network 3 on Router 2 was originally learned from Router 1,
Router 2 updates its route to Network 3 to five hops.
The two routers continue to announce routes to Network 3 with higher and higher
hop counts until infinity (16) is reached.
Then, Network 3 is considered unreachable and the route to Network 3 is eventually
timed out of the routing table.
This is known as the count-to-infinity problem.
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To minimize this exposure, whenever a network is
unavailable, the incrementing of metrics through routing
updates must be halted as soon as it is practical to do so.
In a RIP environment, costs continue to increment until they
reach a maximum value of 16. (This limit is defined in RFC
1058)
A side effect of the metric limit is that it also limits the
number of hops a packet can traverse from source network
to destination network.
In a RIP environment, any path exceeding 15 hops is
considered invalid.
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The routing algorithm will discard these paths
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To help reduce the convergence time of RIP for IP
internetworks and to avoid count-to-infinity and
routing loops in most situations, we can enable the
following modifications to the RIP announcement
mechanism:
 Split
horizon
 Split horizon with poison reverse
 Triggered updates
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Split Horizon
Split horizon helps reduce convergence time by not allowing routers to advertise networks in
the direction from which those networks were learned.
The only information sent in RIP announcements are for those networks that are beyond the
neighboring router in the opposite direction.
Networks learned from the neighboring router are not included.
Split horizon eliminates count-to-infinity and routing loops during convergence in single-path
internetworks and reduces the chances of count-to-infinity in multi-path internetworks.
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Split Horizon with Poison Reverse
Split horizon with poison reverse differs from simple split horizon because it announces all networks.
However, those networks learned in a given direction are announced with a hop count of 16, indicating that the
network is unreachable.
In a single-path internetwork, split horizon with poison reverse has no benefit beyond split horizon.
However, in a multipath internetwork, split horizon with poison reverse greatly reduces count-to-infinity and routing
loops.
Count-to-infinity can still occur in a multipath internetwork because routes to networks can be learned from multiple
sources.
In Figure, split horizon with poison reverse advertises learned routes as unreachable in the direction from which they
are learned.
Split horizon with poison reverse does have the disadvantage of additional RIP message overhead because all
networks are advertised.
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Hold down timers
After hearing a route poisoning, router starts a hold-down
timer for that route.
 If it gets an update with a better metric than the originally
recorded metric within the hold-down timer period, the holddown timer is removed and data can be sent to that
network.
 Also within the hold-down timer, if an update is received
from a different router than the one who performed route
poisoning with an equal or poorer metric, that update is
ignored.
 During the hold-down timer, the “downed” route appears as
“possibly down” in the routing table.

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Triggered Updates
 Triggered updates allow a RIP router to announce changes in metric
values almost immediately rather than waiting for the next periodic
announcement.
 The trigger is a change to a metric in an entry in the routing table.
 For example, networks that become unavailable can be announced with
a hop count of 16 through a triggered update.
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Note that the update is sent almost immediately , where a time interval to
wait is typically specified on the router.
If triggered updates were sent by all routers immediately, each triggered
update could cause a cascade of broadcast traffic across the IP internetwork.
Triggered updates improve the convergence time of RIP internetworks
but at the expense of additional broadcast traffic as the triggered
updates are propagated
RIP limitations
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Path cost limits
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Network-intensive table updates
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Periodic broadcasting of the distance vector table can result in increased
utilization of network resources.
This can be a concern in reduced-capacity segments.
Relatively slow convergence
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The resolution to the counting to infinity problem enforces a maximum cost for a
network path.
Networks requiring paths greater than 15 hops must use an alternate routing
protocol.
RIP, like other distance vector protocols, is relatively slow to converge.
The algorithms rely on timers to initiate routing table advertisements.
No support for variable length subnet masking
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Route advertisements in a RIP environment do not include subnet masking
information.
This makes it impossible for RIP networks to deploy variable length subnet masks.
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Routing Information Protocol Version 2
(RIP-2)
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
RIP Version 1 (RIP-1)
 This

protocol is described in RFC 1058.
RIP Version 2 (RIP-2)
 RIP-2
is also a distance vector protocol designed for
use within an AS.
 It was developed to address the limitations observed in
RIP-1.
 RIP-2 is described in RFC 2453.
 The standard (STD 56) was published in late 1994.
RIP-2 additional benefits
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Support for CIDR and VLSM
Support for multicasting
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Support for authentication
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RIP-2 supports authentication of any node transmitting route
advertisements.
This prevents fraudulent sources from corrupting the routing table.
Support for RIP-1
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This reduces the processing load on hosts not listening for RIP-2 messages
To ensure interoperability with RIP-1 environments, this option is
configured on each network interface.
RIP-2 is fully interoperable with RIP-1. This provides backwardcompatibility between the two standards.
In RIP-1 as well as RIP-2 networks paths with a hop-count greater
than 15 are interpreted as unreachable.
RIP-2 packet format
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The first entry in the
update contains either a
routing entry or an
authentication entry.
If the first entry is an
authentication entry, 24
additional routing
entries can be included
in the message.
If there is no
authentication
information, 25 routing
entries can be provided.
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Version The value contained in this field must be two. This instructs RIP-1
routers to ignore any information contained in the previously unused fields.
AFI (Address Family) A value of x’0002’ indicates the address contained
in the network address field is an IP address. An value of x'FFFF' indicates
an authentication entry.
Authentication Type This field defines the remaining 16 bytes of the
authentication entry. A value of 0 indicates no authentication. A value of
two indicates the authentication data field contains password data.
Authentication Data This field contains a 16-byte password.
Route Tag This field is intended to differentiate between internal and
external routes. Internal routes are learned through RIP-2 within the same
network or AS.
Subnet Mask This field contains the subnet mask of the referenced network.
Next Hop This field contains a recommendation about the next hop the
router should use when sending datagrams to the referenced network.
RIP-2 limitations
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The path cost limits and slow convergence inherent
in RIP-1 networks are also concerns in RIP-2
environments.
There are limitations to the RIP-2 authentication
process.
 The
RIP-2 standard does not encrypt the authentication
password.
 It is transmitted in clear text.
 This makes the network vulnerable to attack by anyone
with direct physical access to the environment.
RIPng for IPv6
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RIPng was developed to allow routers within an IPv6based network to exchange information used to
compute routes.
It is documented in RFC 2080.
is a distance vector protocol designed for use within a
small autonomous system.
RIPng uses the same algorithms, timers, and logic used in
RIP-2.
RIPng has many of the same limitations inherent in other
distance vector protocols.
Path cost restrictions and convergence time remain a
concern in RIPng networks.
Differences between RIPng and RIP-2
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Support for authentication:
RIPng does not include any native authentication support.
 RIPng uses the security features inherent in IPv6
 In addition to authentication, these security features provide
the ability to encrypt each RIPng packet.
 One consequence of using IPv6 security features is that the
AFI field within the RIPng packet is eliminated.
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There is no longer a need to distinguish between authentication
entries and routing entries within an advertisement.
Support for IPv6 addressing formats
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The fields contained in RIPng packets were updated to
support the longer IPv6 address format.