DISTANCE VECTOR ROUTING Flashcards
Describe Distance Vector Routing Protocols
- Advertise networks as vectors of distance
and direction
What are the divisions of Distance Vector Routing Protocols
− Classful routing protocols do not include a subnet
mask in the routing update
− Classless routing protocols include a subnet mask in the routing update
What are the General Characteristics of DV Routing Protocols
- Periodic Updates
* Updates to the routing tables are sent at the end of a certain time
period e.g. 90 seconds for IGRP, 30 seconds for RIP - Triggered Updates
* when changes occur in the topology the router immediately sends
out an update without waiting for the end of the periodic update
interval - Full Routing Table Update
* the entire routing table (not only entries which change) is sent to
their neighbors - Route invalidation timers
* They use various timers to invalidate routing table entries if they
are not refreshed
* e.g. for RIP if no update is received from a specific neighbor
within the 30 seconds, it will use various timers to invalidate the
entries as it waits for a valid update - Have an incomplete view/ knowledge of the network
topology
* Routers do not know the details about the network’s topology
beyond a neighboring router
* “routing by rumor” - They are generally prone to routing loops
* Generally distance vector protocols take longer to converge due to the time taken to calculate routes and forward updates, thus updates about changes may not be processed
in a timely manner.
Describe the routing table and its table entries
- Each router maintains a routing table, where
each node/router in the network has an entry in
the table - Each table entry contains two parts:
1. the outgoing line (interface) for the destination (node)
and
2. the distance (time, cost, whatever metric is used) to
the destination - Each router knows the distance to its neighbors
What is the first step of how DV Protocols build routing tables
. Initialization – each node knows only the distance
between itself and immediate neighbor (those directly
connected to it
* At this point only directly connected networks are initially
placed in the routing table
*See 6
What is the second step of how DV Protocols build routing tables
- Initial Exchange of Routing Information
* If a distance vector routing protocol is configured then the
routers will exchange routing information (each router
passes its routing table to the neighbor)
* Routing updates will be received from other directly
connected routers
What happens at the initial exchange if there is new information
- If there is new information:
the metric is re-calculated to determine the new minimum distance
the outgoing link for the destination is also recorded
and the new information is stored in routing table - If multiple routes to the same destination network exist, the router
chooses the route with the lowest metric
What is the third step of how DV Protocols build routing tables
- Exchange of more Routing Information
* Routers continue to exchange routing information
* If no new information is found then convergence is
reached
What is the third step of how DV Protocols build routing tables
- Exchange of more Routing Information
* Routers continue to exchange routing information
* If no new information is found then convergence is
reached
When is a network said to be converged
The network is converged when all routers have
complete and accurate information about the
entire network:
A network is not completely operable until the
network has converge
What is convergence time
Convergence time is the time it takes routers to
share information, calculate best paths, and
update their routing tables.
What does achieving convergence depend on
a) The speed of propagation of routing information
and
* The speed of propagation refers to the amount
of time it takes for routers within the network to
forward routing information
b) The speed of calculation of optimal paths
Describe Router Information Protocol (RIP)
- Was among the first IGP used over the Internet
- Is designed as an IGP to be used among routers
within an autonomous system - Is a distance vector protocol
What are the various versions of RIP
- RIP version 1-for IP version 4 networks
- RIP version 2- for IP version 4 networks
- RIPng - for IP version 6 networks
What are some common Characteristics of RIP v1 RIP v2 and RIPng:
- Used for routing within an autonomous system
(intradomain routing) - Metric used is - hop count only - i.e. RIP measures
distance in network hops - Maximum hop count is 15 hops - thus is ideal for small
networks - Has a default administrative distance (AD) of 120
− AD measures the preference of one source of routing information
as opposed to another.
− The lower the AD the preferable the source - Marks a network as unreachable if the hop count
reaches16
− Also known as ‘route poisoning’ - Uses Bellman Ford’s Algorithm/Ford Fulkerson’s
Algorithm - Uses two message types: request and response
− Request - used to ask neighboring nodes (routers) for an update
− Response - contains an update - Rely on periodic updates i.e. they periodically send updates every 30 seconds
- Use invalidation timers “garbage collection” timers to
flash unreachable networks from the routing tables
What are some characteristics of RIP Version 1
- Periodically broadcasts it’s routing updates using the address:
255.255.255.255 - Does not support MD5 authentication thus is relatively insecure
* i.e. can only send updates in plain text and the receiving
interface does not authenticate the source - Uses transport layer port 520
- Does not support CIDR
* When supernets are configured, the protocol does not forward
the supernet routes in the update since it cannot forward
updates with a mask smaller than that of the default address
class - Does not support VLSM
* Since the protocol does not include a subnet mask in the routing update, when VLSM is used on a routing domain, some subnets
will not propagated in the update, hence the devices on these networks will not be reachable (see 8 for eg on this) - Does not support discontiguos networks/subnets (see 9 for eg on this)
Give characteristics of RIP 2
- Periodically multicasts it’s routing updates using the address:
224.0.0.9 - Supports MD5 authentication though it is not configured by
default
* RIP version 2 is able to authenticate the source of routing
information so that the router only installs updates from trusted
sources - Uses transport layer port 520
- Supports CIDR
* When supernets are configured, the protocol forwards the
supernet route with its subnet mask - Supports VLSM
* Since it sends a subnet mask in the routing update, all subnets
are propagated in the update regardless of their masks - Can be configured where there are subnets discontiguos
Give characteristics of RIP Next Generation (RIPng)
- Configured in IP version 6 networks
- Uses transport layer port 521
- Periodically multicasts its updates using the IPv6 address:
FF02::9 - RIPng does not include its own authentication mechanism.
* It is assumed that if authentication and/or encryption are
needed, they will be provided using the standard IPSec
features defined for IPv6 at the IP layer
* This is more efficient than having individual protocols like
RIPng perform authentication.
Discuss The Enhanced Interior Gateway
Routing Protocol (EIGRP)
- It is a Cisco-proprietary interior gateway, classless
distance-vector routing protocol - EIGRP is the classless version of IGRP
- It is ideal for large, multiprotocol networks built
primarily on Cisco routers - NB: some of its features are now open and have
been adapted for use on multi-vendor devices
Give some EIGRP Characteristics
- Uses the Diffusing Update Algorithm (DUAL)
* DUAL guarantees loop-free and backup paths throughout the
routing domain - Uses Partial and Bounded updates
* Instead of periodic updates, EIGRP sends partial triggered updates
when a path or metric changes (i.e. partial updates)
* Only those routers that require the information are updated
minimizing bandwidth use (i.e. bounded updates) - Can perform both equal and unequal cost load balancing
* This allows administrators to better distribute traffic flow in their
networks - Uses the Reliable Transport Protocol (RTP) at transport layer
to deliver EIGRP packets to neighbors
* It does not rely on TCP at transport layer
* This ensures multiprotocol support - It uses FIVE message types: Hello, Ack, Update, Query,
and Reply - It generally used bandwidth and delay as the default routing
metrics but can be configured to include load and reliability
as part of the metric calculation - Multicasts updates using the addresses: 224.0.0.10 (for
IPv4) and FF02::A (for IPv6) - At transport layer port 88 is used to identify EIGRP
communication - It is a classless protocol hence supports CIDR, VLSM and
networks with discontiguious subnets - EIGRP supports authentication
―authentication ensures that routers
Discuss the five message types of EIGRP
i. Hello packets
―are used to discover and form adjacencies with neighbors.
―On hearing hellos, a router creates a neighbor table and the
continued receipt of Hellos maintains the table
ii. EIGRP Update packets
―are used to propagate routing information
―Sent to initially exchange topology information or topology change
―EIGRP updates are partial (i.e. only contain needed routing
information and bounded (i.e. are unicast to routers that require it).
iii. EIGRP Acknowledgement (EIGRP Ack)
―Used to acknowledge the receipt of Update packets, Query packets,
and Reply packets
iv. Query Packets – Used by DUAL to search for specific routing
information from EIGRP enabled neighbors
v. Reply Packets – sent in response to a query
