Transcript Dynamic Routing

General Overview
RD-CSY2001
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Review
Routing
• Static Routing
• Dynamic Routing
◦ Characteristics of Distance Vector Routing protocols.
◦ Network discovery process using Routing Information Protocol
(RIP).
◦ Identify the conditions leading to a routing loop and explain the
implications for router performance.
◦ Recognize that distance vector routing protocols are in use
today
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IP routes to subnets, not to
individual hosts
Two hosts on different IP subnets
have to go through one or more
routers.
◦ Even if they are on the same
“physical” network
Two hosts or routers on a common
subnet can send packets “directly” to
one another
Two routers cannot exchange routing
information directly unless they have
one or more IP subnets in common
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IP-A
MAC-A
Packets forwarded using destination host’s IP
address
Packet travel independently
◦ packets between same source-destination pair may
take different paths
IP-B
application
transport
network
data link
physical
MAC-B
1. Send data
R
IP-R
R 1
MAC-R
Which is
the default
gateway ?
R
2. Receive data
IP-R
MAC-R 2
R
Internetwork
R - Router
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application
transport
network
data link
physical
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Routing - Selecting the optimal
path towards destination using
Routing table for path selection
using,
 Static Routing - A network
administrator manually
enters route into the router's
configuration
 Dynamic Routing -Route
knowledge is updated
automatically by a routing
process
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Specifying Outgoing Interface
Specifying the Next-hop IP Address
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LAN
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When a LAN is
connected to a Router,
Default Gateway is the
address of router
interface connected to
Default Gateway
that LAN.
The IP address and
subnet mask is on the
same network as the
hosts on that network
Router
Router
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It is impractical, probably
impossible, and completely
unnecessary for the Company X
routers to have detailed knowledge
of the Internet.
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The default route concept assumes
that data forwarded along the default
route will eventually encounter a
router that DOES know the path to
the destination.
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Function(s) of Dynamic Routing
Protocols:
◦ Dynamically share information
between routers.
 What information and with
whom (next neighbours or all) is
it shared?
◦ Automatically update routing table
when topology changes.
 When (periodicity) are updates
made?
◦ Determine best path to a destination.
 How is best path found?
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There are two basic routing algorithms found on the
Internet.
1. Distance Vector Routing
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Each node knows the distance (=cost) to its directly connected neighbors
A node sends periodically a list of routing updates to its neighbors.
If all nodes update their distances, the routing tables eventually converge
New nodes advertise themselves to their neighbors
2. Link State Routing
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Each node knows the distance to its neighbors
The distance information (=link state) is broadcast to all nodes in the network
Each node calculates the routing tables independently based on topology
information
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Two Main Approaches
 Distance Vector ProtocolsFind route using:
 Distance to final destination
 Vector, or direction, traffic
should be directed
 e.g., RIP (Routing
Information Protocol)
 Link State Protocols - develop
topology information and apply
routing algorithm to find routes to
destination networks
◦ E.g., OSPF (Open Shortest
Path First)
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 Periodic updates to exchange information
with neighbours
 Entire routing table is included with
routing every 30 sec.
 No advertisement for 180 sec:
neighbor/link declared dead
 routes via neighbor invalidated
 Triggered updates: New advertisements
sent to neighbors
 Neighbors in turn send out new
advertisements (if tables changed)
 Link failure info quickly propagates to
entire network
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Initial Exchange of Routing
Information- Routing updates
received from other routers
◦ Router checks update for new
information
 If there is new information
 Metric is updated
 New information is stored
in routing table
◦ Convergence is reached
when all routing tables in the
network contain the same
network information
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RIP uses 4 timers
◦ -Update timer
◦ -Invalid timer
◦ -Holddown timer
◦ -Flush timer
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A
A's Routing Table
to
C
via
(next hop)
B's Routing Table
cost
B
1
B
via
to
2
(next hop)
C
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cost
C
1
-
oo
A
3
now link B-C goes down
C
B
2
C
C
C
-
2
oo
C
C
C
B
C
4
C
oo
C
oo
C
•
3
C
4
•
-
oo
C
The reason for the
count-to-infinity
problem is that each
node only has a “nexthop-view”
For example, in the first
step, A did not realize
that its route (with cost
2) to C went through
node B
How can the Count-toInfinity problem be
solved?
oo
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How can the Count-to-Infinity problem be solved?
Solution 1: Always advertise the entire path in an update
message (Path vectors)
 If routing tables are large, the routing messages require
substantial bandwidth
 BGP uses this solution
Solution 2: Never advertise the cost to a neighbor if this
neighbor is the next hop on the current path (Split Horizon)
 Example: A would not send the first routing update to B,
since B is the next hop on A’s current route to C
 Split Horizon does not solve count-to-infinity in all
cases!
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RIPv2 is an extends
RIPv1:
◦ Subnet masks are carried
in the route information
◦ Authentication of routing
messages
◦ Route information carries
next-hop address
◦ Exploites IP multicasting
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Extensions of RIPv2 are
carried in unused fields
of RIPv1 messages
RIPv2 Message
IP header UDP header
Command Version
Set to 00.00
0xffff
Authentication Type
Password (Bytes 0 - 3)
Password (Bytes 4 - 7)
Password (Bytes 8- 11)
Password (Bytes 12 - 15)
Authetication
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Up to 24 more routes (each 20 bytes)
32 bits
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RIP takes a long time to stabilize
◦ Even for a small network, it takes several minutes until the
routing tables have settled after a change
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RIP has all the problems of distance vector algorithms,
e.g., count-to-Infinity
 RIP uses split horizon to avoid count-to-infinity
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The maximum path in RIP is 15 hops
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debug ip rip
show ip rip database
show ip protocols {summary}
show ip route
debug ip rip {events}
show ip interface brief
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