Dynamic Routing and OSPF (part 1). IP routing zEach router or host makes its own routing decisions...
-
Upload
erica-perry -
Category
Documents
-
view
223 -
download
0
Transcript of Dynamic Routing and OSPF (part 1). IP routing zEach router or host makes its own routing decisions...
Dynamic Routing and OSPF (part 1)
IP routingEach router or host makes its own routing
decisionsSending machine does not have to
determine the entire path to the destinationSending machine just determines the next-
hop along the path. This process is repeated until the destination is
reachedForwarding table consulted to determine
the next-hop
IP routing
Classless routing route entries include
destinationnext-hopmask (prefix-length) indicating size of address space
described by the entry
Longest match for a given destination, find longest prefix
match in the routing table example: destination is 35.35.0.0/19
routing table entries are 35.0.0.0/8 and 35.35.0.0/16
IP routing
Default route where to send packets if don’t have an
entry for the destination in the routing table
most machines have a single default route
often referred to as a default gateway
Static routing
each router manually configured with a list of destinations and the next hop to reach those destinations
ideal for small number of destinations or “stub” networks stub network - network with only one or
two paths to the rest of the network
Dynamic Routing
routers compute routing tables dynamically based on information provided by other routers in the network
routers communicate topology to each other via different protocols
routers then compute one or more next hops for each destination - trying to calculate the most optimal path
Static and Dynamic Routing
Static routing is a simplistic approachShortcomings:
Cumbersome to configure Cannot adapt to link/node failures, addition of new
nodes and links Doesn't scale to large networks
Solution: Dynamic Routing
Desirable Characteristics
Automatically detect and adapt to network topology changes
Optimal routingScalabilityRobustnessSimplicitySpeed of convergenceSome control of routing choices (e.g. which links
we prefer to use)
Convergence - Why do I care?
Convergence is when all the routers have the same routing information
When a network is not converged, there is network downtime Packets don't get to where they are supposed to be
going: routing loops, black holes Occurs when there is a change in the status of a router
or link
Dynamic Protocols
Metrics can be calculated based on a single characteristic of a path or by combining multiple characteristics
Metrics commonly used: Bandwidth Hop count Cost
administratively defined metrics
OSPF magic exercisedelete your static routes
config t no ip route x.x.x.x y.y.y.y z.z.z.z
enter the following: router ospf 1 network x.x.x.x 0.0.0.0 area 0 x.x.x.x = ip address of your backbone
interface redistribute connected subnets
OSPF magic exercise
Verify connectivity to all PCs in the network
Do not save your config
Dynamic Routing Protocols and OSPF (part 2)
Types of Routing Protocols
EGP Exterior Gateway Protocol Example: BGP
IGP Interior Gateway Protocol Example: OSPF, RIP
Types of Routing Protocols
Link-stateDistance-vector
IGP
Used within a single Autonomous System (AS)
Within a single network
Other Interior Gateway Protocols (IGPs)
RIP Lots of scaling problems RIPv1 is classful and officially obsolete RIPv2 is classless
EIGRP Proprietry (Cisco only)
IS/IS The forerunner of OSPF Multiprotocol (OSPF is IP only)
Distance Vector Protocols
Listen to neighboring routesInstall all routes in a tableAdvertise all routes in tableVery simpleVery Stupidexample: RIP
RIP
routing information protocoldistance-vector algorithmcost is hop countbroadcast information to all
neighbors every 30 seconds
RIP
A B
D E
C
ROUTING TABLE for AA -B 1C 2D 3E 2
Why not use RIP?
Distance Vector algorithmBroadcasts everything (not scalable)Metric is hop-count onlyInfinity of 16 (not large enough)Slow convergence (routing loops)Poor robustness
OSPF
Open Shortest Path FirstDynamic IGP (Interior Gateway Protocol)
Use within your own networkLink state algorithm
Shortest Path First
A B
C D
15
3
4 4
7
Metric: Link Cost
Link State Algorithm
Each router maintains a database containing map of the whole topology Links State (including cost)
All routers have the same informationAll routers calculate the best path to
every destination
Link State Algorithm (con)
Any link state changes are flooded across the network
"Global spread of local knowledge”
Link State vs. Distance vectorDistance Vector
views net topology from neighbor’s perspective
adds distance vectors from route to router
frequent, periodic updates; slow convergence
passes copies of routing table to neighbor routers
Link State vs. Distance vectorLink-State
gets common view of entire network topology
calculates the shortest path to other routers
event-triggered updates; faster convergence
passes link-state routing updates to other routers
Distance Vector and Link State ProtocolsDistance vector routers compute the
best path from information passed to them from neighbors
Link State routers each have a copy of the entire network map
Link State routers compute best routes from this local map
Note: Routing is not the same as Forwarding
Forwarding: passing packets along to the next hop There is only one forwarding table Just has prefix and next-hop info
Routing: populating the forwarding table You might have multiple routing databases - e.g.
both OSPF and BGP Routing databases have more information
Routing and Forwarding
OSPF
BGP
Static
ForwardingTable
On Cisco, if the same prefix is received from multiple protocols, the "administrative distance" is used to choose between them
OSPF
open shortest path firstdynamic IGPnot distance vectorLink-State algorithm
OSPF: How it works (1)
"Hello" packets sent periodically on all OSPF-enabled interfaces become "neighbors" establishes that link can carry data used to determine if neighbor is up
Adjacencies (virtual point-to-point links) formed between some neighbors
How it works (2)
Once an adjacency is established, trade information with your neighbor
Topology information is packaged in a "link state announcement"
Announcements are sent ONCE, and only updated if there's a change (or every 30 minutes)
How it works (3)
Each router sends Link State Announcements (LSAs) over all adjacencies LSAs describe router's links, interfaces and state
Each router receives LSAs, adds them into its database, and passes the information along to its neighbors
How it works (4)
Each router builds identical link-state database
Runs SPF algorithm on the database to build SPF tree
Forwarding table built from SPF tree
How it works (5)
When change occurs: Broadcast change All routers run SPF algorithm Install output into forwarding table
HELLO
Broadcast* HELLO on network segmentReceive ACKEstablishes 2-way communicationRepeat periodically
Default: HELLO sent every 10 seconds Default: if no HELLO heard for 40 seconds, link is
assumed to be deadNow establish adjacencies
* Actually uses Multicast addresses (224.0.0.9, 224.0.0.10) sothat non-OSPF devices can ignore the packets
The HELLO packet
Router priority Hello interval Router dead interval Network mask List of neighbors These must match
HELLO
HELLO HELLO
Neighbors
Bi-directional communicationResult of OSPF hello packetsNeed not exchange routing information
Who is adjacent?
"Adjacent" neighbors exchange routing information
Not all neighbors are adjacentOn a point-to-point link
everyoneOn broadcast medium
not everyone why?
Broadcast neighbors
A B
C D
Order of N^2 adjacencies
Broadcast medium
Select a neighbor: Designated Router (DR)All routers become adjacent to DRExchange routing information with the DRDR updates all the other neighborsScales
Adjacencies reduced from N^2 to 2NBackup Designated Router (BDR)
LSAs propagate along adjacencies
DR BDR
Other nice features of OSPFAuthentication (optional)
Equal-cost multipath more than one "best" path - share traffic
Proper classless support (CIDR)Multiple areas
For very large networks (>150 routers) Aggregate routes across area boundaries Keep route flaps within an area Proper use of areas reduce bandwidth and CPU utilisation Backbone is Area 0
Cisco OSPF commands and configurationshow ip routeshow ip ospf neighborshow ip ospf database
Configuring OSPF
router ospf <process-id>network x.x.x.x m.m.m.m area <area-id>m.m.m.m = wildcard mask0 = don’t care bit1 = check bit0.0.0.0 mask for exact matchnetwork 203.167.177.10 0.0.0.0 area 0network 203.167.177.0 0.0.0.255 area 0
A
C
B
FE
I
G
D
H
J
RouterPC
HUB
RouterPC
HUB
RouterPC
HUB
RouterPC
HUB
RouterPC
HUB
Router PC
HUB
Router PC
HUB
Router PC
HUB
Router PC
HUB
Router PC
HUB
SWITCH
Classroom Layout
A
C
B
FE
I
G
D
H
J
133.27.162.96/28
133.27.162.128/28
133.27.162.160/28
133.27.162.192/28
133.27.162.224/28
133.27.162.112/28
133.27.162.144/28
133.27.162.176/28
133.27.162.208/28
133.27.162.240/28
Serial Links for exercise
133.
27.1
62.1
6/28
133.27.162.48/30
133.27.162.52/30
133.27.162.56/30
133.27.162.60/30
133.27.162.64/30