- Basics of Routing
- Static Routing/Dynamic Routing
- Classification of Dynamic Routing
- Administrative Distance and Metric
- Link State Routing and Distance Vector Routing
- Routing Information Protocol (RIP)
- Enhanced Interior Gateway Routing Protocol (EIGRP)
- Open Shortest Path First (OSPF)
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ROUTER BOOTUP PROCESS
Three stage process
Perform POST (Power On Self Test) and load Bootstrap Program
Locate and load the Cisco IOS software
Bootstrap Program performs this task
By default IOS is stored in flash memory
Locate Startup Configuration file or enter in Setup Mode.
Bootstrap searches for startup config (present in NVRAM)
Or else goes in Setup Mode.
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EDITING COMMANDS FOR NETWORK GEEKS
Commands Explanation
Ctrl+A Moves the cursor to the begining of the line
Ctrl+E Moves the cursor to the end of the line
Esc+B Moves back one Word
Esc+F Moves forward one Word
Ctrl+R Redisplays a line
Ctrl+U Erases a line
Ctrl+W Erases a word
Ctrl+Z Ends configuration mode
TAB Finishes command for you
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EDITING COMMANDS FOR NETWORK GEEKS
Commands Explanation
Ctrl+P Shows Last entered command
Ctrl+N Shows Previous commands entered
Show history Show last 10 commands entered by default
Show terminal Shows terminal configurations and history buffer size
Terminal history size Changes buffer size (max 256)
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BASIC CONFIGURATION AND IMPORTANT
COMMANDS
Configuring Hostname:
Router(config)# hostname <NAME>
Configuring Banner:
Router(config)#banner motd ^c <TYPE THE BANNER>
Configuring Password:
Router(config)#enable password
Router(config)#enable secret
Interface configuration
Router(config)#
…and many more
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ROUTING
IMPORTANT STATEMENTS:
For end to end communication, it is not enough to have only forwarding
path, but having reverse path is also important.
If you can reach one host on a network that does not mean you can reach all
hosts on that network.
Initially router knows only about its directly connected neighbors, hence
can only communicate with them.
Router must know the route to reach the destination network
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STATIC vs DYNAMIC
There are two ways to provide routes to the router
Static Routing
Provide the route manually
Not very helpful in larger network
Better when there is a need to provide a specific route
Dynamic Routing
Configuring Routing Protocols on routers
Very useful in larger network
Calculates the best path automatically
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LEARNING A ROUTE THROUGH MULTIPLE
ROUTING PROTOCOLS
PROBLEM:
If a route is learnt from static entry/any routing protocol and same is learnt from
other routing protocol and both paths are different, then which path to prefer?
SOLUTION:
Administrative Distance (AD)
This is a value associated with each routing protocol and also with static route.
Lower the AD value better the path.
So, the router will prefer the path dictated by the one with LOWER AD value.
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ONE ROUTING PROTOCOL MULTIPLE PATHS
PROBLEM:
If there is only one Routing Protocol running on the router and it receives two
different routes to reach same network, then which one to prefer?
SOLUTION:
Metric:
This is a value associated with the path. Every routing protocol has a way to
calculate its own metric. Lower the Metric Better the path and hence preferred.
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DYNAMIC ROUTING- ROUTING PROTOCOLS
Routing protocols can be categorized as
Interior Gateway Protocol
Exterior Gateway Protocol
Routing Protocols can also be categorized as
Classful Protocols
Classless Protocols
…and also as
Link state Routing
Distance Vector Routing
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CLASSFUL ROUTING
Classful Routing Protocols do not send subnet mask with
its route information
Classful Routing automatically summarizes the mask at
Classful boundaries.
May lead to Sub-optimal paths.
Examples of Classful Routing Protocols:
• RIP Version 1 (RIPv1)
• IGRP
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DISTANCE VECTOR ROUTING PROTOCOL
These are the protocols in which…
Updates are sent periodically (every 30/90 seconds)
Entire Routing Table is sent as an update
Updates are broadcasted
Updates are sent to directly connected neighbors only and not to
the entire group
Routers don’t have end-to-end visibility of the entire network,
Directly connected neighbors are the worlds
Convergence is slow.
RIP, IGRP and BGP are DISTANCE VECTOR Routing Protocols
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ROUTING INFORMATION PROTOCOL
Used for smaller networks
Metric: Hop Count
Maximum Hop Count: 15
Administrative Distance: 120
Load Balancing: over equal metric path (4 by default)
RIP versions: RIPv1 and RIPv2
RIPv1 sends updates on Broadcast address
RIPv2 sends updates on Multicast address 224.0.0.9
Encrypted Authentication between 2 RIPv2 routers
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RIP TIMERS
• Update Timer: (default: 30 sec): Indicates how often the router will send out a routing
update
• Invalid Timer: (default: 180 sec): Indicates how long the route will remain in the
routing table before it gets invalid, if no new updates are received
• Hold-Down Timer: (default: 180 sec): Indicate how long RIP will suppress the route
that it has placed in HOLD state.
– HOLD state:
• Invalid timer has expired.
• Update received for a router marking that route with Metric 16 (unreachable)
• An update received from a router, with a higher metric than what is currently in
the routing table. (to prevent loops)
• Flush Timer: (default: 240 sec): Indicates how long a route can stay in the routing table
before its been flushed, if updates are not received
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LINK STATE ROUTING PROTOCOL
These are the protocols in which
Updates are triggered and not periodic
Updates are incremental, entire routing table is not sent as a part of
routing update
Updates are sent as a multicast and not as broadcast
Convergence is fast as updates are triggered
Routers have end to end visibility of entire network through
topology table.
OSPF and ISIS are Link State Routing Protocols
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ADVANCED DISTANCE VECTOR ROUTING PROTOCOL
Combination of both Distance vector and Link State routing
protocols
Best features of both are used in this
Best feature of D.V Simple Configuration
Best feature of L.S triggered updates, convergence is fast
etc.
Used only in EIGRP
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ENHANCED INTERIOR GATEWAY ROUTING PROTOCOL
(EIGRP)
Classless Routing Protocol
Cisco Proprietary Protocol
Advanced Distance Vector Protocol
Uses DUAL (Diffusion Update Algorithm) to determine Best Path to
reach destinations
Neighbors are formed with only directly connected routers
Mostly the EIGRP traffic is sent on Multicast Address 224.0.0.10
Updates are incremental and triggered
Summarization can be done on any router interface manually
Administrative Distance:
Internal EIGRP : Routes originating within local AS- 90
External EIGRP: Routes coming from outside AS- 170
Summary EIGRP: 5
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EIGRP
Maintains 3 tables
Neighbor Table: List of all neighbor routers
Topology Table: List of all routes in the AS
Routing Table: Best routes to reach all networks
Load Balancing:
Load balancing on unequal metric path is possible.
Default value: 4; Max Value: 6
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EIGRP OPERATION
When routers are up, they exchange Multicast Hellos.
Neighbor relation is formed after this exchange. Only directly
connected routers can become neighbors
Topology table will be exchanged, till every router reaches converged
state
After every router reaches converged state…
No more exchange of Topology table, only updates will be sent after
modification in the network topology.
EIGRP DUAL is run to find the best path to reach all networks
Multicast echoes are exchanged to check the connectivity (Keep
Alive)
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EIGRP METRIC
EIGRP is called COMPOSIT METRIC
Bandwidth (K1) and Delay of the line (K3) is used by default to
calculate the distance Metric
Reliability (K4/K5), MTU and Load (K2) are the other 3 parameters
that can be considered
By default:
EIGRP METRIC=
10000000
𝐵𝑊 (𝐾𝑏𝑝𝑠)
+ delay * 256
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EIGRP- EXTRA
EIGRP elects Next Best path at the same time, when it elects BEST
PATH
Best Path is called as SUCCESSOR
Next Best Path is called as FEASIBLE SUCCESSOR
RULES:
For a path to be considered as a feasible successor, its Advertised
Distance should be less than current Feasible Distance.
For a path to be considered as a valid path, its AD should be less
than 2 times current FD
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OPEN SHORTEST PATH FIRST- OSPF
Linked State Routing Protocol
Metric: COST
Higher the Bandwidth Lower the Cost, Lower the Cost Better the Path.
𝐶𝑂𝑆𝑇 =
108
𝐵𝑊
Determine the Best Path to reach all networks: Dijkstra Shortest Path
Algorithm
AD Value: 110
Sends Mask as a part of updates hence supports VLSM
Updates are triggered
Load Balancing on EQUAL costs path
Multicast Address: 224.0.0.5 and 224.0.0.6
Every OSPF router carries the visibility of the entire network with the help of
Link State Database
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THE PROBLEM
OSPF is a highly scalable protocol and can scale up to 1000s of routers.
Has end to end visibility every router knows about all routes to reach
entire networks
Updates are triggered after each Network Modification. This includes
addition/deletion of the route from routing table
High Process Utilization
High Bandwidth Utilization
High Memory Utilization
Solution: Summarization
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CONCEPT OF AREA
All areas must connect to Area 0- One interface of ABR must
be connected to AREA 0
All routers in an area have the same Topology Table
Purpose: Localizing updates within a area
Automatic Summarization is done at the ABR/ASBR only
Hierarchical design is required
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OSPF ROUTER ID
In OSPF neighbors are identified by their RIDs
RID is the highest IP of LOOPBACK interface.
In absence of LOOPBACK interface, it’s the IP of highest ACTIVE
INTERFACE when router starts
RID is also Hardcoded in the configuration using router id command-
Highest Preference
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OSPF NEIGHBOR RELATIONSHIP
Determining own Router ID
Hardcoded
Loopback
Physical
Add interfaces to Link State Database dictated by Network Command
Send Hello on those interfaces
This is DOWN state: Sending Hello but still waiting to hear back
Receives Hello
Goes in INIT state
Checks: Hello/Dead timer, Authentication password, Area ID, Net mask
must be identical
Other parameters: DR/BDR IP address, Router ID and Router Priority
Reaches TWO WAY state.
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OSPF NEIGHBOR RELATIONSHIP
Determines Master-Slave Relationship
Determines who will send the routing first
This is an EX_START ST
Higher priority Masters
Master sends Database Description Packets (DBD) like cliff notes
Slave sends DBDs
DBDs are Acknowledged and Reviewed
Router Goes in LOADING state
First slave reviews the DBD and asks for missing info through LINK STATE REQUEST
to Master
Master sends response in terms of LINK STATE UPDATES
Master sends LSR and Slave responds back with LSU
Neighbors are Synchronized
This is the FULL STATE
Now, Dijkstra’s SPF algorithm is applied to chose the Best Path and put it in the
routing table
Keep Alive are sent
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DR/BDR
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DR/BDR listen on 224.0.0.6
DROTHERS listen on 224.0.0.5
DR/BDR is only for shared segments
DR/BDR election is based on Router
Priority (Default: 1)
If Router Priority is equal then the
Tie Breaker is the Router ID
DROTHERS DO NOT need to
exchange their entire Routing info
with each other. Send it to only DR
and BDR
Hence can stuck in TWO WAY state
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OSPF PACKETS
Hello:
Sent every 10/30 sec
Contains- Router ID, DR/BDR, Subnet Mask, Neighbors, Hello/Dead Intervals,
Auth Password, Router Priority, Area ID
Neighbor relationship formation and keep alive
Data-Base Description:
Cliff notes sent in Ex-start state
Link State Request:
Sent in Loading state for requesting detailed information about a Network
Link State Update:
Response LSR
Contain Multiple LSA
Link State Advertisement:
Response of each sub request
Link State Acknowledgement:
Reliability Mechanism
OSPF itself is a Reliability Protocol
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LSA TYPES
LSA 1 (Router LSA): contains a list of links local to the
router and generated by all the routers in the area
LSA 2 (Network LSA): Generated by DR, contains list of all
routers attached to it
LSA 3 (Network Summary LSA): Generated by ABRs; Used
for inter-area communication
LSA 4 (ASBR Summary LSA): Route to reach ASBR
LSA 5 (External LSA): Generated by ASBR and contains
routes to reach destinations outside local AS.
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