introduction to routing & routing protocol 1. agenda – - router operations – - static route...
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Introduction to Routing & Routing Protocol
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Agenda
– - Router Operations– - Static Route– - Default Route– - Dynamic Route– - Class of Dynamic Routing Protocol– Administrative Distance– - Best Route Selection– - Distance Vector Protocol– - Link State Routing Protocol– - RIP V1 & V2– - RIP Configuration – - Structured approach for Troubleshooting
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Router Operations :- Routing is the process by which items get from one location to another. In networking, a router is the device used to route traffic. Routers can forward packets over static routes or dynamic routes, based on the router
configuration.
A router needs to do the following:– Know the destination address.– Identify the sources from which the router can learn.– Discover possible routes to the intended destination.– Select the best route.– Maintain and verify routing information.
Routers must learn destinations that are not directly connected.
Router Operations (Cont.)
Static Routes :- Static routers use a route that a network administrator enters into the router manually to reach the next hop.
Configure unidirectional static routes to and from a stub network to allow communications to occur.
Static Route Configuration
– Defines a path to an IP destination network or subnet or host– Address = IP address of the next hop router– Interface = outbound interface of the local router
RouterX(config)# ip route network [mask] {address | interface}[distance] [permanent]
Static Route Example
This is a unidirectional route. You must have a route configured in the opposite direction.
RouterA(config)# ip route 172.16.1.0 255.255.255.0 172.16.2.1
RouterA(config)#ip route 172.16.1.0 255.255.255.0 s0/0/0
or
Verifying the Static Route Configuration
RouterA# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default
U - per-user static route
Gateway of last resort is 0.0.0.0 to network 0.0.0.0
10.0.0.0/8 is subnetted, 1 subnets
C 10.1.1.0 is directly connected, Fastethernet 0/0
S* 0.0.0.0/0 is directly connected, Serial0/0/0
Configuring a Default Route• The ip default-network command establishes a default route in networks
using dynamic routing protocols.– Router(config-router)#ip default-network network-number
• Creating an ip route to 0.0.0.0/0 is another way to configure a default route.– Router(config)#ip route 0.0.0.0 0.0.0.0 [next-hop-ip-address | exit-
interface]
Default Routes
This route allows the stub network to reach all known networks beyond Router A.
Configuring Default Route
By default, routers learn paths to destinations in three different ways:
• Static routes: manually defines the static routes as next hop to destination.
• Default routes: manually defines default routes as path when there is no known route to destination.
• Dynamic routes: router learns the path by receiving periodic updates from other routers.
The ip default-network command is usually configured on the routers that connect to a router with a static default route.
HongKong1(config)#ip route 0.0.0.0 0.0.0.0 192.168.20.2
Static vs. Dynamic Routes
• Static Route– Uses a route that a
network administrator enters into the router manually
• Dynamic Route–Uses a route that a
network routing protocol adjusts automatically for topology or traffic changes
What Is a Dynamic Routing Protocol?
Routing protocols are used between routers to determine paths to remote networks and maintain those networks in the routing tables.
After the path is determined, a router can route a routed protocol to the learned networks.
An autonomous system is a collection of networks within a common administrative domain.
Interior gateway protocols operate within an autonomous system.
Exterior gateway protocols connect different autonomous systems.
Autonomous Systems: Interior and Exterior Routing Protocols
Classes of Routing Protocols
Classful Routing Protocol
– Classful routing protocols do not include the subnet mask with the route advertisement.
– Within the same network, consistency of the subnet masks is assumed.– Summary routes are exchanged between foreign networks.– These are examples of classful routing protocols:
• RIPv1• IGRP
Classless Routing Protocol
– Classless routing protocols include the subnet mask with the route advertisement.
– Classless routing protocols support a variable-length subnet mask (VLSM).
– Summary routes can be manually controlled within the network.
– These are examples of classless routing protocols:• RIPv2• EIGRP• OSPF• IS-IS
Administrative Distance :- is used to rate the trustworthiness of the routing protocol.
- Administrative distance is the feature that routers use in order to select the best path when there are multiple routes to the same destination from
different routing protocols.
- Administrative distance defines the reliability of a routing protocol.
- Each routing protocol is prioritized in order of most to least reliable (believable) with the help of an administrative distance value.Lower the AD
vlaue higher is the reliability of that routing protocol.
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Default Distance Value Table
Route Source Default Distance Values
Connected interface 0
Static route 1
Enhanced Interior Gateway Routing Protocol (EIGRP) summary route
5
External Border Gateway Protocol (BGP)
20
Internal EIGRP 90
IGRP 100
OSPF 110
Intermediate System-to-Intermediate System (IS-IS)
115
Routing Information Protocol (RIP)
120
Exterior Gateway Protocol (EGP) 140
On Demand Routing (ODR) 160
External EIGRP 170
Internal BGP 200
Unknown* 255
Administrative Distance: Ranking Routing Sources
Routers choose the routing source with the best administrative distance:
OSPF has an administrative distance of 110.
EIGRP has an administrative distance of 90.
Administrative Distance: Ranking Routes
Selecting the Best Route Using Metrics
Routing protocols use metrics to determine the best route to a destination if there are multiple route to that destination from same Routing Protocol.
Determining the Route Next Hop
• Destination/next hop associations tell a router that a particular destination can be reached optimally by sending the packet to a particular router.
Distance Vector Routing Protocols
Routers pass periodic copies of their routing table to neighboring routers and accumulate distance vectors.
Sources of Information and Discovering Routes
Routers discover the best path to destinations from each neighbor.
Maintaining Routing Information
Updates proceed step by step from router to router.
Inconsistent Routing Entries:Counting to Infinity and Routing
Loops
Each node maintains the distance from itself to each possible destination network.
Counting to Infinity
Slow convergence produces inconsistent routing.
Counting to Infinity (Cont.)
Router C concludes that the best path to network 10.4.0.0 is through router B.
Counting to Infinity (Cont.)
Router A updates its table to reflect the new but erroneous hop count.
Counting to Infinity (Cont.)
The hop count for network 10.4.0.0 counts to infinity.
Solution to Counting to Infinity:Defining a Maximum
A limit is set on the number of hops to prevent infinite loops.
Routing Loops
Packets for network 10.4.0.0 bounce (loop) between routers B and C.
Solution to Routing Loops: Split Horizon
It is never useful to send information about a route back in the direction from which the original information came.
Solution to Routing Loops:Route Poisoning and Poison
Reverse
Routers advertise the distance of routes that have gone down to infinity.
Solution to Routing Loops:Route Poisoning and Poison Reverse (Cont.)
Poison reverse overrides split horizon.
Solution to Routing Loops: Hold-Down Timers
The router keeps an entry for the “possibly down” state in the network, allowing time for other routers to recompute for this topology change.
Triggered Updates
The router sends updates when a change in its routing table occurs.
Eliminating Routing Loops
Eliminating Routing Loops (Cont.)
Eliminating Routing Loops (Cont.)
Link-State Routing Protocols
After an initial flood of LSAs, link-state routers pass small, event-triggered link-state updates to all other routers.
OSPF Hierarchical Routing
Consists of areas and autonomous systems
Minimizes routing update traffic
Link-State Routing Protocol Algorithms
Benefits and Drawbacks of Link-State Routing – Benefits of link-state routing:
• Fast convergence: – Changes are reported immediately by the affected source
• Robustness against routing loops:– Routers know the topology– Link-state packets are sequenced and acknowledged
• Hierarchical network design enables optimization of resources.
– Drawbacks of link-state routing:• Significant demands for resources:
– Memory (three tables: adjacency, topology, forwarding)– CPU (Dijkstra’s algorithm can be intensive, especially when there are many
instabilities)• Requires very strict network design • Configuration can be complex when tuning various parameters and
when design is complex
RIP Overview– is one of the oldest distance-vector routing protocols– Use Hop-count as routing metric selects the path.– prevents routing loops by implementing a limit on the number of hops allowed in a
path from the source to a destination.– The maximum number of hops allowed for RIP is 15.Hop 16 is consider unreachable .– So this max hop count limit the size of network that RIP can support.– RIP implements the split horizon, route poisoning and hold-down mechanisms to
prevent incorrect routing information from being propagated– Routes update every 30 seconds , Routing Information Protocol send complete routing
table every 30 sec. to all its neighboring devices .– This cause a lot of bandwidth utilize on these update flooding . Even If there is no
change in routing information even then these updates are flooded on the network.– As the network size grow there would be massive traffic burst every 30 sec.– This cause high latency & slow convergence of Routing Table .– AD value is 120 , which is least preferred among Dynamic Routing Protocol.
• Split Horizon :- split-horizon route advertisement is a method of preventing routing loops in distance-vector routing protocols by prohibiting a router from advertising a route back onto the interface from which it was learned. Thus when a device that participates in such route advertisements receives an update from an interface, it (the device) does not forward updates through the same interface. By doing so, routing loops are prevented.
• Route Poisoning :- Distance-vector routing protocols in computer networks use route poisoning to indicate to other routers that a route is no longer reachable and should not be considered from their routing tables. RIP, use a maximum hop count to determine how many routers the traffic must go through to reach the destination. Each route has a hop count number assigned to it which is incremented as the routing information is passed from router to router. A route is considered unreachable if the hop count exceeds the maximum allowed. Max limit is 15 hop , 16 is consider unreachable.
• Hold Time :- A router will wait for a particular amount of time before considering a route unreachable , That time is called Hold time for which router wait before flush the route , for RIP Holdtimer is 180 second .
RIPv1 has the following limitations :-
– It does not send subnet mask information in its updates.
– It sends updates as broadcasts on 255.255.255.255.
– It does not support authentication. – It is not able to support VLSM or classless
interdomain routing (CIDR).
RIPv2 Features
RIPv1 and RIPv2 Comparison
RIPv1 RIPv2
Routing protocol Classful Classless
Supports variable-length subnet mask? No Yes
Sends the subnet mask along with the routing update? No Yes
Addressing type BroadcastMulticast Routing
updates over 224.0.0.9
Defined in … RFC 1058RFCs 1721, 1722,
and 2453
Supports manual route summarization? No Yes
Authentication support? No Yes
IP Routing Configuration Tasks
–Router configuration– Select routing protocols– Specify networks or interfaces
RIP Configuration
–Starts the RIP routing process
RouterX(config)# router rip
RouterX(config-router)# network network-number
Selects participating attached networks
Requires a major classful network number
Enables RIP version 2
RouterX(config-router)# version 2
RIP Configuration Example
Verifying the RIP Configuration
Routing Protocol is "rip"Routing Protocol is "rip"
Sending updates every 30 seconds, next due in 6 secondsSending updates every 30 seconds, next due in 6 seconds
Invalid after 180 seconds, hold down 180, flushed after 240Invalid after 180 seconds, hold down 180, flushed after 240
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Redistributing: rip
Default version control: send version 2, receive version 2send version 2, receive version 2
Interface Send Recv Triggered RIP Key-chain
FastEthernet0/0 2 2
Serial0/0/2 2 2
Automatic network summarization is in effect
Maximum path: 4
Routing for Networks:
10.0.0.0
172.16.0.0
Routing Information Sources:
Gateway Distance Last Update
10.1.1.2 120 00:00:25
Distance: (default is 120)Distance: (default is 120)
RouterA#
Displaying the IP Routing Table
Troubleshooting RIPv2
The debug ip rip Command
Summary
– Routing is the process by which items get from one location to another.
– Dynamic routing protocols determine how updates are conveyed, what knowledge is conveyed, when to convey knowledge, and how to locate recipients of the updates.
– A routing protocol that has a lower administrative value is more trustworthy than a protocol that has a higher administrative value.
– There are three classes of routing protocols: distance vector, link-state, and balanced hybrid.
– The ip classless command can be used to prevent a router from dropping a packet that is destined for an unknown subnetwork of a directly attached network if a default route is configured.
Summary (Cont.)
– RIP is a distance vector routing protocol that uses hop count as the matrix for route selection and broadcasts updates every 30 seconds.
– RIPv1 uses classful routing protocol; RIPv2 uses classless routing protocol. RIPv2 supports VLSM, manual route summarization, and authentication; RIPv1 does not support these activities.
– To enable a dynamic routing protocol, first a routing protocol is selected, then IP network numbers are assigned without values being specified (except OSPF).
– The router command starts the routing process. The network command allows the routing process to determine which interfaces will participate in sending and receiving the routing updates.
Summary (Cont.)
– The router RIP command selects RIP as the routing protocol. The network command identifies a participating attached network.
– The show ip command displays information about routing protocols and the routing table.
– The debug ip rip command displays information on RIP routing transactions.
Structured Approach to Troubleshooting
Typical Layer 1 Errors
• Broken cables • Disconnected cables • Cables connected to the wrong ports • Intermittent cable connection • Wrong cables used for the task at hand • Transceiver problems • DCE cable problems • DTE cable problems • Devices turned off
Typical Layer 2 Errors
• Improperly configured serial interfaces
• Improperly configured Ethernet interfaces
• Improper encapsulation set • Improper clock rate settings on
serial interfaces • Network interface card (NIC)
problems
Typical Layer 3 Errors
• Routing protocol not enabled • Wrong routing protocol enabled • Incorrect IP addresses • Incorrect subnet masks
Layer 3 Troubleshooting Using Ping
Layer 7 Troubleshooting Using Telnet
Troubleshooting Layer 1 Using show interfaces
CommandThe show interfaces serial command
Troubleshooting Using show cdp neighbors Command
Troubleshooting Using show cdp neighbors detail Command
Troubleshooting Using traceroute Command
Troubleshooting Routing IssuesThe show ip route Command
Troubleshooting Routing IssuesThe show ip protocols Command
Troubleshooting Using show controllers serial
CommandThe show controllers serial Command
Introduction to debugDebug syntax
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