Switching Basics and Intermediate Routing CCNA 3 Chapter 2

Switching Basics and Intermediate Routing CCNA 3 Chapter 2

Link-State Routing Overview Maintaining Routing Information Via Link States
Link-state routing algorithms, also known as shortest path first (SPF) algorithms, build a complex database of topology information The algorithms compute the shortest path between nodes Maintains full knowledge of distant routers and how they interconnect

Link-state routing uses link-state advertisements (LSAs) A basic building block that describes a router’s local topology and is distributed to all other routers in the area Link-state routing uses a topological database (or link-state database) The set of all links learned from the flooding of LSAs Synchronized with all other routers in the area

OSPF and Intermediate System-to-Intermediate System (IS-IS) are link-state routing protocols Collect routing information from all other routers in the area Each router calculates all the best paths to all destinations in the network Because each router calculates best paths, they are less likely to propagate incorrect information learned from a neighboring router

Link-state routing protocols were designed to overcome the limitations of distance vector routing protocols Respond quickly to network changes Send only triggered updates Send periodic updates at long intervals, such as every 30 minutes A hello mechanism determines reachability of neighbors

Link-State Routing Relies on Complex Mechanisms to Permit Stable, Synchronous and High-Speed Routing

When a failure occurs in a network: Link-state protocols flood LSAs; use a special multicast address Each link-state router takes a copy of the LSA, updates its topological database, and forwards the LSA to neighboring routers All link-state routers in the area recalculate their routing tables using the Dijkstra SPF algorithm A link is similar to an interface on a router The state of the link is a description of the interface and its relation to its neighboring routers

OSPF Uses a Two-Layer Hierarchy
Link-State Routing Overview Maintaining Routing Information Via Link States OSPF Uses a Two-Layer Hierarchy

Two primary elements exist in the two-layer hierarchy Area: A grouping of contiguous networks Areas are logical subdivisions of the autonomous system Each area must be connected directly to the backbone area (known as area 0) Autonomous System (AS): A collection of networks under a common administration Share a common routing strategy Can be logically subdivided into multiple areas

The backbone area is the transition area All other areas communicate through it All non-backbone areas are connected to it These can be configured as a stub area, a totally stubby area, or a not-so-stubby area (NSSA) (not covered in this curriculum) to reduce the sizes of the link-state database and the routing table

Link-State Routing Overview Link-State Routing Protocol Algorithms
Rely on SPF protocols to maintain a complex database of the network topology Develop and maintain a full knowledge of the network routers and how they interconnect Use LSAs to exchange information with other routers Each router that has exchanged LSAs constructs a topological database The SPF algorithm is used to compute reachability to destination networks A routing table is built from this information, containing only lowest-cost routes

(continued): LSA exchanges are triggered events Greatly speed up convergence process No need to wait for a series of timers to expire before the networked routers can begin to converge

Cost Metric Determines Shortest Path for Link-State Routing Protocols

Next Hops and Costs for Destination Routes (Previous Slide)

Link-State Routing Benefits of Link-State Routing
Link-state protocols use cost metrics to choose paths Cost metric reflects the capacity of the links Routing updates are less frequent Network can be segmented into area hierarchies Limits the scope of route changes Link-state protocols send only updates of a topology change Use triggered, flooded updates which lead to faster convergence times

Link-State Routing Benefits of Link-State Routing
Each router has a complete and synchronized picture of the network Difficult for routing loops to occur LSAs are sequenced and aged Routers always base their routing information on the most recent set of information With careful design work, size of link-state databases can be minimized Smaller Dijkstra calculations and faster convergence

Link-State Routing Limitations of Link-State Routing
In addition to a routing table, link-state protocols require: A topological database An adjacency database Lists all the relationships formed between neighboring routers for the purpose of exchanging routing information A forwarding table A data structure of a stripped down association between network prefixes and next hops

Dijkstra’s algorithm requires CPU cycles to calculate best paths through the network If the network is large or unstable, this can require a significant amount of CPU time Not a problem for most modern routers A strict hierarchical network design is required to divide the network into smaller areas Reduces the excessive use of memory and CPU cycles Reduces size of topology tables and Dijkstra calculations Areas must be contiguous at all times

Although configuration of link-state networks is usually simple, configuring a large network can be challenging Trouble-shooting is usually easier, as every router has a copy of the topology However, interpreting the information requires a good understanding of link-state routing concepts Link-state protocols usually scale to bigger networks than distance vector protocols

Link-state routing raises two concerns: During the initial discovery process, link-state routing protocols flood the network with LSAs Significantly decreases the network’s capability to transport data This is temporary, but noticeable Link-state routing is both memory- and processor-intensive Greater demand requires higher-end routers that cost more

Single-Area OSPF Concepts
OSPF was developed by the Interior Gateway Protocol (IGP) group of the Internet Engineering Task Force (IETF) Created in mid 1990s because RIP was unable to serve large, heterogeneous networks OSPF has two primary characteristics: Protocol is an open standard, not proprietary Based on the SPF algorithm

Single-Area OSPF Concepts Comparing OSPF with Distance Vector Routing Protocols
OSPF is a link-state protocol, RIP and IGRP are distance vector protocols Distance vector protocols send all, or a portion of, their routing table in updates to their neighbors A link is an interface on a router The state of the link describes the interface and its relationship to neighboring routers Can include IP address, subnet mask, type of network The collection of link states forms a link-state database

An OSPF router sends LSA packets to periodically advertise its link states instead of sending routing table updates Information about attached interfaces and metrics are included LSAs are flooded to all routers in the area As OSPF routers accumulate link-state information, they use the SPF algorithm to calculate the shortest path to each destination

A topological (link-state) database is an overall picture of networks in relationship to routers Contains the collection of LSAs received from all routers in the same area Database is pieced together from the LSAs Routers in the same area have identical topological databases

OSPF can operate within a hierarchy The largest entity is the Autonomous System (AS): A collection of networks under a common administration that share a common routing strategy An AS can be divided into several areas, which are groups of contiguous networks and attached hosts

Single-Area OSPF Concepts OSPF Hierarchical Routing
OSPF’s capability to separate a large network into multiple areas is known as hierarchical routing Hierarchical routing enables you to separate a large internetwork (AS) into smaller internetworks called areas Routing still occurs between areas Many of the minute internal routing operations, such as recalculating the database, are kept within an area

OSPF Uses Areas to Provide Hierarchy

OSPF’s hierarchical topology possibilities have the following advantages: Reduced frequency of SPF calculations Smaller routing tables Reduced link-state update overhead

Single-Area OSPF Concepts Dijkstra’s Algorithm
In Dijkstra’s algorithm, the best path is the lowest cost path Named for Edsger Wybe Dijkstra, a Dutch computer scientist Each link has a cost Each node has a name Each node has a complete topological database

Dijkstra’s Algorithm Uses Cost Metric

Dijkstra’s algorithm places each router at the root of a tree Calculates the shortest path to each node based on the cumulative cost to reach the destination Each router has its own view of the topology Each router uses the information in its topological database to calculate a shortest-path tree, with itself as the root The router uses this tree to route network traffic

The cost, or metric, of an interface indicates the overhead that is required to send packets across that interface The OSPF cost of an interface is inversely proportional to that interface’s bandwidth Higher bandwidth equals lower cost Cost = 100,000,000 / bandwidth in bps

Shortest Path is Measured from Each Root Node to Build a Shortest Path Tree

Single-Area OSPF Configuration Basic OSPF Configuration
The router ospf command takes a process identifier as an argument: Router (config)# router ospf process-id The process ID is a locally significant number between 1 and 65,535 that you select to identify the routing process It does not need to match the OSPF process ID on other OSPF routers

The network command identifies which IP networks on the router are part of the OSPF network: Router(config-router)#network address wildcard-mask area area-id (all on one command line) Parameters of a network Command

The wildcard mask is sometimes called an inverse mask because it is the inverse of the subnet mask for the network This is not required; many network administrators use the option to match the interface Basis OSPF Network with Each Router in Area 0

Using the network statement in OSPF

A router uses the OSPF hello protocol to establish neighbor relationships Hello packets let other routers know they are still functional On networks supporting more than two routers (multiaccess networks), such as Ethernet networks, the hello protocol elects: A designated router (DR) Generates LSAs Manages link-state synchronization A backup designated router (BDR) Becomes the DR if the existing DR fails

Single-Area OSPF Configuration Loopback Interfaces
The OSPF router ID is the number by which the router is known to OSPF To modify the OSPF router ID to a loopback address use this command: Router(config)#interface loopback number The highest IP address on an active interface of a router at startup can be overridden by using a loopback address OSPF is more reliable if a loopback interface is configured because a loopback interface is always active

Single-Area OSPF Configuration Modifying the OSPF Cost Metric
OSPF uses cost as the metric to determine the best route Cost is associated with the output side of an interface It is calculated with the formula cost = 100,000,000/bandwidth in bps The lower the cost, the more likely the route is to be used

OSPF Cost Values

It is essential for proper OSPF operation that the correct interface bandwidth is set: Router(config)#interface serial 0 Router(config-if)#bandwidth 56 Cost can be changed to influence the outcome of OSPF cost calculation When costs are from different vendors are unequal, might want to make change to match costs Might need to change cost to account for Gigabit Ethernet Use this command to change cost: Router(config-if)#ip ospf cost number

Single-Area OSPF Configuration OSPF Authentication
A router trusts the information that is coming from a router that should be sending it the information To guarantee this trust, routers in a specific area can be configured to authenticate each other with OSPF authentication Each interface can present an authentication key that the router uses to send OSPF information to other routers on the segment The key, known as a password, is a shared secret between the routers The key can be up to eight characters long The key generates the authentication data in the OSPF header

Use the following syntax to configure OSPF authentication: Router(config-if)#ip ospf authentication-key password After the password is configured, authentication must be enabled: Router(config-router)#area area-number authentication With simple authentication, the password is sent as plain text (security risk) Configure encryption of the password

Authentication password encryption syntax: Router(config-if)#ip ospf message-digest-key key-id encryption-type md5 key (all on one line!) The key-id is an identifier with a value of between 1 and 255 The encryption-type refers to the type of encryption, where 0 means none and 7 means proprietary The following is configured in router configuration mode on a router with an interface in the area area-id Router(config-router)#area area-id authentication message-digest MD5 creates a message digest, which is scrambled data based on the password and the message contents If the digests match, the receiving router trusts the data

Single-Area OSPF Configuration OSPF Network Types and OSPF Timers
OSPF interfaces automatically recognize three OSPF network types: Broadcast multiaccess, such as Ethernet Point-to-point networks Nonbroadcast multiaccess networks (NBMA), such as Frame Relay An administrator can manually configure a fourth OSPF network type: point-to-multipoint In a multiaccess network, it is not known in advance how many routers will be connected In point-to-point networks, only two routers will be connected

In a broadcast multiaccess network segment, many routers can be connected If every router has to establish adjacency with every other router, [n * (n-1) / 2] adjacencies need to be formed For 5 routers the formula would be 5*(5-1) / 2 = 5*4 / 2 = 20 / 2 = 10 adjacencies Routers hold an election for a DR router This router becomes adjacent to all other routers in the broadcast segment All other routers send their link-state information to the DR The DR sends link-state information to all other routers on the segment by using the multicast address

Despite the gain in efficiency that electing a DR provides, a disadvantage exists: The DR is a single point of failure A second router is elected the BDR to take over in case the DR fails To make sure that both the DR and BDR see the link states that all routers send on the segment, the multicast address is used On point-to-point networks, no DR or BDR is elected; both routers become fully adjacent

OSPF Network Type, Characteristics, and DR Election

OSPF uses: Hello intervals Default of 10 seconds on broadcast networks Default of 30 seconds on nonbroadcast networks Dead intervals (4 times the hellow interval by default) Default of 40 seconds on broadcast networks Default of 120 seconds on nonbroadcast networks To change the default times: Router(config-if)#ip ospf hello-interval seconds Router(config-if)#ip ospf dead-interval seconds

Single-Area OSPF Configuration Propagating a Default Route
OSPF routing ensures loop-free paths to every network in the routing domain To reach networks outside the domain, either OSPF must know about the network or OSPF must have a default route To have an entry for every network in the world would require enormous resources for each router A practical alternative is to add a default route to the OSPF router connected to the outside network This default route can be redistributed to each router in the AS through normal OSPF updates

Single-Area OSPF Configuration Propagating a Default Route
To configure a static default route: Router(config)#ip route [interface | next hop address] This is referred to as the quad-zero route Any destination network address is matched To propagate this route to all the routers in a normal OSPF area: Router(config-router)#default-information originate All routers in the OSPF area learn a default route provided that the interface of the border router to the gateway router is active

Single-Area OSPF Configuration Verifying OSPF Configuration
Several show commands display information about OSPF configuration: Display parameters about timers, filters, metrics and networks: show ip protocols Display the routes that are known to the router: show ip route Verify that interfaces have been configured in the intended areas: show ip ospf interface Display OSPF neighbor information on a per-interface basis: show ip ospf neighbor

Single-Area OSPF Configuration Troubleshooting OSPF
Output from the debug ip ospf events Command

The debug ip ospf events output might appear if: The IP subnet masks for routers on the same network do not match The OSPF hello interval does not match that configured for a neighbor The OSPF dead interval does not match that configured for a neighbor If a router configured for OSPF does not see a router on an attached network Make sure both routers are configured with the same subnet mask, OSPF hello and dead intervals Make sure both neighbors are part of the same area type

Sample Output from the debug ip ospf packet Command

Fields in debug ip ospf packet Output

Fields in debug ip ospf packet Output (continued)

Summary Link-state routing protocols such as OSPF and IS-IS quickly and reliably propagate routing information within an AS Link-state routing protocols build link-state databases, which are synchronized with link-state advertisements (LSAs) The link-state protocol then applies Dijkstra’s algorithm (SPF) to determine the best path(s) to each destination, which are then installed in the routing table OSPF is the most commonly deployed link-state protocol Employs DRs and BDRs on broadcast segments to optimize propagation of link-state information Each link uses hello and dead interval timers depending on OSPF network type: broadcast multiaccess, NBMA, point-to-point, point-to-multipoint

Summary OSPF is configured by:
Defining which interfaces will participate in a given OSPF process for a specific area Use the network statements coupled with inverse masks Inverse masks are often created to exactly match the subnet mask of the network associated with the given link, or they can be defined simply with a mask to exactly match their interface ID Verifying OSPF configurations is done with these commands: show ip protocol, show ip route, show ip ospf interface, show ip ospf neighbor Troubleshooting OSPF is done with these commands: debug ip ospf events, debug ip ospf packets

Switching Basics and Intermediate Routing CCNA 3 Chapter 2

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Switching Basics and Intermediate Routing CCNA 3 Chapter 2

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