wxes2106 network technology semester 1 2004/2005 chapter 6 routing protocol ccna1: 10.1, 10.2,...
Post on 21-Dec-2015
219 views
TRANSCRIPT
WXES2106Network Technology
Semester 1 2004/2005
Chapter 6
Routing Protocol
CCNA1: 10.1, 10.2, CCNA2: Module 6 and 7
Introduction
A protocol is a set of rules that determines how computers communicate with each other across networks.
A protocol describes the following: The format that a message must conform to The way in which computers must exchange a
message within the context of a particular activity
Routed Protocol A routed protocol allows the router to forward data
between nodes on different networks. Internet Protocol (IP) is the routed protocol of the Internet.
IPX/SPX, Appletalk are types of routed protocol. IP is a connectionless, unreliable, best-effort delivery
protocol. It determines the most efficient route for data based on the routing protocol.
As a packet travels through an internetwork to its final destination, the Layer 2 frame headers and trailers are removed and replaced at every Layer 3 device.
This is because layer 2 data units, frames, are for local addressing while layer 3 data units, packets, are for end-to-end addressing.
Routed Protocol
As a frame is received at a router interface, the destination MAC address is extracted.
The address is checked to see if the frame is directly addressed to the router interface, or if it is a broadcast.
In either of these two cases, the frame is accepted. Otherwise, the frame is discarded
The packet is then checked to see if it is actually destined for the router, or if it is to be routed to another device in the internetwork.
If the destination IP address matches one of the router ports, the Layer 3 header is removed and the data is passed up to the Layer 4.
Routed Protocol
If the packet is to be routed, the destination IP address will be compared to the routing table.
If a match is found or there is a default route, the packet will be sent to the interface specified in the matched routing table statement.
When the packet is switched to the outgoing interface, a new CRC value is added as a frame trailer, and the proper frame header is added to the packet.
The frame is then transmitted to the final destination
Routed Protocol
Two types of delivery services are connectionless and connection-oriented.
In a connectionless system, the destination is not contacted before a packet is sent.
Connectionless network processes are often referred to as packet switched processes
In connection-oriented systems, a connection is established between the sender and the recipient before any data is transferred.
Connection-oriented network processes are often referred to as circuit switched processes.
Routing Protocol Routing is an OSI Layer 3 function. Routing is a hierarchical organizational scheme that allows
individual addresses to be grouped together. Routing is the process of finding the most efficient path from
one device to another. The primary device that performs the routing process is the
router. Two key functions of a router:
Maintain routing tables and make sure other routers know of changes in the network topology.
The router switches the packets to the appropriate interface, adds the necessary framing information for the interface, and then transmits the frame.
Routing Protocol
Routing protocols use various combinations of metrics for determining the best path for data.
Routed protocols transport data across a network. Routing protocols allow routers to choose the best path for data from source to destination.
Routers use routing protocols to exchange routing tables and share routing information. It enable routers to route routed protocols.
Main functions: Provides processes for sharing route information Allows routers to communicate with other routers to
update and maintain the routing tables
Routing Protocol
Example, Routing Information Protocol (RIP), Interior Gateway Routing Protocol (IGRP), Open Shortest Path First (OSPF), Border Gateway Protocol (BGP), and Enhanced IGRP (EIGRP).
Path Determination enables a router to compare the destination address to
the available routes in its routing table, and to select the best path
decide which port an incoming packet should be sent out of to travel on to its destination.
It lets the router to decide which outbound port the packet should be sent.
Routing Protocol
Path Determination Process The destination address is obtained from the packet. The mask of the first entry in the routing table is
applied to the destination address. The masked destination and the routing table entry
are compared. If there is a match, the packet is forwarded to the
port that is associated with that table entry. If there is not a match, the next entry in the table is
checked.
Routing Protocol
If the packet does not match any entries in the table, the router checks to see if a default route has been set.
If a default route has been set, the packet is forwarded to the associated port. A default route is the route to use if there are no matches in the routing table.
If there is no default route, the packet is discarded. Usually a message is sent back to the sending device indicating that the destination was unreachable.
Routing Protocol
Routing Table Routing tables contain the information necessary to
forward data packets across connected networks. Protocol type
The type of routing protocol that created the routing table entry
Destination/next-hop associations Inform a router that a particular destination is either
directly connected to the router, or that it can be reached using another router called the “next-hop”
Routing Protocol
Routing metric Used to determine the desirability of a route. The
Routing Information Protocol (RIP) uses hop count as its only routing metric. Interior Gateway Routing Protocol (IGRP) uses a combination of bandwidth, load, delay, and reliability metrics
Outbound interfaces The interface that the data must be sent out on, in
order to reach the final destination.
Routing Protocol
Routing algorithms Different routing protocols use different algorithms to
decide which port an incoming packet should be sent to. Routing algorithms Design Goal
Optimization Describes the capability of the routing algorithm to
select the best route. The route will depend on the metrics and metric weightings used in the calculation.
Simplicity and low overhead The simpler the algorithm, the more efficiently it will
be processed by the CPU and memory in the router.
Routing Protocol
Robustness and stability A routing algorithm should perform correctly
Flexibility Should quickly adapt to a variety of network changes.
These changes include router availability, router memory, changes in bandwidth, and network delay.
Rapid convergence Convergence is the process of agreement by all
routers on available routes. When a network event causes changes in router availability, updates are needed to reestablish network connectivity.
Routing Protocol
Routing Metric Bandwidth
The data capacity of a link. Delay
The length of time required to move a packet along each link from source to destination.
Load The amount of activity on a network resource such as a
router or a link. Reliability
Usually a reference to the error rate of each network link.
Routing Protocol
Hop count The number of routers that a packet must travel
through before reaching its destination. Ticks
The delay on a data link using IBM PC clock ticks. One tick is approximately 1/18 second.
Cost An arbitrary value, usually based on bandwidth
Routing Protocol
An autonomous system is a network or set of networks under common administrative control, such as the cisco.com domain.
Two families of routing protocols are Interior Gateway Protocols (IGPs) and Exterior Gateway Protocols (EGPs).
EGPs route data between autonomous systems. An example of an EGP is Border Gateway Protocol
(BGP).
Routing Protocol
IGPs route data within an autonomous system. Routing Information Protocol (RIP) and (RIPv2) Interior Gateway Routing Protocol (IGRP) Enhanced Interior Gateway Routing Protocol (EIGRP) Open Shortest Path First (OSPF) Intermediate System-to-Intermediate System protocol
(IS-IS) IGPs can be further categorized as either distance-
vector or link-state protocols.
Routing Protocol
Distance Vector Determines the distance and direction, vector, to
any link in the internetwork. The distance may be the hop count to the link. Routers using distance-vector algorithms send all or
part of their routing table entries to adjacent routers on a periodic basis.
By receiving a routing update, a router can verify all the known routes and make changes to its routing table.
Also known as the Bellman-Ford algorithm Example, RIP, IGRP and EIGRP
Routing Protocol
Link State To overcome limitations of distance vector routing
protocols. Also known as Dijkstra's algorithm or as the shortest
path first (SPF) algorithm. protocols respond quickly to network changes sending
trigger updates only when a network change has occurred.
send periodic updates, known as link-state refreshes, at longer time intervals, such as every 30 minutes.
When a route or link changes, the device that detected the change creates a link-state advertisement (LSA) concerning that link.
Routing Protocol
The LSA is then transmitted to all neighboring devices. Each routing device takes a copy of the LSA, updates its link-state database, and forwards the LSA to all neighboring devices.
Topological database A collection of information gathered from LSAs
SPF algorithm A calculation performed on the database that results
in the SPF tree It computes network reachability.
Routing table A list of the known paths and interfaces
Routing Protocol
Three main concerns related to link-state protocols: Processor overhead Memory requirements Bandwidth consumption
Examples of link-state protocols include Open Shortest Path First (OSPF) and Intermediate System-to-Intermediate System (IS-IS).
Routing Protocol
RIP Distance vector routing protocol Uses hop count as its metric to determine the
direction and distance to any link in the internetwork. Cannot route a packet beyond 15 hops. Routing updates broadcast every 30 seconds
RIPV1 All devices in the network use the same subnet mask Classful routing
Routing Protocol
RIPV2 Provides prefix routing, and does send subnet mask
information in routing updates. Classless routing Different subnets within the same network can have different
subnet masks., variable-length subnet masking (VLSM). IGRP
Distance-vector routing protocol Based on delay, bandwidth, load, and reliability Classful routing Maximum hop 255 Routing updates broadcast every 90 seconds
Routing Protocol
OSPF Link-state routing protocol Address the needs of large, scalable internetworks Open standard routing protocol The SPF algorithm is used to calculate the lowest cost
to a destination. Routing updates are flooded as topology changes
occur. IS-IS
Link-state routing protocol Supports multiple routed protocols including IP
Routing Protocol
EIGRP Proprietary Cisco protocol Provides superior operating efficiency such as fast
convergence and low overhead bandwidth It uses load balancing. It uses a combination of distance vector and link-state
features. Hybrid routing protocol It uses Diffused Update Algorithm (DUAL) to
calculate the shortest path. Routing updates are multicast using 224.0.0.10
every 30 seconds or as triggered by topology changes.
Routing Protocol
BGP An example of an External Gateway Protocol (EGP) Exchanges routing information between autonomous
systems while guaranteeing loop-free path selection. BGP4 is the first version of BGP Supports classless interdomain routing (CIDR) and
route aggregation. Makes routing decisions based on network policies,
or rules using various BGP path attributes
Static Route
A network administrator configures information about remote networks manually.
Static routing is not as scalable as dynamic routing because of the extra administrative requirements.
Static route operations can be divided into these three parts: Network administrator configures the route Router installs the route in the routing table The static route is used to route packets.
Static Route
configure a static route enter global configuration mode Router(config)#ip route destination-network
subnet-mask outgoing-interface Router(config)#ip route destination-network
subnet-mask next-hop-ip-address Router(config)#copy running-config startup-config
save the active configuration to NVRAM. configure default route
Router(config)#ip route 0.0.0.0 0.0.0.0 [next-hop-address | outgoing interface]
Static Route
Router#show running-config Verify that the static route has been correctly entered.
Router#show ip route Verify that the route that was configured is in the
routing table. Use ping and tracert to troubleshoot the static route
configuration
Dynamic Routing
The routing protocol learns all available routes, places the best routes into the routing table, and removes routes when they are no longer valid.
The router uses the information in the routing table to forward routed protocol packets.
Whenever the topology of a network changes because of growth, reconfiguration, or failure, the network knowledgebase must also change.
The network knowledgebase needs to reflect an accurate view of the new topology.
Dynamic Routing
Configure Routing Protocol Router(config)#Router { rip | igrp | eigrp | ospf } option Router(config-router)# Network network-number
Network-number specifies the directly connected network
Dynamic Routing
Routing loops can occur when inconsistent routing tables are not updated due to slow convergence in a changing network.
To reduce routing loops and counting to infinity, RIP uses the following techniques: Split horizon Poison reverse Holddown counters Triggered updates
Dynamic Routing
Example When Network 1 fails, Router E sends an update to
Router A. When Router A sends out its update, Routers B and D
stop routing to Network 1. However, Router C has not received an update. For Router C, Network 1 can still be reached through Router B.
Router C keep sending periodic update to Router D, which indicates a path to Network 1 by way of Router B. Router D changes its routing table to reflect this incorrect information, and sends it to Router A.
Dynamic Routing
Router A sends the information to Routers B and E, and the process continues. Any packet destined for Network 1 will now loop from Router C to B to A to D and back to again to C.
Dynamic Routing
Split horizon can be used to avoid routing loops If a routing update about Network 1 arrives from Router
A, Router B or Router D cannot send information about Network 1 back to Router A.
Split horizon reduces incorrect routing information and routing overhead.
Dynamic Routing
Route poisoning is used by various distance vector protocols to overcome large routing loops and offer detailed information when a subnet or network is not accessible.
The hop count is usually set to one more than the maximum.
When Network 1 goes down, Router E will set a distance of 16 for Network 1 to poison the route.
This indicates that the network is unreachable. After Router B receives a route poisoning from Router E, it
sends an update, which is called a poison reverse, back to Router E. This makes sure all routers on the segment have received the poisoned route information.
Dynamic Routing
Triggered updates A triggered update is sent immediately in response to
some change in the routing table. The router that detects a topology change immediately
sends an update message to adjacent routers. Ensure that all routers know of failed routes before
any holddown timers can expire.
Dynamic Routing
Holddown timers When a router receives an update from a neighbor,
which indicates a network fail, the router marks the route as inaccessible and starts a holddown timer.
Before the holddown timer expires, if an update is received from the same neighbor, which indicates that the network is accessible, the router marks the network as accessible and removes the holddown timer.
if an update arrives from a different neighbor router with a better metric for the network, the router marks the network as accessible and removes the holddown timer.
Dynamic Routing
If an update is received from a different router with a higher metric before the holddown timer expires, the update is ignored
This update is ignored to allow more time for the knowledge of a disruptive change to propagate through the entire network.
RIP
RIP has evolved Classful Routing Protocol, RIP Version 1 (RIP v1) to a Classless Routing Protocol, RIP Version 2 (RIP v2).
RIP v2 enhancements include: Ability to carry additional packet routing information Authentication mechanism to secure table updates Support for variable-length subnet mask (VLSM)
Configuring RIP
RIP
Optional task: Apply offsets to routing metrics Adjust timers Specify a RIP version Enable RIP authentication Configure route summarization on an interface Verify IP route summarization Disable automatic route summarization Run IGRP and RIP concurrently Disable the validation of source IP addresses Enable or disable split horizon Connect RIP to a WAN
RIP
Router(config)#ip classless forward these packets to the best supernet route. Example, if an enterprise uses the entire subnet
10.10.0.0 /16, then a supernet route for 10.10.10.0 /24 would be 10.10.0.0 /16
Router(config-if)#ip split-horizon enable split horizon (default)
Router(config-router)#timers basic update invalid holddown flush [sleeptime] change holddown timer
RIP
Router(config-router)#update-timer seconds Change update interval
Router(config-router)#passive-interface Fa0/0 disable routing updates on specified interfaces
Router(config-router)#neighbor ip-address exchange routing information with neighboring router
Router(config-router)#version { 1|2 } receive and send Version 1 and 2 packets by default send version 1 and 2 packets receive
Version 1 packets
RIP
Router(config-if)#ip rip {receive | send} version { 1|2 } receive or send Version 1 or 2 packets
Router#show ip protocols RIP routing is configured. The correct interfaces send and receive RIP updates. The router advertises the correct networks.
Router#debug ip rip displays RIP routing updates as they are sent and
received. RIP is capable of load balancing over as many as six equal-
cost paths. The default is four paths. RIP performs what is referred to as "round robin" load balancing.
RIP
Router(config-router)#maximum-paths [number] change the maximum number of parallel paths
Router(config)#ip route destination-network subnet-mask next-hop Administrative-Distance Create a static floating route if AD is higher than the
normal RIP route floating route take the place of the RIP route in the
event that the RIP routing process fails.
IGRP Key design of IGRP
Versatility Automatically handle indefinite, complex topologies
Flexibility Segment with different bandwidth and delay
characteristics Scalability
Functioning in very large networks IGRP Metrics
Bandwidth (K1) Delay (K3) Load Reliability
IGRP IGRP Route
Interior Route Routes between subnets of a network attached to a
router interface System routes
Routes to networks within an autonomous system. . Do not include subnet information.
Exterior routes Routes to networks outside the autonomous system
that are considered when a gateway of last resort is identified.
IGRP IGRP maintains many timers such as update timer, an
invalid timer, a holddown timer, and a flush timer. Invalid timer
Specifies how long a router should wait in the absence of routing-update messages about a route before it declares that route invalid.
Flush timer Indicates how much time should pass before a route is
flushed from the routing table Configuring IGRP
Router(config)#router igrp as-number Router(config-router)#network network-number