Routing principle (1)

I. Routing Principle
1.1 Basic knowledge about Route Selection
A route is a relay process that transfers an object from one place to another. The mechanism for learning and maintaining network topology knowledge is considered a routing function. The process in which data passes through the router and is transferred to the outbound interface is another independent function, which is considered as the exchange/forwarding function. A routing device must have both the routing and switching functions to act as a valid relay device.
For routing, the router must know the following three items:
The router must determine whether it has activated support for the Protocol group;
The router must know the destination network;
The router must know which outbound interface is the best route to the destination.
The routing protocol determines the Optimal Path to the destination through the measurement value. A small metric value indicates the preferred path. If two or more paths have the same small metric value, all these paths will be shared equally. Traffic Distribution through multiple paths is called load balancing to the destination.
The information required to perform the routing operation is included in the router routing table, which is generated by one or more routing selection Protocol processes. A route table consists of multiple route entries. Each entry specifies the following content:
Learn the dynamic or manual mechanism used by the route)
Logical destination
Management Distance
It is a measure of the total "overhead" of a path)
The address of the router of the next HOP to the destination;
New and Old Route information
Interface associated with the destination network
Run the show ip route command to view the above content.
The default Management Distance pre-allocation principle is: manually set route entry priority is higher than dynamic learned route entries, the metric Value algorithm complex route selection protocol priority is higher than the metric Value algorithm SIMPLE route selection protocol.
Generally, the router selects a path with the minimum measurement value. If multiple paths with the lowest measurement value and the same value appear in the IP address environment of the CISCO router, load Balancing will be enabled on these multiple paths. c isco supports four paths with the same measurement value by default, you can use the "maximum-paths" command to identify that a CISCO router supports up to six paths with the same measurement value.
RIP is a routing protocol used in small to medium-sized TCP/IP networks. It uses the number of hops as the metric value, and its load balancing function is enabled by default, when RIP decides the optimal path, bandwidth is not taken into account!
IGRP is a routing selection protocol used in medium to large TCP/IP networks. It uses a composite metric value, it takes into account bandwidth, latency, reliability, load, and maximum transmission unit m tu), but uses the bandwidth and latency value by default. IGRP can also perform load balancing. After the router is started, it immediately tries to establish a routing relationship with its adjacent routing device. The initial communication aims to identify adjacent devices and Start Communication and learn the network phase structure. The method for establishing the adjacent relationship and the initial learning of the topology vary with the routing selection protocol.
The routing protocol exchanges regular HELLO messages or regular route update packets to maintain communication between adjacent devices.
After you understand the network topology and the routing table contains the Optimal Path to the known network, you can start to forward data to these destinations.
1.2 route selection Protocol
Classful routing) Overview
The routing selection protocol that does not send subnet mask information with each network address is called the classification selection protocol r12001 and IGRP.) When a classification routing selection protocol is used, all subnetworks must use the same subnet mask. A router running a route selection protocol with a category route selection protocol performs one of the following operations to determine the routing network section:
If the route update information is about the same primary network configured on the receiving interface, the router uses the subnet mask configured on the interface;
If the route update is about the networks of different primary classes configured on the receiving interface, the router uses the default subnet mask according to the address category.
The generation of categorized route entries is automatically processed by the classless routing protocol.
Classless routing Overview
Classless routing protocols include Open Shortest Path First (OSPF), OSPF, RIPV2, intermediate system to intermediate system IS-IS), and Border Gateway Protocol version 4BGP4 ).
The subnet mask VLSM with different mask lengths is called variable lengths in the same primary network ). The classless routing (classless Routing) protocol supports VLSM. Therefore, you can set a subnet mask more effectively to meet the needs of different subnets for different numbers of hosts and make full use of host addresses.
Most regular and routine route Updates generated from the vector routing selection protocol are transmitted only to directly connected routing devices.
In a pure distance vector routing environment, route updates include a complete route table. By receiving the full route table of neighboring devices, the route can check all known routes, then, modify the local route table based on the received update information. The Distance Vector Method for Solving routing problems is sometimes called "rumor routing by rumor )"
Cisco ios supports several distance vector routing protocols, such as r00001, r00002, and IGRP. CISCO also directly holds the Network VPN protocol, which is an advanced distance vector routing protocol.
The routing protocol is usually associated with the network layer of the Protocol group.
Most distance vector routing protocols use the Bellman-Ford algorithm to calculate routes. VPN is an advanced distance vector routing protocol. It uses the dispersion Correction Algorithm d ual)
Comparison of Cisco IP Distance Vector Routing Protocol
Feature r00001r00002igrpeigrp
Count to infinite XXX
Horizontal Distance XXXX
Suppression timer XXX
Trigger update, route reverse XXXX
Server Load balancer-equal cost path XXXX
Server Load balancer-non-equal cost path XX
VLSM supports XX
Routing Algorithm Bellman-Ford DUAL
Measure, Skip, skip, and composite
The maximum number of hops is 1515100100.
Easy scalability
Note: The maximum number of hops for IGRP and VPN is 100 by default, but the maximum value can be 255.
A route update packet is generated only when the network topology changes. When the link status changes, the device that detects this change generates a link state notice about this link route ). Then, LSA is spread to all adjacent devices through a special multicast address. Each routing device retains the LSA copy and forwards the LSA to its adjacent devices. This process is called diffusion f looding) and then updates its topology database. This is a information table that contains all links of the network ). LSA diffusion is used to ensure that all routing devices are aware of this change, so that they can update their data and generate an updated route table that reflects the new network topology.
Comparison of Cisco link-State routing selection protocols
Features OSPFIS-ISEIGRP
Required systematic topology XX
Keep an understanding of all possible routes XXX
Route induction-manual XXX
Route induction-Automatic X
Event triggered announcement XXX
Server Load balancer-equal cost path XXX
Server Load balancer-non-equal cost path X
VLSM supports XXX
Routing algorithm routing strais-ISDUAL
Measurement Value Link Cost bandwidth) Composite
The number of hops cannot exceed 1024100
High scalability
The routing process in each vro must have a non-circular path to the logical network of all possible destinations. When all route tables are synchronized, when each route table contains an available route entry to the destination network, the network converges. Convergence is a synchronization activity associated with a route table when the network topology structure changes, for example, when the status of a new route or existing route changes.
The convergence time is the time required for the current topology to be recognized by all routes in the network, the network size, the used route selection protocol, and many configurable timers can affect the convergence time.
There are two detection methods:
When the physical layer or data link layer fails to receive a certain number of consecutive keepalive messages, the link is considered invalid.
When the route selection Protocol fails to receive a certain number of consecutive Hello messages, route updates, or similar messages, the link is considered invalid.
Most routing selection protocols have timers used to prevent the generation of topology loops during link state transition.