IPV4-IGP

1、 What underlying protocol is the routing protocol based on , And what are the characteristics of the way they work based on these underlying protocols ？
（1）BGP Is based on TCP Of , Port number 179,TCP It's a reliable transport layer protocol , therefore BGP Sending update When the news , There is no need for a confirmation mechanism （ Depend on TCP Of ACK To confirm receipt of ）. be based on TCP The protocol can also establish neighbors across devices （ unicast ）, Just promise TCP Source IP Address and purpose IP The address can reach .

（2）OSPF Is based on IP Of , The agreement number is 89.IP The agreement is unreliable , therefore OSPF Many reliability mechanisms have been developed （Hello List of neighbors of the package ,DD The message uses the serial number for implicit confirmation ,LSR/LSU/LSACK Mutual confirmation between ）.OSPF be based on IP layer , Can only support IP The Internet .
OSPFV2 be used for IPv4,OSPFV3 be used for IPv6.

（3）ISIS It is based on the data link layer , So it can support a variety of network layer protocols , Not restricted by network layer protocol ISIS Version of

Whether there is a neighbor relationship, which is better ？
Having a neighbor relationship does not require frequent and periodic transmission of routing information ; If a neighbor exists, the route passed by the neighbor is saved , Periodic update is mainly data synchronization ;
1、 Having neighbors is equivalent to establishing a channel in advance , Perceived failure , Fast route convergence
2、 There is no need to send route updates frequently and periodically , Just maintain good neighbors .

2、 Classification of routing protocols , What are their characteristics, advantages and disadvantages ？
（1） Scope of work of the agreement ：IGP/EGP
IGP： Working in AS Inside , be used for AS Internal interactive information ; It's mainly used to discover , Select and calculate the optimal route . Such as ：RIP/ISIS/OSPF
EGP： Working in AS between , be used for AS Interactive routing information between ; It is mainly used to deliver and select the best route .

（2） Whether to carry mask ： Some kind / No class agreement
Some kind ： Pass information without mask , Such as RIPv1
No class ： Carry a mask when passing a route , Such as ISIS/OSPF,BGP

（3） The algorithm of the protocol ：DV/LS
DV（ Distance vector ）： Only care about the distance and direction to reach the destination , No network topology ; In the route update message , Contains only routing information , No topology information ; Article by article convergence , Calculate the routing table according to the routing information transmitted by neighbors . Such as ：RIP/BGP LS（ Link state ）： Care about the topology of the whole network ; In the message of route update , Including routing information and topology information ; According to oneself SPF The algorithm calculates the route . Such as ：OSPF/ISIS;

How to understand distance vector and link state ？
Distance vector ： Route delivery only contains route information , No topology information ; Only care about the direction to the destination address ;
Link state ： Routing delivery includes both routing information and topology information ; Will establish the whole network topology with itself as the root ;
The main points of ：1、 Transmit information ;2、 Whether the equipment has the whole network topology .

In distance vector routing protocol RIP/BGP, What is the difference between them when calculating routes ？
RIP Measure distance by hops , The next hop address indicates the direction .（ If the next jump is not filled , with response The source address of the message confirms the direction ）BGP Use rich path attributes to know the distance and direction （ Generally accepted compliance property next-hop Field ）

3、 What is the routing priority of each routing protocol ？ Why is this design ？
The router defines external priority and internal priority respectively , The smaller the value, the higher the priority . External priority refers to the priority that users can manually configure for each routing protocol .
Direct link routing （DIRECT） The priority of cannot be modified .

Internal priority ： It can't be modified by hand , When the external priority cannot be compared , Will compare according to the internal priority .

Why design the priority of various protocols so ？
（1） There are two reasons for prioritization ： When different routing protocols are configured with the same priority , The system can determine which routing protocol will be the best route through internal priority .
（2） Reasons for designing external routing priorities ：
1、rip And static ： Static routing is manually configured by the administrator, so it is better than rip Routing learned from neighbors .
2、rip and ospf、isis：RIP Is a distance vector routing protocol , Low reliability ;isis and ospf Are link state routing protocols , High reliability .
3、ospf and isis：ospf More popular than ISIS high
4、 Static and ospf： Static routing has a large amount of configuration , Not easy to manage ;ospf It's dynamic learning routing , It can ensure that the route is acyclic , And perceiving network changes .
5、BGP： It is mainly used for routing between autonomous systems , Other routing protocols are used for AS Internal ;AS Route learned in , Reliability is better than AS The route learned from outside . therefore BGP The priority of is designed to 255.

（3）ISIS and BGP Reasons for internal routing settings ：
ISIS Design two priorities , It is used to distinguish routes within regions and routes between regions , Can prevent suboptimal .
BGP Design two priorities , The following is an example ：

The routers in the lower left corner are from EBGP Neighbors and IBGP The neighbors received information about 100.1.1.0/24 The routing . here , You should choose EBGP Routing from neighbors . Because this route does not go through Local AS, Reduced in Local AS Transmission overhead .

OSPF Design 2 What are the benefits of this priority ？
osfp Design two priorities , Used to distinguish AS Inside you and AS External routing , It can prevent sub optimization when routing is introduced .

R3 take 172.16.0.0/22 The route introduced to OSPF After the domain ,R4 From RIP、OSPF All received information about 172.16.0.0/22 The routing .
here , without OSPF AES、NSSA The priority of the . that R4 On OSPF The priority of 10, be better than RIP Of 100. therefore R4 Will choose from OSPF Go to the target network segment , Cause suboptimal path .
If there is OSPF AES、NSSA The priority of the .R4 Routing on the target network segment ,OSPF by 150、RIP by 100;R4 choice RIP Go to the target network segment .

When routers from different manufacturers are interconnected , What should I pay attention to about routing priority ？
Routing protocol priority is inconsistent , Be careful to adjust it to be consistent , Otherwise, the problem of suboptimal path may occur .（ Cisco equipment OSPF110, Static routing priority is 1）

How the router looks up the routing table ？
First, the longest matching is preferred , Then compare the routing priority （ The subnet is consistent with the mask, and the different priority will only show the optimal route in the routing table ）, Last comparison Cost value .

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