1、When to use the multi-route protocol?
When two different routing protocols want to exchange routing information, a multi-route protocol is used. Of course, route redistribution also allows the exchange of routing information. The following cases do not require the use of multiple routing protocols.
Upgrade from the older version of the Interior Gateway Protocol (I G P) to the newer version of I G P.
You want to use another routing protocol but have to keep the original one.
You want to terminate internal routing to avoid interference from other routers that do not have strict filtering supervision.
You are in an environment consisting of routers from multiple manufacturers.
What is distance vector routing protocol?
Distance vector routing protocols are designed for small network environments. In large network environments, this type of protocol will generate large traffic in learning routes and maintaining them, taking up too much bandwidth. It only considers an adjacent site unreachable if it does not receive a routing table update from the adjacent site within 9 0 seconds. Every 30 seconds, the distance vector routing protocol has to send the entire routing table to the neighboring site so that the routing table of the neighboring site is updated. This allows it to collect a list of networks from other sites (directly connected or otherwise connected) for routing purposes. Distance vector routing protocols use hop count as a metric to calculate the number of routers to pass through to reach a destination.
For example, R I P uses the B e l l m a n – F o r d algorithm to determine the shortest path, i.e., the line that can reach the destination as long as it passes through the smallest number of hops. The maximum number of hops allowed is usually set at 1 5. Those terminals that must pass through more than 1 5 routers are considered unreachable.
The distance vector routing protocols are as follows: IP RIP, IPX RIP, A p p l e Talk RT M P, and I G R P.
What is the Link State Routing Protocol?
The link-state routing protocol is more suitable for large networks, but its complexity makes routers require more C P U resources. Its ability to discover already broken links or newly connected routers in a shorter period of time makes the protocol’s convergence time shorter than that of distance vector routing protocols. Typically, it considers a neighbor to be unreachable when no H E L LO message is received from the neighbor within 1 0 seconds. A link-state router sends update messages to its neighbors notifying all the links it knows about. The metric it uses to determine the optimal path is a numerical cost, the value of which is generally determined by the bandwidth of the link. The link with the smallest cost is considered to be optimal. In the shortest path priority algorithm, the value of the maximum possible cost can be almost infinite.
If nothing changes in the network, the router simply refreshes the routing tables that are not updated periodically (the period can be from 3 0 minutes to 2 hours).
The following link-state routing protocols are available: IP OSPF, IPX NLSP, and I S – I S.
Can a router use both distance vector routing protocols and link-state routing protocols?
Yes. Each interface can be configured to use a different routing protocol; however, they must be able to exchange routing information by redistributing routes. (Redistribution of routes is discussed later in this chapter.)
2、What is the access table?
The access table is a set of rules added by the administrator to control the input and output of packets in the router. It is not generated by the router itself. The access table is capable of allowing or disallowing packets to enter or exit to their destinations. The table entries of the access table are executed sequentially, i.e., when a packet arrives, it is first looked at to see if it is subject to the first table entry, if not, it is then executed sequentially down the list; if it matches the first table entry, whether it is allowed or forbidden, there is no need to execute the following table entry checks.
There can be only one access table per protocol per interface.
What types of access tables are supported?
An access table can be identified by its number. The specific protocols and their corresponding access table numbers are as follows.
◎ I P standard access form number: 1 to 9 9
◎ I P extended access table number: 1 0 0 to 1 9 9
◎ I P X standard access table number: 8 0 0 to 8 9 9
◎ I P X extended access table number: 1 0 0 0 to 1 0 9 9
◎AppleTa l k access table number: 6 0 0 to 6 9 9
Tip In Cisco IOS Release 11.2 or above, you can identify access tables numbered 1 to 199 with a famous access table.
How do I create an IP standard access table?
An I P standard access table can be created by the following command: access-list access list number source [source-mask]
In this order.
◎access list number: Identifies which access table this entry belongs to. It is a number from 1 to 9 9.
◎permit | deny: Indicates whether this portal allows or blocks message traffic from a specific address.
Â- source: Determine the source I P address.
◎ s o u r c e – m a s k : Determines which bits of the address are used for matching. If a bit is “1″, it means that the bit in the address is left alone, and if it is “0″, it means that the bit in the address will be used for matching. Wildcards can be used.
The following is an example of an access table in a router configuration file.
Router# show access-lists
Standard IP access list 1
deny 188.8.131.52, wildcard bits 0.0.0.255
3、When to use route redistribution?
Route redistribution is typically configured on routers that are responsible for learning routes from one autonomous system and then broadcasting them to another autonomous system. If you are using I G R P or E I G R P, route redistribution is usually performed automatically.
4、What is management distance?
Administrative distance refers to the routing trust level of a routing protocol. Each routing protocol is assigned a trust level in order from highest to lowest reliability, and this trust level is called the administrative distance. For routing information from two different routing protocols to a destination, the router first decides which protocol to trust based on the administrative distance.
5、How to configure the redistribution?
Before you can perform a route redistribution, you must first.
1) Decide where to add the new protocol.
2) Determine the Autonomous System Boundary Router (ASBR).
3) Decide which protocol is at the core and which is at the boundary.
4) Decide on the direction to perform route redistribution.
Routing updates can be redistributed (this example is for OSPF) using the following command.
router(config-router)#redistribute protocol [process-id] [metric metric – value ] [metric-type type – value ] [subnets]
In this command.
◎protocol: Specify the source routing protocol for the router to redistribute routes.
The main values are: bgp, eqp, igrp, isis, ospf, static [ ip ], connected, and rip.
◎ process-id: Specify the process ID of OSPF.
◎ metric: is an optional parameter that specifies the metric value of the redistributed route. The default metric value is 0.
6、Why is it important to determine the adjacent router?
Identifying adjacent routers in a small network is not a major problem. This is because when one router fails, the other routers are able to converge within an acceptable time. In a large network, however, the latency to discover a failed router can be significant. Knowing the neighboring routers can speed up convergence because the routers can know the faulty router faster because the hello message interval is shorter than the interval between routers exchanging messages.
Routers using distance vector routing protocols can discover that an adjacent router is unreachable only when the adjacent router does not send routing update information, and this time is generally 10 to 90 seconds. While routers using link-state routing protocols can discover that the adjacent router is unreachable without receiving a hello message, and this interval is generally 10 seconds.
How do distance vector routing protocols and link-state routing protocols discover neighboring routers?
A router using the distance vector routing protocol creates a routing table (which includes the networks it is directly connected to), and it sends this routing table to the routers it is directly connected to. The neighboring router merges the received routing table into its own routing table, and it also sends its own routing table to its neighboring router. A router using the link-state routing protocol creates a link-state table that includes a list of destinations throughout the network. In an update message, each router sends its entire list. When the neighboring router receives this update message, it copies its contents and sends the information to its neighbors. There is no need for recalculation when forwarding the routing table contents.
Note that routers using IGRP and EIGRP broadcast hello messages to discover neighboring stations while exchanging routing update information like OSPF. eIGRP maintains a neighbor table for each network layer protocol that includes the address of the neighboring station, the number of messages waiting to be sent in the queue, the average time it takes to receive a message from or send a message to the neighbor, and the time it takes to determine that the link is broken before The time that no messages are received from the neighboring station before the link is determined to be broken.
7、What is an autonomous system?
An autonomous system is a cluster of routers and networks that are under the control of a governing body. It can be a router directly connected to a LAN and also to the Internet; it can be a multiple LAN interconnected by an enterprise backbone. All routers in an autonomous system must be connected to each other, run the same routing protocol, and be assigned the same autonomous system number. Links between autonomous systems use external routing protocols, such as B G P.
8、What is BGP?
BGP (Border GatewayProtocol) is a routing protocol that dynamically exchanges routing information between autonomous systems. The classical definition of an autonomous system is a group of routers under the control of a governing body that forwards messages to other autonomous systems using IGP and common metric values.
The term autonomous system is used in BGP to emphasize the fact that the administration of an autonomous system provides a uniform internal routing plan for other autonomous systems, and it provides a consistent description of those networks that are reachable through it.
9、What kinds of sessions does BGP support?
Sessions between neighboring BGP routers are built on top of the TCP protocol, which provides a reliable transport mechanism and supports two types of sessions.
o External BGP (EBGP): is a session between routers that belong to two different autonomous systems. These routers are adjacent and share the same media and subnet.
o Internal BGP (IBGP): is a session between routers within an autonomous system. BGP routers can be located anywhere in the autonomous system, even several routers apart.
Note that “the contents of the initial data stream are the entire BGP routing table. BGP does not need to update the entire routing table on a periodic basis. A BGP router periodically sends a Keep Alive message to confirm that the connection is active. When an error or special condition occurs, the router sends a Notification message. When a connection has an error, a notification message is generated and the connection is disconnected.” -from RFC11654, BGP Operations.
10、Does BGP allow route redistribution?
Allowed. Because BGP is primarily used for routing between autonomous systems, it must support a combination of the routing tables of RIP, OSPF, and IGRP in order to transfer their routing tables into an autonomous system. bgp is an external routing protocol, so it operates differently than an internal routing protocol. In BGP, a route can be created in the BGP routing table with the NETWORK command only if a route already exists in the IP routing table.
11、How to display all the BGP routes in the database?
To display all BGP routes in the database, simply enter at the EXEC command line.
show ip bgp paths
The output of this command might be.
Address Hash Refcount MetricPath
0 x 2 9 7 A 9 C 0 2 0 i
12、What is horizontal segmentation?
Horizontal partitioning is a technique to avoid the emergence of routing loops and to speed up route aggregation. Since a router may receive routing information from itself, which is useless, the horizontal splitting technique does not back-announce any routing updates received from endpoints, but only those routes that will not be cleared due to counting to infinity.
13、How is the routing ring generated?
Due to the presence of route aggregation time in the network, new or changed routes in the routing table do not stabilize quickly across the network, allowing inconsistent routes to exist, and routing loops are created.
14、What is the metric value?
The metric values represent distances. They are used to determine the optimal route when searching for a route. Each routing algorithm generates a value (metric) for each path through the network when generating a routing table, with the smallest value representing the optimal path. The metric can be calculated by considering only one characteristic of the path, but more complex metrics are generated by combining multiple characteristics of the path. Some common metric values are.
◎ Hop steps: The number of router output ports through which the message will pass.
Â- Ticks: the delay of the data link (about 1/18th per second).
Â- Cost: can be an arbitrary value that is obtained based on bandwidth, cost or other calculation methods defined by the network manager.
◎ Bandwidth: The capacity of the data link.
Â- Time delay: the length of time for a message to travel from the source to the destination.
Â- Load: The size of the portion of the network resource or link that is already in use.
Â- Reliability: The ratio of error bits in the network link.
Â- Maximum Transmission Unit (MTU): the maximum message length (in bytes) that can be accepted by all links on a path.
What type of routing metric does IGRP use? What does this metric consist of?
IGRP uses multiple routing metrics. It consists of the following components.
◎ Bandwidth: The minimum bandwidth value between source and destination.
Â- Time delay: the interface delay accumulated in the path.
Â- Reliability: the worst possible reliability between source and destination, based on the state of the link hold.
◎ Load: The worst-case load of the link between the source and the destination, expressed in bits per second.
◎ MTU: The smallest M T U value in the path.