Summary of router FAQs in daily applications

Source: Internet
Author: User
Tags keep alive

  When will route redistribution be used?

Route redistribution is usually configured on routers responsible for learning routes from one autonomous system and then broadcasting to another autonomous system. If you are using I g r p or e I G R P, route redistribution is generally automatically executed.

  What is Management Distance?

Management Distance refers to the route reliability of a routing protocol. Each routing protocol is assigned a level of trust from high to low reliability. This level of trust is called the Management Distance. For the routing information from two different routing protocols to one destination, the router first determines which Protocol to trust based on the Management Distance.

 How to configure redistribution?

Before route redistribution, you must first:

1) decide where to add a new protocol.

2) determine the border router (ASBR) of the autonomous system ).

3) decide which protocol is at the core and which is at the boundary.

4) determines the route redistribution direction.

You can use the following command to reallocate route updates (this example is for OSPF ):

Router (config-router) # redistribute protocol [process-id] [metric-value] [metric-type-value] [subnets]

In this command:

Protocol: Specifies the source route protocol for Route redistribution on the vro.

The main values are bgp, eqp, igrp, isis, ospf, static [ip], connected, and rip.

Process-id: the ID of the OSPF process.

Metric: an optional parameter used to specify the metric value of the route for redistribution. The default measurement value is 0.

  Why is it important to determine the adjacent router?

Determining the adjacent router in a small network is not a major problem. When a router fails, other routers can converge within an acceptable period of time. However, in a large network, the latency of a faulty router may be high. Knowing that the adjacent router can accelerate convergence, because the router can know the faulty router more quickly, because the interval of hello messages is shorter than the interval of router information exchange.

When a router using the distance vector routing protocol does not send route update information to the adjacent router, it can be found that the adjacent router is no longer reachable. This time is generally 10 ~ 90 seconds. The router that uses the link status routing protocol does not receive the hello message, it can be found that the adjacent router is not reachable, this interval is generally 10 seconds.

  How can I find the adjacent router through distance vector routing protocol and link status routing protocol?

A Router using the Distance Vector Routing Protocol creates a route table (including the network directly connected to it), and sends the route table to the router directly connected to it. The adjacent router combines the received route table into its own route table, and also sends its route table to its adjacent router. A connection status table is created for the router that uses the link status routing protocol, including the list of the entire network destination site. In the UPDATE message, Each router sends its entire list. When the adjacent router receives the update packet, it copies the content and sends the information to its neighbor. It is not necessary to recalculate the content of the route table.

Note that the hello message is broadcast by the routers of IGRP and VPN to discover the adjacent station and exchange route update information like OSPF. For each network layer protocol, it includes the address of the neighboring station, the number of packets waiting to be sent in the queue, and the average time required to receive or send packets to the neighboring station, and the time when no message is received from the neighboring station before the link is confirmed to be disconnected.

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 What is an autonomous system?

An autonomous system is a vro and network group under the control of a management organization. It can be a vro that is directly connected to a LAN and connected to the Internet. It can be a LAN that is interconnected by an enterprise backbone network. All routers in an autonomous system must connect to each other, run the same routing protocol, and assign the same autonomous system number. Links between autonomous systems use external routing protocols, such as B G P.

  What is BGP?

Border GatewayProtocol is a routing protocol that dynamically exchanges route information between autonomous systems. A classic definition of an autonomous system is a group of routers under the control of a management organization. It uses IGP and common measurement values to forward packets to other autonomous systems.

The term autonomous systems are used in BGP to emphasize the fact that the management of an autonomous system provides a unified internal routing plan for other autonomous systems, it provides a consistent description for networks that can be reached through it.

  What session types does BGP support?

The session between BGP neighboring routers is based on the TCP protocol. TCP provides a reliable transmission mechanism that supports two types of sessions:

External BGP (EBGP): refers to the session between routers belonging to two different Autonomous Systems. These routers are adjacent and share the same media and subnet.

Internal BGP (IBGP): refers to the session between routers in an autonomous system. It is used to coordinate and synchronize routing processes in autonomous systems. BGP routers can be separated by several routers at any location in the autonomous system.

Note: "The content of the initial data stream is the entire BGP Route table. However, when the route table changes later, the router only transmits the changed part. BGP does not need to update the entire route table periodically. Therefore, when a connection has been established, a BGP sender must save the entire BGP Route table that is currently common to all vrouters at the same level. The BGP router periodically sends the Keep Alive message to confirm that the connection is activated. When an error or special situation occurs, the router sends a Notification message. When a connection error occurs, a notification message is generated and the connection is disconnected. "-From rfc000054 and BGP.

  Does BGP allow route redistribution?

Yes. Because BGP is mainly used to select routes between autonomous systems, it must support the synthesis of routes selection tables for RIP, OSPF, and IGRP, so that their route tables can be transferred to an autonomous system. BGP is an external routing protocol, so its * operation is different from an internal routing protocol. In BGP, only when a route already exists in the IP route table can you use the NETWORK command to create a route in the BGP Route table.

  How do I display all BGP routes in the database?

To display all BGP routes in the database, enter the following command line in the EXEC command line:

Show ip bgp paths

The output of this command may be:

Address Hash Refcount MetricPath

0x2 9 7 A 9 C 0 2 0 I

 What is horizontal segmentation?

Horizontal segmentation is a technology that avoids the appearance of routing rings and accelerates route aggregation. Because the router may receive the route information sent by itself, and this information is useless, the horizontal segmentation technology does not reverse notice any route update information received from the terminal, instead, only route entries that are not cleared due to infinite counts are advertised.

 How is a routing ring generated?

Due to the existence of the route aggregation time of the network, the new or changed route in the route table cannot be stable in the whole network quickly, so that there are inconsistent routes, so a routing ring will be generated.

 What is a metric value?

The measurement value indicates the distance. They are used to determine the optimal route when looking for a route. When a route table is generated by each routing algorithm, a value (measurement value) is generated for each route through the network. The minimum value indicates the optimal path. The Calculation of measurement values can only consider one feature of the path, but more complex measurement values are generated by combining multiple features of the path. Some common metric values include:

Hop count: the number of router output ports to pass through.

Ticks: latency of data links (about 1/18 per second ).

Cost: it can be an arbitrary value, which is calculated based on bandwidth, fees, or other network manager-defined calculation methods.

Bandwidth: the capacity of the data link.

Latency: the length of time the message is sent from the source to the destination.

Load: the size of the used part of the network resource or link.

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Reliability: the ratio of error bits on the network link.

Maximum Transmission Unit (MTU): The maximum message length (in bytes) acceptable to all links in a path ).

What type of Route metric value does IGRP use? What is the composition of this metric value?

IGRP uses multiple route metric values. It includes the following parts:

Bandwidth: The minimum bandwidth value between the source and destination.

Latency: The interface latency accumulated in the path.

Reliability: The worst possible reliability between the source and target, based on the state of link persistence.

Load: the load of the link between the source and target in the worst case, expressed in bits per second.

MTU: The minimum m t u value in the path.

  Can the metric value be modified or adjusted?

Add a positive offset. The complete structure of this command is as follows: You can use the OFFSET-list router subcommand.

Add a positive offset for the network input and output metric values in the access table.

Offset-list {in | out} offset [access-list] no offset-list {in | out} offset [access-list]

If the value of the parameter LIST is 0, the OFFSET parameter is added to all measurement values. If the value of OFFSET is 0, there is no effect. For IGRP, the offset value is only added to the latency. This sub-command also applies to the RIP and hello routing protocols.

Use the no offset-LIST command with the appropriate parameter to clear this OFFSET.

In the following example, a router using IGRP adds offset 10: offset-list out 10 to the latency of all output metric values.

Here is an example of adding the same offset to Access Table 121:

Offset-list out 10 121

 Which of the following information does a vro need to know when searching for a route?

All routers need the following information to find routes for packets:

Destination Address: the target host for sending packets.

Determine the neighboring station: specify who is directly connected to the router interface.

Discovery of routes: Discover the networks that the neighboring sites know.

Select route: Provides the optimal path (related to the metric value) to reach the destination based on the information learned from the neighboring station.

Keep route info: the router saves a route table, which stores

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