Router basic knowledge question and answer

Router basic knowledge question and answer

1. When to use multiple routing protocols?

When two different routing protocols want to exchange routing information, multiple routing protocols are used. Of course, routing redistribution can also exchange routing information. There is no need to use multiple routing protocols in the following situations:

Upgrade from the old version of the Interior Gateway Protocol (IGP) to the new version of IGP.

You want to use another routing protocol but you must keep the original protocol.

You want to terminate internal routing so as not to be disturbed by other routers that do not have strict filtering supervision functions.

You are in an environment made up of routers from multiple manufacturers.

What is the distance vector routing protocol?

The distance vector routing protocol is designed for small network environments. In a large-scale network environment, this type of protocol will generate a large amount of traffic and consume too much bandwidth when learning and maintaining routes. If the routing table update sent by the neighboring station is not received within 90 seconds, it considers the neighboring station unreachable. Every 30 seconds, the distance vector routing protocol sends the entire routing table to the neighboring site, so that the routing table of the neighboring site is updated. In this way, it can collect a list of networks from other sites (directly connected or connected in other ways) for routing purposes. The distance vector routing protocol uses the number of hops as a metric to calculate the number of routers to pass through to reach the destination.

For example, RIP uses Bellman-Ford algorithm to determine the shortest path, that is, the line that can reach the destination as long as it passes the smallest number of hops. The maximum allowable hop count is usually set to 15. Those terminals that must pass more than 15 routers are considered unreachable.

The distance vector routing protocols are as follows: IP RIP, IPX RIP, Apple Talk RT MP and IGR P.

What is a link state routing protocol?

Link state routing protocol is more suitable for large networks, but due to its complexity, routers require more CPU resources. It can discover the broken link or newly connected router in a shorter time, making the convergence time of the protocol shorter than the distance vector routing protocol. Normally, if it does not receive the HEL LO message from the neighboring station within 10 seconds, it considers that the neighboring station is unreachable. A link state router sends an update message to its neighbors, informing all the links it knows about. It determines that the metric value of the optimal path is a numerical cost, and the value of this cost is generally determined by the bandwidth of the link. The link with the smallest cost is considered to be optimal. In the shortest path first algorithm, the value of the maximum possible cost can be almost unlimited.

If there is no change in the network, the router only needs to refresh the routing table that has not been updated periodically (the length of the cycle can be from 30 minutes to 2 hours).

There are several types of link state routing protocols: IP OSPF, IPX NLSP and IS-IS.

Can a router use both distance vector routing protocol and link state routing protocol?

can. Each interface can be configured to use different routing protocols; however, they must be able to exchange routing information by redistributing routes. (Routing redistribution will be discussed later in this chapter.)

2. What is an access list?

The access list is a series of rules added by the administrator to control the input and output of data packets in the router. It is not generated by the router itself. The access table can allow or prohibit the entry or output of data packets to the destination. The entries of the access table are executed sequentially, that is, when the data packet arrives, it is first checked whether it is bound by the first entry, if not, it is executed sequentially down; Whether it is allowed or forbidden, there is no need to perform the check of the following items.

There can only be one access list per protocol per interface.

What types of access tables are supported?

An access list can be determined by its number. The specific agreement and the corresponding access table number are as follows:

◎ IP standard access table number: 1 ~ 9 9

◎ IP extended access table number: 1 0 0 ~ 1 9 9

◎ IPX standard access table number: 8 0 0 ~ 8 9 9

◎ IPX extended access table number: 1 0 0 0 ~ 1 0 9 9

◎ AppleTa lk access table number: 6 0 0 ~ 6 9 9

TIP: In Cisco IOS Release 11.2 or above, you can use the well-known access table to determine the access table numbered 1 to 199.

How to create IP standard access table?

An IP standard access list can be created by the following command: Access-list access list number {permit | deny} source [source-mask]

In this command:

â—Ž access list number: Determine which access list this entry belongs to. It is a number from 1 to 9 9.

â—Ž permit | deny: indicates whether this entry allows or blocks information traffic from a specific address.

â—Ž source: Determine the source IP address.

â—Ž source-mask: Determine which bits in the address are used for matching. If a bit is "1", it means that the bit in the address does not matter, 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, wildcard bits

permit any

3. When to use route redistribution?

Route redistribution is usually configured on routers that are responsible for learning routes from one autonomous system and then broadcasting to another autonomous system. If you are using IGRP or EIGRP, route redistribution is usually performed automatically.

4. What is the management distance?

Administrative distance refers to the routing credibility of a routing protocol. Each routing protocol is assigned a trust level in order from high to low reliability. This trust level is called the management 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 redistribution?

Before performing route redistribution, you must first:

1) Decide where to add a new agreement.

2) Determine the autonomous system border router (ASBR).

3) Decide which protocol is at the core and which is at the border.

4) Decide the direction of route redistribution.

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

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

In this command:

â—Ž protocol: indicates the source routing protocol that the router wants to redistribute.

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

â—Ž process-id: indicates the process ID of OSPF.

â—Ž metric: is an optional parameter used to indicate the metric value of the redistributed route. The default metric value is 0.

6. Why is it important to identify adjacent routers?

Determining adjacent routers in a small network is not a major problem. Because when one router fails, other routers can converge within an acceptable time. But in a large network, the delay of discovering a faulty router may be large. Knowing the neighboring router can speed up the convergence, because the router can know the failed router faster because the interval between hello messages is shorter than the interval between routers exchanging information.

A router using the distance vector routing protocol can find that the adjacent router is unreachable when the adjacent router does not send routing update information. This time is generally 10 to 90 seconds. The router using the link state routing protocol can find that the adjacent router is unreachable without receiving the hello message. This interval is generally 10 seconds.

How do distance vector routing protocols and link state routing protocols discover adjacent routers?

A router using the distance vector routing protocol will create a routing table (including the network directly connected to it), and at the same time it will send this routing table to the router directly connected to it. The adjacent router merges the received routing table into its own routing table, and at the same time it also sends its own routing table to its adjacent router. Routers using link-state routing protocols need to create a link-state table, including a list of destination stations for the entire network. In the update message, each router sends its entire list. When the neighboring router receives the update message, it copies the contents and sends the information to its neighboring station. No recalculation is necessary when forwarding the contents of the routing table.

Note that routers using IGRP and EIGRP broadcast hello messages to discover neighbors, and at the same time exchange routing update information like OSPF. EIGRP maintains a list of neighbors for each network layer protocol, which includes the address of the neighbor, the number of packets waiting to be sent in the queue, the average time required to receive or send messages to or from the neighbor, and The time when no message was received from the neighboring station before the link was determined to be broken.

7. What is an autonomous system?

An autonomous system is a group of routers and networks under the control of a regulatory agency. It can be a router connected directly to a LAN and also to the Internet; it can be a multiple local area network interconnected by an enterprise backbone network. All routers in an autonomous system must be connected to each other, run the same routing protocol, and assign the same autonomous system number at the same time. Links between autonomous systems use external routing protocols, such as BG P.

8. What is BGP?

BGP (Border Gateway Protocol) is a routing protocol that dynamically exchanges routing information between autonomous systems. The classic definition of an autonomous system is a group of routers under the control of a regulatory agency, which uses IGP and common metrics to forward packets to other autonomous systems.

The term autonomous system is used in BGP to emphasize the fact that the management of an autonomous system provides a unified internal routing plan for other autonomous systems, which provides a consistent description.

9. What kinds of sessions does BGP support?

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

o External BGP (EBGP): is a session between routers belonging to two different autonomous systems. These routers are adjacent and share the same medium and subnet.

o Internal BGP (IBGP): is a conversation between routers within an autonomous system. It is used to coordinate and synchronize the routing process within the autonomous system. The BGP router can be anywhere in the autonomous system, and even several routers can be separated in the middle.

Note that the content of the initial data flow is the entire BGP routing table. However, when the routing table changes in the future, the router only transmits the changed part. BGP does not need to periodically update the entire routing table. The BGP sender must keep the entire BGP routing table common to all peer routers. The BGP router periodically sends Keep Alive messages to confirm that the connection is active. 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 RFC11654, BGP *.

10. Does BGP allow route redistribution?

allow. Because BGP is mainly used for routing between autonomous systems, it must support the synthesis of RIP, OSPF, and IGRP routing tables in order to transfer their routing tables to an autonomous system. BGP is an external routing protocol, so its performance is different from an internal routing protocol. In BGP, only when a route already exists in the IP routing table, you can use the NETWORK command to create a route in the BGP routing table.

11. How to display all BGP routes in the database?

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

show ip bgp paths

The output of this command may be:

Address Hash Refcount MetricPath

0 x 2 9 7 A 9 C 0 2 0 i

12. What is split horizon?

Split horizon is a technology that avoids the appearance of routing loops and speeds up route aggregation. Since the router may receive the routing information sent by itself, and this information is useless, the split horizon technology does not reversely advertise any routing update information received from the terminal, but only advertises those that will not be cleared due to infinity. routing.

13. How is the routing loop generated?

Due to the existence of the routing convergence time of the network, new routes or changed routes in the routing table cannot be stabilized in the entire network quickly, so that there are inconsistent routes, and a routing loop will be generated.

14. What is a metric?

The metric value represents the distance. They are used to determine the optimal route when looking for a route. When each routing algorithm generates a routing table, it will generate a value (metric value) for each path through the network. The smallest value indicates the optimal path. The calculation of the metric value can only consider one characteristic of the path, but more complex metric values ​​are generated by combining multiple characteristics of the path. Some commonly used metrics are:

â—Ž Number of hops: the number of router output ports through which the packet passes.

â—Ž Ticks: Delay of the data link (about 1/18 per second).

â—Ž Cost: It can be an arbitrary value, which is obtained based on bandwidth, cost or other calculation methods defined by the network administrator.

â—Ž Bandwidth: The capacity of the data link.

â—Ž Time delay: the length of time for the message to pass from the source to the destination.

â—Ž Load: The size of the part of the network resource or link that has been used.

â—Ž Reliability: The ratio of erroneous bits of the network link.

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

What type of routing metrics does IGRP use? What constitutes this measure?

IGRP uses multiple routing metrics. It includes the following parts:

â—Ž Bandwidth: The smallest bandwidth value between source and destination.

â—Ž Delay: The interface delay accumulated in the path.

â—Ž Reliability: The worst possible reliability between source and destination is based on the state of link maintenance.

â—Ž Load: The load of the link between the source and the destination in the worst case, expressed in bits per second.

â—Ž MTU: The smallest MTU value in the path.

15. 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 to add a positive offset to the network input and output metrics 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, then the OFFSET parameter will be added to all measurement values. If the value of OFFSET is 0, then it has no effect. For IGRP, the offset value is only added to the delay. This subcommand also applies to RIP and hello routing protocols.

Use the NO OFFSET-LIST command with appropriate parameters to clear this offset.

In the following example, a router using IGRP adds an offset of 10 to the delay of all output metrics: offset-list out 10

The following is an example of adding the same offset to the access table 121:

offset-list out 10 121

16. What five pieces of information does each router need to know when searching for a route?

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

â—Ž Destination address: the destination host for sending the message.

â—Ž Determining the neighboring station: indicate who is directly connected to the interface of the router.

â—Ž Routing discovery: Discover which networks the neighbors know.

â—Ž Choose route: Provide the best (related to the metric value) route to the destination through the information learned from the neighbor station.

â—Ž Keep routing information: The router keeps a routing table, which stores all the routing information it knows.

17. Are the routing protocols supported by Cisco routers compatible with the protocols of other manufacturers' equipment?

With the exception of IGRP and EIGRP, all routing protocols supported by Cisco routers are compatible with the same protocols implemented by other manufacturers. IGRP and EIGRP are Cisco's patented products.

18. What does the information in the RIP routing table entries show?

Each entry in the RIP routing table provides certain information, including the final destination address, the next hop address to the destination, and the metric value. This metric value represents the distance to the destination terminal (number of steps). Other information can also be included.

Router issues added:

1. Does the Cisco3600 series router currently support WAN interface cards WIC-2T and WIC-2A / S?

Cisco3600 series routers support WIC-2T and WIC-2A / S two WAN interface cards in 12.007XK and above.

But it should be noted that:

Only the Fast Ethernet hybrid network module can support these two WAN interface cards.

The network modules that support these two interface cards are as follows:

NM-1FE2W, NM-2FE2W, NM-1FE1R2W, NM-2W.

The Ethernet hybrid network module does not support it, as follows:

NM-1E2W, NM-2E2W, NM1E1R2W.

2. What are the maximum different / synchronization rates supported by NM (4A / S, NM (8A / S network module and WIC (2A / S WAN interface card of Cisco3600 series routers?

These network modules and WAN interface cards can support both asynchronous and synchronous. The maximum asynchronous rate supported is 115.2Kbps, and the maximum synchronous rate is 128Kbps.

3. What are the cables of WIC-2T and WIC-1T?

WIC-1T: DB60 to V35 or RS232, 449 and other cables. Such as: CAB-V35-MT.

WIC-2T: SMART to V35 or RS232, 449 and other cables. Such as: CAB-SS-V35-MT.

4. What is the difference between MCE1 on Cisco 7000 series and E1 and CE1 on Cisco 2600/3600?

MCE1 on Cisco 7000 can be configured as E1 and CE1, while E1 and CE1 on Cisco 2600/3600 only support their own functions.

5. Does the Cisco 2600 series router support inter-VLAN routing, what are the requirements for IOS software?

Cisco (2600 series routers, only Cisco2620 and Cisco2621 can support inter-VLAN routing (100M ports only support inter-VLAN routing). And if inter-VLAN routing is supported, the IOS software must include the IP Plus feature set.

6. What are the hardware differences between the Cisco 3660 router and the 3620/3640 router?

The differences are as follows:

The basic configuration of the Cisco 3660 router includes 1 or 2 10 / 100M adaptive fast Ethernet interfaces; the Cisco 3620/3640 basic configuration does not include an Ethernet interface.

The Cisco 3660 router supports hot swap of network modules, while the Cisco 3620/3640 does not support hot swap of network modules.

The redundant power supply of the Cisco 3660 is built-in, while the redundant power supply of the Cisco 3620/3640 is external.

7. Why can't the 3640 recognize NM-1FE2W?

IOS needs to be upgraded to 12.0.7T

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