44 router problems that network engineers should be aware of (1)

1. When to use the multi-route protocol?
When two different routing protocols need to exchange routing information, they need to use multiple routing protocols. Of course, route redistribution can also exchange route information. Multi-route protocol is not required in the following cases:
From the old version of the Internal Gateway Protocol Interior Gateway Protocol, I g p) upgrade to the new version of the I G P.
To use another routing protocol, you must retain the original one.
You want to stop the internal route to avoid interference from other routers that do not strictly filter the monitoring function.
You are in an environment composed of routers from multiple manufacturers.
What is distance vector routing protocol?
The distance vector routing protocol is designed for small network environments. In a large network environment, such protocols will generate large traffic when learning routes and maintaining routes, occupying excessive bandwidth. If the route selection table update is not received from the adjacent site within 9 0 seconds, the adjacent site is considered inaccessible. Every 30 seconds, the distance vector routing protocol sends the entire route selection table to the adjacent site to update the route selection table of the adjacent site. In this way, it can be directly connected from other sites or other methods) to collect a list of networks for route selection. Distance Vector Routing uses the number of hops as the metric value to calculate the number of routers that need to pass to the destination.
For example, r I p uses the B e l m a n-F o r d algorithm to determine the shortest path, that is, the line that can reach the destination as long as it passes through the smallest hop count. The maximum number of hops allowed is usually set to 1 5. Terminals that must pass through more than five vrouters are considered inaccessible.
Distance Vector routing protocols include ip rip, ipx rip, A p l e Talk rt m p, And I G R P.
What is the link status routing protocol?
The link status routing protocol is more suitable for large networks. However, due to its complexity, the router needs more c p u resources. It can detect broken links or new routers in a shorter period of time, so that the convergence time of the Protocol is shorter than that of the Distance Vector routing protocol. Generally, if you do not receive the he l lo packet from the neighboring station within one 0 second, it is deemed that the neighboring station is no longer reachable. A connection status router sends an update message to its neighbor to notify it of all the links it knows. It determines that the measurement value of the optimal path is a numerical cost, which is generally determined by the bandwidth of the link. Links with the minimum cost are considered optimal. In the Shortest Path priority algorithm, the maximum possible cost is almost unlimited.
If the network does not change, as long as the router periodically refreshes the route selection table that has not been updated, the duration of the cycle can be from 3 0 minutes to 2 hours ).
The connection status routing protocols include ip ospf, ipx nlsp, And I s-I S.
Can a vro use the distance vector routing protocol and the link status routing protocol?
Yes. Each interface can be configured to use different routing protocols. However, they must be able to exchange routing information by allocating routes. Route redistribution will be discussed later in this chapter .)
2. What is an access table?
The access table is a series of rules that the manager adds to control data packets input and output in the vro. It is not generated by the router itself. The Access Table allows or disables data packets from entering or outputting to the destination. The table items for accessing the table are executed sequentially. When the data packet arrives, the system first checks whether it is restricted by the first table item. If not, it is executed sequentially; if it matches the first table item, you do not have to perform the following table item check whether it is allowed or disabled.
Each Protocol of each interface can have only one access table.
What types of access tables are supported?
The number of an access table can be determined. The specific protocol and its corresponding access table number are as follows:
◎ I P standard Access Table No.: 1 ~ 9 9
◎ I P extended access table No.: 1 0 0 ~ 1 9 9
◎ I P X standard Access Table No.: 8 0 0 ~ 8 9 9
◎ I P X extended access table No.: 1 0 0 0 ~ 1 0 9 9
◎ AppleTa l k Access Table No.: 6 0 0 ~ 6 9 9
Note: In Cisco IOS Release11.2 or later versions, you can use the famous access table to determine the number between 1 and later ~ 199 access table.
How do I create an IP standard access table?
The following command creates an I P standard Access table: access-list Access list number {permit | deny} source [source-mask]
In this command:
◎ Access list number: determines which access table the entry belongs. It is a number ranging from 1 to 9.
◎ Permit | deny: indicates whether the entry permits or blocks information traffic from a specific address.
◎ Source: determine the source I P address.
◎ S o u r c e-m a s k: determines which bits in the address are used for matching. If a bit is "1", it indicates that the bit in the address is unnecessary. If it is "0", it indicates that the bit in the address will be used for matching. Wildcard characters can be used.
The following is an example of an access table in the vro configuration file:
Router # show access-lists
Standard IP access list 1
Deny 204.59.144.0, wildcard bits 0.0.0.255
Ermit any
3. 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.
4. 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.
5. 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: indicates the OSPF process ID.
◎ Metric: it is an optional parameter used to specify the metric value of the redistribution route. The default measurement value is 0.
6. 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 needs to create a route table, including the network directly connected to it), and it will send 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.
7. 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.
8. 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.
9. 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:
O external BGPEBGP): refers to the session between routers belonging to two different Autonomous Systems. These routers are adjacent and share the same media and subnet.
O internal BGPIBGP): 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 operations.
10. 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.