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Static routing and dynamic routing

I. the basic concept of static routing

The route manually set by the system administrator is called static (static) route, which is generally preset according to the configuration of the network when the system is installed, and it will not change automatically with the change of network topology in the future. Its advantage is that it does not occupy the network, system resources and security; its disadvantage is that the network administrator needs to configure one by one by hand, and can not automatically adjust the change of the network state. In networks with no redundant connections, static routes may be the best choice. Whether a static route appears in the routing table depends on whether the next hop is reachable. The static route is generated in the routing table (onwer) is static (static), the priority of the route is 1, and its Metricvalue is 0.

Basic configuration commands

III. BFD technology

1. Background

In order to protect critical applications, redundant backup links are designed in the network. In case of network failure, network devices are required to quickly detect faults and switch traffic to backup links to speed up network convergence. At present, some links (such as POS) use hardware detection mechanism to achieve fast fault detection. However, some links, such as Ethernet links, do not have such a detection mechanism. At this time, the application has to rely on the mechanism of the upper layer protocol for fault detection, and the detection time of the upper layer protocol is more than 1 second, which is intolerable for some applications. Some routing protocols such as OSPF and IS-IS have Fast Hello function to speed up the detection speed, but the detection time can only reach the accuracy of 1 second. Moreover, the Fast Hello function is only for this protocol and cannot provide fast fault detection for other protocols.

two。 Technical advantages

Fault detection is carried out for any type of bi-directional forwarding path between network devices, including directly connected physical links, virtual circuits, tunnels, MPLS LSP, multi-hop routing paths and unidirectional links.

It can provide consistent fast fault detection time for different upper layer applications.

Provide a detection time of less than 1 second, so as to speed up the network convergence, reduce the application interruption time, and improve the reliability of the network.

3. Brief introduction of Technology implementation

BFD establishes sessions on two network devices to detect two-way forwarding paths between network devices and serves the upper layer. BFD itself does not have a neighbor discovery mechanism, but relies on the upper application being served to notify its neighbor information to establish a session. After the session is established, the BFD message will be sent periodically and quickly. If the BFD message is not received within the detection time, it is considered that the two-way forwarding path has failed, and the upper application served is notified to deal with it accordingly.

4.BFD session Establishment proc

OSPF discovers neighbors and establishes connections through its own Hello mechanism

After establishing a new neighbor relationship, OSPF advertises neighbor information (including destination address and source address, etc.) to BFD; BFD to establish a session based on the neighbor information received.

5.BFD Fault Discovery process

The detected link failed

BFD detects link failure and dismantles BFD neighbor session

BFD notifies the local OSPF process that the BFD neighbor is unreachable

The local OSPF process interrupts the OSPF neighbor relationship

IV. RIP

1. Basic concept

RIP is a protocol based on distance vector (Distance-Vector) algorithm. It exchanges routing information through UDP packets and uses port number 520.

RIP uses hop count to measure the distance to the destination address, which is called a metric. In RIP, the number of hops from a router to a network directly connected to it is 0, the number of hops to a network that can be reached through a router is 1, and so on. In order to limit the convergence time, RIP stipulates that the metric takes an integer between 015 and 15, and the hop count greater than or equal to 16 is defined as infinity, that is, the destination network or host is unreachable. Because of this limitation, RIP is not suitable for large networks.

To improve performance and prevent routing loops, RIP supports split horizon (Split Horizon) and toxicity reversal (Poison Reverse) functions

two。 Anti-loop mechanism

Count to infinity (Counting to infinity): defines a route with a metric equal to 16 as infinity. When a routing loop occurs, the metric for a route increases to 16, and the route is considered unreachable.

Triggered updates (Triggered Updates): by triggering updates, RIP avoids the possibility of forming routing loops between multiple routers and accelerates the convergence of the network. As soon as the metric of a route changes, an update message is issued to the neighboring router instead of waiting for the update cycle.

Split horizon (Split Horizon): routes learned by RIP from an interface are not sent back to neighboring routers from that interface. This not only reduces bandwidth consumption, but also prevents routing loops.

Toxicity reversal (Poison Reverse): after RIP learns a route from an interface, it sets the metric of that route to 16 (unreachable) and sends it back to the neighboring router from the original interface. In this way, you can clear the useless information in the routing table of the other party.

3.RIP running process

(1) after the router starts RIP, it sends a request message (Request message) to the neighboring router. After receiving the request message, the neighboring RIP router responds to the request and sends back a response message (Response message) containing local routing table information.

(2) after receiving the response message, the router updates the local routing table and sends a trigger update message to the neighboring routers to advertise the routing update information. After receiving the trigger update message, the neighboring routers send the trigger update message to their respective neighboring routers. After a series of triggered update broadcasts, each router can get and maintain the latest routing information.

(3) the router periodically sends the local routing table to the neighboring routers. After receiving the message, the neighboring routers running RIP protocol maintain the local route, select the best route, and then send the update information to their respective neighboring networks, so that the updated route can be globally effective. At the same time, RIP uses aging mechanism to aging time-out routes to ensure the real-time and effectiveness of routing.

4.RIP version differences

RIP-1 is a classful routing protocol (Classful Routing Protocol), which only supports broadcasting protocol messages. RIP-1 protocol packets can not carry mask information, it can only identify the routes of natural network segments such as A, B, C, so RIP-1 does not support discontiguous subnets (Discontiguous Subnet).

RIP-2 is a classless routing protocol (Classless Routing Protocol) that has the following advantages over RIP-1:

Routing tags are supported, and routing can be flexibly controlled according to routing tags in routing policies.

The message carries mask information and supports route aggregation and CIDR (Classless Inter-Domain Routing, classless inter-domain routing).

The next hop can be specified, and the optimal next hop address can be selected on the broadcast network.

Multicast routing is supported to send update messages, and only RIP-2 routers can receive update messages to reduce resource consumption.

Support the verification of protocol messages, and provide plaintext authentication and MD5 authentication to enhance security.

RIP-2 has two modes of message transmission: broadcast and multicast. By default, multicast will be used to send messages, and the multicast address used is 224.0.0.9. When the interface runs RIP-2 broadcast mode, it can also receive RIP-1 messages.

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