Network Basic Skills Series: elaborate routing (bottom)

Introduction:

Static routing refers to routing information that is manually configured by the network administrator. When the topology of the network or the state of the link changes, the network administrator needs to manually modify the relevant static routing information in the routing table. Dynamic routing is the process by which routers communicate with each other, pass routing information, and update routing tables with received routing information. is implemented based on some kind of protocol. This paper elaborates the realization process of both.

Static routes:

Static routing refers to a table entry that is manually configured on the router by a network administrator. For a specific destination address, and in a small or stable network environment, manually configuring static routes can be very successful to apply. By using static routing, the network administrator determines the path to a target network.

An important concept is that the core of a route is the next hop. The next hop is a router that is a step closer to the target address in terms of a particular router. Shows a medium-sized routing topology. From R1 's point of view, R2 is also the next hop to 192.168.3.0 and 192.168.4.0.

in the initial state, nothing is configured except the interface that has been started and the given IP address. The router's routing table will only contain direct-attached routes. Each router knows only two networks that its interface is connected to. The following table shows the routing tables for this moment.

as can be seen from the above table, routers do not know the whole network situation. For example, node A connects to switch 1 and tries to access node B of Switch 4. After the host routing table is processed, a forwards the data to R1 's default gateway (192.168.1.254), R1 queries its own routing table and discovers no information about the target network. The R1 then sends an ICMP Destination Unreachable message.

How can we solve this problem? For small networks like this, network administrators can enter routing commands on the router to configure additional forwarding information:

IP route destination-network destination-network-mask next-hop-ip-address (forwarding router interface)

For example, the following command tells R1 how to reach 192.168.3.0 and 192.168.4.0:

IP route 192.168.3.0 255.255.255.0 192.168.2.254

IP route 192.168.4.0 255.255.255.0 192.168.2.254

After you enter a command on R1, the routing table update looks like this:

      

now R1 understand that reaching these networks requires R2, but what about R2? Because 192.168.3.0 directly connects to the R2,R2 can be directly ARP hosts. For 192.168.4.0,R2, however, the following commands are required by the administrator to assist:

IP route 192.168.4.0 255.255.255.0 192.168.3.254

The routing table is updated accordingly:

      

The message needs to be returned in half of the current success. View the routing table for the R3, and find that the router does not know how to find 192.168.1.0. After node A's message arrives, Node B tries to reply, but receives an ICMP Destination Unreachable message from R3. In Node A, it seems like the transmission never received a reply. To complete this process, you need to update the routing table for all unknown network input IP route commands on all routers.

R2 The true routing table and the IP route command entered on R2 as shown in:

Dynamic Routing:

routing protocols allow routers to dynamically share information from remote networks and automatically add this information to their routing tables. One of the great benefits of a dynamic routing protocol is that when the topology changes, the router exchanges routing information, which automatically learns the new network and, in the case of a link failure, finds a replacement path.

the way the routing protocol accomplishes this functionality depends on the algorithm it uses and the operational characteristics of the Protocol. In general, the dynamic routing protocol executes as follows:

1. The router sends and receives routing messages on the port.

2. Routers share routing information with other routers that use the same routing protocol.

3. Routers Exchange routing information to learn the remote network.

4. When the router detects a topology change, the routing protocol notifies the other routers of this change.

Network Discovery:

For example, a topology between R1,R2,R3:

R1: Sends updates for 10.1.0.0 and 10.2.0.0, receives 10.3.0.0 from R2, jumps by 1, stores 10.3.0.0 information in the routing table, and metric is set to 1.

R2: Sends updates for 10.3.0.0 and 10.2.0.0, receives 10.1.0.0 from R1, jumps by 1, stores 10.1.0.0 information in the routing table, and metric is set to 1. Receive 10.4.0.0 of information from R3, hop number plus 1, store 10.4.0.0 information in the routing table, metric set to 1.

R3: Sends updates for 10.3.0.0 and 10.4.0.0, receives 10.2.0.0 from R2, jumps by 1, stores 10.2.0.0 information in the routing table, and metric is set to 1.

To Exchange routing information:

The router periodically updates the information. After the initial network discovery has ended, each router continues the convergence process by sending and receiving the following updates:

R1: Send updates for 10.1.0.0,10.2.0.0 and 10.3.0.0, receive 10.4.0.0 from R2, hop number plus 1, store 10.4.0.0 information in the routing table, metric set to 2, and receive the same R2 update from 10.3.0.0 , metric is 1, not updated.

R2: Send 10.1.0.0,10.2.0.0,10.3.0.0 and 10.4.0.0 updates, receive 10.1.0.0 of the information from R1, do not update, receive 10.4.0.0 from R3 and do not update.

R3: Send updates for 10.2.0.0,10.3.0.0 and 10.4.0.0, receive 10.1.0.0 from R2, hop number plus 1, store 10.1.0.0 information in the routing table, metric set to 2, and receive the same R2 update from 10.2.0.0 , metric is 1, not updated.

the distance vector routing protocol cuts off the loop between neighbor routes, also known as horizontal splitting. The horizontal split block information is sent out after it is received from the same port. For example, R2 does not send network 10.1.0.0 information from the serial 0/0/0 port because R2 learned from serial 0/0/0 10.1.0.0.

after the router in the network converges the information, the router can use the routing table to determine the best path to reach the destination. Different routing protocols have different methods for calculating the best path.

Route Convergence:

When all routers have an accurate update to the entire network, the routing convergence State is reached, as shown in:

Convergence Time is the time that the router shares information, calculates the best path, and updates the routing table. Convergence is collaborative and independent at the same time. Routers share information with each other but must independently calculate the impact of their routing topology changes. As they agreed on the new topology independently, they converged on that agreement.

Network Basic Skills Series: elaborate routing (bottom)