Dynamic Routing 1

1. Dynamic routing

1) Basis of routing protocol

All routing protocols are built around an algorithm. Usually, an algorithm is a step-by-phase solution, so an algorithm should at least indicate the following:

• The process of transmitting network-accessible information to other routers;
• The process of receiving accessibility information from other routers;
• The process of the optimal routing based on the existing information decision making and the process of recording the information in the routing table;
• The process of responding, correcting and communicating topological changes in the network;

For all routing protocols, the common problems that need to be solved are path decision, measure, convergence and load balancing.

Path decision:

The so-called path decision is the process of selecting the optimal reachable path for the host in the network, or selecting the optimal path for the access target through a certain comparative calculation when the Multilink is reachable.

1.4.1, in the process of network communication, if the router R1 the host 192.168.2.1/24 need to access the router R3 the host 192.168.7.1/24 need to be implemented by the following process:

• First, the routing r1/r2/r3 can and can only know its direct-attached network through its interface address and mask when there is no routing protocol, so its direct-attached network routing information is as follows:

R1:192.168.1.0/24 (direct) 192.168.2.0/24 (direct) 192.168.3.0/24 (direct)

R2:192.168.3.0/24 (direct) 192.168.4.0/24 (direct) 192.168.5.0/24 (direct) 192.168.6.0/24 (direct)

R3:192.168.1.0/24 (direct) 192.168.6.0/24 (direct) 192.168.7.0/24 (direct)

• By Figure 1.4.1 We know that client A (192.168.2.1/24) is going to access client B (192.168.7.1/24) with the following two physical paths to go:

Client a---"r1-àr3-àclient B, or client a---" R1---"R2-ÀR3---" Client B;

• Regardless of the above approach, it is necessary to have a direct routing path to the client A to B route delivery, and in the current environment, router R1/R2/R3 can only obtain the network information connected to its own interface, so in order to implement the routing path, we need to r1/r2/ R3 each other to each other to communicate their direct routing table information to each other;
• When the routing synchronization announcement is complete so far, we can physically see that the two paths that we have previously said could enable client A to communicate with client B, but should we choose that path as the optimal path in the specific communication process? How do we measure the selected path as optimal?

Figure 1.4.1:

Routing r1/r2/r3 can know its direct-attached network through its interface address and mask, then its direct-attached network routing information is as follows:

R1:192.168.1.0/24 (direct) 192.168.2.0/24 (direct) 192.168.3.0/24 (direct)

R2:192.168.3.0/24 (direct) 192.168.4.0/24 (direct) 192.168.5.0/24 (direct) 192.168.6.0/24 (direct)

R3:192.168.1.0/24 (direct) 192.168.6.0/24 (direct) 192.168.7.0/24 (direct)

Metric (Metric ):

The so-called measure is that when there are multiple paths to the same target network, we choose a certain mechanism to calculate the optimal route as a good, sequential, priority of the standard or variable;

Routing protocol: The so-called routing protocol is one of the computing mechanisms for choosing a path; Different routing protocols use different metrics because of the different calculation rules and standards for each routing protocol.

For example: RIP defines the optimal path for the least number of route hops, and EIGRP defines the optimal path based on the minimum bandwidth and total delay along the path.

Some metric parameters:

• The hop count metric can simply calculate the hop count of the router, as shown in 1.4.1, if the packet has two selectable paths from client A to client B (hereinafter referred to as A/b), and is 2 hops from aàr1àr3àb If passing through Aàr1àr2àr3àb, it is 3 hops. If the optimal path is selected only in the routing protocol and only by the number of hops, it is obvious that the preceding route is optimal.
• The bandwidth (bandwidth) metric will select the path of the highly available bandwidth rather than the path of the low available bandwidth. That is, when bandwidth is measured, a 100M bandwidth path of 70% bandwidth utilization is clearly not necessarily better than a path of 50M bandwidth with a bandwidth utilization of 0.
• The load metric reflects the amount of link bandwidth that the traffic occupies along the way. The optimal path should be the least loaded path. The measurement changes as the load on the path changes. When metrics change frequently, routing fluctuations (where the best paths change frequently) can occur, negatively impacting the CPU, data link bandwidth, and network stability of the router.
• The delay measure is the amount of time a packet takes to go through a path. The routing protocol will select the shortest delay path as the optimal path when using the delay metric. (delay includes chain delay, network equipment processing delay, queue delay, etc.).
• Reliability (reliability) measures are used to measure the likelihood that a link will fail in a certain situation, and the reliability can be fixed or varied. The path with the highest reliability will be selected as the optimal path. For example: The number of link failures in a certain period of time, network administrator customized reliability standards.
• The overhead (cost) metric can respond to better or worse routing, which is a cost that is set by the administrator or that is required by the standard to reach the destination through this path packet.

Convergence (Convergence ):

The so-called convergence refers to the process of the routing table of all routers in the network reaching a consistent state. The convergence time is the sum of time that the whole network realizes information sharing and all routers calculate the optimal path.

Dynamic routing protocols include the process of advertising local direct-attached networks to other routers, receiving and processing advertisement information from other routers, and forwarding information received from other routers. For routing protocols, the information in the routing table for all routers on the network must be consistent.

Routing loop (routing loop): We refer to the continuous loop of traffic between two or more target networks as a routing loop.

As shown in 1.4.1, when in the network we put aàr1àr3àb as the optimal path, and determine R3 to reach the optimal path to B is through the router R1, then when the AÀB packets through R1 sent to R3, R3 will send the packet to R1, and then transferred to R1 by R3, thus caught in the reciprocating cycle, The AÀB data transfer is a routing loop between R1 and R3 and cannot be effectively delivered.

About convergence time, 1.4.2 When the topology of the network changes, only the total time of the whole network is calculated according to the latest topology to calculate the optimal path is the convergence time, that is, convergence time t=t0+t1+t2+t3+t4.

Networks that are not in a convergent state can cause data forwarding to be unreachable due to incorrect routing information. In the topology, the

The faster the convergence of a network after a change of life, the better!

Figure 1.4.2:

load balancing (load balance ):

The so-called load balancing is when there are multiple paths on the network to reach the same target server, according to a certain weighted algorithm to make multiple access through multiple paths to achieve access to increase the bandwidth, throughput, etc., to reduce the single-path load pressure and reliability of the impact of data interaction quality.

Dynamic Routing 1