What Is a Router?

Routing Table

A routing table is a data table stored on each router. It is like a map, on which you can search for the path to a specific destination network address. A routing table has many entries, each indicating a route that corresponds to a destination on a network. The following figure shows a routing table.

When a router receives an IP packet, it checks the destination address in the packet header and searches its routing table for an entry matching the destination address. If a matching entry is found, the router forwards the packet based on the outbound interface and next hop specified in the entry. If no matching entry is found, the router checks whether a default route exists; if no default route exists, the router discards the IP packet. At the same time, the router sends an Internet Control Message Protocol (ICMP) error message to the source device of the packet, reporting that the destination address or network is unreachable.

Each router stores one local core routing table, which stores the routes discovered by various routing protocols. Among these routes, the optimal route is selected. The routing table may contain the following types of routes:

• Direct route: indicates a route to the network segment where a local interface of the router resides. Such routes are automatically discovered and added to the routing table by the router.
• Static route: indicates a route manually configured by the network administrator.
• Dynamic route: indicates a route discovered by the router through a dynamic routing protocol, such as Border Gateway Protocol (BGP), Intermediate System to Intermediate System (IS-IS), Open Shortest Path First (OSPF), or Routing Information Protocol (RIP).

Routes can also be classified into the following two types by the destination address type:

• Host route: indicates a route destined for a host address. Through this route, data packets are sent only to this host address.
• Network segment route: indicates a route destined for a network segment address. Through this route, data packets are sent to all hosts on the network segment.

On NetEngine series routers, you can run the display ip routing-table command to view information about the routing table. For example:

<HUAWEI> display ip routing-table
Route Flags: R - relay, D - download to fib, T - to vpn-instance, B - black hole route
------------------------------------------------------------------------------
Routing Table: Public
         Destinations : 8        Routes : 8

Destination/Mask     Proto  Pre  Cost   Flags  NextHop        Interface

       0.0.0.0/0      Static 60   0      D     10.1.4.2       GigabitEthernet1/0/0
       10.1.4.0/30    OSPF   10   0      D     10.1.4.1       GigabitEthernet1/0/0
       10.1.4.1/32    Direct 0    0      D     127.0.0.1      InLoopBack0
       10.1.4.2/32    OSPF   10   0      D     10.1.4.2       GigabitEthernet1/0/0
     127.0.0.0/8      Direct 0    0      D     127.0.0.1      InLoopBack0
     127.0.0.1/32     Direct 0    0      D     127.0.0.1      InLoopBack0
127.255.255.255/32    Direct 0    0      D     127.0.0.1      InLoopBack0
255.255.255.255/32    Direct 0    0      D     127.0.0.1      InLoopBack0

The routing table contains the following key information:

• Destination: indicates the destination address or destination network segment of the route.
• Mask: indicates the network mask. It is used together with the destination address (through an AND operation) to identify the address of the network segment where a destination host or router resides. For example, if the destination address is 1.1.1.1 and the mask is 255.255.255.0, the address of the network segment where the host or router resides is 1.1.1.0.
• Proto: indicates the method by which the route is obtained. For example, Direct indicates a direct route, and Static indicates a static route.
• Pre: indicates the preference of the route in the IP routing table. Multiple routes to the same destination may exist and have different next hops or outbound interfaces. These routes may be discovered by different routing protocols or manually configured static routes. Among these routes, the one with the highest priority (smallest preference value) is selected as the optimal route.
• Cost: indicates the cost of the route. When multiple routes to the same destination have the same preference, the route with the smallest cost is selected as the optimal route. The route cost is a value used to evaluate the quality of a route path. It is also called the route metric and indicates the cost of the route path from the source node to the destination node. The cost depends on multiple factors such as the network topology, bandwidth, delay, reliability, and security.
• Flags: indicates the source and destination of a data packet and how the router processes the packet.
• Nexthop: indicates the IP address of the next hop.
• Interface: indicates the outbound interface through which IP packets are forwarded.

Routing Protocols

Routing protocols are rules used by routers to maintain routing tables, such as discovering routes, generating routing entries, and guiding packet forwarding.

Routers support not only static routes, but also dynamic routes. Dynamic routing protocols have their own routing algorithms, which enable them to automatically adapt to network topology changes. They are particularly suitable for networks with a large number of Layer 3 devices. Generally, these protocols can be classified into the following types:

• Interior Gateway Protocol (IGP): routing protocol used within an AS. This protocol is irrelevant to routing protocol selection by other ASs on the Internet. Common IGPs include RIP, OSPF, and IS-IS.
• Exterior Gateway Protocol (EGP): protocol used to transmit data packets across the boundary of an AS when the source and destination devices are located in different ASs. Currently, BGP is the most widely used EGP.

The following figure shows the differences between static and dynamic routes, along with their respective application scenarios.

Differences between static and dynamic routes, along with their respective application scenarios

In real-world situations, a router can have both static and dynamic routes configured. By default, static routes take precedence over dynamic ones. If there are two routes available to the same destination — one dynamic route and another static route — the static route is preferred.

Addressing

After receiving a packet, a router parses the destination IP address of the packet, searches the routing table for the best path to the destination network, and processes the packet based on the search result.

As shown in the following figure, PC1 needs to forward a packet to PC2 through two routers. The addressing process is as follows:

Router's addressing process

1. PC1 sends a packet to RouterA. After receiving the packet, Router A parses the destination IP address of the packet.
2. Router A searches its routing table based on the destination IP address and processes the packet based on the search result.
   • If the destination IP address is found in the routing table, RouterA forwards the packet to RouterB through the corresponding outbound interface.
   • If the destination IP address is not found in the routing table, RouterA checks whether a default route exists. If a default route exists, RouterA forwards the packet to RouterB through the outbound interface for the default route.
   • If the destination IP address is not found in the routing table and no default route exists, RouterA sends an error ICMP message to the source IP address, indicating that the packet cannot be transmitted. At the same time, RouterA discards the packet.
3. After receiving the packet, RouterB processes the packet in the same way as RouterA and forwards the packet to PC2, the destination host.