IP routing basics· 1

Routing table· 1

Route preference· 2

Route backup· 2

Displaying and maintaining a routing table· 2

The term "router" in this document refers to both routers and access points.

IP routing directs IP packet forwarding on routers based on a routing table. This book focuses on unicast routing protocols. For more information about multicast routing protocols, see IP Multicast Configuration Guide.

Routing table

A router maintains at least two routing tables: a global routing table and a FIB. The FIB table contains only the optimal routes, and the global routing table contains all routes. The router uses the FIB table to forward packets. For more information about the FIB table, see Layer 3 Configuration Guide.

Table 1 categorizes routes by different criteria.

Table 1 Route categories

To view brief information about a routing table, use the display ip routing-table command:

<Sysname> display ip routing-table

Routing Tables:

         Destinations : 7       Routes : 7

Destination/Mask    Proto  Pre  Cost         NextHop         Interface

1.1.1.0/24          Direct 0    0            1.1.1.1         Vlan1

2.2.2.0/24          Static 60   0            12.2.2.2        Vlan2

80.1.1.0/24         Static 60   0            80.1.1.1        Vlan3

…

A route entry includes the following key items:

·          Destination—IP address of the destination host or network.

·          Mask—Mask length of the IP address.

·          Pre—Preference of the route. Among routes to the same destination, the one with the highest preference is optimal.

·          Cost—If multiple routes to a destination have the same preference, the one with the smallest cost becomes the optimal route.

·          NextHop—Next hop.

·          Interface—Output interface.

Route preference 

Routing protocols (including static and direct routing) each by default have a preference. If they find multiple routes to the same destination, the router selects the route with the highest preference as the optimal route.

The preference of a direct route is always 0 and cannot be changed. You can configure a preference for each static route and each dynamic routing protocol as required. The following table lists the route types and default preferences. The smaller the value, the higher the preference.

Table 2 Route types and default route preferences

Route backup 

Route backup can improve network availability. Among multiple routes to the same destination, the route with the highest priority is the primary route and others are secondary routes.

The router forwards matching packets through the primary route. When the primary route fails, the route with the highest preference among the secondary routes is selected to forward packets. When the primary route recovers, the router uses it to forward packets.

Displaying and maintaining a routing table