IP routing basics 1

Routing table· 1

Dynamic routing protocols 2

Route preference· 2

Load sharing· 3

Route backup· 3

Route recursion· 3

Route redistribution· 3

Displaying and maintaining a routing table· 4

IP routing directs IP packet forwarding on routers based on a routing table. This chapter 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—IP Services 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

Destinations : 19        Routes : 19

Destination/Mask    Proto  Pre  Cost         NextHop         Interface

0.0.0.0/32          Direct 0    0            127.0.0.1       InLoop0

1.1.1.0/24          Direct 0    0            1.1.1.1         GE3/0/1

1.1.1.0/32          Direct 0    0            1.1.1.1         GE3/0/1

1.1.1.1/32          Direct 0    0            127.0.0.1       InLoop0

1.1.1.255/32        Direct 0    0            1.1.1.1         GE3/0/1

2.2.2.0/24          Static 60   0            12.2.2.2        GE3/0/2

80.1.1.0/24         OSPF   10   2            80.1.1.1        GE3/0/3

...

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 route with the highest preference is optimal.

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

·           NextHop—Next hop.

·           Interface—Output interface.

Dynamic routing protocols

Static routes work well in small, stable networks. They are easy to configure and require fewer system resources. However, in networks where topology changes occur frequently, a typical practice is to configure a dynamic routing protocol. Compared with static routing, a dynamic routing protocol is complicated to configure, requires more routers resources, and consumes more network resources.

Dynamic routing protocols dynamically collect and report reachability information to adapt to topology changes. They are suitable for large networks.

Dynamic routing protocols can be classified by different criteria, as shown in Table 2.

Table 2 Categories of dynamic routing protocols

An AS refers to a group of routers that use the same routing policy and work under the same administration.

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. The following table lists the route types and default preferences. The smaller the value, the higher the preference.

Table 3 Route types and default route preferences

Load sharing

A routing protocol may find multiple optimal equal-cost routes to the same destination. You can use these routes to implement equal-cost multi-path (ECMP) load sharing.

Static routing, IPv6 static routing, RIP, RIPng, OSPF, OSPFv3, BGP, IPv6 BGP, IS-IS, and IPv6 IS-IS support ECMP load sharing.

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.

Route recursion

To use a BGP, static, or RIP route that has an indirectly connected next hop, a router must perform route recursion to find the outgoing interface to reach the next hop.

Link-state routing protocols, such as OSPF and IS-IS, do not need route recursion, because they obtain directly connected next hops through route calculation.

Route redistribution

Route redistribution enables routing protocols to learn route information from each other. A dynamic routing protocol can redistribute routes from other routing protocols, including direct and static routing. For more information, see the respective chapters on those routing protocols in this configuration guide.

Displaying and maintaining a routing table

Execute display commands in any view and reset commands in user view.