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Configuration Guide - IP Unicast Routing

S7700 and S9700 V200R013C00

This document describes the configurations of IP Unicast Routing, including IP Routing, Static Route, RIP, RIPng, OSPF, OSPFv3, IPv4 IS-IS, IPv6 IS-IS, BGP, Routing Policy, IP Routing Table Management, and PBR.
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Configuring OSPF to Advertise Default Routes to an OSPF Area

Configuring OSPF to Advertise Default Routes to an OSPF Area

Context

On an OSPF network, multiple switches often reside on the area border and AS border for next-hop backup or traffic load balancing. To reduce routing entries and improve resource usage on the OSPF network, you can configure a default route in the following scenarios:

  1. An ABR advertises a Type 3 LSA carrying a default route for instructing switches to forward inter-area packets.
  2. An ASBR advertises a Type 5 or Type 7 LSA carrying a default route for instructing switches to forward AS external packets.

When no matching route is discovered, the switch can forward packets through a default route.

The preference of a default route in Type 3 LSAs is higher than that of the route in Type 5 or Type 7 LSAs.

Advertisement of a default route is determined by the type of the area to which the default route needs to be imported. Table 5-20 lists the default route advertisement by area type.

Table 5-20  Default route advertising mode

Area Type

Generated By

Advertised By

LSA Type

Flooding Area

Common area

A default route is generated using the default-route-advertise command.

ASBR

Type 5 LSA

Common area

Stub area

A default route is generated automatically.

ABR

Type 3 LSA

Stub area

NSSA

A default route is generated using the nssa [ default-route-advertise ] command.

ASBR

Type 7 LSA

NSSA

Totally NSSA

A default route is generated automatically.

ABR

Type 3 LSA

NSSA

Perform the following steps on an ASBR running OSPF.

Procedure

  • Configure OSPF to advertise a default route to an OSPF area.
    1. Run system-view

      The system view is displayed.

    2. Run ospf [ process-id ]

      The OSPF process view is displayed.

    3. Run default-route-advertise [ [ always | permit-calculate-other ] | cost cost | type type | route-policy route-policy-name [ match-any ] ] *

      OSPF is configured to advertise a default route to the OSPF area.

      • The parameter always indicates that an LSA describing a default route is generated and then advertised regardless of whether there are active default routes of other OSPF processes in the routing table of the local device.
      • The parameter permit-calculate-other indicates that the local router is allowed to calculate the default routes advertised by other switches after adverting its own default route.
      • The parameter route-policy route-policy-name indicates that the local device advertises default routes according to the parameters of the configured routing policy when there are matched default routing entries generated by other OSPF processes.
      NOTE:
      • An ASE LSA describing a default route is generated and then advertised only when there is an active default route of another OSPF process in the routing table of the local device.
      • Before advertising default routes, OSPF compares the preferences of default routes. Therefore, if a static default route is configured on an OSPF switch, the default route advertised by OSPF can be added to the current routing table only when it has a higher preference than the static route.

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Updated: 2019-04-08

Document ID: EDOC1100065744

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