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ME60 V800R010C10SPC500 Feature Description - WAN Access 01

This is ME60 V800R010C10SPC500 Feature Description - WAN Access
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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
OSPF Route Control

OSPF Route Control

You can use the following features to control the advertising and receiving of OSPF routes:

  • Route summarization

    Route summarization enables a ME device to summarize routes with the same prefix into a single route and to advertise only the summarized route to other areas. Route summarization reduces the size of a routing table and improves ME device performance.

  • Route filtering

    OSPF can use routing policies to filter routes. By default, OSPF does not filter routes.

  • OSPF Database Overflow

    Set the maximum number of external routes supported by the LSDB to dynamically limit the LSDB's size.

These features meet requirements for network planning and traffic management.

Route Summarization

When a large OSPF network is deployed, an OSPF routing table includes a large number of routing entries. To accelerate route lookup and simplify management, configure route summarization to reduce the size of the OSPF routing table. If a link frequently alternates between Up and Down, the links not involved in the route summarization are not affected. This process prevents route flapping and improves network stability.

Route summarization can be carried out on an ABR or ASBR.

  • ABR summarization

    When an ABR transmits routing information to other areas, it generates Type 3 LSAs for each network segment. If consecutive network segments exist in this area, you can summarize these network segments into a single network segment. The ABR generates one LSA for the summarized network segment and advertises only that LSA.

  • ASBR summarization

    If route summarization has been configured and the local ME device is an ASBR, the local ME device summarizes imported Type 5 LSAs within the summarized address range. If an NSSA has been configured, the local ME device also summarizes imported Type 7 LSAs within the summarized address range.

    If the local ME device is both an ASBR and an ABR, it summarizes Type 5 LSAs translated from Type 7 LSAs.

Route Filtering

OSPF routing policies include access control lists (ACLs), IP prefix lists, and route-policies. For details about these policies, see the section "Routing Policy" in the ME60Feature Description - IP Routing.

OSPF route filtering applies in the following aspects:

  • Route import

    OSPF can import the routes learned by other routing protocols. A ME device uses a configured routing policy to filter routes and imports only the routes matching the routing policy. Only an ASBR can import routes, and therefore a routing policy for importing routes must be configured on the ASBR.

  • Advertising of imported routes

    A ME device advertises imported routes to its neighbors. Only an ASBR can import routes, and therefore a routing policy for the advertising of imported routes must be configured on the ASBR.

    If OSPF imports a large number of external routes and advertises them to a device with a smaller routing table capacity, the device may restart unexpectedly. To address this problem, configure a limit on the number of LSAs generated when an OSPF process imports external routes.

  • Route learning

    A ME device uses a routing policy to filter received intra-area, inter-area, and AS external routes. The ME device adds only the routes matching the routing policy to its routing table. All routes can still be advertised from an OSPF routing table.

    The ME device filters only routes calculated based on LSAs, and therefore learned LSAs are complete.

  • Inter-area LSA learning

    An ABR in an area can be configured to filter Type 3 LSAs advertised to the area. The ABR can advertise only Type 3 LSAs, and therefore a routing policy for inter-area LSA learning must be configured on the ABR.

    During inter-area LSA learning, the ABR directly filters Type 3 LSAs advertised to the area.

  • Inter-area LSA advertising

    An ABR in an area can be configured to filter Type 3 LSAs advertised to other areas. The ABR can advertise only Type 3 LSAs, and therefore a routing policy for inter-area LSA advertising must be configured on the ABR.

OSPF Database Overflow

OSPF requires that devices in the same area have the same LSDB. As the number of routes increase continually, some devices cannot carry excess routing information due to limited system resources. This situation is called an OSPF database overflow.

You can configure stub areas or NSSAs to prevent resource exhaustion caused by continually increasing routing information. However, configuring stub areas or NSSAs cannot prevent an OSPF database overflow caused by a sharp increase in dynamic routes. To resolve this issue, set the maximum number of external routes supported by the LSDB to dynamically limit the LSDB's size.

NOTE:
The maximum number of external routes configured for all devices in the OSPF AS must be the same.

When the number of external routes in the LSDB reaches the maximum number, the device enters the overload state and starts the overflow timer at the same time. The device automatically exits from the overflow state after the overflow timer expires. Table 6-11 describes the operations performed by the device after it enters or exits from the overload state.

Table 6-11 Operations performed by the device after it enters or exits from the overload state

Phase

OSPF Processing Procedure

Staying at overload state

Deletes self-generated non-default external routes and stops advertising non-default external routes.

Discards newly received non-default external routes and does not reply with a Link State Acknowledgment (LSAck) packet.

Checks whether the number of external routes is still greater than the configured maximum number when the overflow timer expires.

  • Restarts the timer if the number of external routes is greater than or equal to the configured maximum number.
  • Exits from the overflow state if the number of external routes is less than the configured maximum number.

Exiting from the overflow state

Ends the overflow timer.

Advertises non-default external routes.

Accepts newly received non-default external routes and replies with LSAck packets.

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Updated: 2019-01-04

Document ID: EDOC1100059473

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