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ME60 V800R010C10SPC500 Feature Description - WAN Access 01

This is ME60 V800R010C10SPC500 Feature Description - WAN Access
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OSPF NSSA

OSPF NSSA

Background

As defined in OSPF, stub areas cannot import external routes. This mechanism prevents external routes from consuming the bandwidth and storage resources of ME devices in stub areas. If you need to both import external routes and prevent resource consumption caused by external routes, you can configure not-so-stubby areas (NSSAs).

There are many similarities between NSSAs and stub areas. However, different from stub areas, NSSAs can import AS external routes into the OSPF AS and advertise the imported routes in the OSPF AS without learning external routes from other areas on the OSPF network.

Related Concepts

  • N-bit

    A ME device uses the N-bit carried in a Hello packet to identify the area type that it supports. The same area type must be configured for all ME devices in an area. If ME devices have different area types, they cannot establish OSPF neighbor relationships. Some vendors' devices do not comply with standard protocols, but the N-bit is also set in OSPF Database Description (DD) packets. You can manually set the N-bit on a ME device to interwork with the vendors' devices.

  • Type 7 LSA

    Type 7 LSAs, which describe imported external routes, are introduced to support NSSAs. Type 7 LSAs are generated by an ASBR in an NSSA and advertised only within the NSSA. After an ABR in an NSSA receives Type 7 LSAs, it selectively translates Type 7 LSAs into Type 5 LSAs to advertise external routes to other areas on an OSPF network.

Principles

To advertise external routes imported by an NSSA to other areas, a translator must translate Type 7 LSAs into Type 5 LSAs. Notes for an NSSA are as follows:

  • By default, the translator is the ABR with the largest router ID in the NSSA.

  • The propagate bit (P-bit) is used to notify a translator whether Type 7 LSAs need to be translated.

  • Only Type 7 LSAs with the P-bit set and a non-zero forwarding address (FA) can be translated into Type 5 LSAs. An FA indicates that packets to a destination address will be forwarded to the address specified by the FA.

    NOTE:

    FA indicates that the packet to a specific destination address is to be forwarded to the address specified by.

    The loopback interface address in an area is preferentially selected as the FA. If no loopback interface exists, the address of the interface that is Up and has the largest logical index in the area is selected as the FA.

  • The P-bit is not set for default routes in Type 7 LSAs generated by an ABR.

Figure 6-17 shows an NSSA.
Figure 6-17 NSSA

Advantages

Multiple ABRs may be deployed in an NSSA. To prevent routing loops caused by default routes, ABRs do not calculate the default routes advertised by each other.

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Updated: 2019-01-04

Document ID: EDOC1100059473

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