Type 7 NotSoStubby Area LSAs

Type 7 LSAs are generated by ASBRs. These LSAs describe routes within a not-so-stubby-area (NSSA). Type 7 LSAs can be summarized and converted into Type 5 LSAs by the ABRs for transmission into other OSPF areas. After Type 7 LSAs are converted to Type 5 LSAs, they are distributed to areas that can support Type 5 LSAs. Figure 3-11 illustrates the operation of Type 7 LSAs, and Figure 3-12 illustrates the format of Type 7 LSA packets.

Figure 3-11 Type 7 LSA Operation

Figure 3-11 Type 7 LSA Operation

makes the stub area router ID) translates an NSSA. Type 7 to Type 5

LSAs for the NSSA.

makes the stub area router ID) translates an NSSA. Type 7 to Type 5

LSAs for the NSSA.

Figure 3-12 Type 7 LSA Packet Format

0 1234567890 1234567890 1234567890 1

LS Age

Options

Link State ID

Advertising Router

LS Sequence Number

LS Checksum

Length

Network Mask

Metric

Forwarding Address

External Route Tag

TOS Metric

Forwarding Address

External Route Tag

NOTE

When discussing NSSA operation and structure, be aware that NSSAs have their own RFC (1587). Refer to this RFC for more detailed information.

One of the most interesting features of NSSAs is how they convert Type 7 LSAs, which describe the external routes to all the routers in the NSSA, into the more commonly seen Type 5 LSAs so that all the rest of the OSPF autonomous systems know about the external routes. The RFC describes this process, but the flow chart in Figure 3-13 makes it easier to understand.

112 Chapter 3: OSPF Communication

Figure 3-13 NSSA Type 7-to-Type 5 LSA Translation Process

(1) The Type 7 LSA has been tagged for advertisement. The address/mask pair is found on this router and then translate if the following is true:

(1.1) The translation has not already occurred

(1.2) Path or metric is different between the Type 7 and 5

(1.3) Forwarding address is different between the Type 7 and Type 5.

If forwarded, all route characteristics will remain the same except the advertising router will be shown as the ABR.

The router with the highest RID (i.e. ABR) within an NSSA will be responsible for translating Type 7 LSAs into Type 5 LSAs as part of the SPF calculation; after the original Type 7 and 5 LSAs have been processed, the following can occur:

(2) The Type 7 LSA has been tagged for advertisement.

The route's address or mask indicates a more specific route because of a longer mask for example; then a Type 5 LSA is generated with link-state ID equal to the range's address. The advertising router field will be the router ID of this Area Border Router.

(3) When the route's status indicates DoNotAdvertise, then the router will suppress the Type 5 translation.

However, if the P-bit is set and the LSA has a nonzero forwarding address and if the route is not configured, then the translation will occur if one of the following is true:

(3.1) No Type 5 LSA has already been translated from the Type 7 LSA in question.

(3.2) The path type or the metric in the corresponding type 5 LSA is different from the Type 7 LSA.

(3.3) The forwarding address in the corresponding Type 5 LSA is different from the Type 7 LSA.

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