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4.4.3.6. AS-External-LSAs

The LS type of an AS-external-LSA is set to the value 0x4005. AS-external-LSAs have AS flooding scope. Each AS-external-LSA describes a path to a prefix external to the Autonomous System.

The procedure for originating AS-external-LSAs in IPv6 is the same as the IPv4 procedure documented in Section 12.4.4 of [OSPFV2], with the following exceptions:

  • The Link State ID of an AS-external-LSA has lost all of its addressing semantics and simply serves to distinguish multiple AS-external-LSAs that are originated by the same router.

  • The prefix is described by the PrefixLength, PrefixOptions, and Address Prefix fields embedded within the LSA body. Network Mask is no longer specified.

  • The NU-bit in the PrefixOptions field should be clear.

  • Link-local addresses can never be advertised in AS-external-LSAs.

  • The forwarding address is present in the AS-external-LSA if and only if the AS-external-LSA's bit F is set.

  • The external route tag is present in the AS-external-LSA if and only if the AS-external-LSA's bit T is set.

  • The capability for an AS-external-LSA to reference another LSA has been supported through the inclusion of the Referenced LS Type field and the optional Referenced Link State ID field (the latter present if and only if the Referenced LS Type is non-zero). This capability is for future use; the Referenced LS Type should be set to 0, and received non-zero values for this field should be ignored until its use is defined.

As an example, consider the OSPF Autonomous System depicted in Figure 6 of [OSPFV2]. Assume that RT7 has learned its route to N12 via BGP and that it wishes to advertise a Type 2 metric into the AS. Also assume that the IPv6 prefix for N12 is the value 2001:0db8:0a00::/40. RT7 would then originate the following AS-external-LSA for the external network N12. Note that within the AS-external-LSA, N12's prefix occupies 64 bits of space in order to maintain 32-bit alignment.

; AS-external-LSA for Network N12,
; originated by Router RT7

LS age = 0                  ; newly (re)originated
LS type = 0x4005            ; AS-external-LSA
Link State ID = 123         ; LSA type/scope unique identifier
Advertising Router = Router RT7's ID
bit E = 1                   ; Type 2 metric
bit F = 0                   ; no forwarding address
bit T = 1                   ; external route tag included
Metric = 2
PrefixLength = 40
PrefixOptions = 0
Referenced LS Type = 0      ; no Referenced Link State ID
Address Prefix = 2001:0db8:0a00 ; padded to 64-bits
External Route Tag = as per BGP/OSPF interaction
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