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4.4.3.8. Link-LSAs

The LS type of a link-LSA is set to the value 0x0008. Link-LSAs have link-local flooding scope. A router originates a separate link-LSA for each attached link that supports two or more (including the originating router itself) routers. Link-LSAs SHOULD NOT be originated for virtual links.

Link-LSAs have three purposes:

  1. They provide the router's link-local address to all other routers attached to the link.

  2. They inform other routers attached to the link of a list of IPv6 prefixes to associate with the link.

  3. They allow the router to advertise a collection of Options bits in the network-LSA originated by the Designated Router on a broadcast or NBMA link.

A link-LSA for a given Link L is built in the following fashion:

  • The Link State ID is set to the router's Interface ID on Link L.

  • The Router Priority of the router's interface to Link L is inserted into the link-LSA.

  • The link-LSA's Options field is set to reflect the router's capabilities. On multi-access links, the Designated Router will logically OR the link-LSA Options fields for all fully adjacent neighbors in Link L's network-LSA.

  • The router inserts its link-local address on Link L into the link-LSA. This information will be used when the other routers on Link L do their next-hop calculations (see Section 4.8.2).

  • Each IPv6 address prefix that has been configured on Link L is added to the link-LSA by specifying values for the PrefixLength, PrefixOptions, and Address Prefix fields.

After building a link-LSA for a given link, the router installs the link-LSA into the associated interface data structure and floods the link-LSA on the link. All other routers on the link will receive the link-LSA, but they will not flood the link-LSA on other links.

If LinkLSASuppression is configured for the interface and the interface type is not broadcast or NBMA, origination of the link-LSA may be suppressed. This implies that other routers on the link will ascertain the router's next-hop address using a mechanism other than the link-LSA (see Section 4.8.2). Refer to Appendix C.3 for a description of the LinkLSASuppression interface configuration parameter.

As an example, consider the link-LSA that RT3 will build for N3 in Figure 1. Suppose that the prefix 2001:0db8:c001:0100::/56 has been configured within RT3 for N3. This will result in the following link-LSA that RT3 will flood only on N3. Note that not all routers on N3 need be configured with the prefix; those not configured will learn the prefix when receiving RT3's link-LSA.

; RT3's link-LSA for N3

LS age = 0                  ; newly (re)originated
LS type = 0x0008            ; link-LSA
Link State ID = 1           ; RT3's Interface ID on N3
Advertising Router = 192.0.2.3 ; RT3's Router ID
Rtr Priority = 1            ; RT3's N3 Router Priority
Options = (V6-bit|E-bit|R-bit)
Link-local Interface Address = fe80:0001::RT3
# prefixes = 1
PrefixLength = 56
PrefixOptions = 0
Address Prefix = 2001:0db8:c001:0100 ; pad to 64-bits
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