Skip to main content

4.4.3.9. Intra-Area-Prefix-LSAs

The LS type of an intra-area-prefix-LSA is set to the value 0x2009. Intra-area-prefix-LSAs have area flooding scope. An intra-area-prefix-LSA has one of two functions. It either associates a list of IPv6 address prefixes with a transit network link by referencing a network-LSA, or associates a list of IPv6 address prefixes with a router by referencing a router-LSA. A stub link's prefixes are associated with its attached router.

A router MAY originate multiple intra-area-prefix-LSAs for a given area. Each intra-area-prefix-LSA has a unique Link State ID and contains an integral number of prefix descriptions.

Designated Router Origination​

A link's Designated Router originates one or more intra-area-prefix-LSAs to advertise the link's prefixes throughout the area. For a link L, L's Designated Router builds an intra-area-prefix-LSA in the following fashion:

  • In order to indicate that the prefixes are to be associated with the Link L, the fields Referenced LS Type, Referenced Link State ID, and Referenced Advertising Router are set to the corresponding fields in Link L's network-LSA. This means that the Referenced LS Type is set to 0x2002, the Referenced Link State ID is set to the Designated Router's Interface ID on Link L, and the Referenced Advertising Router is set to the Designated Router's Router ID.

  • Each link-LSA associated with Link L is examined. If the link-LSA's Advertising Router is fully adjacent to the Designated Router and the Link State ID matches the neighbor's interface ID, the list of prefixes in the link-LSA is copied into the intra-area-prefix-LSA. Prefixes having the NU-bit and/or LA-bit set in their Options field SHOULD NOT be copied, nor should link-local addresses be copied. Each prefix is described by the PrefixLength, PrefixOptions, and Address Prefix fields. Multiple prefixes having the same PrefixLength and Address Prefix are considered to be duplicates. In this case, their PrefixOptions fields should be logically OR'ed together, and a single instance of the duplicate prefix should be included in the intra-area-prefix-LSA. The Metric field for all prefixes is set to 0.

  • The "# prefixes" field is set to the number of prefixes that the router has copied into the LSA. If necessary, the list of prefixes can be spread across multiple intra-area-prefix-LSAs in order to keep the LSA size small.

Router Origination​

A router builds an intra-area-prefix-LSA to advertise prefixes for its attached stub links, looped-back interfaces, and hosts. A Router RTX would build its intra-area-prefix-LSA in the following fashion:

  • In order to indicate that the prefixes are to be associated with the Router RTX itself, RTX sets the Referenced LS Type to 0x2001, the Referenced Link State ID to 0, and the Referenced Advertising Router to RTX's own Router ID.

  • Router RTX examines its list of interfaces to the area. If the interface is in the state Down, its prefixes are not included. If the interface has been reported in RTX's router-LSA as a Type 2 link description (link to transit network), prefixes that will be included in the intra-area-prefix-LSA for the link are skipped. However, any prefixes that would normally have the LA-bit set SHOULD be advertised independent of whether or not the interface is advertised as a transit link. If the interface type is point-to-multipoint or the interface is in the state Loopback, the global scope IPv6 addresses associated with the interface (if any) are copied into the intra-area-prefix-LSA with the PrefixOptions LA-bit set, the PrefixLength set to 128, and the metric set to 0. Otherwise, the list of global prefixes configured in RTX for the link are copied into the intra-area-prefix-LSA by specifying the PrefixLength, PrefixOptions, and Address Prefix fields. The Metric field for each of these prefixes is set to the interface's output cost.

  • RTX adds the IPv6 prefixes for any directly attached hosts belonging to the area to the intra-area-prefix-LSA.

  • If RTX has one or more virtual links configured through the area, it includes one of its global scope IPv6 interface addresses in the LSA (if it hasn't already), setting the LA-bit in the PrefixOptions field, the PrefixLength to 128, and the Metric to 0. This information will be used later in the routing calculation so that the two ends of the virtual link can discover each other's IPv6 addresses.

  • The "# prefixes" field is set to the number of prefixes that the router has copied into the LSA. If necessary, the list of prefixes can be spread across multiple intra-area-prefix-LSAs.

Examples​

For example, the intra-area-prefix-LSA originated by RT4 for Network N3 (assuming that RT4 is N3's Designated Router) and the intra-area-prefix-LSA originated into Area 1 by Router RT3 for its own prefixes are pictured below:

; RT4's Intra-area-prefix-LSA for network link N3

LS age = 0                  ; newly (re)originated
LS type = 0x2009            ; Intra-area-prefix-LSA
Link State ID = 5           ; LSA type/scope unique identifier
Advertising Router = 192.0.2.4 ; RT4's Router ID
# prefixes = 1
Referenced LS Type = 0x2002 ; network-LSA reference
Referenced Link State ID = 1
Referenced Advertising Router = 192.0.2.4
PrefixLength = 56           ; N3's prefix
PrefixOptions = 0
Metric = 0
Address Prefix = 2001:0db8:c001:0100 ; pad

; RT3's Intra-area-prefix-LSA for its own prefixes

LS age = 0                  ; newly (re)originated
LS type = 0x2009            ; Intra-area-prefix-LSA
Link State ID = 177         ; LSA type/scope unique identifier
Advertising Router = 192.0.2.3 ; RT3's Router ID
# prefixes = 1
Referenced LS Type = 0x2001 ; router-LSA reference
Referenced Link State ID = 0
Referenced Advertising Router = 192.0.2.3
PrefixLength = 56           ; N4's prefix
PrefixOptions = 0
Metric = 2                  ; N4 interface cost
Address Prefix = 2001:0db8:c001:0400 ; pad

When network conditions change, it may be necessary for a router to move prefixes from one intra-area-prefix-LSA to another. For example, if the router is the Designated Router for a link but the link has no other attached routers, the link's prefixes are advertised in an intra-area-prefix-LSA referring to the Designated Router's router-LSA. When additional routers appear on the link, a network-LSA is originated for the link and the link's prefixes are moved to an intra-area-prefix-LSA referring to the network-LSA.

Note that in the intra-area-prefix-LSA, the Referenced Advertising Router is always equal to the router that is originating the intra-area-prefix-LSA (i.e., the LSA's Advertising Router). The reason the Referenced Advertising Router field appears is that, even though it is currently redundant, it may not be in the future.

Last updated on