Virtual Link And Transit Area For Ospf - Cisco ASR 9000 Series Routing Configuration Manual

Implementing OSPF
• Network LSA (Type 2)—Describes the link state and cost information for all routers attached to a
• Interarea-prefix LSA for ABRs (Type 3)—Advertises internal networks to routers in other areas (interarea
• Interarea-router LSA for ASBRs (Type 4)—Advertises an ASBR and the cost to reach it. Routers that
• Autonomous system external LSA (Type 5)—Redistributes routes from another autonomous system,
• Autonomous system external LSA (Type 7)—Provides for carrying external route information within
• Link LSA (Type 8)—Has link-local flooding scope and is never flooded beyond the link with which it
• Intra-area-prefix LSAs (Type 9)—A router can originate multiple intra-area-prefix LSAs for every router
An address prefix occurs in almost all newly defined LSAs. The prefix is represented by three fields: Prefix
Length, Prefix Options, and Address Prefix. In OSPFv3, addresses for these LSAs are expressed as "prefix
and prefix length" instead of "address and mask." The default route is expressed as a prefix with length 0.
Inter-area-prefix and intra-area-prefix LSAs carry all IPv6 prefix information that, in IPv4, is included in
router LSAs and network LSAs. The Options field in certain LSAs (router LSAs, network LSAs, interarea-router
LSAs, and link LSAs) has been expanded to 24 bits to provide support for OSPF in IPv6.
In OSPFv3, the sole function of link-state ID in interarea-prefix LSAs, interarea-router LSAs, and autonomous
system external LSAs is to identify individual pieces of the link-state database. All addresses or router IDs
that are expressed by the link-state ID in OSPF Version 2 are carried in the body of the LSA in OSPFv3.

Virtual Link and Transit Area for OSPF

In OSPF, routing information from all areas is first summarized to the backbone area by ABRs. The same
ABRs, in turn, propagate such received information to their attached areas. Such hierarchical distribution of
routing information requires that all areas be connected to the backbone area (Area 0). Occasions might exist
for which an area must be defined, but it cannot be physically connected to Area 0. Examples of such an
multiaccess network segment. This LSA lists all OSPF routers that have interfaces attached to the network
segment. Only the elected designated router for the network segment can generate and track the network
LSA for the segment. In OSPFv3, network LSAs have no address information and are
network-protocol-independent.
routes). Type 3 LSAs may represent a single network or set of networks aggregated into one prefix.
Only ABRs generate Type 3 LSAs. In OSPFv3, addresses for these LSAs are expressed as "prefix and
prefix length" instead of "address and mask." The default route is expressed as a prefix with length 0.
are trying to reach an external network use these advertisements to determine the best path to the next
hop. ABRs generate Type 4 LSAs.
usually from a different routing protocol into OSPF. In OSPFv3, addresses for these LSAs are expressed
as "prefix and prefix length" instead of "address and mask." The default route is expressed as a prefix
with length 0.
an NSSA. Type 7 LSAs may be originated by and advertised throughout an NSSA. NSSAs do not receive
or originate Type 5 LSAs. Type 7 LSAs are advertised only within a single NSSA. They are not flooded
into the backbone area or into any other area by border routers.
is associated. Link LSAs provide the link-local address of the router to all other routers attached to the
link or network segment, inform other routers attached to the link of a list of IPv6 prefixes to associate
with the link, and allow the router to assert a collection of Options bits to associate with the network
LSA that is originated for the link.
or transit network, each with a unique link-state ID. The link-state ID for each intra-area-prefix LSA
describes its association to either the router LSA or network LSA and contains prefixes for stub and
transit networks.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
Virtual Link and Transit Area for OSPF
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