Section B.9. OSPFv3 Messages and the Link State Database (Chapter 8)


B.9. OSPFv3 Messages and the Link State Database (Chapter 8)

Table B-17 lists all possible OSPF for IPv6 packet types.

Table B-17. OSPF for IPv6 packet types

Packet type

Name

Description

1

Hello

Initializes and maintains adjacencies. Also used to elect DR and BDR.

See the section "Forming Adjacencies."

2

Database Description

Exchanges database description during the formation of adjacencies.

See the section "Forming Adjacencies."

3

Link State Request

Requests missing or changed LSAs.

See the section "Forming Adjacencies."

4

Link State Update

Transmits LSAs either responding to requests when forming adjacencies or during LSA flooding.

See the sections "Forming Adjacencies" and "LSA Flooding."

5

Link State Acknowledgment

Acknowledges the reception of an LSA. Every LSA must be acknowledged.

See the sections "Forming Adjacencies" and "LSA Flooding."


The Options field describes the optional capabilities of the router. Table B-18 explains the bits used in the Options field.

Table B-18. The Options field

Bit

Name

Description

0-17

Not used

Reserved for future use.

18

DC

Handling of Demand Circuits, as described in RFC 1793.

19

R

Indicates that the originator of the Hello packet is an active router. If this bit is set to 0, the originator will not forward packets: for example, a multihomed host that wants to build an OSPF routing table without actually routing packets.

20

N

All routers within an NSSA must set this bit. In addition, the E bit must be set to 0 (see RFC 3101).

21

MC

Multicast capability, as defined in RFC 1584.

22

E

External-routes capability of the router. All members of an area must agree on the external capability. In a stub area, all routers must set this bit to 0 to achieve adjacency. The E bit is meaningful only in Hello packets (similar to the N bit).

23

V6

Indicates that the router supports OSPF for IPv6. If set to 0, this router/link should be excluded from IPv6 routing calculation.


Table B-19 shows possible link state types. The LS type is represented in hexadecimal notation to reflect the flooding scope.

Table B-19. Link state (LS) types

LS type

Name

Flooding scope

Advertised by

Link State ID

0x2001

Router-LSA

Area

Each router

Router ID

0x2002

Network-LSA

Area

DR

DR's Interface ID of the transit link

0x2003

Inter-Area-Prefix-LSA

Area

ABR

A locally unique ID set by ABR

0x2004

Inter-Area-Router-LSA

Area

ABR

A locally unique ID set by ABR

0x4005

AS-External-LSA

AS

ASBR

A locally unique ID set by ASBR

0x2006

Group-Membership-LSA

Area

See RFC 1584

See RFC 1584

0x2007

Type-7-LSA

Area

See RFC 3101

See RFC 3101

0x0008

Link-LSA

Link

Each router for each link

Locally unique Interface ID

0x2009

Intra-Area-Prefix-LSA

Area

Each router

A locally unique ID set by router


The Flags field indicates the special function of the router originating the LSA. Table B-20 shows the possible values and their corresponding functions

Table B-20. Flags in Router-LSA

Bit

Name

W bit

The router is a wildcard multicast receiver. See RFC 1584 for more information.

V bit

The router is the endpoint of a virtual link using this area as a transit area.

E bit

The router is an ASBR.

B bit

The router is an ABR.


The link types shown in Table B-21 describe the possible link types of an interface in a Router-LSA.

Table B-21. Link type supported in a Router-LSA

Link type

Name

Neighbor Interface ID

Neighbor Router ID

1

Point-to-point

Interface ID of the neighbor on the other end of the point-to-point link

Router-ID of the neighbor on the other end of the point-to-point link

2

Transit

Interface ID of the DR on this link

Router ID of the DR on this link

3

Reserved

N/A

N/A

4

Virtual

Interface ID of the neighbor on the other end of the virtual link

Router ID of the neighbor on the other end of the virtual link


Table 2-22 explains the prefix options starting with the leftmost bit in an Inter-Area-Prefix-LSA.

Table 2-22. Prefix options starting with the leftmost bit

Bit

Name

Description

0-3

Reserved

N/A

4

P bit

Propagate bit: if set, the NSSA ABR readvertises the prefix into the backbone. Used only in Type-7-LSA.

5

MC bit

Multicast bit: if set, the prefix should be included in IPv6 multicast routing calculations.

6

LA bit

Local address bit: if set, the prefix is actually a local IPv6 address of the originating router.

7

NU bit

No unicast bit: if set, the prefix should be excluded from IPv6 unicast calculation.




IPv6 Essentials
IPv6 Essentials
ISBN: 0596100582
EAN: 2147483647
Year: 2004
Pages: 156
Authors: Silvia Hagen

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