RFC5340 OSPFv3 for IPv6

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OSPFv3 for IPv6 Cheat sheet

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EGP versus IGP

OSPFv3 is an IGP and it is designed to run within an IPv6 Autonomous system, provide a maximum of information to build a consistent

topology of the Autonomous system. Scalability is achieved by the use of Area.

Each area router has a common Topological Database.

Differences in OSPFv3

• Permit to run 256 instances on the same shared media.

• Security is managed by IPSec and is no longer part of OSPF Process.

• As many addresses can be configured on each link, OSPFv3 run per link and not per subnet.

• OSPF Database has been redesigned with 2 new LSAs. One LSA to advertise addresses in an area and one LSA to advertise Link-Local Addresses. The Type 1 and 2 LSA do not carry any more addresses but topological addresses only. A new Intra-Area LSAhas been introduced to carry these addresses.

• Two LSA have been renamed with more explicit names.

• Two new bits have been added to Option bits: R-Bit and V6-bit. The R-bit allows a router to signal that it should not be used in the routing process. The V6-bit bit signals that the router is able to carry IPv6 Traffic.

• MOSPF has been deprecated in OSPFv3.

Common Characteristics with OSPFv2

• It runs directly over IPv6 Protocol number 89.

• The router ID still has the format of an IPv4 address.

• It uses multicast so non OSPFv3 nodes are not impacted by OSPFv3 traffic.

• Multiaccess networks are optimised with a Designated router.

• No filtering of routing information in an area. Only ASBR and ABR can summarise Routes.

• It still uses multicast addresses: ff02::5, ff02::6.

Remarks about OSPFv3 design

Routing is operated with Link-Local addresses. The Global addresses are only needed for remote routing for user traffic.

If OSPF is a Link-State routing protocol inside an area, it is a Distance-Vector between Area. The Area Border Router transmit routes and not Link-states informations. Same for ASBR, they transmit External Routes and not Link States. This is why 1) ABR should be able to summarise routing information between Area.2) It is not recommended to redistribute BGP routes into OSPF but to run iBGP between routers. Always control with routes-maps the routes that you redistribute in OSPF.

HELLO PROTOCOL

With OSPF exchanges start with Hello and all OSPF packets starts with a common headers.They are 5 types of packets in OSPF.

Two routers are sending Hellos using the AllOspfRouters Multicast address ff02::5.

The Finite State Machine for the inteface goes:DOWN -> INIT

During the INIT state they check that the parameters found in Hello are compatible. For instance if they are not in the same Area or do both not send hello every 10 seconds, they will not form Neighbour relationship.

When a router sees its Router-ID in a Neighbour advertisement, their states becomes TWO-WAY.

DR/BDR Election. In addition, they will elect a Designated Router and a Backup Designated Router on Multipoint, Broadcast or Not, interface. They start waiting 40 seconds listening if there are already a DR/BDR on this link. Then they compare their Router Priority with the Priority of the others received. This is not preemptive, if a DR is elected you would not replace it either should you have a better priority. With the same priority, highest Router ID wins. Priority 0 is ineligible.

The next state is Database synchronisation with Adjacent routers. On a Point-to-Point you should be systematically adjacent. On a multiaccess you only become adjacent with the DR/BDR. This is to avoid that everybody need to synchronise with everybody!

EXSTART Once, routers are neighbours, they may become adjacent.Each router send a DBD with the Master bit set (MS) pretending it is the Master. Highest Router-ID wins.

DBD other bits are: I-bit The Init bit. When set to 1, this packet is the first in the sequence of Database Description packets.

M-bit The More bit. When set to 1, it indicates that more Database Description packets are to follow.

MS-bit The Master/Slave bit. When set to 1, it indicates that the router is the master during the Database Exchange process. Otherwise, the router is the slave.

The Master will then be responsible to synchronise the communication. This exchange looks like a simple TFTP file transfer. Each router sends its Database, LSA Headers for each router to check that it has the latest version for each LSA.

EXLOADIf a LSA is not up to date, the router will send a Link State Request. It's neighbour should now reply with a Link-State Reply which will be Acked with a Link State Acknowledgement. When both Databases are up to date, the status is now FULL.

Packet Length3 2

Router-ID

Area-ID

Checksum Instance ID 0

Database Description Packet

0 Options

Interface MTU 0 0 0 0 0 0 I M MS

DD Sequence Number

An LSA Header

Other OSPF packets are needed to exchange the Database Records called LSA or Link State Acknowledgement.

Packet Length3 3

Router-ID

Area-ID

Checksum Instance ID 0

Link State Request

0 LS Type

Link-State ID

Advertising Router

Link-State ID....

Packet Length3 4

Router-ID

Area-ID

Checksum Instance ID 0

Link State Update

# Link State

LSAs....

Packet Length3 5

Router-ID

Area-ID

Checksum Instance ID 0

Link State Acknowledgement

An LSA Header

LS Age LS Type

Link State ID

Advertising Router

LS Sequence Number

LS Checksum Length

LSA Header

LSA Type

U S2 S1 LSA Function CodeU=0 Treat the LSA as if it had link-local flooding scopeU=1 Store and Flood the LSA as if the type is understood

S2 S1 Flooding Scope0 0 Link-Local Scoping0 1 Area Scoping1 0 AS Scoping1 1 Reserved

LSA Function Code LS-Type1 0x2001 Router-LSA2 0x2002 Network-LSA3 0x2003 Inter-Area-Prefix-LSA4 0x2004 Inter-Area_Router-LSA5 0x2005 AS-External-LSA6 Deprecated7 0x2007 NSSA-LSA9 0x2009 Intra-Area-Prefix-LSA