Cisco CCNA ICND2 (Interconnecting Cisco Networking Devices Part 2)Exam Cram

2. Routing Technologies

2.6 OSPFv3 for IPv6

OSPF for IPv6 is also known by OSPFv3. OSPF is an interior gateway protocol, widely used in both IPv4 and IPv6 networks.

The following are true about OSPFv3:

1. Multiple OSPF processes are supported as in OSPFv2

2. OSPFv3 supports IPv6 whereas OSPFv2 doesn't.

3. Uses multicast addresses FF02::5 and FF02::6

4. Type 3 and type 9 LSAs carry IPv6 prefix information, whereas IPv4 prefix information is carried in type 1 and type 2 LSAs

5. OSPFv3 uses the IPv6 multicast addresses FF02::5 (for all OSPF routers) and FF02::6

224.0.0.5 and 224.0.0.6 are the multicast addresses used by OSPFv2.

6. OSPF for IPv6 requires the use of IPsec to enable authentication. The IPv6 AH (Authentication Hearder) and ESP extension headers are used to provide authentication and confidentiality to OSPF for Ipv6.

The correct sequence of steps required for enabling ospf in area 0 are (assuming that the interface Ethernet 0/0 is being configured):

Router>enable
Router#configure terminal
Router(config)#interface
ethernet 0/0
Router(config-if)#ipv6 ospf
1 area 0

Note: The command ipv6 ospf <process-id> area <area-id> enables OSPF for IPv6 on an interface.

Configuration of OSPFv3 can also be accomplished through following commands

Router(config)#ipv6 router ospf <area-id>
Router(config-router)#router-id <router-id>

OSPF Packet Types:

1. LSA (Link State Advertisement): LSAs are included in the database description packets (DDPs or DBDs). LSA entries include link-state type, the address of the advertising router, the cost of the link, and the sequence number.

2. LSR ( Link State Request): When a slave router receives an DDP (Database Description Packet), it sends and LSAck packet. Then it compares the received information with the information it has. If the DDP has more recent information, the slave router sends a link-state request (LSR) to the master router.

3. LSU ( Link State Update): LSU packet is sent in response to LSR (Link-State Request) packet sent from a slave router to a master router. LSU contains complete information about the requested entry.

OSPF Area Types :

1. Area backbone LSAs: The LSAs generated by Area Backbone Routers are LSA1, LSA2, LSA3, LSA4, and LSA5. Note that LSA6 is not supported by Cisco, and LSA7 is generated by NSSA router.

2. Stub area LSAs: The Stub area router generates LSA types 1, 2, and 3. i.e. Router LSA, Network LSA, and Summary LSA.

3. Totally Stubby LSAs: The Totally Stubby area routers generate LSA types 1 and 2

4. NSSA LSAs: A NSSA (Not So Stubby Area) router generates LSA types 1, 2, and 7. . LSA 7 is translated into LSA 5 as it leaves the NSSA

Different LSA types

a. LSA 1 (Router LSA): Generated by all routers in an area to describe their directly attached links (Intra-area routes). These do not leave the area.

b. LSA 2 (Network LSA): Generated by the DR of a broadcast or Nonbroadcast segment to describe the neighbors connected to the segment. These do not leave the area.

c. LSA 3 (Summary LSA): Generated by the ABR to describe a route to neighbors outside the area. (Inter-area routes)

d. LSA 4 (Summary LSA): Generated by the ABR to describe a route to an ASBR to neighbors outside the area.

e. LSA 5 (External LSA): Generated by ASBR to describe routes redistributed into the area. These routes appear as E1 or E2 in the routing table. E2 (default) uses a static cost throughout the OSPF domain as it only takes the cost into account that is reported at redistribution. E1 uses a cumulative cost of the cost reported into the OSPF domain at redistribution plus the local cost to the ASBR.

f. LSA 6 (Multicast LSA): Not supported on Cisco routers.

g. LSA 7 (NSSA External LSA): Generated by an ASBR inside a NSSA to describe routes redistributed into the NSSA. LSA 7 is translated into LSA 5 as it leaves the NSSA. These routes appear as N1 or N2 in the ip routing table inside the NSSA. Much like LSA 5, N2 is a static cost while N1 is a cumulative cost that includes the cost upto the ASBR.

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In an OSPF network, when a packet need to traverse from one area to another area to reach its destination, it is routed as below:

Source Area -> Source ABR -> Backbone Area -> Destination ABR -> Destination Area Routers

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