CCDA 640-864 Official Cert Guide - Chapter 11 Summary

OSPFv2 Areas

You can limit the CPU and memory requirements that OSPFv2 puts on the routers by dividing an OSPFv2 network into areas. An area is a logical division of routers and links within an OSPFv2 domain that creates separate subdomains. LSA flooding is contained within an area, and the link-state database is limited to links within the area. You can assign an area ID to the interfaces within the defined area. The Area ID is a 32-bit value that you can enter as a number or in dotted decimal notation, such as 10.2.3.1.

If you define more than one area in an OSPFv2 network, you must also define the backbone area, which has the reserved area ID of 0. If you have more than one area, then one or more routers become area border routers (ABRs). An ABR connects to both the backbone area and at least one other defined area.

Stub Area

You can limit the amount of external routing information that floods an area by making it a stub area. A stub area is an area that does not allow AS External (type 5) LSAs. These LSAs are usually flooded throughout the local autonomous system to propagate external route information. Stub areas have the following requirements:

•All routers in the stub area are stub routers.

•No ASBR routers exist in the stub area.

•You cannot configure virtual links in the stub area.

Stub areas use a default route for all traffic that needs to go through the backbone area to the external autonomous system. The default route is 0.0.0.0 for IPv4.

Not-So-Stubby Area

A Not-so-Stubby Area (NSSA) is similar to a stub area, except that an NSSA allows you to import autonomous system external routes within an NSSA using redistribution. The NSSA ASBR redistributes these routes and generates NSSA External (type 7) LSAs that it floods throughout the NSSA. You can optionally configure the ABR that connects the NSSA to other areas to translate this NSSA External LSA to AS External (type 5) LSAs. The ABR then floods these AS External LSAs throughout the OSPFv2 autonomous system. The backbone Area 0 cannot be an NSSA.

OSPF Router Types

The ABR has a separate link-state database for each area to which it connects. The ABR sends Network Summary (type 3) LSAs from one connected area to the backbone area. The backbone area sends summarized information about one area to another area.

OSPFv2 defines one other router type: the autonomous system boundary router (ASBR). This router connects an OSPFv2 area to another autonomous system. An autonomous system is a network controlled by a single technical administration entity. OSPFv2 can redistribute its routing information into another autonomous system or receive redistributed routes from another autonomous system.

LSA Types

Type	Name	Description
1	Router LSA	LSA sent by every router. This LSA includes the state and the cost of all links and a list of all OSPFv2 neighbors on the link. Router LSAs trigger an SPF recalculation. Router LSAs are flooded to local OSPFv2 area.
2	Network LSA	LSA sent by the DR. This LSA lists all routers in the multi-access network. Network LSAs trigger an SPF recalculation.
3	Network Summary LSA	LSA sent by the area border router to an external area for each destination in the local area. This LSA includes the link cost from the area border router to the local destination.
4	ASBR Summary LSA	LSA sent by the area border router to an external area. This LSA advertises the link cost to the ASBR only.
5	AS External LSA	LSA generated by the ASBR. This LSA includes the link cost to an external autonomous system destination. AS External LSAs are flooded throughout the autonomous system.
7	NSSA External LSA	LSA generated by the ASBR within a not-so-stubby area (NSSA). This LSA includes the link cost to an external autonomous system destination. NSSA External LSAs are flooded only within the local NSSA.
9-11	Opaque LSAs	LSA used to extend OSPF.

OSPFv2 Summary

OSPFv2 is used in large enterprise IPv4 networks. The network topology must be hierarchy. The characteristics of OSPFv2 follow:

• Link-state routing protocol.
• Uses IP protocol 89.
• Classless protocol (supports VLSMs and CIDR).
• Metric is cost (based on interface bandwidth by default).
• Fast convergence. Uses link-state updates and SPF calculation.
• Reduced bandwidth use. Sends partial route updates only when changes occur.
• Routes are labeled as intra-area, interarea, external Type 1, or external Type 2.
• Support for authentication.
• Uses the Dijkstra algorithm to calculate the SPF tree.
• Default administrative distance is 110.
• Uses multicast address 224.0.0.5 (ALLSPFRouters).
• Uses multicast address 224.0.0.6 (ALLDRouters).
• Good scalability. Recommended for large networks.

OSPFv3 Changes from OSPFv2:

The following are the major changes for OSPFv3:

• Version number is 3: Obviously, this is a newer version of OSPF, and it runs over IPv6 only.
• Support for IPv6 addressing: New LSAs created to carry IPv6 addresses and prefixes.
• Per-link processing: OSPFv2 uses per-subnet processing. With link processing, routers in the same link can belong to multiple subnets.
• Address semantics removed: Addresses are removed from the router and network LSAs. These LSAs now provide topology information.
• No authentication in the OSPFv3 protocol: schemes inherited in IPv6.
• New link LSA: For local-link flooding scope.
• New intra-area-prefix LSA: Carries all the IPv6 prefix information. Similar to OSPFv2 router and network LSAs.
• Identifying neighbors by router ID: Neighbors are always identified by the router ID. This does not occur in OSPFv2 point-to-point and broadcast networks.
• Options field changes: Two Options bits, the R-bit and the V6-bit, have been added to the Options field for processing router LSAs during the SPF calculation.

OSPFv3 LSA Types

Router LSA:
Each OSPF router originates Router LSAs indicating the state and cost of the router's interfaces to the area. Router LSAs are flooded throughout the single area only.
A router may originate one or more Router LSAs, distinguished by their Link State IDs. The receiving router concatenates the Router LSAs if it receives more than one Router LSA from a single router.
The Router LSA indicates if the router is an ASBR or an ABR or if it is one end-point of a virtual link. These LSAs have no address information.


Network LSA:
Network LSAs are originated by the DR for a broadcast or NBMA network in the area which supports two or more routers. The LSA describes all routers connected to the link, including the DR. The LSA's Link State ID field is set to the Interface ID that the DR has been using in Hello packets. No address information is carried in the Network LSA.


Inter-Area Prefix LSA:
These LSAs are IPv6 equivalent of IPv4's Type-3 Summary LSAs. These LSAs are originated by the ABR to specify IPv6 prefixes that belong to other areas. A separate LSA is originated for each address prefix.
For Stub areas, the Inter-area Prefix LSA is used to describe a default route. The prefix length of the default route is set to 0.


Inter-Area Router LSA:
These LSAs are IPv6 equivalent of IPv4's Type-4 Summary LSAs. Originated by the ABR, the Inter-Area Router LSA describes the route to the ASBR. Each LSA describes a route to a single router.


AS-External LSA:
These LSAs are IPv6 equivalent of IPv4's Type-5 External LSAs. These LSAs are originated by ASBRs describing the destinations external to the AS. Each LSA describe a route to a single IPv6 prefix external to the AS.
AS-External LSAs can be used to describe a default route. Default routes are used when no specific route exists for a destination.


Link LSA:
A router originates a separate Link LSA for each link it is attached to. These LSAs have link-local flooding scope and are never flooded beyond a link that they are associated with. These LSAs have three purposes-
    - notify the link-local address of the router's interface to the routers attached to the link
    - inform other routers attached to the link of the list of IPv6 prefixes to associate with the link
    - allow the router to assert the collection of Option bits to associate with the Network LSA that will be originated for the link.
The Link-State ID is set to the Interface ID of link of the originating router.

Intra-Area Prefix LSA:
A router uses Intra-Area Prefix LSA to advertise IPv6 prefixes that are associated with
    a) the router itself (in IPv4, this was carried in Router LSA)
    b) an attached stub network segment (in IPv4, this was carried in Router LSA)
    c) an attached transit network segment (in IPv4, this was carried in Network LSA)
A router can originate multiple Intra-Area Prefix LSAs for each router or transit network; each LSA is distinguished by its Link State ID.


Options field:

The 24-bit Options field is included in Hello and DBD packets, and Router, Network and Inter-area Router LSAs. It enables OSPF routers to support optional capabilities, and to communicate their capabilities to other OSPF routers in the network. 

OSPFv3 Summary

OSPFv3 is used in large enterprise IPv6 networks. The network topology must be hierarchical. OSPF is used in the enterprise campus building access, distribution, and core layers. OSPF is also used in the enterprise data center, WAN/MAN, and branch offices.

The characteristics of OSPFv3 follow:

• Link-state routing protocol for IPv6.
• Uses IPv6 Next Header 89.
• Metric is cost (based on interface bandwidth by default).
• Sends partial route updates only when changes occur.
• Routes are labeled as intra-area, interarea, external Type 1, or external Type 2.
• Uses IPv6 for authentication.
• Uses the Dijkstra algorithm to calculate the SPF tree.
• Default administrative distance is 110.
• Uses multicast address FF02::5 (ALLSPFRouters).
• Uses multicast address FF02::6 (ALLDRouters).
• Fast convergence, scalable, and reduces bandwidth.
• Recommended for large IPv6 networks.

BGP Summary

The characteristics of BGP follow:

• BGP is an Exterior Gateway Protocol (EGP) used in routing in the Internet. It is an interdomain routing protocol.
• BGP is a path-vector routing protocol suited for strategic routing policies.
• It uses TCP port 179 to establish connections with neighbors.
• BGPv4 implements CIDR.
• eBGP is used for external neighbors. It is used between different autonomous systems.
• iBGP is used for internal neighbors. It is used within an autonomous system.
• BGP uses several attributes in the routing-decision algorithm.
• It uses confederations and route reflectors to reduce BGP peering overhead.

Route Manipulation

            PBR

            Route Summarization

            Route Redistribution

            Route Filtering

           

IP Multicast Review

Multicast Addresses

List of Multicast addresses

IGMP

IGMP is a standard defined in RFC1112 for IGMPv1, in RFC2236 for IGMPv2 and in RFC3376 for IGMPv3. IGMP specifies how a host can register with a router in order to receive specific multicast traffic. 

CGMP

CGMP was first implemented by Cisco to restrain multicast traffic in a L2 network. Because a switch is, by essence, not capable of looking at L3 packets, it cannot distinguish an IGMP packet. With CGMP, the router provides the interface between the hosts. The routers "talk" IGMP, and the switches "talk" CGMP.

IGMP Snooping

IGMP snooping is another feature that allows you to directly capture IGMP frames. 

IGMP snooping, as implied by the name, is a feature that allows the switch to "listen in" on the IGMP conversation between hosts and routers. When a switch hears an IGMP report from a host for a given multicast group, the switch adds the host's port number to the GDA list for that group. And, when the switch hears an IGMP Leave, it removes the host's port from the CAM table entry.

Sparse Versus Dense Multicast 

The Dense approach assumes that the multicast group members are densely distributed throughout the network (many of the subnets contain at least one group member), all devices want to receive multicast traffic, and that bandwidth is plentiful. The Sparse approach to multicast routing assumes that multicast group members are sparsely distributed throughout the network, assumes received do not want to receive multicast traffic, and that bandwidth is not necessarily widely available.

Multicast Source and Shared Trees 

With source trees, the tree roots from the source of the multicast group and then expands throughout the network in spanning-tree fashion to the destination hosts. Source trees are also called shortest-path trees (SPT) because they create paths without having to go through a rendezvous point (RP).

Shared trees create the distribution tree’s root somewhere between the network’s source and receivers. The root is called the RP. The tree is created from the RP in spanning-tree fashion with no loops.

PIM 

PIM comes in two flavors: sparse mode (PIM-SM) and dense mode (PIM-DM). The first uses shared trees and RPs to reach widely dispersed group members with reasonable protocol bandwidth efficiency. The second uses source trees and reverse path forwarding (RPF) to reach relatively close group members with reasonable processor and memory efficiency in the network devices of the distribution trees.

DVMRP 

RFC 1075 describes DVMRP. It is the primary multicast routing protocol used in the multicast backbone (MBONE). The MBONE is used in the research community.

IPv6 Multicast Addresses