Cisco Router OSPF Design and Implementation Guide William Parkhurst, PhD, CCIE $54.95 0-07-048626-3 |
![]() ![]() |
Chapter: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 |
Reserve your copy at a Beta Bookstore near you! |
Contact Bet@books © 1998 The McGraw-Hill Companies, Inc. All rights reserved. Any use of this Beta Book is subject to the rules stated in the Terms of Use. |
OSPF overcomes all of the limitations of RIP that were identified in Chapter 5. This chapter is an overview of OSPF as specified in RFC 2178, which specifies version 2 of the OSPF protocol. RFC 2178 is a rather formidable document containing over two hundred pages whereas the RIP RFC contains thirty-two pages. As with many RFCs they are sometimes confusing and difficult to understand. The intention of this chapter is to clearly explain the terminology, concepts, and operations of OSPF in order to provide a solid foundation and understanding of the protocol for the design and implementation on Cisco routers. This chapter does not attempt to present or explain every detail of the RFC but only introduce those concepts that are necessary to understand the operation of the OSPF protocol.
Overview
OSPF is an interior gateway protocol (IGP) and is used for routing decisions in a single autonomous system (AS). OSPF is a link-state routing protocol as opposed to RIP, which is a distance-vector routing protocol. A link is just another word for router interface so OSPF could be called an interface-state routing protocol. The term state refers to the parameters of the router interface or link. These parameters are the physical condition of the interface, is it up or down, the IP address of the interface, the subnet mask assigned to the interface, the network to which the interface is connected, and the associated cost for using the routers network connection. We have seen that RIP operates by exchanging its entire routing table with its neighbors by broadcasting the routing table onto interfaces that are running RIP. OSPF exchanges information with other OSPF routers but the information exchanged is not routes but link-states. Instead of informing other routers which networks it can reach and what the distance is, OSPF routers inform each other of the state of their network interfaces, the networks these interfaces are attached to and the cost of using the interfaces. Obviously every router has a different link-state than every other router. Each router’s link-state can be referred to as a local link-state. These local link-states are propagated (flooded) throughout the OSPF network until every OSPF router has a complete and identical link-state database.
Once every router has received all the local link-states then each router can build a tree, with itself as the root, and the branches representing the shortest, or least cost routes to all the networks in the AS. Each OSPF router will use these shortest paths to build the routing table, hence the name shortest path first. The O in OSPF simply means that the standard defining OSPF is open to the public and not a closed proprietary routing scheme like Cisco’s IGRP and EIGRP. Figure 6.1 illustrates some of the basic OSPF terms. Router A has three active interfaces or links. Each interface has an IP address and a subnet mask. The IP address and subnet mask uniquely identify which interface is connected to which network. The cost of each link is a dimensionless quantity that is usually related to the bandwidth of the link. A faster link is a "better" link and should therefore have a lower associated cost. One method of calculating the cost is to divide one hundred million by the bandwidth of the interface. Assuming the ethernet networks in figure 6.1 are 10BaseT with a bandwidth of 10 Mbits/sec then the cost associated with the ethernet networks would be 10. The ISDN links would have a cost of 1786 and the frame relay connections cost would be 333 (All costs are rounded to an integer). The local state of each router can now be determined and the link-state for router A is contained in Table 6.1. Router A will exchange it’s local link-state with every attached router and each router will flood this information throughout the network until all five routers have a complete and identical link-state database.
Table 6.1. Link-State for Router A in Figure 6.1.
Interface |
Interface State |
IP Address |
Subnet Mask |
Cost |
Ethernet |
UP |
156.26.1.1 |
255.255.255.0 |
10 |
Serial – Frame |
UP |
156.26.2.1 |
255.255.255.0 |
333 |
Serial-ISDN |
UP |
156.26.3.1 |
255.255.255.0 |
1786 |
Once every router has a complete link-state database, each router can calculate a shortest-path tree. For example, there are many paths for router A to choose from when trying to get to network 5 (Figure 6.2). For the four paths shown the cost would be:
The shortest path is path 3 so this is the path that router A would insert into its routing table for destination network 5. The network of figures 6.1 and 6.2 contains many loops. RIP cannot detect these loops but the Shortest Path First algorithm can detect loops and creates a routing scheme that is loop free, another improvement over RIP. Which path would RIP choose from router A to network 5? As we have seen in chapter 5 the answer would depend on the order of the routing table updates received by router A from the other routers. Assume router A’s frame relay link becomes disabled. Paths 3 and 4 in figure 6.2 become the shortest paths to network 5. Which one does OSPF choose to insert in the routing table? Both! OSPF can route traffic over equal cost paths while RIP can handle only one path to a destination network.
Figure 6.2. Some Possible Paths From Router A to network 5.
As the network grows, so does the size of the complete link-state database that each router needs to maintain. As the size of the link-state database grows, the time required to compute the shortest path tree also increases. Many corporate networks contain hundreds of routers and networks. Imagine flooding hundreds of local link-states to every router in a large network and then requiring each router to compute a shortest path tree. It would seem that OSPF does not scale well but there is an elegant solution, break the network into smaller pieces or areas and require the routers only to exchange link-states with routers in the same area. This requires less information that needs to be flooded and the computational intensity of calculating the shortest path tree will also be reduced. This concept of OSPF areas is shown in figure 6.3. Area 0 is a required area if more than one area is being implemented and it is typically referred to as the backbone area. All non-zero or non-backbone areas must touch the backbone area either physically or logically. A physical connection is the type that the non-backbone areas in figure 6.3 have with the backbone. This physical connection is through a router having one interface in the backbone and one in the non-backbone area. A logical, or virtual, link to the backbone can be used either when the backbone becomes discontiguous or when it is not possible for a non-backbone area to physically connect to the backbone area. A virtual link is defining by two endpoints and a transit area. One of the endpoints is a router interface that is part of the backbone and the other endpoint is a router interface in a area that does not have a physical connection to the backbone. The transit area is the area between the backbone and the non-backbone area that does not have a physical connection to the backbone (figure 6.4). A backbone can become disjointed due to a link failure. The backbone is required to be contiguous for the proper operation of OSPF. A virtual link can be used to repair a disjointed backbone in the same way that a virtual link can connect a non-backbone area to the backbone when a physical connection is not possible.
Figure 6.3. Partitioning a Network Into Multiple OSPF Areas.
Definitions
An understanding of the terms used when describing an OSPF network is essential, so here we go.
Router ID – A 32-bit number used to identify each router. Typically the highest IP address assigned to the router. If loopback interfaces are used on the router than the router ID is the highest IP address of the loopback interfaces regardless of the value of the IP address for the physical interfaces
Interface – Connection between a router and a network with a single IP address and subnet mask. Also referred to as a link.
Neighboring routers – Routers with an interface to a common network.
Broadcast network – A network that supports broadcasts. Ethernet is a broadcast network.
Non-broadcast network – Supports more than two attached routers but has no broadcast capability. Frame-relay and X.25 are two examples of non-broadcast networks. There are two general types of non-broadcast networks, Non-Broadcast Multi-Access (NBMA) and point-to-multi-point. The differences between the two depend more on the configuration of router than on the physical network. Consider the networks in figure 6.6 a and b. The first shows a router with frame relay connections to three other routers. All the connections are using the same IP subnet therefore this is an NBMA network. The second configuration is physically the same except the hub router has three logical sub-interface frame relay connections configured on different IP subnets, making this a point-to-multi-point configuration. On an NBMA network a designated router is elected while on a point-to-multi-point network there is no designated router. The differences are subtle but important, as we shall see when we begin to configure routers.
Designated Router (DR) - On broadcast and NBMA networks the designated router is used to flood link-state information to the common network.
Backup Designated Router (BDR) - Router that will take over the role of DR in the event the DR fails.
Area Border Router (ABR) – A router that has an interface in more than one OSPF area.
Autonomous System Border Router (ASBR) – AN OSPF router that connects to another AS or to an area of the network in the same AS which is running a different IGP than OSPF.
Adjacency – An adjacency is formed between the DR and all non-designated routers on a broadcast or NBMA network. An adjacency is also formed between the BDR and all non-designated routers. OSPF routing updates are only sent and received on adjacencies.
Flooding – used to distribute and synchronize link-state databases between routers.
Link State Advertisement (LSA) – Describes the local state of a router.
External Routes – Routes learned from another AS or from another routing protocol can be injected into OSPF as external routes. There are two types of external routes. A type 1 external route has a cost that includes the OSPF cost plus the cost from the ASBR to the network. A type 2 external route has a cost equal only to the cost from the ASBR to the external network.
Intra-area Routing – Routes between networks in the same area. These routes are based only on information received from within the area.
Inter-area Routing – Routes between two different OSPF non-backbone areas. The inter-area path consists of three parts: the intra-area path from the area to the ABR of the source area, the backbone path from the source ABR to the destination ABR, and finally from the destination ABR to the destination network.
Route Summarization – Routes within an area, routes from another AS, and routes learned from another routing protocol can be summarized by OSPF into a single route advertisement if the IP network was designed properly. Figure 6.7 shows an OSPF area with 14 class C networks. Without summarization the ABR will advertise 14 routes. With summarization the ABR can advertise 1 route, significantly reducing the size of the routing update and downstream routers will have smaller routing tables. Notice that summarization can only occur on an ABR or an ASBR. Summarization is not automatic as is mentioned in the RFC.
Stub Area – An area with a single path point from the area. Virtual links cannot be used in a stub area.
Figure 6.6. Non-broadcast networks
OSPF Routing Protocol Packets
OSPF uses five type of routing protocol packets as listed in table 6.2. The hello protocol is used to find neighbors on a routers’ attached networks. The hello protocol is also used to determine if a neighboring routers’ interface is still functional by periodically sending out hello packets. Designated routers are elected on broadcast and NBMA networks through the use of the hello protocol. Hello packets are sent to the multicast address
AllSPFRouters (224.0.0.5). During flooding some protocol packets are also sent to this address. Designated and backup designated routers send and receive protocol packets to
the multicast address AllDRouters (224.0.0.6). Packets sent to the multicast address have the TTL field in the IP header to 1 so the packets will travel only a single hop.
Figure 6.7. Route Summarization Example.
Table 6.2. OSPF Routing Protocol packet Types
Packet Type |
Purpose |
Hello |
Discover and Maintain Neighbors |
Database Description |
Summarize Database Contents |
Link State Request |
Database Download |
Link State Update |
Database Upload |
Link State Acknowledgement |
Flooding Acknowledgement |
Packet Formats
All OSPF packets share a common 24-byte header as shown in figure 6.8. The version number is set to the version of OSPF that a router is running. The current version number is 2. The type field has a value in the range 1 to 5 depending on the type of the OSPF packet. The packet length is the length in bytes of the OSPF packet plus the OSPF header. Router ID is the highest IP address assigned to the router transmitting the packet. Area ID is the 32-bit designator for the area assigned to the routers’ transmitting interface. If the packet is sent over a virtual link then the area ID is 0. The checksum is calculated using the one’s complement sum as we have seen with the FCS in an ethernet frame and the checksum for the IP header. If authentication is being used the AuType identifies the authentication method and the following 64-bit field contains data required by the authentication type being used. The Hello protocol uses the packet format shown in figure 6.9
0 7 8 15 16 31
Version Number |
Type |
Packet length |
Router ID |
||
Area ID |
||
Checksum |
AuType |
|
Authentication |
||
Authentication |
The network mask is the subnet mask of the interface that transmitted the Hello packet. The Hello interval is the time in seconds between transmissions of a Hello packet. The router priority filed can be used to influence the election of the DR. The default router priority value is 1. If all routers use this default value then the DR will be the one with the highest IP address. A value of 0 indicates that the route cannot be elected DR. If values other than 0 or 1 are used, then the router with the highest priority will be elected DR. The RouterDeadInterval is used to determine if a neighbor has died. If a hello packet has not been received from an established neighbor during the number of seconds specified by the RouterDeadInterval, then the neighbor is declared down. The designated router and backup designated router fields contain the IP address of the DR and BDR. This field is set to 0 if there is no DR or BDR (point-to-point network). Finally the packet contains a list of neighbors that the router has on this network. If a hello packet is not received from a neighbor in RouterDeadInterval seconds then the
0 7 8 15 16 31
Version Number |
Type = 1 |
Packet Length |
|
Router ID |
|||
Area ID |
|||
Checksum |
AuType |
||
Authentication |
|||
Authentication |
|||
Network Mask |
|||
HelloInterval |
Options |
Router Priority |
|
RouterDeadInterval |
|||
Designated Router |
|||
Backup Designated Router |
|||
Neighbor Router ID |
|||
Neighbor Router ID |
.
.
.
Neighbor Router ID |
neighbor is removed from this list. The options field contains five assigned bits but only 1 bit is described in RFC 2178, the E bit, which determines how external link-state advertisements are flooded.
Designated Router Election
The algorithm for electing the DR and BDR is rather complex because the protocol requires a smooth transition from BDR to DR in the event the DR fails. Understanding of the procedure for the election of the DR and the BDR to DR transition is essential for designing a stable and robust OSPF network. Initially a network’s DR and BDR are set to 0.0.0.0 indicating that a DR and BDR have not yet been elected. A router will examine the list of neighbors and eliminate any router with a priority of zero indicating that the router is not eligible to become DR or BDR. The algorithm is then run with the routers having zero priority removed from the eligibility list.
The state diagram in figure 6.10 shows the transitions from a router interface being down through the state of two routers forming a full adjacency on their common network. The down state signifies that either a router interface is physically down or that the OSPF protocol has not yet been enabled on the interface. When OSPF has been enabled on the interface Hello packets will be transmitted. When a Hello packet is received from a neighbor but the router does not appear in the list of neighbors the interface enters the Init state. All interfaces in this state and attached to the network will be contained in the list of neighbors. When a router sees it’s interface listed in a neighbor’s list of neighbors the state 2-way is entered. Any routers in the state 2-way or greater are eligible to be elected DR or BDR. Also, routers can start exchanging link-state information by entering the ExStart state. In this state the routers decide who is the master and who is the slave and what the initial database description sequence number will be. When the exchange state is entered the router is sending database description packets to neighboring routers and link-state request packets requesting the neighbor’s most up to date information. In the loading state the neighbor is waiting for the request database updates. When the exchange of link-state
databases is complete the two routers will enter the full state. If an adjacency has been formed adjacent routers will exchange Hello packets periodically as a way of informing and detecting if adjacent neighbors are still functioning properly.
Link-State Database Packet Formats
The different types of link-state advertisements that routers can generate are listed in table 6.3. Router and network LSAs contain the information necessary to determine how the networks and routers are interconnected. Summary LSAs are used to transmit networks that have been aggregated into a single network advertisement. The summaries can be created either at an ABR (type 3) or ASBR (type 4). External routing information can be propagated into the OSPF domain by an ASBR using the type 5 LSA packet.
Every LSA packet has a common 20-byte header (Figure 6.11).
Table 6.3. Link State Types
Type |
Description |
1 |
Router LSA |
2 |
Network LSA |
3 |
Summary LSA |
4 |
Summary LSA (ASBR) |
5 |
AS-external LSA |
Link State Age |
Options |
Link State Type |
Link State ID |
||
Advertising Router |
||
Link State Sequence Number |
||
Checksum |
Length |
Figure 6.11. LSA header format.
Typically there are many LSA being exchanged by routers in the OSPF domain and part of the function of the LSA header is to uniquely identify each LSA packet and determine which one is the most recent. This is accomplished by using the LS sequence, LS checksum, and LS age fields. The LS age field is initially set to 0 by the originating router and is incremented by an amount referred to as InfTransDelay which represents the time required to transmit the LSA to the next hop. A parameter, MaxAge, is using to retire LSAs and the LS age value is never incremented past the MaxAge value. Newer LSAs will be used in the database and older versions will be deleted from the database when they reach maxAge.
The options field is used to denote the capabilities of the router. The only bit currently used is the E-bit (external) and is 0 for stub areas and 1 for all other areas. LS type has a value of 1 to 5 and signals the type of LSA packet and listed in table 6.3. The Link State ID values depends on the LSA packet type and these values are listed in table 6.4. The advertising router field contains the router ID of the originator of the LSA. In a router LSA the advertising router field is the same as the link state
Table 6.4. LSA link state ID values.
LS Type |
Link State ID Value |
1 |
The originating router’s ID |
2 |
IP interface address of this networks DR |
3 |
Destination networks IP address |
4 |
Router ID of the ASBR |
5 |
Destination networks IP address |
ID field. The network’s DR is contained in this field for network LSAs, the ID of the ABR is used in summary LSAs and the ID of the ASBR is used in AS-external-LSAs. Sequence numbers are used to detect old and duplicate LSAs and the checksum is used to detect corrupted packets. The length field indicates the length of the LSA including the header.
Link State Database
A router maintains a separate link state database for every area in which it has an active interface. Every router than has an interface in an area will have an identical link state database for that area. The shortest path tree is calculated for each area separately by each router with itself as the root of the tree. A router’s portion of the link state database for an area is only flooded through that area. This area database is composed of router LSAs, network LSAs and summary LSAs. If the area is not a stub area the database will also include an AS-external LSA. A router will add an LSA to its’ database when it is received during flooding or when the router initiates the LSA. Routers delete LSAs from the database when a newer LSA is received, when the router generates a new LSA or when a LSA becomes to old. Any LSAs deleted from a router’s database will also be removed from every neighbors retransmission list.
Every router will originate a router LSA. If the router is also the DR then the router will generate a network LSA for the network that it is the DR. Area border routers will generate summary-LSAs for the inter-area routes and ASBR will generate AS-external LSAs for external routes. Ten events have been identified which will trigger a new instance of an LSA to be generated.
Router Link State Advertisements
A router will generate a router LSA for every area in which it hs an active OSPF interface. The information contained in the router LSA is the states of the router’s interfaces in that area and the LSA is flooded throughout the area. The format of a router LSA is shown in figure 6.12. All of the router’s interfaces into an area must be described in a single router LSA. The Link State ID field is the router’s OSPF ID. The V E B bits are used to determine the types of links that the router may have. The V bit indicates that the router is the endpoint of a virtual link. If the router is an ASBR then the E bit will be set. If the router is an ABR then the B bit will be set. The number of links indicates how many OSPF interfaces are active on the router. Each link is described by the following fields:
Type 2 Connection to a transit network
Type 3 Connection to a stub network
Type 4 Virtual link
2. Link ID For type 1 the neighboring router’s ID
For type 2 the IP address of the DR
For type 3 the IP network/subnet number
For type 4 the neighboring router’s ID.
Link State Age |
Options |
Link State Type = 1 |
||||||||
Link State ID |
||||||||||
Advertising Router |
||||||||||
Link State Sequence Number |
||||||||||
Checksum |
Length |
|||||||||
0 |
0 |
0 |
0 |
0 |
V |
E |
B |
0 |
Number of Links |
|
Link ID |
||||||||||
Link Data |
||||||||||
Type |
# TOS |
Metric |
.
.
.
TOS |
0 |
TOS Metric |
Link ID |
||
Link Data |
.
.
.
Figure 6.12. Router LSA packet format.
The link ID identifies what the router’s interface is connected to. The link ID is typically equal to the neighboring router’s link state ID. The contents of the link data field depend on the link type field. If the router has a connection to a stub network then this field will contain the IP address mask for the network For other link types this field contains the IP address assigned to the interface. The type of service field (TOS) is usually set to 0. The final value is the metric, or cost of the link which should always be non-zero except if the link is attached to a stub network.
Network Link State Advertisements
The format of a network LSA is shown in figure 6.13.
Link State Age |
Options |
Link State Type=2 |
Link State ID |
||
Advertising Router |
||
Link State Sequence Number |
||
Checksum |
Length |
|
Network mask |
||
Attached Router |
||
Attached Router |
.
.
.
A network LSA is a type 2 LSA and they are generated for each broadcast and NBMA network in an area which supports two or more routers. Network LSAs are originated by the DR for the network and the link state ID field contains the IP address of the DR’s interface to the area. A cost or metric is not needed in the advertisement since the routers are directly connected to the network resulting in a cost of zero. The network mask field contains the network mask for the common network and attached router fields contain the IP address of the router interfaces that are connected to this common network and that are also adjacent with the DR.
Summary Link State Advertisements.
Type 3 and 4 LSAs are summary link state advertisements having the format shown in figure 6.14. Summary LSAs are generated by area border routers and the describe inter-area destinations. A type 3 summary LSA has a destination of an IP address and the link state ID field is an IP network number. A type 4 summary LSA has as its destination an autonomous system border router and the link state ID field is the OSPF router ID. The link state ID field is the only difference between the two types of summary LSA packets.
Link State Age |
Options |
Link State Type=3,4 |
|
Link State ID |
|||
Advertising Router |
|||
Link State Sequence Number |
|||
Checksum |
Length |
||
Network mask |
|||
0 |
Metric |
||
TOS |
TOS metric |
.
.
.
Figure 6.14. Summary LSA packet format.
Type 3 LSAs can also be used to describe a default route into a stub area. For this case the link state ID field and network mask field is set to 0.0.0.0. The network mask field is the destinations network’s Ip address mask for a type 3 LSA and should be set to 0 for a type 4 LSA.
AS-external-LSAs
Type 5 LSAs are AS-external LSAs and are used by ASBRs to describe networks external to the autonomous system and have the packet format shown in figure 6.15.
LS age |
Options |
Type = 5 |
||
Link State ID |
||||
Advertising Router |
||||
LS Sequence Number |
||||
LS Checksum |
Length |
|||
Network Mask |
||||
E |
0 |
Metric |
||
Forwarding Address |
||||
External Route Tag |
||||
E |
TOS |
TOS Metric |
||
Forwarding Address |
||||
External Route Tag |
. . .
Figure 6.15. AS-external link state advertisement packet format.
As-external LSAs are used to describe routes to an external network. The link state ID field will contain an IP network number or 0.0.0.0 if describing a default route. If the LSA is describing a default route then the network mask field is also set to 0.0.0.0. The E bit is used to indicate whether the external route is a type 2 (E = 1) or a type 1 (E = 0) external route. Recall that a type 1 route has a metric using the same units as the link state metric and a type 2 route is larger than any link state metric. The forwarding address contains the address to send any traffic destined for the external network. The external route tag field is not used by OSPF.
Database Description Packet
A database description packet is a type 2 OSPF packet with the format shown in figure
Database description packets are exchanged between routers that in the process of forming an adjacency and they describe the link-state database. Depending on the number of interfaces and networks more than one database description packet may be needed to transmit the entire link-state database. The routers involved in the exchange enter a master/slave relationship. The master sends packets and the slave acknowledges packets through the use of the database description (DD) sequence numbers. The interface MTU field indicates the size of the largest IP packet that can be sent through the interface. This field is set to 0 when sent over a virtual link.
The options field contains 3 bits that are used to indicate the capabilities of the router.
The I-bit is the Init bit and it is set to 1 for the first packet in the database sequence. The M-bit is set to 1 to indicate there are more database description packets in the sequence. The MS-bit is the master/slave bit. A 1 indicates that the router is the master and a 0 indicates the router is the slave during the exchange of database description packets.
0 7 8 15 16 31
Version Number |
Type = 2 |
Packet length |
|||||||||
Router ID |
|||||||||||
Area ID |
|||||||||||
Checksum |
AuType |
||||||||||
Authentication |
|||||||||||
Authentication |
|||||||||||
Interface MTU |
Options |
0 |
0 |
0 |
0 |
0 |
I |
M |
MS |
||
Database Description Sequence Number |
|||||||||||
LSA Header |
|||||||||||
.
.
.
The remaining portion of the packet is one of more LSAs with the formats described in previous sections of this chapter.
Link State Request Packets
Type 3 OSPF packets are link state request packets and have the format shown in figure 6.17. When two routers have completed exchanging database description packets a router may detect that portions of the link state database are out-of-date. When this situation occurs the router can request newer database description packets that are more up-to-date.
0 7 8 15 16 31
Version Number |
Type = 3 |
Packet length |
Router ID |
||
Area ID |
||
Checksum |
AuType |
|
Authentication |
||
Authentication |
||
LS Type |
||
Link State ID |
||
Advertising Router |
Link State Update Packet
OSPF packet type 4 is a link state update packet and are used to implement the flooding of LSAs. The format of a link state update packet is shown in figure 6.18. Each link state update packet will contain one or more LSAs and each packet is acknowledged by using link state acknowledgement packets.
0 7 8 15 16 31
Version Number |
Type = 4 |
Packet length |
Router ID |
||
Area ID |
||
Checksum |
AuType |
|
Authentication |
||
Authentication |
||
Number of LSAs |
||
LSAs |
||
Link State Acknowledgement Packets
Type 5 OSPF packets are link state acknowledgement packets. The format of a link state acknowledgement packet is shown in figure 6.19. These packets can be sent to one of three addresses: the multicast address ALLDRouters, the multicast address ALLSPFRouters or to a unicast address.
0 7 8 15 16 31
Version Number |
Type = 5 |
Packet length |
Router ID |
||
Area ID |
||
Checksum |
AuType |
|
Authentication |
||
Authentication |
||
LSA header |
||
Table 6.5. OSPF LSA Summary
LSA Type |
Router Type |
Router |
All Routers |
Network |
Designated Routers |
Summary |
Area Border Routers |
AS-external |
Autonomous System Border Routers |
OPSF Not-So-Stubby-Area (NSSA)
RFC 1587 describes the OSPF NSSA option, which was developed to address the scenario depicted in figure 6.20.
From the point of view of the wide area transit network the sites are stub areas having the following properties:
The problem with this configuration is that routers r1 and r2 (and routers r3 and r4) cannot both be members of the stub area because a stub area cannot import routes external to OSPF. If OSPF is running on routers r1 and r3 then routers r1 and r3 must be connected to the backbone or not be in a stub area. There is no reason for routers r1 and r3 to maintain all the external routes for the transit network so we must run a different routing protocol, such as RIP, on routers r1 and r3 and redistribute routes from OSPF into RIP. NSSAs were developed so that AS external routes can be imported into a stub area in a limited manner using a new type of LSA which is referred to as a type 7 AS external. Type 7 LSAs have the following properties:
NSSA border routers can originate a default type 7 LSA into the NSSA and a limited exchange of external routing information can be sent into an NSSA area by translating type 7 LSAs into type 5 LSAs. One major difference between stub area and NSSA area is that OSPF type 3 summary LSAs are imported into the NSSA. NSSAs allow both routers r1 and r2 (and r3 and r4) to be members of the NSSA. Router r2 is an NSSA area border router and router r1 is an autonomous system border router internal to the NSSA. Router r1 will import the subnets of site 1 into the NSSA as type 7 LSAs. Router r2 will translate the type 7 LSAs into type 5 summary LSAs and flood them into the OSPF backbone.
![]() ![]() Chapter: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 |
Reserve your copy at a Beta Bookstore near you! |
Contact Bet@books © 1998 The McGraw-Hill Companies, Inc. All rights reserved. Any use of this Beta Book is subject to the rules stated in the Terms of Use. |