When OSPF neighbors become adjacent, the LSDBs synchronize between the OSPF routers. As an OSPF router adds or removes a directly connected network link to or from its database, the router floods the LSA out all active OSPF interfaces.
The OSPF LSA contains a complete list of networks advertised from that router.
LSA Types 1, 2, and 3 are used for building the SPF tree for intra-area and interarea route routes.
LSA Types 4, 5, and 7 are related to external OSPF routes (that is, routes that were redistributed into the OSPF routing domain).
|Type 1, router LSA||LSA that advertises network prefixes within an Area.|
|Type 2, network LSA||LSA that indicates the router attached to a broadcast segment within an Area.|
|Type 3, summary||LSA that advertises network prefixes located in a different Area.|
|Type 4, ASBR Summary||LSA used to locate the ASBR from a different Area.|
|Type 5, AS External||LSA that advertises network prefixes that were redistributed into OSPF.|
|Type 7, NSSA External||LSA for external network prefixes that were redistributed in a local NSSA Area.|
Each LSA has a 20-byte common LSA header.
|LS Age||Gives the time, in seconds, since the LSA originated.
The maximum age of the LSA is 3600 seconds; the refresh time is 1800 seconds.
If the LS age reaches 3600 seconds, the LSA must be removed from the database.
|LS Type||Represents the types of LSA.|
||Identifies the portion of the network that is being described by the LSA.
This ﬁeld changes according to the LS type.
|Adv. Router||RID of the router originating the LSA.|
|LS Sequence Number||Detects old or duplicate LSAs.|
|LS Checksum||Performs checksum on the LSA, not including LS age.
An LSA can be corrupted during ﬂooding or while kept in the memory, so this checksum is necessary.
- OSPF uses the sequence number to overcome problems caused by delays in LSA propagation in a network.
- The LSA sequence number is a 32-bit number for controlling versioning.
- When the originating router sends out LSAs, the LSA sequence number is incremented.
- If a router receives an LSA sequence that is greater than the one in the LSDB, it processes the LSA.
- If the LSA sequence number is lower than the one in the LSDB, the router deems the LSA old and discards the LSA.
LSA Age and Flooding
- Every OSPF LSA includes an age that is entered into the local LSDB and that will increment by 1 every second.
- When a router’s OSPF LSA age exceeds 1800 seconds (30 minutes) for its networks, the originating router advertises a new LSA with the LSA age set to 0.
- As each router forwards the LSA, the LSA age is incremented with a calculated (minimal) delay that reflects the link.
- If the LSA age reaches 3600 (60 minutes), the LSA is deemed invalid and is purged from the LSDB.
- The repetitive flooding of LSAs is a secondary safety mechanism to ensure that all routers maintain a consistent LSDB within an area.
Type 1 Router LSA
- Generated by each router for each area to which the router belongs.
- These packets describe the states of the router’s link to the area and are ﬂooded only within a particular area.
- A Type 1 LSA entry exists for each OSPF-enabled link (that is, every interface and its attached networks).
- All the router’s links in an area must be described in a single LSA.
- Type 1 LSAs are the essential building blocks within the LSDB.
- The Router LSA indicates whether it's an ABR, ASBR or an endpoint of a Virtual Link.
During the SPF tree calculation, network link types are one of the following:
|Transit||A transit network indicates that an adjacency was formed and that a DR was elected on that link.|
|Point-to-Point||P2P links indicate that an adjacency was formed on a network type that does not use a DR.
Interfaces using the OSPF point-to-point network type advertise two links:
One Link: P2P link type that identifies the OSPF neighbor RID for that segment.
Second Link: Stub network link that provides the subnet mask for that network.
|Stub||A stub network indicates that no neighbor adjacencies were established on that link.
Point-to-point and transit link types that did not become adjacent with another OSPF router are classified as a stub network link type.
When an OSPF adjacency forms, the link type changes to the appropriate type: point-to-point or transit.
|Link State ID /
|Should have the same value - RID of advertising router.|
|Flags||Bit V - Determine whether it's an endpoint of a virtual link.
Bit E - Determine whether this router is an ASBR
Bit B - Determine whether this router is an ABR
|Number of Links||Includes the number of router links.|
|Link ID, Link Data
|Link ID and Link Data - represents the 4-byte IP address value, depending on the network type
Type - represent the 4 types of router links
|Metric||Contains the OSPF cost of a specific link.|
Router LSA Example
A few important things to note:
- In normal situations, the LS Age field should be less than 1800.
- In the case of a Router LSA, the Link State ID field and Advertising Router should have the same value.
- This router is an ABR and has three router links.
With every point-to-point link, there is a stub link to provide the subnet mask of the link. In this example, two point-to-point links and one stub link are associated with these two point-to-point links because the network type is point-to-multipoint. So, if there are 300 point-to-point links, the router will generate 300 point-to-point links as well as 300 stub links to address the subnet associated with each point-to-point link.
The point-to-multipoint network type is a better choice in this case, for two reasons:
- Only one subnet is required per point-to-multipoint network.
- The size of the router LSA is cut in half because there will be only one stub link
to address the subnet on a point-to-multipoint network.
Type 2 Network LSA
- A Type 2 LSA represents a multi-access network segment that uses a DR.
- The DR always advertises the Type 2 LSA and identifies all the routers attached to that network segment.
- If a DR has not been elected, a Type 2 LSA is not present in the LSDB because the corresponding Type 1 transit link type LSA is a stub.
- Like Type 1 LSAs, Type 2 LSAs are not flooded outside the originating OSPF area.
- When the DR changes for a network segment, a new Type 2 LSA is created, causing SPF to run again within the OSPF area.
Network LSA Packet Format
|Link State ID||Interface IP Address of DR|
|Adv. Router||RID of DR|
|Network Mask||Indicates the network mask associated with the transit link|
|Attached Router||Includes the RIDs of each router attached to the transit link.
The DR router also lists itself in attached routers.
Network LSA Example
Two important things to remember here:
- The Link State ID field always contains the IP address of the DR.
- The Advertising Router field always contains the RID of the DR.
Type 3 Summary LSA
- Type 3 LSAs represent networks from other areas.
- The role of the ABRs is to participate in multiple OSPF areas and ensure that the networks associated with Type 1 LSAs are reachable in the non-originating OSPF areas.
- ABRs do not forward Type 1 or Type 2 LSAs into other areas.
- When an ABR receives a Type 1 LSA, it creates a Type 3 LSA referencing the network in the original Type 1 LSA; the Type 2 LSA is used to determine the network mask of the multi-access network. The ABR then advertises the Type 3 LSA into other areas.
- If an ABR receives a Type 3 LSA from Area 0 (the backbone), it regenerates a new Type 3 LSA for the nonbackbone area and lists itself as the advertising router, with the additional cost metric.
- An ABR advertises only one Type 3 LSA for a prefix, even if it is aware of multiple paths from within its area (Type 1 LSAs) or from outside its area (Type 3 LSAs). The metric for the best path is used when the LSA is advertised into a different area.
How metric is calculated?
The advertising router for Type 3 LSAs is the last ABR that advertises the prefix. The metric in the Type 3 LSA uses the following logic:
- If the Type 3 LSA is created from a Type 1 LSA, it is the total path metric to reach the originating router in the Type 1 LSA.
- If the Type 3 LSA is created from a Type 3 LSA from Area 0, it is the total path metric to the ABR plus the metric in the original Type 3 LSA.
- R4 does not know if the 10.56.1.0/24 network is directly attached to the ABR (R5) or if it is multiple hops away.
- R4 knows that its metric to the ABR (R5) is 1 and that the Type 3 LSA already has a metric of 1, so its total path metric is 2 to reach the 10.56.1.0/24 network.
- R3 does not know if the 10.56.1.0/24 network is directly attached to the ABR (R4) or if it is multiple hops away.
- R3 knows that its metric to the ABR (R4) is 65 and that the Type 3 LSA already has a metric of 2, so its total path metric is 67 to reach the 10.56.1.0/24 network.
Summary LSA Packet Format
|Link State ID||Network ID|
|Adv. Router||RID of ABR|
|Network Mask||This fields contains the network mask associated with the network|
|Metric||Represents the cost of the network|
|TOS||Normally set to 0|
Summary LSA Example
Two things to remember:
- The Link State ID ﬁeld is the network 18.104.22.168, and the network mask is /24.
- The Link State ID ﬁeld in summary LSAs Type 3 will always contain the network number that the summary LSA is generated for, along with the network mask.
Type 4 ASBR Summary LSA
- A Type 4 LSA locates the ASBR for a Type 5 LSA.
- A Type 5 LSA is flooded through the OSPF domain, and the only mechanism to identify the ASBR is the RID. Routers examine the Type 5 LSA, check to see whether the RID is in the local area, and if the ASBR is not local, they require a mechanism to locate the ASBR. Type 4 LSAs provide a way for routers to locate the ASBR when the router is in a different area from the ASBR.
- A Type 4 LSA is created by the first ABR, and it provides a summary route strictly for the ASBR of a Type 5 LSA.
Metric's logic to reach ASBR
The metric for a Type 4 LSA uses the following logic:
- When the Type 5 LSA crosses the first ABR, the ABR creates a Type 4 LSA with a metric set to the total path metric to the ASBR.
- When an ABR receives a Type 4 LSA from Area 0, the ABR creates a new Type 4 LSA with a metric set to the total path metric of the first ABR plus the metric in the original Type 4 LSA.
An ABR advertises only one Type 4 LSA for every ASBR, even if the ASBR advertises thousands of Type 5 LSAs.
ASBR Summary Packet Format
|Link State ID||RID of ASBR|
|Adv. Router||RID of ABR|
|Network Mask||Field must be 0|
|Metric||Cost to reach ASBR|
ASBR Summary LSA Example
Two important things to remember:
- The Link State ID is always the RID of the ASBR.
- The Network Mask field must always be 0 because this is the information about a router (ASBR), not a network,
Type 5 External LSA
- The external LSA defines routes to destinations external to the AS.
- The external route is flooded throughout the entire OSPF domain as a Type 5 LSA, except to stubby areas.
- Type 5 LSAs are not associated with a specific area and are flooded throughout the OSPF domain.
- Only the LSA age is modified during flooding for Type 2 external OSPF routes.
To install an external LSA in the RIB, two essential things must take place:
- The calculation router must see the ASBR through the intra-area or inter-area route, which means that it should have either a Type 1 Router LSA or a Type 4 ASBR Summary LSA, in case of multiple areas.
- The forwarding address must be known through an intra- or inter-area route.
External Route Calculation
|Intra-Area Externals||Inter-Area Externals|
|- ASBR can reach Link A in cost X
- I can reach ASBR via SPT in cost Y
- Implies I can reach Link A via SPT in cost X + Y
|- ASBR can reach Link A in cost X
- ABR can reach ASBR via SPT in cost Y
- I can reach ABR via SPT in cost Z
- Implies I can reach Link A via SPT in cost X + Y + Z
External LSA Packet Format
|Link State ID||External network number.|
|Adv. Router||RID of ASBR|
|Network Mask||Network mask of the external network.|
|Bit E||Specifies the external type.
If set, it's an External Type 2; otherwise, it's a Type 1.
|Forwarding Address||Indicates the address to which data traffic to the advertised network should be forwarded.
0.0.0.0 means that the traffic should be forwarded to the ASBR.
|External Route Tag||Not used by OSPF|
- Link State ID: External network number.
- Advertising Router: RID of ASBR.
- Netmask: Network mask of the external network.
- Bit E: Specifies the external type.
- If set, it’s an External Type 2; otherwise, it’s a Type 1.
- Forwarding Address: Indicates the address to which data traffic to the advertised network should be forwarded.
- 0.0.0.0 means that the traffic should be forwarded to the ASBR.
- External Route Tag: Not used by OSPF
External LSA Example
Things to remember:
- The Link State ID represents the external network number.
- The advertising router contains the RID of the ASBR.
- Metric Type: 2 means that the metric 20, remains the same throughout the OSPF domain.
- A forwarding address of 0.0.0.0 means that the trafﬁc should be forwarded directly to the ASBR.
- The route to the nonzero forwarding address must be known through an intra-area or interarea route; otherwise, the external route will not get installed in the routing table.
- Forwarding address is selected on ASBR using the following rules:
- If there is a loopback configured in the area, then the IP address of looback is selected as forwarding address.
- If first condition is not met, then IP address of first interface on the OSPF interface list is selected as forwarding address.
Type 7 NSSA External LSA
- An ASBR injects external routes as Type 7 LSAs in an NSSA.
- The ABR does not advertise Type 7 LSAs outside the originating NSSA, but it converts the Type 7 LSA into a Type 5 LSA for the other OSPF areas.
- If the Type 5 LSA crosses Area 0, the second ABR creates a Type 4 LSA for the Type 5 LSA.
R4# show ip ospf database ! Output omitted for brevity OSPF Router with ID (192.168.4.4) (Process ID 1) .. Summary ASB Link States (Area 1234) Link ID ADV Router Age Seq# Checksum 192.168.5.5 192.168.4.4 193 0x80000001 0x002A2C Type-5 AS External Link States Link ID ADV Router Age Seq# Checksum Tag 172.16.6.0 192.168.5.5 176 0x80000001 0x0045DB0
R5# show ip ospf database nssa-external OSPF Router with ID (192.168.5.5) (Process ID 1) Type-7 AS External Link States (Area 56) LS age: 122 Options: (No TOS-capability, Type 7/5 translation, DC, Upward) LS Type: AS External Link Link State ID: 172.16.6.0 (External Network Number ) Advertising Router: 192.168.6.6 LS Seq Number: 80000001 Checksum: 0xA371 Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) MTID: 0 Metric: 20 Forward Address: 10.56.1.6 External Route Tag: 0
NSSA LSA Packet Format
- Link State ID: External network number
- Advertising Router: RID of ASBR
Packet format for Type 7 LSA is very similar of Type 5 LSA.
NSSA LSA Example
Few things to remember:
- The P bit is used to tell the NSSA ABR whether to translate Type 7 LSAs into Type 5 LSAs.
- No Type 7/5 translation means P bit = 0
- Type 7/5 translation means P bit = 1
- If P bit = 0, the NSSA ABR must not translate this LSA into a Type 5 LSA. This happens when the NSSA ASBR is also an NSSA ABR.
- If P bit = 1, the NSSA ABR (if multiple NSSA ABRs exist, the one with the lowest
router ID) must translate this Type 7 LSA into a Type 5 LSA.
P stands for propagation. Basically, this is used for propagation control. The ABR makes the decision based on the value of this bit.
LSA Type 1
- Generated by every router in the OSPF domain
- Describes its directly connected links
- Used to build the graph for intra-area SPF
LSA Type 2
- Generated byDR on broadcast and non-broadcast network types
- Not flooded outside area
- Describes who is adjacent with the DR
- Link cost to the DR
- Implies link cost to all other adjacent to that DR
- Used to reduce redundant information in the database
LSA Type 5
- Generated by ASBR
- Flooded to all non-stub areas
- Describes routes ASBR is redistributing
LSA Type 3
- Generated by ABR
- Flooded from Area 0 into non-backbone areas and vice-versa
- Describes ABR’s reachability to links in other areas
- Includes cost, but hides ABR’s actual path to destination
- SPF not run for ABR advertised routes
- ABR can reach link A via SPT in cost X
- I can reach ABR via SPT in cost Y
- Implies I can reach link A via SPT in cost X + Y
LSA Type 4
- Generated by ABR
- Describes ABR’s reachability to ASBRs in other areas
- SPF not run to reach inter-area ASBR
- ABR can reach ASBR via SPT in cost X
- I can reach ABR via SPT in cost Y
- Implies I can reach ASBR via SPT in cost X + Y