Let’s Review: OSPF LSA Types
With my Written exam for CCIE: R/S coming up soon (Cisco Live 2015) I figured this would be a good time to back over some basics, and when it comes to OSPF what is more basic than OSPF LSA Types!
LSA Type 1 – These are also known as Router LSA’s, every router participating in OSPF will generate one of these, and Type 1 LSA’s are flooded within the area that the router belongs. So, what is it the Type 1 LSA tells us:
- Certain capabilities, is the router a Border router or ASBR. (Respective E-bit and B-bit)
- Does the router terminate a virtual link (V-bit)
- Number of links
- Link types – PtP, Virtual, etc
- Link Cost/Metric
So lot’s of good information in LSA-1, think of these as a ‘blue-print’ of the router, describing what is has and what it is doing.
LSA Type 2 – Network LSA’s. These LSA types are generated by the DR (Designated Router) and one type-2 LSA is generated for every network with two or more OSPF neighbors. You can expect to find these when you multiple devices connected to a broadcast network (A.K.A. LAN) or on an NMBA network (Frame Relay/DMVPN). Type-2 LSA’s will include information about the attached routers along with information of the network segment. This LSA is just an informative one, letting you know what other Routers you are connected to along, no metric/cost information is included in a Type-2 LSA.
LSA Type 3 – Summary LSA’s. Type-3’s are generated by ABR’s and are used to extend reachable to other OSPF areas. Remember LSA Type-1’s are limited to the area that they originate them in. They also designate Inter-Area connectivity as opposed to Intra-Area connectivity. A good rule of thumb is for every route in the routing table, there should be an associated Type-3 LSA in the OSPF LSDB.
LSA Type 4 – Think of these as Type-3 LSAs however instead of being generated by ABRs they are generated from ASBRs. Type-3 & Type-4 LSA’s use the exact same format, however the differentiating factor is the fact, a Type-4 Link State ID is the Router-ID of the ASBR. (Where-as a Type 3 LSA will/may have a Link State ID of the IP Network)
Similarities in Type-3 & 4 LSAs
- Cost/Metric information is included in these LSAs (unlike Type-2 LSAs but those do serve a different purpose)
- Contain routing prefix/host information for reach-ability purposes.
LSA Type 5 – AS-External LSAs. Type 5 LSAs are also originated by ASBRs and represent external routes from another Autonomous System (or routing protocol) typically done via redistribution. Type-5 LSAs will contain metric/cost information similar to that of Type 3s & 4s, however because this information external from the OSPF routing domain there is some additional information included within this type of LSA.
- External Metric (E-bit) – Designates the external metric of the route. Used for Type-2 External routes, where as Type-1 external routes do not include this information (IE: Set to 0, so the field is included just the value is set to 0)
- Forwarding Address – Specifies where transit traffic should be sent, in some cases this can be (intentionally) set to all 0’s in which case the forwarding address will be that of the Router-ID of the originating router.
- Route Tag – Used to set tagging information of the route itself (typically for mutual redistribution purposes). 32-bit field.
- Remember redistributing static or connected routes also generates Type-5 LSAs, the routes do not always have to originate from another routing protocol.
LSA Type 7 – External LSAs, only found when using NSSA Areas (Not So Stubby Area). So if Type-7 LSAs are only found in NSSAs then how does this reach-ability information make it to other areas? Well the NSSA Border router will translate these Type-7 LSAs to Type-5s for the rest OSPF network/Areas, this is designated by the P-bit / Propagation bit.
Back to studying I go!