In OSPF, an ASBR inside area 1 redistributes 198.51.100.0/24; which LSAs carry it to a router in area 2, and what changes if area 1 is an NSSA?
answer
- one LSA for prefix, one for owner
- the E bit stays home
- ABRs vouch for the ASBR
- translation changes the advertising router
- forwarding address survives translation
basics
~20 sThe ASBR's type 5 floods unchanged into area 2, while ABRs originate type 4 LSAs so area 2 can reach that ASBR. In an NSSA the ASBR originates a type 7, and a translating ABR originates the type 5 itself.
solid answer
~50 sIn a normal area the ASBR originates a **type 5** for `198.51.100.0/24`, and it floods unchanged through areas 1, 0 and 2 with the ASBR as advertising router. Routers in area 1 locate the ASBR from its router-LSA, whose **E bit** is set. That router-LSA stays in area 1, so area 1's ABR originates a **type 4** for the ASBR into area 0, and area 2's ABR originates its own type 4 into area 2. A router in area 2 adds its cost to that ABR to the type 4's cost; with no route to the ASBR it ignores the type 5. If area 1 is an **NSSA**, the ASBR originates a **type 7** with the P-bit set. The translating NSSA ABR originates the type 5 with **itself** as advertising router, keeping the forwarding address, so area 2 needs a type 4 for that ABR and a route to the forwarding address.
go deeper
Recall that external routes use their own LSA type and that other areas need to know where the router announcing them is.
Trace type 5 flooding unchanged, the E bit inside the ASBR's area, and the type 4 each ABR originates for the next area.
Diagnose a missing external route by checking the type 4, the forwarding address, and in an NSSA the P-bit and which ABR translates.
Judge where redistribution points belong in an area design, and what NSSA translation and aggregation do to exit choice for the rest of the network.
## The setup A 300-router network runs area 0 and areas 1, 2 and 3. Router **E1**, inside area 1, learns `198.51.100.0/24` from a static route toward a partner and redistributes it into OSPF, which makes E1 an **ASBR**. **B1** is the ABR between area 1 and area 0; **B2** is the ABR between area 0 and area 2; **R2** is an internal router in area 2. The question is what R2 needs in its database to forward to `198.51.100.0/24`. ## Case 1: area 1 is a normal area 1. **E1 originates one type 5** AS-external-LSA: Link State ID `198.51.100.0`, mask `/24`, an external metric of type 1 or 2, advertising router E1. Here the forwarding address is `0.0.0.0`, meaning "send it to E1". 2. **Type 5 floods AS-wide unchanged.** B1 and B2 pass it on as-is; R2 sees E1 as its advertising router. 3. **Area 1 finds E1 from its router-LSA**, where E1 sets the **E bit**. Nobody outside area 1 sees that LSA. 4. **B1 originates a type 4** summary-LSA into area 0, Link State ID = E1's Router ID, metric = B1's cost to E1. 5. **B2 originates its own type 4** into area 2, with its own Router ID and its own cost to E1: the backbone path to B1 plus B1's advertised cost. 6. **R2 computes its route.** RFC 2328 section 16.4 first looks up a routing table entry for the ASBR named in the type 5; with none, the LSA is skipped. With the type 4 from B2, R2's cost to E1 is its cost to B2 plus the type 4 metric. | LSA | Originated by | Seen in area 2? | Role for R2 | |---|---|---|---| | Type 1 from E1, E bit set | E1 | No | Locates E1 inside area 1 only | | Type 4 into area 0 | B1 | No | Lets backbone routers reach E1 | | Type 4 into area 2 | B2 | Yes | Lets R2 reach E1 | | Type 5 for `198.51.100.0/24` | E1 | Yes | The external destination | If E1 sets a **non-zero forwarding address**, R2 must also hold an intra-area or inter-area route to that address, and traffic goes there rather than to E1. The external metric type then decides the final cost: for **type 1** it is R2's internal cost to E1 plus the external metric; for **type 2** the external metric is compared first, and the internal cost only breaks ties. ## Case 2: area 1 is an NSSA Type 5 LSAs may not originate inside an NSSA, so RFC 3101 changes the chain: 1. **E1 originates a type 7** for `198.51.100.0/24`, flooded inside area 1 only. To have it leave the area, E1 sets the **P-bit** and must give a **non-zero forwarding address**: the next hop toward the partner when that sits on a network attached to the NSSA, otherwise one of E1's own active interface addresses. 2. **One NSSA ABR translates.** With a single ABR, B1 does it. With several, an ABR configured with `NSSATranslatorRole` = Always translates; otherwise the candidate with the highest Router ID is elected and announces itself with the **Nt** bit in its router-LSA. 3. **B1 originates a new type 5**: same prefix, mask, metric, metric type, route tag and forwarding address, but **B1** is the advertising router. The type 7 never leaves area 1. 4. **B1 now acts as an ASBR** for the backbone: RFC 3101 has every NSSA ABR set the E bit in its router-LSAs for its attached non-stub areas such as area 0, translating or not. B2 originates a type 4 for **B1** into area 2; nobody originates a type 4 for E1, because RFC 3101 says an NSSA's border routers never do. 5. **R2 needs two routes:** one to B1 (via the type 4) so the type 5 is usable, and one to the forwarding address, which lies in area 1 and is reachable through B2's type 3 summary. Traffic follows the forwarding address back toward E1. If B1 aggregates several type 7s with a **type 7 address range**, the translated type 5 carries forwarding address `0.0.0.0`, and traffic goes to B1 first. ## What goes wrong in practice - **External route missing in area 2 while the type 5 is present:** look for the type 4. A missing type 4 means no ABR has a route to the ASBR. - **Route missing everywhere outside an NSSA:** check the P-bit and the forwarding address of the type 7; either one stops translation. - **Area 2 is a stub area:** neither type 5 nor type 4 enters it, and R2 reaches `198.51.100.0/24` through its default route.
- Why does an OSPF router need a route to the ASBR even when the type 5 LSA has a non-zero forwarding address?RFC 2328's external route calculation first looks up a routing table entry for the advertising ASBR and skips the LSA when there is none. Only then does it resolve the forwarding address, which must be reachable by an intra-area or inter-area route. Both lookups must succeed before the external route is installed.
- In an OSPF NSSA with two ABRs, which one translates type 7 LSAs into type 5?Under RFC 3101, an ABR configured with NSSATranslatorRole set to Always translates unconditionally. Among Candidate ABRs, one stays disabled if another reachable border router has the Nt bit set or a higher Router ID; otherwise it is elected. A displaced translator keeps translating for TranslatorStabilityInterval, 40 seconds by default.
saying these in an interview costs you the question
- The ABR turns a type 5 LSA into a type 3 summary for the next area.
- Type 4 LSAs carry the external prefix itself to other areas.
- A router can use a type 5 LSA without any route to its ASBR.
- An NSSA's type 7 LSA is flooded into area 0 and translated there.
- Every NSSA ABR translates every type 7 LSA, whatever its P-bit.
- After NSSA translation the type 5 still names the internal ASBR as advertising router.