Why does OSPF need a Type 4 LSA at all?
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A Type 5 LSA says "prefix X, reach it via ASBR 3.3.3.3", but routers in other areas do not have 3.3.3.3's Type 1 LSA, so they cannot compute their cost to it. The ABR fills the gap with a Type 4 that advertises the ASBR's router ID and the ABR's cost to it. Routers then add their cost to the ABR.
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Example
R3 (3.3.3.3) is an ASBR in area 0 originating a default route. R2 (2.2.2.2) is the ABR to area 1.
R1 (1.1.1.1) is inside area 1.
R1# show ip ospf database
OSPF Router with ID (1.1.1.1) (Process ID 1)
Router Link States (Area 1)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 312 0x80000004 0x00B1C2 2
2.2.2.2 2.2.2.2 298 0x80000003 0x0047E1 1
Summary Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
10.0.23.0 2.2.2.2 301 0x80000001 0x00D4A8
Summary ASB Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
3.3.3.3 2.2.2.2 301 0x80000001 0x00C6B3
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
0.0.0.0 3.3.3.3 640 0x80000002 0x00E5F1 1The Type 5 names 3.3.3.3. The only thing in area 1 that mentions 3.3.3.3 is the Type 4 from R2.
Without it, R1 would ignore the external route.
Where Type 4 is not needed
- Inside the ASBR's own area, where its Type 1 LSA is present.
- In stub and NSSA areas, which do not get Type 5 LSAs at all.
- For an NSSA, the translating ABR becomes the advertising router of the Type 5. Other areas then get a Type 4 for that ABR as the ASBR.
They will ask next
What is the forwarding address in a Type 5 LSA?
A field that can tell routers to send traffic to a next hop other than the ASBR, for example another router on a shared segment. When it is 0.0.0.0, traffic goes to the ASBR itself. When it is set, the address must be reachable through an OSPF intra- or inter-area route or the LSA is not used.
Traps
- Saying Type 4 carries external prefixes. It only carries the ASBR's router ID and a cost.