Difference between LSA 5 and LSA 7
Difference between LSA 5 and LSA 7
To provide the ability of carrying external routes originated in the NSSA, a new LSA type was defined, Type 7 LSA. It has the structure and semantics of a Type 5 (External) LSA, with a two differences:
1.Type 7 LSAs can be originated and propagated within the NSSA, they do not cross area borders like Type 5 LSAs do.
2.Type 5 LSAs are not supported in NSSA; they can be neither originated nor propagated in NSSA.
Translation of Type 7 to Type 5 LSA at the ABR
An ASBR in a NSSA injects its external routes as Type 7 LSAs (The NSSA does not support Type 5 Externals). The ABR can (optionally) translate those to regular Type 5 LSAs and inject them into the backbone as Type 5 Externals. At this point the these LSAs are indistinguishable from other Type 5 LSAs that may have come from other source. As shown on the figure below, these Type 5 LSAs, along with those from other sources, are flooded on all other non-stub areas.
When the NSSA is connected to the backbone via more than one ABR, the ABR with the highest Router ID is selected to perform the Type 7 à Type 5 translation. If that ABR fails, the ABR with next-highest Router ID is selected.
TYPE 7–>TYPE 5
r2#sh ip ospf data
OSPF Router with ID (2.2.2.2) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 1247 0×80000006 0xAFDD 3
2.2.2.2 2.2.2.2 437 0×80000003 0×3668 2
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
3.3.3.3 1.1.1.1 1473 0×80000001 0x6D7E
4.4.4.4 2.2.2.2 674 0×80000001 0x21C2
4.4.4.4 151.1.1.1 1440 0×80000001 0xF35D
5.5.5.5 2.2.2.2 232 0×80000001 0xD441
5.5.5.5 151.1.1.1 1397 0×80000001 0xA7DB
192.168.13.0 1.1.1.1 1639 0×80000001 0xACD5
192.168.24.0 2.2.2.2 988 0×80000004 0xF61
192.168.24.0 151.1.1.1 1435 0×80000004 0xE1FB
192.168.25.0 2.2.2.2 236 0×80000007 0xDFC2
192.168.25.0 151.1.1.1 1401 0×80000003 0xBA59
Router Link States (Area 1)
Link ID ADV Router Age Seq# Checksum Link count
2.2.2.2 2.2.2.2 684 0×80000008 0×4435 2
4.4.4.4 4.4.4.4 679 0×80000009 0x4FFE 3
Summary Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
0.0.0.0 2.2.2.2 812 0×80000002 0x73C1
3.3.3.3 151.1.1.1 1474 0×80000001 0xA470
5.5.5.5 151.1.1.1 1398 0×80000001 0xA7DB
192.168.13.0 151.1.1.1 1541 0×80000001 0xE3C7
192.168.25.0 151.1.1.1 1408 0×80000001 0xBE57
Router Link States (Area 3)
Link ID ADV Router Age Seq# Checksum Link count
2.2.2.2 2.2.2.2 245 0×80000005 0xAF54 1
5.5.5.5 5.5.5.5 241 0×80000008 0x15AC 2
Net Link States (Area 3)
Link ID ADV Router Age Seq# Checksum
192.168.25.2 2.2.2.2 245 0×80000001 0x6E1D
Summary Net Link States (Area 3)
Link ID ADV Router Age Seq# Checksum
1.1.1.1 2.2.2.2 440 0×80000002 0x4F99
3.3.3.3 2.2.2.2 440 0×80000002 0x752B
3.3.3.3 151.1.1.1 1475 0×80000001 0xA470
4.4.4.4 2.2.2.2 440 0×80000002 0xC418
4.4.4.4 151.1.1.1 1441 0×80000001 0xF35D
192.168.12.0 2.2.2.2 440 0×80000005 0x373E
192.168.13.0 2.2.2.2 440 0×80000002 0xB482
192.168.13.0 151.1.1.1 1573 0×80000001 0xE3C7
192.168.24.0 2.2.2.2 440 0×80000005 0xB2B6
192.168.24.0 151.1.1.1 1436 0×80000004 0xE1FB
Type-7 AS External Link States (Area 3)
Link ID ADV Router Age Seq# Checksum Tag
25.25.24.0 5.5.5.5 159 0×80000001 0x7CA4 0
55.55.55.0 5.5.5.5 238 0×80000003 0×5468 0
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
25.25.24.0 2.2.2.2 160 0×80000001 0x6BCB 0
55.55.55.0 2.2.2.2 234 0×80000001 0x478D 0
Note: In the works , usually in order to reduce the depth of the routing table, we often have to restrain LSA7 on the ABR to LSA5 conversion, in the past to have a small trick, and now after the 12.0 ios, there is a direct command to the curb.