OSPF: LSA Type-4
· Last week, I attended one interview and came across one interesting scenario in OSPF.
· He gave me one topology and asked me how LSA Type-4 gets generated, its Link-ID and Router-ID, what is the P-Bite value in LSA-Type-4 and what is the Forwarding address value etc..
· I came home and labbed it up…
Topology:
EIGRP ßà OSPF redistribution configs on R4:
R4#show run | begin router
router eigrp 100
redistribute ospf 1 metric 1 1 1 1 1
network 100.1.45.4 0.0.0.0
no auto-summary
eigrp router-id 4.4.4.4
!
router ospf 1
router-id 4.4.4.4
log-adjacency-changes
area 14 nssa
redistribute eigrp 100 subnets
network 100.1.14.4 0.0.0.0 area 14
!
R4#show ip route 5.5.5.5
Routing entry for 5.0.0.0/8
Known via "eigrp 100", distance 90, metric 2297856, type internal
Redistributing via ospf 1, eigrp 100
Advertised by ospf 1 subnets
Last update from 100.1.45.5 on Serial1/0, 00:01:45 ago
Routing Descriptor Blocks:
* 100.1.45.5, from 100.1.45.5, 00:01:45 ago, via Serial1/0
Route metric is 2297856, traffic share count is 1
Total delay is 25000 microseconds, minimum bandwidth is 1544 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1
R4#ping 5.5.5.5
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 5.5.5.5, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 16/47/100 ms
R4#
Now, let’s check OSPF Database in R4 (NSSA ASBR):
R4#show ip ospf database nssa-external
OSPF Router with ID (4.4.4.4) (Process ID 1)
Type-7 AS External Link States (Area 14)
LS age: 50
Options: (No TOS-capability, Type 7/5 translation, DC)
LS Type: AS External Link
Link State ID: 5.5.5.5 (External Network Number )
Advertising Router: 4.4.4.4
LS Seq Number: 80000001
Checksum: 0x3FBB
Length: 36
Network Mask: /32
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 100.1.14.4
External Route Tag: 0
LS age: 279
Options: (No TOS-capability, Type 7/5 translation, DC)
LS Type: AS External Link
Link State ID: 100.1.45.0 (External Network Number )
Advertising Router: 4.4.4.4
LS Seq Number: 80000001
Checksum: 0x106C
Length: 36
Network Mask: /24
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 100.1.14.4
External Route Tag: 0
R4#
Observation:
· P - bit is set for the NSSA external routes (Type 7/5 translation)
· Forward Address is set to R4-R1 link address 100.1.14.4
· RFC 1587 - The OSPF NSSA Option
When an NSSA internal AS boundary router originates a type-7 LSA that it wants to be translated into a type-5 LSA by the NSSA area border router (and subsequently flooded into the backbone), it must set the propagate (P)- bit in the LS header's options field and add a valid forwarding address in the type-7 LSA.
Now let’s see the OSPF Database in NSSA ABR, R1:
R1#show ip ospf database nssa-external
OSPF Router with ID (1.1.1.1) (Process ID 1)
Type-7 AS External Link States (Area 14)
Routing Bit Set on this LSA
LS age: 96
Options: (No TOS-capability, Type 7/5 translation, DC)
LS Type: AS External Link
Link State ID: 5.5.5.5 (External Network Number )
Advertising Router: 4.4.4.4
LS Seq Number: 80000001
Checksum: 0x3FBB
Length: 36
Network Mask: /32
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 100.1.14.4
External Route Tag: 0
Routing Bit Set on this LSA
LS age: 325
Options: (No TOS-capability, Type 7/5 translation, DC)
LS Type: AS External Link
Link State ID: 100.1.45.0 (External Network Number )
Advertising Router: 4.4.4.4
LS Seq Number: 80000001
Checksum: 0x106C
Length: 36
Network Mask: /24
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 100.1.14.4
External Route Tag: 0
R1#
Observation:
· Forwarding address remains the same, 100.1.14.4
Let’s see whether R1 NSSA ABR is advertising LSA-Type-4 (we know that LSA-Type 4 is generated by an ABR):
R1#show ip ospf database asbr-summary
OSPF Router with ID (1.1.1.1) (Process ID 1)
R1#
· Surprisingly no Type-4 LSA generated by R1
R2#show ip ospf database asbr-summary
OSPF Router with ID (2.2.2.2) (Process ID 1)
Summary ASB Link States (Area 23)
LS age: 805
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(AS Boundary Router)
Link State ID: 1.1.1.1 (AS Boundary Router address)
Advertising Router: 2.2.2.2
LS Seq Number: 80000001
Checksum: 0x935C
Length: 28
Network Mask: /0
TOS: 0 Metric: 64
R2#
R3#show ip ospf database asbr-summary
OSPF Router with ID (3.3.3.3) (Process ID 1)
Summary ASB Link States (Area 23)
Routing Bit Set on this LSA
LS age: 972
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(AS Boundary Router)
Link State ID: 1.1.1.1 (AS Boundary Router address)
Advertising Router: 2.2.2.2
LS Seq Number: 80000001
Checksum: 0x935C
Length: 28
Network Mask: /0
TOS: 0 Metric: 64
R3#
Observation:
· R2 (ABR for Area 23) is generating an asbr-summary LSA for Area 23
· Link-ID is 1.1.1.1 (AS Boundary Router Address)
· That means, since R1 is converting Type7 into Type-5, it is acting as an ASBR also!!!
· Since R1 itself is an ASBR, it need not generate LSA Type-4 for area 0, that’s why I didn’t see any asbr-LSAs in R1’s OSPF database.
R2#show ip ospf database external
OSPF Router with ID (2.2.2.2) (Process ID 1)
Type-5 AS External Link States
Routing Bit Set on this LSA
LS age: 511
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 5.5.5.5 (External Network Number )
Advertising Router: 1.1.1.1
LS Seq Number: 80000001
Checksum: 0x2EE2
Length: 36
Network Mask: /32
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 100.1.14.4
External Route Tag: 0
Routing Bit Set on this LSA
LS age: 734
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 100.1.45.0 (External Network Number )
Advertising Router: 1.1.1.1
LS Seq Number: 80000001
Checksum: 0xFE93
Length: 36
Network Mask: /24
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 100.1.14.4
External Route Tag: 0
R2#
R3#show ip ospf database external
OSPF Router with ID (3.3.3.3) (Process ID 1)
Type-5 AS External Link States
Routing Bit Set on this LSA
LS age: 677
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 5.5.5.5 (External Network Number )
Advertising Router: 1.1.1.1
LS Seq Number: 80000001
Checksum: 0x2EE2
Length: 36
Network Mask: /32
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 100.1.14.4
External Route Tag: 0
Routing Bit Set on this LSA
LS age: 901
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 100.1.45.0 (External Network Number )
Advertising Router: 1.1.1.1
LS Seq Number: 80000001
Checksum: 0xFE93
Length: 36
Network Mask: /24
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 100.1.14.4
External Route Tag: 0
R3#
Observation:
· Forwarding address remains the same, 100.1.14.4
R4# show ip ospf border-routers
OSPF Process 1 internal Routing Table
Codes: i - Intra-area route, I - Inter-area route
i 1.1.1.1 [64] via 100.1.14.1, Serial1/1, ABR/ASBR, Area 14, SPF 3
R4#
R1#show ip ospf border-routers
OSPF Process 1 internal Routing Table
Codes: i - Intra-area route, I - Inter-area route
i 4.4.4.4 [64] via 100.1.14.4, Serial1/1, ASBR, Area 14, SPF 6
i 2.2.2.2 [64] via 100.1.12.2, Serial1/0, ABR, Area 0, SPF 7
R1#
R2#show ip ospf border-routers
OSPF Process 1 internal Routing Table
Codes: i - Intra-area route, I - Inter-area route
i 1.1.1.1 [64] via 100.1.12.1, Serial1/0, ABR/ASBR, Area 0, SPF 7
R2#
R3#show ip ospf border-routers
OSPF Process 1 internal Routing Table
Codes: i - Intra-area route, I - Inter-area route
I 1.1.1.1 [128] via 100.1.23.2, Serial1/1, ASBR, Area 23, SPF 5
i 2.2.2.2 [64] via 100.1.23.2, Serial1/1, ABR, Area 23, SPF 5
R3#
Observation:
· The output clearly shows that R1 is acting an ASBR also!
Hi Usha,
ReplyDeleteThanks for the post! I learned a few things:
* R1 is technically both an ABR and an ASBR;
* the forward address & Type 7/5 translation data are actually called out in the "show ip ospf database nssa-external" command; and
* a good reason to use the "show ip ospf border-routers" command.
Thanks!
fmj/vrs
One thought...
ReplyDeleteYou may want to change the title on your post from
OPSF: LSA Type-4
to
OSPF: LSA Type-4
Just sayin'... ;-)
fjm/vrs