Wednesday 20 June 2012

OSPF: LSA Type-4

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!



2 comments:

  1. Hi Usha,

    Thanks 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

    ReplyDelete
  2. One thought...

    You may want to change the title on your post from

    OPSF: LSA Type-4

    to

    OSPF: LSA Type-4


    Just sayin'... ;-)


    fjm/vrs

    ReplyDelete