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Community Member

Anyone explain Forward address in NSSA...

Hi,

Was going though some of the Blog like (IPexpert,http://lostintransit.se), as per the blog below the category to choos Forward address in OSPF database:

" first it will choose the highest loopback that is on the ASBR and that is also being advertised into the area. If there is no loopback, it will go with the highest interface that is running OSPF and also advertised into the NSSA".


While doing practice i found one wierd ouput which opposes the selection of NSSA forward address of Highest Physical interface ID as per blog. Below is the excercise i have done and as per my excercise " It Chose the interface with lower IP address ".

Can any one help me out to overcome my observation....

------------------------------------------------------------------------------------

NSSA between 16-18 router with 2 fastethernet link:

-----------------------------------------------------------------------------------

16-CAP-1#sh ip ospf int brief

Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C

Fa0/0        1     22              192.168.68.6/24    1     DR    1/1

Fa0/1        1     22              192.168.86.6/24    1     DR    1/1

SBI-BLR-7206-16-CAP-1#sh run | sec osp

router ospf 1

log-adjacency-changes

area 22 nssa

redistribute rip subnets route-map RED-RIP-2-OSPF

network 192.168.68.0 0.0.0.255 area 22

network 192.168.86.0 0.0.0.255 area 22

SBI-BLR-7206-16-CAP-1#sh route-map

route-map RED-RIP-2-OSPF, permit, sequence 10

  Match clauses:

    ip address (access-lists): 1

  Set clauses:

  Policy routing matches: 0 packets, 0 bytes

SBI-BLR-7206-16-CAP-1#sh access-li

SBI-BLR-7206-16-CAP-1#sh access-lists 1

Standard IP access list 1

    10 permit 172.16.5.0, wildcard bits 0.0.0.255 (8 matches)

18-CAP-1#sh ip ospf int brief                            

Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C

Se1/2        1     0               192.168.48.8/24    64    P2P   1/1

Lo0          1     0               172.16.8.8/24      1     LOOP  0/0

Fa0/0        1     22              192.168.68.8/24    1     BDR   1/1

Fa0/1        1     22              192.168.86.8/24    1     BDR   1/1

18-CAP-1#sh ip route ospf

     172.16.0.0/16 is variably subnetted, 4 subnets, 2 masks

O N2    172.16.5.0/24 [110/20] via 192.168.68.6, 00:31:52, FastEthernet0/0

O       172.16.4.4/32 [110/65] via 192.168.48.4, 00:32:07, Serial1/2

O IA    172.16.1.1/32 [110/66] via 192.168.48.4, 00:32:07, Serial1/2

     10.0.0.0/24 is subnetted, 1 subnets

O IA    10.0.0.0 [110/65] via 192.168.48.4, 00:32:07, Serial1/2

18-CAP-1#sh ip ospf dat

18-CAP-1#sh ip ospf database nss

18-CAP-1#sh ip ospf database nssa-external

            OSPF Router with ID (172.16.8.8) (Process ID 1)

        Type-7 AS External Link States (Area 22)

  Routing Bit Set on this LSA

  LS age: 69

  Options: (No TOS-capability, Type 7/5 translation, DC)

  LS Type: AS External Link

  Link State ID: 172.16.5.0 (External Network Number )

  Advertising Router: 172.16.6.6

  LS Seq Number: 80000019

  Checksum: 0x81BE

  Length: 36

  Network Mask: /24

    Metric Type: 2 (Larger than any link state path)

    TOS: 0

    Metric: 20

    Forward Address: 192.168.68.6

    External Route Tag: 0

18-CAP-1#sh ip ospf database ext          

18-CAP-1#sh ip ospf database external

            OSPF Router with ID (172.16.8.8) (Process ID 1)

        Type-5 AS External Link States

  LS age: 106

  Options: (No TOS-capability, DC)

  LS Type: AS External Link

  Link State ID: 172.16.5.0 (External Network Number )

  Advertising Router: 172.16.8.8

  LS Seq Number: 80000002

  Checksum: 0x2A33

  Length: 36

  Network Mask: /24

    Metric Type: 2 (Larger than any link state path)

    TOS: 0

    Metric: 20

    Forward Address: 192.168.68.6

    External Route Tag: 0

-----------------------------------------------------------------

Working fine for Loopback interface:

Created one loopback interface @ R16 were RIP is going to redistribute one external route:

16-CAP-1#sh ip ospf int brief

Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C

Lo16         1     22              172.16.16.16/24    1     LOOP  0/0

Fa0/0        1     22              192.168.68.6/24    1     DR    1/1

Fa0/1        1     22              192.168.86.6/24    1     DR    1/1

18-CAP-1#sh ip route ospf

     172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks

O N2    172.16.5.0/24 [110/20] via 192.168.86.6, 00:00:47, FastEthernet0/1

                      [110/20] via 192.168.68.6, 00:34:15, FastEthernet0/0

O       172.16.16.16/32 [110/2] via 192.168.86.6, 00:00:47, FastEthernet0/1

                        [110/2] via 192.168.68.6, 00:00:47, FastEthernet0/0

O       172.16.4.4/32 [110/65] via 192.168.48.4, 00:34:30, Serial1/2

O IA    172.16.1.1/32 [110/66] via 192.168.48.4, 00:34:30, Serial1/2

     10.0.0.0/24 is subnetted, 1 subnets

O IA    10.0.0.0 [110/65] via 192.168.48.4, 00:34:30, Serial1/2

18-CAP-1#sh ip ospf dat

18-CAP-1#sh ip ospf database ns

18-CAP-1#sh ip ospf database nssa-external

            OSPF Router with ID (172.16.8.8) (Process ID 1)

        Type-7 AS External Link States (Area 22)

  Routing Bit Set on this LSA

  LS age: 52

  Options: (No TOS-capability, Type 7/5 translation, DC)

  LS Type: AS External Link

  Link State ID: 172.16.5.0 (External Network Number )

  Advertising Router: 172.16.6.6

  LS Seq Number: 8000001A

  Checksum: 0x61B4

  Length: 36

  Network Mask: /24

    Metric Type: 2 (Larger than any link state path)

    TOS: 0

    Metric: 20

   Forward Address: 172.16.16.16

    External Route Tag: 0

18-CAP-1#sh ip ospf database ex           

            OSPF Router with ID (172.16.8.8) (Process ID 1)

        Type-5 AS External Link States

  LS age: 52

  Options: (No TOS-capability, DC)

  LS Type: AS External Link

  Link State ID: 172.16.5.0 (External Network Number )

  Advertising Router: 172.16.8.8

  LS Seq Number: 80000003

  Checksum: 0xA29

  Length: 36

  Network Mask: /24

    Metric Type: 2 (Larger than any link state path)

    TOS: 0

    Metric: 20

   Forward Address: 172.16.16.16

    External Route Tag: 0

----------------------------------------------------------

After tweaking the ip address of it choose the lowest IP address as per my output:

SBI-BLR-7206-18-CAP-1#sh ip ospf int brief

Fa0/1        1     22              192.168.68.8/24    1     DR    1/1

Fa0/0        1     22              192.168.86.8/24    1     DR    1/1

SBI-BLR-7206-18-CAP-1#sh ip ospf database external     

            OSPF Router with ID (172.16.8.8) (Process ID 1)

        Type-5 AS External Link States

  LS age: 701

  Options: (No TOS-capability, DC)

  LS Type: AS External Link

  Link State ID: 172.16.5.0 (External Network Number )

  Advertising Router: 172.16.8.8

  LS Seq Number: 80000001

  Checksum: 0x2C32

  Length: 36

  Network Mask: /24

    Metric Type: 2 (Larger than any link state path)

    TOS: 0

    Metric: 20

    Forward Address: 192.168.68.6

    External Route Tag: 0

SBI-BLR-7206-18-CAP-1#sh ip ospf database nssa-external

            OSPF Router with ID (172.16.8.8) (Process ID 1)

        Type-7 AS External Link States (Area 22)

  Routing Bit Set on this LSA

  LS age: 717

  Options: (No TOS-capability, Type 7/5 translation, DC)

  LS Type: AS External Link

  Link State ID: 172.16.5.0 (External Network Number )

  Advertising Router: 172.16.6.6

  LS Seq Number: 80000001

  Checksum: 0xB1A6

  Length: 36

  Network Mask: /24

    Metric Type: 2 (Larger than any link state path)

    TOS: 0

    Metric: 20

    Forward Address: 192.168.68.6

    External Route Tag: 0

1 REPLY
Cisco Employee

Anyone explain Forward address in NSSA...

Hi Afeez,

RFC 1587 that codifies the NSSA area type has the following to say about the forwarding address selection:

   If the network between the NSSA AS boundary router and the adjacent
   AS is advertised into OSPF as an internal OSPF route, the forwarding
   address should be the next hop address but if the intervening network
   is not advertised into OSPF as an internal OSPF route, the forwarding
   address should be any one of the router's active OSPF interface
   addresses.

Note that the RFC 1587 does not mandate which exact address out of potentially many IP addresses an NSSA ASBR has will be used as the forwarding address. Hence, according to the standard, this is left to individual implementors' choice.

Support for updated NSSA RFC 3101 has been added only to most recent IOSes. Even this RFC is not more specific in this regard:

   When a router is forced to pick a forwarding address for a Type-7
   LSA, preference should be given first to the router's internal
   addresses (provided internal addressing is supported).  If internal
   addresses are not available, preference should be given to the
   router's active OSPF stub network addresses.  These choices avoid the
   possible extra hop that may happen when a transit network's address
   is used.  When the interface whose IP address is the LSA's forwarding
   address transitions to a Down state (see [OSPF] Section 9.3), the
   router must select a new forwarding address for the LSA and then re-
   originate it.  If one is not available the LSA should be flushed.

So even though different articles out there about Cisco's NSSA may state different things, RFCs as the authoritative source do not mandate any particular way of selecting the highest or the lowest ASBR's IP address. The only mandated requirement is that if the network connecting the ASBR and the next hop of the redistributed routes is added to OSPF as an internal network, the next hop will be preserved. Otherwise, any IP address on the ASBR's active interfaces advertised in OSPF shall be used.

Best regards,

Peter

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