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New Member

OSPF Point to Point network question

I just have a quesiton about the linkstate database. I have a point-to-point link between a couple routers(makes sense) and when I look at the database for the lsa type 1, it shows the router has 2 links in the area and one is a stub. I don't under stand how thats possible.

 

            OSPF Router with ID (4.4.4.4) (Process ID 1)

                Router Link States (Area 1)

  LS age: 1136
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 4.4.4.4
  Advertising Router: 4.4.4.4
  LS Seq Number: 80000007
  Checksum: 0xC738
  Length: 60
  Number of Links: 3

    Link connected to: another Router (point-to-point)
     (Link ID) Neighboring Router ID: 1.1.1.1
     (Link Data) Router Interface address: 192.168.41.2
      Number of TOS metrics: 0
       TOS 0 Metrics: 10

    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 192.168.41.0
     (Link Data) Network Mask: 255.255.255.0
      Number of TOS metrics: 0
       TOS 0 Metrics: 10

 

Can anyone explain to me what I'm seeing here? Thanks

1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Hi,What you see is perfectly

Hi,

What you see is perfectly normal - it is the way OSPF describes point-to-point links in LSA1. In short, an LSA1 can carry information about four types of adjacencies: unnumbered point-to-point link to another router identified by its RID; a link to a multi-access network identified by the IP address of its DR; a stub network identified by its IP address and netmask; a virtual link identified by the RID of the other virtual link endpoint. Note that out of these four types of adjacencies, the only one that actually carries addressing information is the stub network. All other adjacency types either refer to the adjacent router's RID or the IP of the DR. A point-to-point link with addressing is represented in OSPF as a pair of adjacencies: an unnumbered point-to-point link to the other router, and a stub network representing the IP subnet on the point-to-point link.

There was a thread about this topic some time ago where I explained this OSPF behavior in more detail. You may be interested to read it - beware, though, it is somewhat large.

https://supportforums.cisco.com/discussion/11608991/ospf-termonilogy-stub-network

Best regards,
Peter

3 REPLIES
New Member

Full config hostname R4!boot

Full config

 

hostname R4
!
boot-start-marker
boot-end-marker
!
!
no aaa new-model
memory-size iomem 5
no ip icmp rate-limit unreachable
ip cef
!
!
!
!
no ip domain lookup
!
multilink bundle-name authenticated
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
archive
 log config
  hidekeys
!
!
!
!
ip tcp synwait-time 5
!
!
!
!
interface FastEthernet0/0
 ip address 192.168.104.1 255.255.255.0
 ip ospf authentication message-digest
 ip ospf message-digest-key 1 md5 dallas
 ip ospf 1 area 1
 duplex auto
 speed auto
!
interface FastEthernet0/1
 ip address 192.168.41.2 255.255.255.0
 ip ospf authentication message-digest
 ip ospf message-digest-key 1 md5 dallas
 ip ospf 1 area 1
 duplex auto
 speed auto
!
router ospf 1
 router-id 4.4.4.4
 log-adjacency-changes
!
ip forward-protocol nd
!
!
no ip http server
no ip http secure-server
!
!
!
!
!
!
!
control-plane
!
!
!
!
!
!
!
!
!
!
line con 0
 exec-timeout 0 0
 privilege level 15
 logging synchronous
line aux 0
 exec-timeout 0 0
 privilege level 15
 logging synchronous
line vty 0 4
 login
!
!
end

R4#

 

Cisco Employee

Hi,What you see is perfectly

Hi,

What you see is perfectly normal - it is the way OSPF describes point-to-point links in LSA1. In short, an LSA1 can carry information about four types of adjacencies: unnumbered point-to-point link to another router identified by its RID; a link to a multi-access network identified by the IP address of its DR; a stub network identified by its IP address and netmask; a virtual link identified by the RID of the other virtual link endpoint. Note that out of these four types of adjacencies, the only one that actually carries addressing information is the stub network. All other adjacency types either refer to the adjacent router's RID or the IP of the DR. A point-to-point link with addressing is represented in OSPF as a pair of adjacencies: an unnumbered point-to-point link to the other router, and a stub network representing the IP subnet on the point-to-point link.

There was a thread about this topic some time ago where I explained this OSPF behavior in more detail. You may be interested to read it - beware, though, it is somewhat large.

https://supportforums.cisco.com/discussion/11608991/ospf-termonilogy-stub-network

Best regards,
Peter

New Member

Thank you, that answer was

Thank you, that answer was outstanding.

Dallas

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