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I've consulted the spec https://www.ietf.org/rfc/rfc2328.txt and have googled to the end of the internet but I'm confused on OSPF Point-To-Point.

If I have the following:

 Link State ID: 4.4.4.4 Advertising Router: 4.4.4.4

And then in my Router-LSA Update I have the following:

 (Link ID) Neighboring Router ID: 2.2.2.2 (Link Data) Router Interface address: 10.0.1.1 Number of TOS metrics: 0 TOS 0 Metrics: 64

I know this means that OSPF Router 4.4.4.4 has a point-to-point connection to 2.2.2.2 via the interface 10.0.1.1 However, my question is as follows: Does the 10.0.1.1. interface reside on the 4.4.4.4 router or the 2.2.2.2 router?

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  • Need more info. See OSPF Implementation By Diane Teare, Rick Graziani, Bob Vachon. Commented Sep 23, 2016 at 1:47

2 Answers 2

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What you are seeing is the neighbor Router ID and interface IP address. This is explained in the RFC on pages 12 and 13, particularly the point-to-point link of figure 1a:

 **FROM** * |RT1|RT2| +---+Ia +---+ * ------------ |RT1|------|RT2| T RT1| | X | +---+ Ib+---+ O RT2| X | | * Ia| | X | * Ib| X | | Physical point-to-point networks

A router will know its own interface IP addresses, but it needs to know the interface IP address of its neighbor. If it knows the interface IP address of its neighbor, then it knows which interface to use to reach that neighbor.

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This is the IP address on router 4.4.4.4, the advertising router

With OSPF P2P links, you should see two links for each P2P adjacent router within the router LSA.

The first link is of link type P2P with a Link ID of the other router's router ID (2.2.2.2) and Link Data should be the IP address of the advertising router on the adjacent subnet (10.0.1.1)

You will also see a second link, which is a Stub Network link to the subnet on the adjacent interface. This will have a link ID of the network (not sure but could be 10.0.1.0 in this case) and Link Data will contain the subnet mask (255.255.255.0?)

This is all in the RFC 2328 page 129

This may seem confusing as you would expect the IP address to be that of the adjacent router. This information is not needed when calculating the SPF as OSPF only needs to see a connection to the other router and any onward networks reachable by that router. When the router needs to actually forward a packet to an adjacent router it uses the neighbor IP address's associated MAC address to forward the packet onwards. This IP address can be seen using show ip ospf neighbor on a Cisco router.

Set this up in the lab:

R1: interface GigabitEthernet0/0 ip address 10.0.1.1 255.255.255.252 ip ospf network point-to-point router ospf 1 router-id 4.4.4.4 network 10.0.0.0 0.255.255.255 area 0 R2: interface GigabitEthernet0/1 ip address 10.0.1.2 255.255.255.252 ip ospf network point-to-point router ospf 1 router-id 2.2.2.2 network 10.0.0.0 0.255.255.255 area 0 R1#sh ip ospf data router self-originate OSPF Router with ID (4.4.4.4) (Process ID 1) Router Link States (Area 0) LS age: 66 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: 80000004 Checksum: 0x865D Length: 48 Number of Links: 2 Link connected to: another Router (point-to-point) (Link ID) Neighboring Router ID: 2.2.2.2 (Link Data) Router Interface address: 10.0.1.1 Number of MTID metrics: 0 TOS 0 Metrics: 1 Link connected to: a Stub Network (Link ID) Network/subnet number: 10.0.1.0 (Link Data) Network Mask: 255.255.255.252 Number of MTID metrics: 0 TOS 0 Metrics: 1 R2#sh ip ospf data router self-originate OSPF Router with ID (2.2.2.2) (Process ID 1) Router Link States (Area 0) LS age: 105 Options: (No TOS-capability, DC) LS Type: Router Links Link State ID: 2.2.2.2 Advertising Router: 2.2.2.2 LS Seq Number: 80000004 Checksum: 0x7179 Length: 48 Number of Links: 2 Link connected to: another Router (point-to-point) (Link ID) Neighboring Router ID: 4.4.4.4 (Link Data) Router Interface address: 10.0.1.2 Number of MTID metrics: 0 TOS 0 Metrics: 1 Link connected to: a Stub Network (Link ID) Network/subnet number: 10.0.1.0 (Link Data) Network Mask: 255.255.255.252 Number of MTID metrics: 0 TOS 0 Metrics: 1
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  • "...it uses the neighbor IP address's associated MAC address to forward the packet onwards [sic]." Unfortunately, that isn't true. In many cases of a point-to-point link, the interface protocol doesn't use MAC addresses. Commented Aug 23, 2016 at 15:26
  • True, was assuming Ethernet Commented Aug 23, 2016 at 15:33
  • By default, ethernet is a broadcast medium, and it must be specifically configured to be point-to-point for OSPF. Commented Aug 23, 2016 at 15:35
  • That is correct, but many people do it, see RFC 5309 Commented Aug 23, 2016 at 15:50

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