JUNOS OSPF ROUTER LSA

OSPF LSAs

OSPF routers use Link-State Advertisement to send updates to other routers about thier network connectivity. These LSAs are flooded into the network with different scopes i.e. flooded into an area, different areas etc.

LSA Header

Even though there are multiple LSA types, all of them have a common 20-octet LSA header. Fields in this common header include:

Link-State age [2-octets]: carries time since the LSA was first originated in the network. This age is incremented in seconds beginning at zero and maxes out to 3600 seconds or 1 hour. After Max-Age [3600seconds], LSA is removed from the database, unless the originated router resends this information before Max-Age.

Options [1-octet]: The originating router of this LSA advertises its capabilities in this field. e.g Bit 1 = E bit = support of Type 5 external LSAs. Similarly, Bit 3 = N/P bit = support for not-so-stubby LSAs.

Link-State Type [1-octet]: 1=Router LSA, 2=Network, 3=Network Summary, 4=ASBR summary, 5=AS External LSA, 6=Group Membership, 7=NSSA external, 9-11=Opaque LSAs.

Link-State ID [4-octets]: describes the part of network advertised by the LSA.

Link-State Sequence Number [4-octets]: a signed 4byte file used to uniquely define the version number of LSA. Each new LSA has this field set to 0x80000001, which gets incremented upto 0x7fffffff. For example for a newly generated LSA that is about to reach max-age, the originating router will re-issue this LSA with sequence number 0x80000002.

Link-State Checksum [2-octets]: IP checksum for entire LSA excluding the Age field.

Length [2-octet]: length of the entire LSA i.e. header length [20] + rest of the LSA length.

The OSPF Router LSA [LSA Type 1]

In the extensive ospf database output link type-field, such as (3), is followed by comments for explanation.
Each point-to-point link is advertised as two links: one stub and the other point-to-point. This is because on a pt-to-pt link, an OSPF router alwasys forms an adjacency with its peer over an unnumbered connection. Hence, the link ID = the neighbor's router ID
Data = IP address on pt-to-pt interface of this router. [see below]

jtac@CHXGAR1-re0.186> show ospf interface
Interface           State   Area            DR ID           BDR ID          Nbrs
ae0.0               PtToPt  0.0.0.4         0.0.0.0         0.0.0.0            1
ge-1/1/0.0          PtToPt  0.0.0.4         0.0.0.0         0.0.0.0            0
lo0.0               DRother 0.0.0.4         0.0.0.0         0.0.0.0            0
lo0.0               DRother 0.0.0.4         0.0.0.0         0.0.0.0            0
so-4/0/0.0          PtToPt  0.0.0.4         0.0.0.0         0.0.0.0            1

jtac@CHXGAR1-re0.186> show interfaces terse ae0
Interface               Admin Link Proto    Local                 Remote
ae0                     up    up
ae0.0                   up    up   inet     10.1.185.25/30
                                   inet6    fe80::219:e2ff:fe65:87f0/64
                                   mpls
                                   multiservice

{master}
jtac@CHXGAR1-re0.186> show interfaces terse ge-1/1/0.0
Interface               Admin Link Proto    Local                 Remote
ge-1/1/0.0              up    up   inet     10.1.185.18/30
                                   multiservice

{master}
jtac@CHXGAR1-re0.186> show interfaces terse lo0.0
Interface               Admin Link Proto    Local                 Remote
lo0.0                   up    up   inet     10.0.0.186          --> 0/0
                                            10.0.8.186          --> 0/0
                                            127.0.0.1           --> 0/0
                                   inet6    cafe:506:10:2::3
                                            fe80::2a0:a5ff:fe5f:53b

{master}
jtac@CHXGAR1-re0.186> show interfaces terse so-4/0/0.0
Interface               Admin Link Proto    Local                 Remote
so-4/0/0.0              up    up   inet     10.1.185.30/30
                                   inet6    fe80::2a0:a5ff:fe5f:6ea/64
                                   mpls

jtac@CHXGAR1-re0.186> show ospf database advertising-router self extensive router lsa-id 10.0.0.186

OSPF database, Area 0.0.0.4
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len
Router  *10.0.0.186       10.0.0.186       0x8000001c  1023  0x22 0x5eca 108
  bits 0x0, link count 7
  id 10.0.0.116, data 10.1.185.25, Type PointToPoint (1) ae0.0 ID = farside IP data = this router side  IP of pt-to-pt link.
    Topology count: 0, Default metric: 50
  id 10.1.185.24, data 255.255.255.252, Type Stub (3) ae0.0  ID = network data= mask this pt-to-pt interface is using 
    Topology count: 0, Default metric: 50
  id 10.1.185.16, data 255.255.255.252, Type Stub (3) ge-1/1/0 ID = network + mask [data] this interface is using
    Topology count: 0, Default metric: 65500
  id 10.0.8.186, data 255.255.255.255, Type Stub (3) LO ID = network + mask [data] this interface is using
    Topology count: 0, Default metric: 1
  id 10.0.0.186, data 255.255.255.255, Type Stub (3) LO  ID = network + mask [data] - THis router has a second address on lo0.0 interface.
    Topology count: 0, Default metric: 1
  id 10.0.0.117, data 10.1.185.30, Type PointToPoint (1)  so-4/0/0 data = this router side IP of pt-to-pt link.
    Topology count: 0, Default metric: 500
  id 10.1.185.28, data 255.255.255.252, Type Stub (3) so-4/0/0 ID = network + mask
    Topology count: 0, Default metric: 500
  Topology default (ID 0)
    Type: PointToPoint, Node ID: 10.0.0.117
      Metric: 500, Bidirectional
    Type: PointToPoint, Node ID: 10.0.0.116
      Metric: 50, Bidirectional
  Gen timer 00:32:56
  Aging timer 00:42:56
  Installed 00:17:03 ago, expires in 00:42:57, sent 00:17:01 ago
  Last changed 01:57:03 ago, Change count: 10, Ours

From RFC 2328. Following is the definition of Link Types:

Type   Description
                 __________________________________________________
                 1      Point-to-point connection to another router
                 2      Connection to a transit network [Broadcast and NBMA]
                 3      Connection to a stub network
                 4      Virtual link

Here is a router-LSA on a router with ethernet interfaces:

juniper@m10i:ospf> show interfaces lo* terse
Interface               Admin Link Proto    Local                 Remote
lo0
lo0.100                 up    up   inet     192.168.0.1         --> 0/0
                                            192.168.1.1         --> 0/0
                                            192.168.2.1         --> 0/0
                                            192.168.3.1         --> 0/0

juniper@m10i:ospf> show interfaces fe* terse
Interface               Admin Link Proto    Local                 Remote
fe-0/0/3
fe-0/0/3.100            up    up   inet     172.16.40.1/30
fe-0/0/3.200            up    up   inet     172.16.40.5/30

juniper@m10i:ospf> show ospf interface
Interface           State   Area            DR ID           BDR ID          Nbrs
fe-0/0/3.100        BDR     0.0.0.1         10.0.9.6        192.168.0.1        1
fe-0/0/3.200        DR      0.0.0.1         192.168.0.1     10.0.9.7           1

juniper@m10i:ospf> show ospf database router advertising-router self extensive

OSPF database, Area 0.0.0.1
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len
Router  *192.168.0.1      192.168.0.1      0x80000083   401  0x22 0xcb5d  48
  bits 0x2, link count 2
  id 172.16.40.2, data 172.16.40.1, Type Transit (2)
    Topology count: 0, Default metric: 1
  id 172.16.40.5, data 172.16.40.5, Type Transit (2)
    Topology count: 0, Default metric: 1
  Topology default (ID 0)
    Type: Transit, Node ID: 172.16.40.5
      Metric: 1, Bidirectional
    Type: Transit, Node ID: 172.16.40.2
      Metric: 1, Bidirectional
  Gen timer 00:43:18
  Aging timer 00:53:18
  Installed 00:06:41 ago, expires in 00:53:19, sent 00:06:39 ago
  Last changed 01:40:37 ago, Change count: 11, Ours

Type of links is 2 or Broadcast/NBMA. First IP address is that of DR on that segment. Data is the IP address of this router that is advertising this LSA.