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Topic review - OSPF Multi area scenario with isolated areas cisco & Juniper - part 1.
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Post subject: OSPF Multi area scenario with isolated areas cisco & Juniper - part 1.  |  Post Posted: Thu Nov 13, 2014 9:51 am
OSPF Multi area scenario with isolated areas cisco & Juniper - part 1.

Before I start this is a list of very good references on this subject, imho:

Lately I've went through a hand full of articles describing OSPF loop avoidance architecture and about it being a link state protocol. Hoewever it's hard to find a good read that answers questions like "what if ?".

OSPF is both a link state protocol with a distance vector behavior when it comes to inter-area routes.

First thing I want to mention is that, inside one area, all routers have the same link state database. Based on this they build a loop-free shortest path tree to every destination inside the area.

Routers with one link in one area a second link in another area are called AreaBorderRouters - they identify themselves by setting the B bit (last bit 0x1) in Type 1 LSAs - OSPF Router LSAs. Another thing that you can read everywhere is that all ABRs have to have at least one link in area 0 - the OSPF backbone area. Why is that ? What if it doesn't ? And so on... I hope I can answer this question later in this series.

The way OSPF areas ideally work is that they need to be designed in a star / hub-and-spoke like topology, Area 0 - backbone - being the center, HUB and all other areas, the spokes.

In this way, OSPF is desgined to route all inter-area traffic (let's say from area 10 to area 20) via area 0. This way loops are avoided and suboptimal routing is introduced in some cases when spoke areas are adjacent - I hope I can explain this later how this statement differs from Cisco to Juniper and how it can be changed (only on Cisco to my knowledge).

An ABR has one separate database for each area it resides in. It calculates a separate shortest path three for all destinations in each area. It then uses type 3 LSAs - OSPF Summary and Router LSAs - OSPF Ttype 1 LSA and information from routing table and "translates" them to the other area changing the "Advertising Router" field to it's own ID. This way routers in the second area know that traffic for destinations in first area or an area behind the first one have to go through the ABR. This is the distance-vector behavior in OSPF.

Example on Junos output:

vlan.4/ - R1>---Area 1---<R2>---Area 0---<R3>---Area 3---<R4>

R1 ID:
R2 ID:
R3 ID:

R1 uses a passive interface vlan.4 within network and adds it to OSPF area 1 as a stub network under it's router LSA.

R1> show ospf database router advertising-router self extensive

    OSPF database, Area
Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len
Router  *       0x80000021   333  0x22 0x5461  60    --> R1 ID
  bits 0x2, link count 3
  id, data, Type Stub (3)
    Topology count: 0, Default metric: 0
  id, data, Type Transit (2)
    Topology count: 0, Default metric: 1
  id, data, Type Stub (3)          -> STUB Network - Passive interface
    Topology count: 0, Default metric: 1
  Topology default (ID 0)
    Type: Transit, Node ID:
      Metric: 1, Bidirectional
  Gen timer 00:44:27
  Aging timer 00:54:27
  Installed 00:05:33 ago, expires in 00:54:27, sent 00:06:13 ago
  Last changed 00:06:13 ago, Change count: 3, Ours

Then we look at R2 output, first ABR from area 1 to area 0:
R2> show ospf database lsa-id   

    OSPF database, Area
Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len
Summary *       0x80000001   503  0x22 0x19f4  28

R2/ABR uses the STUB destination network from R1's Type 1 LSA and advertises it in backbone as a summary (all OSPF domain destinations from one area are advertised into other areas as type 3 - summary LSA).

The other ABR R3 uses the summary received from R2 and floods another type 3 LSA into area 3 with same ID, but it's own ID as advertising router:
R3> show ospf database lsa-id

    OSPF database, Area
Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len
Summary       0x80000001     1* 0x22 0x19f4  28       --> R2 ID advertising router

    OSPF database, Area
Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len
Summary *       0x80000001   650  0x22 0x1dee  28       --> R3 ID advertising router

So traffic from an Area 3 router to an Area 1 router travels area 3, area 0 and area 1.

Saved by the bell. There will be multiple parts on this story.
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