Linux, FreeBSD, Juniper, Cisco / Network security articles and troubleshooting guides

FAQ
It is currently Sat Aug 19, 2017 2:44 am


Author Message
mandrei99
Post  Post subject: OSPF Multi area scenario with isolated areas cisco & Juniper - part 1.  |  Posted: Thu Nov 13, 2014 9:51 am

Joined: Tue Aug 04, 2009 9:16 am
Posts: 245

Offline
 

OSPF Multi area scenario with isolated areas cisco & Juniper - part 1.

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:
http://blog.ine.com/wp-content/uploads/2011/01/Loop-Prevention-in-OSPF.pdf
http://blog.ipexpert.com/what-makes-a-router-an-ospf-abr-cisco-and-juniper-comparison/
https://tools.ietf.org/html/rfc3509#ref-Ref1
http://www.research.att.com/techdocs/TD_100766.pptx


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.4/192.168.4.0/24 - R1>---Area 1---<R2>---Area 0---<R3>---Area 3---<R4>

R1 ID: 172.16.2.6
R2 ID: 172.16.2.1
R3 ID: 172.16.2.2

R1 uses a passive interface vlan.4 within network 192.168.4.0 and adds it to OSPF area 1 as a stub network under it's router LSA.

Code:
R1> 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  *172.16.2.6       172.16.2.6       0x80000021   333  0x22 0x5461  60    --> R1 ID
  bits 0x2, link count 3
  id 172.16.2.6, data 255.255.255.255, Type Stub (3)
    Topology count: 0, Default metric: 0
  id 10.1.1.51, data 10.1.1.51, Type Transit (2)
    Topology count: 0, Default metric: 1
  id 192.168.4.0, data 255.255.255.0, Type Stub (3)          -> STUB Network - Passive interface
    Topology count: 0, Default metric: 1
  Topology default (ID 0)
    Type: Transit, Node ID: 10.1.1.51
      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:
Code:
R2> show ospf database lsa-id 192.168.4.0   

    OSPF database, Area 0.0.0.0
Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len
Summary *192.168.4.0      172.16.2.1       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:
Code:
R3> show ospf database lsa-id 192.168.4.0

    OSPF database, Area 0.0.0.0
Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len
Summary  192.168.4.0      172.16.2.1       0x80000001     1* 0x22 0x19f4  28       --> R2 ID advertising router

    OSPF database, Area 0.0.0.3
Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len
Summary *192.168.4.0      172.16.2.2       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.





Top
Display posts from previous:  Sort by  
E-mail friendPrint view

Topics related to - "OSPF Multi area scenario with isolated areas cisco & Juniper - part 1."
 Topics   Author   Replies   Views   Last post 
There are no new unread posts for this topic. OSPF areas - how to achieve optimal routing - Cisco way

mandrei99

1

834

Tue Nov 25, 2014 8:46 pm

Exsosus View the latest post

There are no new unread posts for this topic. Attachment(s) OSPF areas: Analysing an apprently redundant design - Cisco way

mandrei99

0

720

Tue Nov 18, 2014 11:14 am

mandrei99 View the latest post

There are no new unread posts for this topic. Attachment(s) OSPF: Dangers of non-standard area design - Juniper way

mandrei99

0

879

Thu Nov 13, 2014 12:29 pm

mandrei99 View the latest post

There are no new unread posts for this topic. Injecting a default route in an OSPF NSSA area from a Juniper device

mandrei99

0

2982

Sun Mar 15, 2015 5:24 pm

mandrei99 View the latest post

There are no new unread posts for this topic. Attachment(s) Twisted LDP signaled VPLS scenario using Juniper SRX selective packet services

mandrei99

1

1457

Mon Apr 13, 2015 3:25 am

Tripple26 View the latest post

There are no new unread posts for this topic. OSPF neighbors stuck in ExStart - case 1 - interface type mismatch

mandrei99

0

3845

Wed Dec 10, 2014 5:42 am

mandrei99 View the latest post

There are no new unread posts for this topic. Understanding the OSPF External NSSA LSA Metric Type 1 with JunOS examples

mandrei99

0

1464

Sun Mar 15, 2015 1:51 pm

mandrei99 View the latest post

 

Who is online
Users browsing this forum: No registered users and 0 guests
You can post new topics in this forum
You can reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum
Jump to:  
News News Site map Site map SitemapIndex SitemapIndex RSS Feed RSS Feed Channel list Channel list


Delete all board cookies | The team | All times are UTC - 5 hours [ DST ]



phpBB SEO