In the previous post, we discussed about the single-area OSPF and Type-1, Type-2 LSAs. This article focuses on multi-area OSPF and Type-3 Summary LSAs. I highly recommend you to read my previous post here:

Juniper OSPF Configuration Example
In this blog post, I will go through Junos Single area OSPF configuration and verification using Juniper vLAB Sandbox. Interfaces and OSPF configurations are already done for us, so we can jump straight into exploring and understanding various Junos OSPF features. Kudos to Juniper for providing su…

Diagram

juniper vlab

Assumptions

  • You are already familiar with Junos CLI
  • You have a basic understanding of OSPF
  • You have read the previous single-area OSPF post

Summary

  • Multi-area OSPF
  • OSPF Database
  • Type-3 Summary LSA

Multi-area OSPF

OSPF uses areas to simplify administration and reduce CPU/memory resources. Having a multi-area OSPF decreases the size of the Link State Database and limits the propagation of link state updates when a topology change is detected. All the routers in the same area should have the same LSDB.

In the diagram above, multi-area OSPF is shown. vMX1, vMX2, vMX3 and vMX4 are ABRs because they have interfaces in area-0 and area-1 or 2.

Type-3 Summary LSA

Type-3 Summary LSAs are generated by ABR and flooded into directly connected Areas. Summary LSAs describe Inter-Area destinations. As we discussed on the previous post, Type 1 LSAs are not flooded into other areas. So, ABR uses Type-3 LSAs to inform other areas about prefixes learnt in its area.

Let's look at the OSPF database from vMX1 perspective. vMX1 is an ABR connected to both Area-0 and Area-1. Since its an ABR, it creates and floods Summary LSAs into both area-0 and area-1.

jcluser@vMX1> show ospf database 

    OSPF database, Area 0.0.0.0
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
Router  *10.100.100.1     10.100.100.1     0x80000005   120  0x22 0x5b43  60
Router   10.100.100.2     10.100.100.2     0x80000004   126  0x22 0x1d6c  60
Router   10.100.100.3     10.100.100.3     0x80000004   136  0x22 0x2139  60
Router   10.100.100.4     10.100.100.4     0x80000004   126  0x22 0xa1c6  60
Network  10.100.12.2      10.100.100.2     0x80000001   126  0x22 0xd669  32
Network  10.100.14.2      10.100.100.4     0x80000001   126  0x22 0xc871  32
Network  10.100.23.2      10.100.100.3     0x80000001   146  0x22 0x6fc2  32
Network  10.100.34.2      10.100.100.4     0x80000001   140  0x22 0x81c   32
Summary *10.100.13.0      10.100.100.1     0x80000003   110  0x22 0x8664  28
Summary  10.100.13.0      10.100.100.3     0x80000003   110  0x22 0x7a6e  28
Summary *10.100.15.0      10.100.100.1     0x80000004   115  0x22 0x6e79  28
Summary  10.100.15.0      10.100.100.3     0x80000003   110  0x22 0x6e77  28
Summary  10.100.24.0      10.100.100.2     0x80000004   119  0x22 0x5d8   28
Summary  10.100.24.0      10.100.100.4     0x80000004   120  0x22 0xf8e2  28
Summary  10.100.26.0      10.100.100.2     0x80000005   119  0x22 0xeced  28
Summary  10.100.26.0      10.100.100.4     0x80000004   120  0x22 0xeceb  28
Summary *10.100.35.0      10.100.100.1     0x80000002   110  0x22 0x9f35  28
Summary  10.100.35.0      10.100.100.3     0x80000004   125  0x22 0x854c  28
Summary  10.100.46.0      10.100.100.2     0x80000003   119  0x22 0x1ea9  28
Summary  10.100.46.0      10.100.100.4     0x80000006   120  0x22 0x2c1   28
Summary *10.100.100.5     10.100.100.1     0x80000001   115  0x22 0x97f8  28
Summary  10.100.100.5     10.100.100.3     0x80000001   125  0x22 0x8b03  28
Summary  10.100.100.6     10.100.100.2     0x80000002   124  0x22 0x8508  28
Summary  10.100.100.6     10.100.100.4     0x80000001   138  0x22 0x7b11  28

    OSPF database, Area 0.0.0.1
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
Router  *10.100.100.1     10.100.100.1     0x80000005   112  0x22 0x3549  48
Router   10.100.100.3     10.100.100.3     0x80000005   113  0x22 0xbe92  48
Router   10.100.100.5     10.100.100.5     0x80000004   118  0x22 0xd68b  60
Network  10.100.13.2      10.100.100.3     0x80000001   113  0x22 0xcf6d  32
Network  10.100.15.2      10.100.100.5     0x80000001   118  0x22 0xc175  32
Network  10.100.35.2      10.100.100.5     0x80000001   131  0x22 0x120   32
Summary *10.100.12.0      10.100.100.1     0x80000006   110  0x22 0x8b5d  28
Summary  10.100.12.0      10.100.100.3     0x80000003   109  0x22 0x8f59  28
Summary *10.100.14.0      10.100.100.1     0x80000006   110  0x22 0x7571  28
Summary  10.100.14.0      10.100.100.3     0x80000003   109  0x22 0x796d  28
Summary *10.100.23.0      10.100.100.1     0x80000003   110  0x22 0x22bd  28
Summary  10.100.23.0      10.100.100.3     0x80000005   109  0x22 0x8d4   28
Summary *10.100.24.0      10.100.100.1     0x80000002   120  0x22 0x19c6  28
Summary  10.100.24.0      10.100.100.3     0x80000001   137  0x22 0xfcf   28
Summary *10.100.26.0      10.100.100.1     0x80000001   120  0x22 0x5d9   28
Summary  10.100.26.0      10.100.100.3     0x80000001   137  0x22 0xf8e3  28
Summary *10.100.34.0      10.100.100.1     0x80000004   110  0x22 0xa62d  28
Summary  10.100.34.0      10.100.100.3     0x80000005   109  0x22 0x8e43  28
Summary *10.100.46.0      10.100.100.1     0x80000002   120  0x22 0x26a3  28
Summary  10.100.46.0      10.100.100.3     0x80000001   137  0x22 0x1cac  28
Summary *10.100.100.1     10.100.100.1     0x80000002   179  0x22 0xb3e0  28
Summary  10.100.100.1     10.100.100.3     0x80000002   114  0x22 0xbbd4  28
Summary *10.100.100.2     10.100.100.1     0x80000001   120  0x22 0xb5dd  28
Summary  10.100.100.2     10.100.100.3     0x80000001   137  0x22 0xa9e7  28
Summary *10.100.100.3     10.100.100.1     0x80000002   120  0x22 0xb3dc  28
Summary  10.100.100.3     10.100.100.3     0x80000002   182  0x22 0x93fc  28
Summary *10.100.100.4     10.100.100.1     0x80000002   120  0x22 0x9ff0  28
Summary  10.100.100.4     10.100.100.3     0x80000001   137  0x22 0x95f9  28
Summary *10.100.100.6     10.100.100.1     0x80000002   120  0x22 0x95f7  28
Summary  10.100.100.6     10.100.100.3     0x80000002   120  0x22 0x8902  28

I'm jumping straight into Summary LSAs, as we covered Type-1 & 2 in the previous post. We can see 16 X Summary LSAs in Area-0 and 24 x LSAs in Area-1. All the Summary LSAs are generated by vMX1, vMX2, vMX3 and vMX4 and flooded into areas they are connected to.

Summary LSAs in Area-0

Area-1 and Area-2 each has 3 x  physical links/networks and one loopback interface.  So, each ABR creates 4 x Summary LSA and flood it into Area-0.  

jcluser@vMX1> show ospf database summary 
Area 0.0.0.0:
   4 Router LSAs
   4 Network LSAs
   16 Summary LSAs

For example, vMX5 creates a router LSA and floods within area-1. vMX1 is part of area-1 so, vMX1 will store this in its OSPF Database. Other Routers in this OSPF domain need to know about the networks connected to vMX5. So, vMX1 will create a Type-3 summary LSA for each network and flood it into Area-0. This LSA will also flood into other OSPF areas. On the other hand, vMX3 would do the same thing by creating and flooding Type-3 LSA for the networks connected to vMX5.

Summary LSAs in Area-1

jcluser@vMX1> show ospf database summary area 0.0.0.1 
Area 0.0.0.1:
   3 Router LSAs
   3 Network LSAs
   24 Summary LSAs

There are 24 Summary LSAs in Area-1.

  • 4 x Loopback interfaces in area-0 (First column)
  • 4 x physical interfaces in area-0 (Second column)
  • 1 x loopback interface in area-2 (Third column, highlighted in Grey)
  • 3 x physical interfaces in area-2 (Third column, highlighted in Grey)

So, vMX1 & vMX2 each creates 12 Summary LSAs and flood it into Area-1. Now, vMX5 knows how to reach all the other networks in Area-0 and Area-2.

Let's verify the Summary LSAs being sent out to vMX5 by vMX1.

jcluser@vMX5> show ospf database netsummary advertising-router 10.100.100.1 

    OSPF database, Area 0.0.0.1
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
Summary  10.100.12.0      10.100.100.1     0x80000001   168  0x22 0x9558  28
Summary  10.100.14.0      10.100.100.1     0x80000001   168  0x22 0x7f6c  28
Summary  10.100.23.0      10.100.100.1     0x80000001   168  0x22 0x26bb  28
Summary  10.100.24.0      10.100.100.1     0x80000002   357  0x22 0x19c6  28
Summary  10.100.26.0      10.100.100.1     0x80000001   368  0x22 0x5d9   28
Summary  10.100.34.0      10.100.100.1     0x80000001   168  0x22 0xac2a  28
Summary  10.100.46.0      10.100.100.1     0x80000002   357  0x22 0x26a3  28
Summary  10.100.100.1     10.100.100.1     0x80000001   168  0x22 0xb5df  28
Summary  10.100.100.2     10.100.100.1     0x80000001   168  0x22 0xb5dd  28
Summary  10.100.100.3     10.100.100.1     0x80000001   168  0x22 0xb5db  28
Summary  10.100.100.4     10.100.100.1     0x80000001   168  0x22 0xa1ef  28
Summary  10.100.100.6     10.100.100.1     0x80000002   357  0x22 0x95f7  28

Reference

Configuring OSPF Areas - TechLibrary - Juniper Networks
Understanding OSPF Areas, OSPF Designated Router Overview, Example: Configuring an OSPF Router Identifier, Example: Controlling OSPF Designated Router Election, Understanding OSPF Areas and Backbone Areas, Example: Configuring a Single-Area OSPF Network, Example: Configuring a Multiarea OSPF Network…

Thanks for reading

As always, your feedback and comments are more than welcome