In the previous posts, we discussed about Type-1, Type-2 and Type-3 LSAs. This article focuses on OSPF Route summarization. I highly recommend you to read my previous posts 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…
OSPF Type-3 Summary LSA
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 ExampleIn this blog post, I will go through Junos Single area OSPF…

OSPF Route Summarization

  • OSPF allows route summarization at ABR and ASBR only.
  • Summarization is applied for LSAs, not routes.
  • Benefits of summarization includes, reduce the size of the LSDB, save memory/CPU resources of the router and path manipulation.

We can configure summarization by using the command set protocols ospf area 0.0.0.0 area-range x.x.x.x/x

  • The area number refers to the area where the subnets exist. This summary will be advertised into all other areas connected to the ABR.
  • There should be at least one subnet that falls within the summary range.

Diagram

As we discussed on the previous post, vMX5 receives 24 Summary LSAs. We can reduce the number of Summary LSAs advertised by the ABR using route summarization. Please note that the summarization is applied to the LSAs not the routes.

jcluser@vMX5> show ospf database summary 
Area 0.0.0.1:
   3 Router LSAs
   3 Network LSAs
   24 Summary LSAs
   

jcluser@vMX5> show ospf database netsummary 

    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     0x80000008   800  0x22 0x875f  28
Summary  10.100.12.0      10.100.100.3     0x80000005  1053  0x22 0x8b5b  28
Summary  10.100.14.0      10.100.100.1     0x80000008   467  0x22 0x7173  28
Summary  10.100.14.0      10.100.100.3     0x80000005   903  0x22 0x756f  28
Summary  10.100.23.0      10.100.100.1     0x80000005  1300  0x22 0x1ebf  28
Summary  10.100.23.0      10.100.100.3     0x80000007   453  0x22 0x4d6   28
Summary  10.100.24.0      10.100.100.1     0x80000004  2133  0x22 0x15c8  28
Summary  10.100.24.0      10.100.100.3     0x80000004  2102  0x22 0x9d2   28
Summary  10.100.26.0      10.100.100.1     0x80000004  1966  0x22 0xfedc  28
Summary  10.100.26.0      10.100.100.3     0x80000003  2852  0x22 0xf4e5  28
Summary  10.100.34.0      10.100.100.1     0x80000005  1133  0x22 0xa42e  28
Summary  10.100.34.0      10.100.100.3     0x80000006   303  0x22 0x8c44  28
Summary  10.100.46.0      10.100.100.1     0x80000003  2966  0x22 0x24a4  28
Summary  10.100.46.0      10.100.100.3     0x80000003  1952  0x22 0x18ae  28
Summary  10.100.100.1     10.100.100.1     0x80000005   134  0x22 0xade3  28
Summary  10.100.100.1     10.100.100.3     0x80000004  1502  0x22 0xb7d6  28
Summary  10.100.100.2     10.100.100.1     0x80000004  1799  0x22 0xafe0  28
Summary  10.100.100.2     10.100.100.3     0x80000003  2702  0x22 0xa5e9  28
Summary  10.100.100.3     10.100.100.1     0x80000004  1633  0x22 0xafde  28
Summary  10.100.100.3     10.100.100.3     0x80000005   153  0x22 0x8dff  28
Summary  10.100.100.4     10.100.100.1     0x80000003  2799  0x22 0x9df1  28
Summary  10.100.100.4     10.100.100.3     0x80000003  1802  0x22 0x91fb  28
Summary  10.100.100.6     10.100.100.1     0x80000004  1466  0x22 0x91f9  28
Summary  10.100.100.6     10.100.100.3     0x80000004  1652  0x22 0x8504  28

Let's configure summarisation on vMX1 and vMX3 for area-0. After we done that, we should only see a summary route of 10.100.0.0/16 instead of 10.100.12.0, 10.100.14.0, 10.100.23.0, 10.100.34.0, 10.100.100.1, 10.100.100.2, 10.100.100.3 and 10.100.100.4. (Networks which are part of area-0)

jcluser@vMX1# set protocols ospf area 0.0.0.0 area-range 10.100.0.0/16 

jcluser@vMX3# set protocols ospf area 0.0.0.0 area-range 10.100.0.0/16
jcluser@vMX5> show ospf database summary    
Area 0.0.0.1:
   3 Router LSAs
   3 Network LSAs
   10 Summary LSAs
   

jcluser@vMX5> show ospf database netsummary 

    OSPF database, Area 0.0.0.1
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
Summary  10.100.0.0       10.100.100.1     0x80000001    66  0x22 0x24d4  28
Summary  10.100.0.0       10.100.100.3     0x80000001    54  0x22 0x18de  28
Summary  10.100.24.0      10.100.100.1     0x80000004  1678  0x22 0x15c8  28
Summary  10.100.24.0      10.100.100.3     0x80000004   435  0x22 0x9d2   28
Summary  10.100.26.0      10.100.100.1     0x80000003  1844  0x22 0x1db   28
Summary  10.100.26.0      10.100.100.3     0x80000003  2235  0x22 0xf4e5  28
Summary  10.100.46.0      10.100.100.1     0x80000004  1511  0x22 0x22a5  28
Summary  10.100.46.0      10.100.100.3     0x80000004   285  0x22 0x16af  28
Summary  10.100.100.6     10.100.100.1     0x80000004  1344  0x22 0x91f9  28
Summary  10.100.100.6     10.100.100.3     0x80000004   136  0x22 0x8504  28

Perfect, but why are we still seeing area-2 individual prefixes? Well, we haven't configured summarization on area-2. Let's do that now.

jcluser@vMX2# set protocols ospf area 0.0.0.2 area-range 10.100.0.0/16 

jcluser@vMX4# set protocols ospf area 0.0.0.2 area-range 10.100.0.0/16
jcluser@vMX5> show ospf database summary    
Area 0.0.0.1:
   3 Router LSAs
   3 Network LSAs
   2 Summary LSAs

jcluser@vMX5> show ospf database netsummary    

    OSPF database, Area 0.0.0.1
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
Summary  10.100.0.0       10.100.100.1     0x80000001   901  0x22 0x24d4  28
Summary  10.100.0.0       10.100.100.3     0x80000001   889  0x22 0x18de  28

As you can see above, we now only receives 2 x Type-3 LSAs from vMX1 & vMX2.

Discard route

ABR which creates the summary route will also create a discard route to prevent loops.

Reference

Configuring OSPF Route Control - TechLibrary - Juniper Networks
Understanding OSPF Route Summarization, Example: Summarizing Ranges of Routes in OSPF Link-State Advertisements Sent into the Backbone Area, Example: Limiting the Number of Prefixes Exported to OSPF, Understanding OSPF Traffic Control, Example: Controlling the Cost of Individual OSPF Network Segment…