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OSPF Inter-area route summarizationDocument

Sat, 09/06/2014 - 11:12

ashirkar Oct 26th, 2012

Introduction:

OSPF is link state routing protocol that works on the concept of areas. All

areas must have same LSDB (link state database); hence OSPF summarization

can only done on the border routers i.e. on ABR (Area border router) and

ASBR (Autonomous system boundary router).In this document we discussed about

route summarization between the areas.Background:

Summarization between areas can be done on ABR by using single command under

OSPF process:

area [area-id] range [ ip-address ] [mask ] [advertise | not-advertise |cost

{cost}]

A) area-id= Identifier of the area about which routes are to be summarized

B) [ ip-address ] [mask]= Summary route to be advertise in areas

Summary route calculation:Only 3 steps are required to calculate summary route

1) Convert the addresses to binary format

2) Locate the bit where the common pattern of digits ends

3) The number of common bits is subnet mask and the summary route

number is represented by the first IP address in the block.

Let’s assume the following networks in routing table:

192.168.0.0/24

192.168.1.0/24

192.168.2.0/24

Prefix’s Binary Splitting

8 16 24 32

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192.168.0.0 11000000 10101000 0000 0000 000000000

192.168.1.0 11000000 10101000 0000 0001 000000000

192.168.2.0 11000000 10101000 0000 0010 000000000

From the above table it shows first 22 bits are common, so summary route

subnet mask is /22 i.e. 255.255.252.0 and summarized route is 192.168.0.0/22

C) advertise = By default type 3 LSA is advertise

D) not-advertise =Its optional and used to filter type 3 LSA component of

the networks specify in range command.

E) Cost =It’s optional When we have multiple ABRs we can Advertise summary

route with cost for route manipulation.

Configuration overview:

In following example router ABR1 is directly connected to R1 in area 1 and

to R2 in backbone area0.

Topology Diagram:

On R1 loopbacks are created loopbacks for generating network prefixes and

advertise them into OSPF as follow:

R1#sh ip interface brief | include Lo

Loopback0 192.168.0.1 YES manual up

up

Loopback1 192.168.1.1 YES manual upup

Loopback2 192.168.2.1 YES manual up


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up

R2 in area 0 learns Type3 LSA for loopbacks created on R1 via ABR1 and

installed into routing table as shown in below output:

R2#sh ip ospf database | Beg Summary

Summary Net Link States (Area 0)

Link ID ADV Router Age Seq# Checksum

10.1.1.0 10.1.1.6 260 0x80000001 0x00B463

192.168.0.0 10.1.1.6 102 0x80000001 0x00B8FD

192.168.1.0 10.1.1.6 102 0x80000001 0x00AD08

192.168.2.0 10.1.1.6 102 0x80000001 0x00A212

R2#sh ip ospf database summary 192.168.0.0

OSPF Router with ID (10.1.1.5) (Process ID 100)

Summary Net Link States (Area 0)

Routing Bit Set on this LSA

LS age: 7

Options: (No TOS-capability, DC, Upward)

LS Type: Summary Links(Network)

Link State ID: 192.168.0.0 (summary Network Number)

Advertising Router: 10.1.1.6

LS Seq Number: 80000002

Checksum: 0x5C62

Length: 28


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Network Mask: /24

TOS: 0 Metric: 2

R2#sh ip route | Begin Gateway

Gateway of last resort is not set

10.0.0.0/30 is subnetted, 2 subnets

O IA 10.1.1.0 [110/2] via 10.1.1.6, 00:00:36, FastEthernet0/0

C 10.1.1.4 is directly connected, FastEthernet0/0

O IA 192.168.0.0/24 [110/3] via 10.1.1.6, 00:00:36, FastEthernet0/0



Configuring ABR1 to summarized loopback subnets available on R1 i.e.

192.168.0.0/24

192.168.1.0/24

192.168.2.0/24

ABR1(config)#router ospf 100

ABR1(config-router)#area 1 range 192.168.0.0 255.255.252.0

ABR1(config-router)#end

After configuring summarization on ABR1, R2 receives Type 3

i.e.192.168.0.0/22 not individual type 3 LSA for each loopback address from

ABR1 shown below:

R2#sh ip route | Begin Gateway

Gateway of last resort is not set

10.0.0.0/30 is subnetted, 2 subnets


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O IA 10.1.1.0 [110/2] via 10.1.1.6, 00:16:36, FastEthernet0/0

C 10.1.1.4 is directly connected, FastEthernet0/0


Discard route:

It's interesting to look at these summary routes on the ABR, which is

responsible for doing all of the summarization. The ABR also includes the

summary routes, but they aren't real routes, so it simply points them to its

Null0 interface called discard routes. They help to prevent routing loops

during summarization:

ABR1#sh ip route ospf

O 192.168.0.0/24 [110/2] via 10.1.1.1, 00:15:01, FastEthernet0/0

O 192.168.1.0/24 [110/2] via 10.1.1.1, 00:15:01, FastEthernet0/0

O 192.168.2.0/24 [110/2] via 10.1.1.1, 00:15:01, FastEthernet0/0O 192.168.0.0/22 is a summary, 00:15:01, Null0

Discard route is generated by default, and you don't need to create it. If

you want to disable creation of the discard route, you can use the no

discard-route command as follows:

ABR1(config)#router ospf 100

ABR1(config-router)#discard-route internal

ABR1(config-router)#end

Benefits of OSPF Summarization:

1) Reduce the number of routing table entries.

2) Reduce Type3 flooding and saves router CPU resource.

3) Summarization can also be used for route manipulation by using longestmatch prefix concept

Related Information:

OSPF Design Guide

OSPF Vesion 2

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cisco support community - ospf inter-area route summarization - 2014-09-06

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