Route summarization

[M.Sultan]

Hi guys

How to summarize these routes on ASBR router ? :

These routes belong to EIGRP I already redistributed to OPSF as well.

140.15.0.0 255.255.0.0

140.16.0.0 255.255.0.0

140.16.0.0 255.255.0.0

140.17.0.0 255.255.0.0

When i summarized on ASBR

#Router ospf 1

#summary-address 140.15.0.0 255.252.0.0

on the other router in OSPF It has All routes and new summarized route 140.14.0.0 

Although i enterd

#Clear ip ospf process #clear ip eigrp neighbours

but no changes made.

[Harold Ritter · Accepted solution]

Hi @M.Sultan ,

140.15.0.0 is not a subnet address for a 255.252.0.0 mask. The

summary-address 140.12.0.0 255.252.0.0

statement will be generated by IOS instead.

It means that 140.12.0.0/16, 140.13.0.0/16, 140.14.0.0/16 and 140.15.0.0/16 will be summarized by this statement. 140.16.0.0/16 and 140.17.0.0/16 will not be summarized.

Regards,

[M.Sultan]

Thank you, but i don't want to include 140.12.0.0 the subnets my router has 140.15-16-17-18 but now the summary is taken started from 140.12.0.0.

[Harold Ritter · Accepted solution]

Hi @M.Sultan ,

You could summarize 140.16/16 to 140.19/16 using 140.16.0.0 255.252.0.0, but 140.15/16 will not be included in the summary.

Regards,

[Joseph W. Doherty]

For 15..18, and nothing else, you can only aggregate 16 & 17, as earlier noted by @paul driver .

[Joseph W. Doherty]

If it helps any, remember, address aggregation is performed along the same address blocks for the prefix, i.e. the "right" number of consecutive IPs won't always work.

For example:

192.168.1.0/31 comprises 192.168.1.0 and 192.168.1.1

and

192.168.1.2/31 comprises 192.168.1.2 and 192.168.1.3

but 

192.168.1.1 and 192.168.1.2 cannot be in the same /31.

192.168.1.1/31 is really 192.168.1.0/31

and

192.168.1.2/31 is really 192.168.1.2/31

[paul driver]

Hello
Those subnets are discontiguous as such to capture all three /16 subnets you could use a single summary however this will encompass additional subnets that you may/or not own and do not need to be advertised

Summary-address 140.0.0.0 255.224.0.0 

Alternatively you could just advertise the 140.15.0.0/16 and summarize 140.16.0.0/140.17.0.0 subnets

Summary-address  140.16.0.0  255.254.0.0

[MHM Cisco World · Accepted solution]

this summary address not cover all subnet this make your router advertise summary and subnet that not cover by summary let start

summary-address 140.-0.0.0 255.240.0.0

140.15.0.0 255.255.0.0 <<- include 

140.16.0.0 255.255.0.0 <<- NOT include

140.16.0.0 255.255.0.0<<- NOT include

140.17.0.0 255.255.0.0<<- NOT include

summary-address 140.0.0.0 255.224.0.0

140.15.0.0 255.255.0.0 <<- include 

140.16.0.0 255.255.0.0 <<- include 

140.16.0.0 255.255.0.0<<- include 

140.17.0.0 255.255.0.0<<- include 

summary-address 140.12.0.0 255.252.0.0

140.15.0.0 255.255.0.0 <<- include 

140.16.0.0 255.255.0.0 <<- NOT include

140.16.0.0 255.255.0.0<<- NOT include

140.17.0.0 255.255.0.0<<- NOT include

[M.Sultan]

Thank you friend very clear example i understand, it is same as suppernetting FLSM subnets should start on fixed lenght.

[Joseph W. Doherty]

Are your network prefixes, as posted, correct?  I ask since .16. is listed twice.

As the other posters have already noted, those 3 prefixes cannot be summarized into a single prefix, unless you want to summarize .0. through .31.

Lastly, I recall (???) the OSPF summary-address is to summarize non area zero topology prefixes going to area zero on an ABR.  I.e., might not apply to external prefixes.  If true, you might need to aggregate such prefixes as part of EIGRP redistribution into OSPF.

[Harold Ritter]

Hi @Joseph W. Doherty ,

summary-address

is used to summarize routes redistributed into OSPF (ASBR).

area x range 

is used to summarize routes between areas (ABR).

Regards,

[Joseph W. Doherty]

@Harold Ritter thanks for correcting my faulty recollection.  I did confuse the ABR summary syntax with the ASBR summary syntax.

[M.Sultan]

On ASBR router

#router ospf 1

#summary-address x.x.x.x x.x.x.x

On ABR router

#router ospf 1

#area 0 range x.x.x.x x.x.x.x

remember summarizing routes on ASBR only works if you have do the redistribution among eigrp and ospf domains.

#summary-address x.x.x.x x.x.x.x only summarized the redistributed routes.