Cisco Tips & Tricks

May 12, 2006

Route Summarization

Filed under: Technology and Software — ciscotips @ 9:04 pm

When you summarize routes in RIP, IGRP, EIGRP, or OSPF, you're replacing a series of routes with a summary route and mask. With RIP, IGRP, and EIGRP, this actually lessens the size of the routing update packet itself – multiple routes are replaced with the summary route. For instance, the routes 8.0.0.0/8, 9.0.0.0/8, 10.0.0.0/8, and 11.0.0.0/8 can be summarized as 8.0.0.0 252.0.0.0. Only the summary address will be found in the update packet, making it concise yet complete.

Summarizing routes can also make the routing table smaller, yet still allow for complete IP connectivity when done correctly. Using the above example, the four more-specific routes will be replaced by a single summary route. Since the entire routing table is parsed before the routing process is complete, keeping the routing table as small as possible does help speed the routing process as a whole.

Here are some additional tips on route summarization.

With RIP version 2 and EIGRP, manual route summarization is configured on the interface that will be advertising the summary. This is done with the route summarization command "ip summary-address."

RIP version 2 and EIGRP also both perform autosummarization on routes that are advertised across classful network boundaries. This is disabled with the protocol-level command "no auto-summary".

OSPF offers two different route summarization commands. To summarize routes from one OSPF area to another, use the "area range" command to summarize routes learned via redistribution, use the "summary-address" command on the ASBR.

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33 Comments »

  1. [...] Understanding OSPF Default Route Generation Récupère les infos de tes routeurs Cisco avec un super script TCL (Retrieve information about Cisco routers with a super TCL script – Translate to English Route Summarization Network Redundancy, Integrity and Reliability…Oh My! QoS Classification in a Nutshell [...]

    Pingback by CCNP Recertification » Network Blog Roundup #1 — May 14, 2006 @ 2:40 pm

  2. your stuff is really great, but l think it would really be fantastic if you include some detail on how you get the summary route and its subnet mask.

    Comment by zibusiso ndlovu — October 16, 2007 @ 7:40 am

  3. very interesting, but I don’t agree with you
    Idetrorce

    Comment by Idetrorce — December 15, 2007 @ 1:37 pm

  4. Please give easy tips & tricks for route summarization

    Comment by Chandan Debnath — December 30, 2007 @ 9:04 pm

  5. In the first paragraph, wouldn’t the route mask be 248.0.0.0?

    Comment by Josef — January 23, 2008 @ 4:20 pm

    • Because the first 6 digits (including leading zeros) in the first octet are all the same for each network (8,9,10,11), the mask would be

      11111100.00000000.00000000.00000000 or
      252.0.0.0

      Comment by JB — October 8, 2009 @ 4:40 pm

  6. There is a tutorial on how to get summary routes at http://searchwarp.com/swa45047.htm

    Comment by Joao Costa — February 5, 2008 @ 1:26 am

  7. Josef,
    252.0.0.0 is the correct mask. If you used 248.0.0.0, it would also include the networks 12.0.0.0/8, 13.0.0.0/8, 14.0.0.0/8, and 15.0.0.0/8.

    Comment by Jesse — March 20, 2008 @ 9:41 pm

  8. Hi,

    Curious as to what I’m missing here as I set up a simulation of this in my lab. I have a router 7 (3640-ios 12.4) configured with 4 Ethernet interfaces with each connecting to a corresponding Ethernet interface on another router (routers 1-4) for five routers total.

    For simplicity all routers are running
    router rip
    no auto
    vers 2
    net 192.168.0.0
    net xxxx (as appropriate for the ip of lo0)

    and router R7 has the addition of
    net 172.16.0.0 to handle the serial connection to R21

    On routers 1-4 I configured loopback0 as
    R1: 8.8.8.8/8
    R2: 9.9.9.9/8
    R3: 10.10.10.10/8
    R4: 11.11.11.11/8

    I then have S1/0 (IP 172.16.1.2/30) on R7 acting as the outbound connection to S0 (IP 172.16.1.1/30) on my primary router (R21) that connects to my LAN and is the gateway router to the firewall and Internet.

    I have the routers R1-R4 divided as various subnets of the address space 192.168.0.0/24. When I summarize this on the S1/0 interface as: #ip summary-address rip 192.168.0.0 255.255.255.0

    it acts as expected and the routing table on R21 shows

    R 192.168.0.0/24 [120/1] via 172.16.1.2, 00:00:09, Serial0

    and all the various subnetted interfaces can be successfully pinged from R21.

    However:
    If I also try to summarize the loopback interfaces on S1/0 I get:
    p1R4(config-if)#ip summary-address rip 8.0.0.0 252.0.0.0
    Summary mask must be greater or equal to major net

    If I redo this using
    p1R4(config-if)#ip summary-address rip 8.0.0.0 255.0.0.0

    the router is happy but of course this doesn’t summarize the address space of 8.0.0.0/8-11.0.0.0/8

    What am I missing/need to change to get this working?

    TIA!

    Cheers!

    Mike Shafer

    Comment by Mike Shafer — May 26, 2008 @ 6:33 pm

  9. Small Correction: (this should not be on R4)
    If I redo this using
    p1R4(config-if)#ip summary-address rip 8.0.0.0 255.0.0.0

    Should read: (but on R7)
    If I redo this using
    p1R7(config-if)#ip summary-address rip 8.0.0.0 255.0.0.0

    Comment by Mike Shafer — May 26, 2008 @ 6:39 pm

  10. To continue on the above it appears that it’s a RIP thing in not allowing summarization less than the classful boundary.

    I took my above configuration and changed it over to EIGRP and all worked just dandy! That is I was able to summarize the Class A addresses on the S1/0 interface using:

    p1R7(config-if)#ip summary-address eigrp 100 8.0.0.0 252.0.0.0

    and the routes were summarized on the routing table on R21 as expected showing:

    D 8.0.0.0/6 [90/2323456] via 172.16.1.2, 00:00:15, Serial0

    If anyone knows how to accomplish this using RIP v2 I would appreciate you’re letting me know. I’m taking the BSCI exam in the near future and it’s *my* $150 down the drain if I crash and burn! :)

    Cheers!

    MS

    Comment by Mike Shafer — May 26, 2008 @ 7:50 pm

  11. kindly favour me to understand the summerization of rputer IP

    Comment by faisal — July 8, 2008 @ 8:52 am

  12. What is route summerization and how is work on routing protocol.

    Comment by Shyam kumar Pundir — September 25, 2008 @ 6:44 am

  13. is root summarization and supernetting same????

    if not then why????

    Comment by amit — September 2, 2009 @ 5:40 am

    • was the egg or the chicken first?

      Comment by maria — June 3, 2010 @ 2:41 pm

      • both were came together …………..

        Comment by Rahul khokhar — August 19, 2012 @ 7:37 am

      • sorry ,i mean
        both came together ..and no one was single first .both were first…

        Comment by Rahul khokhar — August 19, 2012 @ 7:40 am

    • no ,because supernetting is different concept which is basically know as Vlsm where we use the space which was not using in other subnet after doing another subnetting in that subnet…..
      while summarization is only use for summarizing the route so that routeing table route can be reduce only by giving one route to more than 1 ip address…….

      Comment by Rahul khokhar — August 19, 2012 @ 7:31 am

  14. this matter is not for fully satisfied…please explain how summrization is done????
    when & where is it use in the network ?????please explain about it & send me mail……thanks

    Comment by amit sachdeva — October 7, 2010 @ 5:08 pm

  15. is root summarization and supernetting same????

    if not then why????

    Comment by amit sachdeva — October 7, 2010 @ 5:11 pm

  16. [...] Route Summarization May 2006 17 comments 3 [...]

    Pingback by 2010 in review « Cisco Tips & Tricks — January 2, 2011 @ 6:55 am

  17. yup, this right you can make this as below:-

    1- we have 8.0.0.0, 9.0.0.0, 10.0.0.0 and 11.0.0.0
    2- you will found four different of IP in first Octet.
    3- number 4 its place in the last number one 11111100
    4- you can type the subnet mask 11111100.00000000.00000000.00000000
    or 252.0.0.0
    5- and choose the lowest number of ips” it 8″
    6- finally you finish of summarization 8.0.0.0 252.0.0.0
    or 8.0.0.0/6

    thanks,

    Ahmed Abdallah
    Network Engineer
    CCNA & JNCIA-EX
    Mob.: +20114085063
    a.abdellah10@gmail.com

    Comment by Ahmed Abdallah Ibrahim — January 6, 2011 @ 10:00 am

  18. please refer http://ciscohowtos.blogspot.com/, fantastic description with very easy language. you can find more configuration about cisco. really good blog

    Comment by kundan — July 14, 2011 @ 10:15 am

  19. Hello, I want to know about loopbacks in detail.Please help.Thanks

    Comment by Ravinder — March 31, 2012 @ 12:56 pm

  20. Interesting and precised

    Comment by matthew — July 24, 2012 @ 9:30 pm

  21. the below are outputs of my two routers…

    here is a small doubt, 10.1.1.0., 10.1.2.0 , 172.16.1.0,172.16.2.0 networks are summarized where as 200.1.1.0 and 200.1.2.0 networka are not summarized… we can see the summarized routes in router2…… please clear my doubt..

    Router1#sh ip route
    Codes: C – connected, S – static, I – IGRP, R – RIP, M – mobile, B – BGP
    D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
    N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
    E1 – OSPF external type 1, E2 – OSPF external type 2, E – EGP
    i – IS-IS, L1 – IS-IS level-1, L2 – IS-IS level-2, ia – IS-IS inter area
    * – candidate default, U – per-user static route, o – ODR
    P – periodic downloaded static route

    Gateway of last resort is not set

    10.0.0.0/24 is subnetted, 2 subnets
    C 10.1.1.0 is directly connected, Loopback3
    C 10.1.2.0 is directly connected, Loopback4
    172.16.0.0/24 is subnetted, 2 subnets
    C 172.16.1.0 is directly connected, Loopback10
    C 172.16.2.0 is directly connected, Loopback11
    C 192.168.1.0/24 is directly connected, FastEthernet0/0
    C 200.1.1.0/24 is directly connected, Loopback1
    C 200.1.2.0/24 is directly connected, Loopback2

    *******************************************************************************************************************

    Route2r#SH IP ROUte
    Codes: C – connected, S – static, I – IGRP, R – RIP, M – mobile, B – BGP
    D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
    N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
    E1 – OSPF external type 1, E2 – OSPF external type 2, E – EGP
    i – IS-IS, L1 – IS-IS level-1, L2 – IS-IS level-2, ia – IS-IS inter area
    * – candidate default, U – per-user static route, o – ODR
    P – periodic downloaded static route

    Gateway of last resort is not set

    R 10.0.0.0/8 [120/1] via 192.168.1.1, 00:00:00, FastEthernet0/0
    R 172.16.0.0/16 [120/1] via 192.168.1.1, 00:00:00, FastEthernet0/0
    C 192.168.1.0/24 is directly connected, FastEthernet0/0
    R 200.1.1.0/24 [120/1] via 192.168.1.1, 00:00:00, FastEthernet0/0
    R 200.1.2.0/24 [120/1] via 192.168.1.1, 00:00:00, FastEthernet0/0

    Comment by nagendra — November 20, 2012 @ 8:07 pm

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