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Eigrp: Null0 routes, Automatic summarization Lab question.

Mar 18, 2012 4:42 PM

chemilio 243 posts since
May 31, 2009

Hi,

 

This is my topology, where R1-HUB, R2-SPOKE, R3-SPOKE. Eigrp AS 100 is on all 3 routers. No ip split horizon command on s0/0 on R1-HUB.

 

Sorry: Disregard the serial connection from R1 to R3 and the ethernet segment from R3 to R2. They are all shut down.

 

eigrp-topology2.PNG

 

 

 

Automatic summarization is enabled on R1-HUB, R2-SPOKE and R3-SPOKE, because i wanted to test how eigrp autosummary behaves across discontigous networks.(i.e from 20.0.0.0/16 network through 172.12.0.0/16 network to 20.0.0.0/16).

 

As expected on the spokes eigrp Null0's the summary routes because  there alrealdy exist more specific Loopback0&1 routes on the routing table.

 

R2-SPOKE#sh ip route | begin Gateway

Gateway of last resort is not set

 

     20.0.0.0/8 is variably subnetted, 3 subnets, 2 masks

D       20.0.0.0/8 is a summary, 01:50:40, Null0

C       20.1.0.0/16 is directly connected, Loopback0

C       20.2.0.0/16 is directly connected, Loopback1

     172.12.0.0/16 is variably subnetted, 2 subnets, 2 masks

D       172.12.0.0/16 is a summary, 02:38:33, Null0

C       172.12.13.0/24 is directly connected, Serial0/0

     172.23.0.0/24 is subnetted, 1 subnets

C       172.23.23.0 is directly connected, FastEthernet0/0

 

R3-SPOKE#sh ip route | begin Gateway

Gateway of last resort is not set

 

     20.0.0.0/8 is variably subnetted, 3 subnets, 2 masks

C       20.4.0.0/16 is directly connected, Loopback1

D       20.0.0.0/8 is a summary, 01:58:18, Null0

C       20.3.0.0/16 is directly connected, Loopback0

     172.12.0.0/16 is variably subnetted, 2 subnets, 2 masks

D       172.12.0.0/16 is a summary, 02:45:47, Null0

C       172.12.13.0/24 is directly connected, Serial0/0

     172.23.0.0/24 is subnetted, 1 subnets

C       172.23.23.0 is directly connected, FastEthernet0/0

 

On the R1-HUB

 

R1-HUB#sh ip int brief

Interface                  IP-Address      OK? Method Status                Protocol

FastEthernet0/0            unassigned      YES NVRAM  administratively down down

Serial0/0                  172.12.13.1     YES NVRAM  up                    up

FastEthernet0/1            unassigned      YES NVRAM  administratively down down

Serial0/1                  unassigned      YES NVRAM  administratively down down

Loopback0                  20.5.5.5        YES manual up                    up

 

But on the hub eigrp does not NULL0 the 20.0.0.0/8 or the 172.12.0.0/16. Instead eigrp performs equal cost load balancing with the spokes despite the existance the more specific directly connected loopback0 route(20.5.5.0/16) in the routing table.

 

It is obvious that eigrp is autosummarizing but it does not NULL0 the routes.

R1-HUB#sh ip route | begin Gateway

Gateway of last resort is not set

 

     20.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

C       20.5.0.0/16 is directly connected, Loopback0

D       20.0.0.0/8 [90/2297856] via 172.12.13.3, 00:12:29, Serial0/0

                   [90/2297856] via 172.12.13.2, 00:12:29, Serial0/0

     172.12.0.0/24 is subnetted, 1 subnets

C       172.12.13.0 is directly connected, Serial0/0

 

It is only after advertising  the loopback0 does eigrp NULL0's the summary routes on the R1-HUB.

 

R1-HUB(config)#router eigrp 100

R1-HUB(config-router)#network 20.5.0.0 0.0.255.255

 

R1-HUB#sh ip route | begin Gateway

Gateway of last resort is not set

 

     20.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

C       20.5.0.0/16 is directly connected, Loopback0

D       20.0.0.0/8 is a summary, 00:01:39, Null0

     172.12.0.0/16 is variably subnetted, 2 subnets, 2 masks

D       172.12.0.0/16 is a summary, 00:01:39, Null0

C       172.12.13.0/24 is directly connected, Serial0/0

 

Questions

What i dont undestand is how comes eigrp NULL0's the summary routes on the R1-HUB only _AFTER_ advertising the loopback0 and not _BEFORE_ ?

 

Also, why I don't see R1-HUB loopback0(20.5.5.5/16) 'route on the R2-SPOKE, R3-SPOKE routing tables despite it being advertised out?

 

R3-SPOKE#sh ip route | begin Gateway

Gateway of last resort is not set

 

     20.0.0.0/8 is variably subnetted, 3 subnets, 2 masks

C       20.4.0.0/16 is directly connected, Loopback1

D       20.0.0.0/8 is a summary, 01:45:40, Null0

C       20.3.0.0/16 is directly connected, Loopback0

     172.12.0.0/16 is variably subnetted, 2 subnets, 2 masks

D       172.12.0.0/16 is a summary, 05:06:18, Null0

C       172.12.13.0/24 is directly connected, Serial0/0

     172.23.0.0/24 is subnetted, 1 subnets

C       172.23.23.0 is directly connected, FastEthernet0/0

 

 

Thanks,

 

Chemillio.

 

Message was edited by: chemilio

 

Message was edited by: chemilio Typo on the topology: Changed the Loopback's to  /16

  • lp4nb 520 posts since
    Jun 27, 2009


    Automatic summarization is enabled on R1-HUB, R2-SPOKE and R3-SPOKE, because i wanted to test how eigrp autosummary behaves across discontigous networks.(i.e from 20.0.0.0/16 network through 172.12.0.0/16 network to 20.0.0.0/16)

     

     

    Questions

    What i dont undestand is how comes eigrp NULL0's the summary routes on the R1-HUB only _AFTER_ advertising the loopback0 and not _BEFORE_ ?

     

     

     

    A discard route (route to Null0) is generated either when eigrp does autosummarization or summary is configured manually. And auto-summary is the topic of interest here,  Auto summary is generated at the classful boundaries to prevent advertising subnets of a major network to another major network. We know that unless the loopback is advertised into the eigrp process, there is no other network other than the connected 172.12.0.0 to be advertised. But, it is not being summarzed here,

    why? because there is no other major network active in eigrp process, and untill there is no other major network there won't be any autosummarization and hence there wont be any discard route. When the loop back is configured and advertised into the eigrp process, What happens is the router gets a different major network boundary over which it has to send update, and therefore a discard route is created.

     

    So, this means for a discard route (route to Null0) to be created, there has to be atleast 2 major networks on the router and any of there subset/subnet networks has to be active on eigrp.

     

    e.g

     

        R1 -------------11.1.1.0/24

         |

        Lo 1.1.1.0

     

    all interface are active and advertised in eigrp and Autosummary is used

    Now two discard routes are there, 11.0.0.0/8, and 1.0.0.0/8.

     

    because when an update for 11.0.0.0/8  is sent out loopback, its a different network, and when 1.0.0.0/8 is advertised out physical link its again a diff major network therefore out of both interface the summaries for the network are sent.

    on lo--  11.0.0.0/8

    on fasteth-  1.0.0.0/8

     

    now a simple verification test would be using passive-interface for loopback. When the router is configured

     

    router ei 1

    pass loop0

    end

     

    the Null route for 11.0.0.0/8 is removed from the routing table, because it is not summarizing 11.1.1.0/24 to major network any more because it is not advertised to any one now.

    At this point, say two interfaces comes up and are advertised in eigrp process.

     

    First--  say- 11.2.1.0/24  is brought up, no new discard route is created as still the major networks are same. (1.0.0.0, and 11.0.0.0)

     

    Second, 12.1.1.0/24 comes up, and as this interface comes up and is advetised into eigrp, there will be 2 new discard routes, one for 11.0.0.0/8 (earlier it was removed when loopback0  was made passive)  and one for the new network 12.0.0.0/8.

     

     

     

    Also, why I don't see R1-HUB loopback0(20.5.5.5/16) 'route on the R2-SPOKE, R3-SPOKE routing tables despite it being advertised out?

     

     

    R1 do send the summarized update to R2 and R3, prefix sent is 20.0.0.0/8. But when the router R2/3 receive this update over a different major network they discard it because they already have this major network as a directly connected in their routing table. (this is the same reason why discontiguous networks are not supported into rip or classful Eigrp)

     

    the spciefic interfaces are advertised if the updates are not summarised, that means if the link was connected with ip address fro 20.0.0/8 instead of 172.12/16 network, there would be no discard routes, and all routers would have reachability to all subnetworks.

     

    Regards

     

    Message was edited by: lp4nb

  • lp4nb 520 posts since
    Jun 27, 2009

     

    What I understand now is that from Router1's perspective it will   have _TWO_ summary routes to choose from to advertise through the 172.12.13.0/24 major network boundary.  One summary route coming from the 20.1.0.0/16 network and the other from 20.5.0.0/16 network.

     

    Which one will Router1 choose as the one to advertise to Router2?

     

    It will choose to advertise the summary route derived from the 20.5.0.0/16 network. Why? Because from routers1 perspective the directly connected is a more specific network (EIGRP has a  AD value of 5, for summary routes, directly connected route has AD 0). The summary route derived from the 20.1.0.0/16 is therefore put as null0 "discarded".

    At the time when the summary 20.0.0.0/8 arrives from Router 0 to Router 1 and if that time router 1 is connected to 20.0.0.0/8 network and It wont accept this summary. (Null0 is a little different- grenerated when the router 1 itself creates a eigrp summary)

    But in the absence of the eigrp 20.5.0.0/16 network statement, Router1 will have only _ONE_ summary route to advertise to Router2 and thats the one derived from 20.1.0.0/16 network.(As you accomplished by using the passive interface command). That is why i think it did _not_ Null0 it before adding the loopback0 to eigrp, but I might be wrong.

    It didnt create a discard route for the summary received from R0 when R1 has only one network defined (172.x.x), because it already is getting a summary route from R0. It will generate a discard route only if it was summarizing  the prefixes. for eg- when f0/0 on R1 is shutdown and 20.x.x. network is advertised into eigrp, then it will generate a Null route.

    So, its not created upon receiving but while generating a summary route.

     

    Regards

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