What's the prupose of "ip route null0"?

We often saw configuration of putting the summary address of an internal private network in "ip route .... null0". What's the purpose of it? Why the traffic to the internal network doesn't get dropped on the router it is configured?
Would the null0 statis route show up in the "sh ip route?
Thanks
Gary

Gary
There are several reasons why some routers are configured with ip route ... null 0. One of those reasons has to do with route summarization. Essentially configuring the null 0 route is to provide protection from "black hole" developing. An example might help. Lets assume that a network is using subnets of 172.17.0.0. Lets assume that subnets 172.17.2.0, 172.17.4.0, 172.17.6.0, 172.17.8.0, and 172.17.10.0 are in the routing table. Lets assume that the router is going to advertise a summary route to some of its neighbors advertising 172.17.0.0/16. So the router is telling its neighbors "if you want to reach anything in 172.17.0.0, then send it to me". So what happens if a neighbor sends a packet with destination address 172.17.5.5. What should the router do? We do not know where that subnet is, so what should we do? Should we forward it to the default route? What if the router that is the next hop for the default route received our summary advertisement for 172.17.0.0 and forwards the packet back to us? As you can see this can get messy !
So configuring ip route 172.17.0.0 255.255.0.0 null 0 is a way to assure that if the packet can not be routed it will be dropped.
There are some other situations where a route may be configured to null 0. One of those is on routers that run BGP. The router may be configured with a BGP network statement, so if the route is in the routing table that BGP will advertise it. And we want to be sure that the route is in the routing table. So we configure a route to null 0 as a way to be sure that the route remains in the table and will be advertised by BGP.
HTH
Rick

Similar Messages

  • What's the difference between transport route and transport layer

    what's the difference between transport route and transport layer,Can somebody give me some explaination? thks in advance!

    Hi,
    Transport Layer in ABAP Workbench
        The Change and Transport System supports the distribution of development
        work on large projects across multiple SAP Systems.
        The packages in each development system are grouped into one transport
        layer.
        The transport layer determines whether objects are assigned to a local
        or transportable change request.
    Use
        Each of your SAP development systems is assigned a transport layer as
        its standard transport layer. If you use Extended Transport Control, you
        can assign different standard transport layers to certain clients.
        You can define at the most one consolidation target for each SAP System
    and transport layer.
    When you create a package, it is assigned the standard transport layer
    of the SAP System.
    If you want to assign a different transport layer to a package, you
    require the administration authorization for the Change and Transport
    System.
    The objects in a package automatically have the transport attributes
    defined for the corresponding transport layer.
    o   If a consolidation route originating in their SAP System is defined,
         then the objects are assigned to a transportable request, and
         transported into the consolidation target when it is released.
    o   If a consolidation route is not defined, the objects are assigned to
         a local request, and are not transported.
    Customizing settings are not assigned to a package. They have the
    transport attributes of the standard transport layer of the system or
    client.
    It is best to assign a package a standard transport layer for which a
    consolidation route originating in the development system is defined.
    To display and maintain the transport layers and routes, use the
    Transport Management System (transaction STMS).
    Only the system adminstrator can make changes.
    Caution:
         The tables TSYST, DEVL, TWSYS, TASYS are no longer productive as of
         Release 4.0A and cannot be maintained.
    Regards
    Ben

  • What Is The Difference Between A Router Extender & A Bridge?

    Since I was here, I just thought I'd ask what is the difference between a router extender like the RE 1000 and what is known as a bridge?  Does one work better than the other?  Is one better for one thing, like a wired DVD player, than the other?

    The RE1000 can act as range extender as well as bridge where as the versatile Wireless Ethernet Bridge can make a wired Ethernet-enabled device a part of your wireless network.
    If we talk abuot the functionality in the bridge mode, then in that case both will be same the only difference will be, WET610N will be getting DUAL frequency i.e. 2.4GHz and 5GHz from the wireless network if that is also a DUAL band. 

  • What is the prupose abap query? configuaration process?

    what is the prupose abap query? configuaration process?

    Hi Balaji,
    Here is the details of ABAP Query
    ABAP Query
    An ABAP Workbench tool that enables users without knowledge of the ABAP programming language to define and execute their own reports.
    In ABAP Query, you enter texts and select fields and options to determine the structure of the reports. Fields are selected from functional areas and can be assigned a sequence by numbering.
    ABAP Query offers the following types of reports:
    Basic lists
    Statistics
    Ranked lists
    I hope it will clear for you,
    Regards,
    Murali.

  • What is the best 802.11G router for both B and G laptops?

    I have an older laptop with 802.11B wireless.  This is a work provided computer, so I can not make any mods.  I have a personal one that is G.  I read that the WRT54GS does not play well with B's and the WRT54G has connection problems.  I don't know if I capable of doing the flash that I keep reading about.  Did Version 8 fix any of the problems?  What about the GX, GX2 or GX4?
    Thanks

    well WRT54GS should work with 802.11B, if you go for WRT54G router then i would suggest you to buy a version WRT54Gv6...

  • What's the difference between Routing and Service Callout in ALSB?

    I am puzzled by that.
    what's the difference between them.
    Or what's the situation should use Routing, and that of service callout?

    There is little difference between routing and service callout.
    Routing is symbolizes the transition of request thread to a response thread starts. (in OSB every invocation of proxy is undertaken by different request and response threads ). So by this definition, there can be only one route node in a pipeline.
    Service call out is also used for similar actions as route node , but in pipeline. So service callout can be either in request pipeline (request thread) or in response pipeline (response thread ) . So in a proxy you can have multiple ServiceCallouts but only one route node.
    There are some other slight differences.
    Manoj

  • What's the purpose of SQL Developer's remote debug?

    AS per SQL Developer's "Remote debuggin", the user guide says "To debug a function or procedure for a
    connection where the database is on a different
    host than the one on which you are running SQL Developer, you can perform remote debugging."
    I'm confused what's the prupose to do a remote debugging? Could you please give me a
    scenario to help me clarify this term?
    Thank you!
    Jeff

    Mmm, don't know if the database should really be on another host, but anyway it would be a characteristic, not a definition.
    Remote debugging differs from normal debugging in the origin of the debugged session:
    For normal debugging, you start a new session from within sqldev (local) and hook into the debugger (pretty easy and automated). For remote debugging, you hook whatever existing session (started from whatever program) on whatever host (remote) into the debugger (a bit more manual as you have to include the call to hook into the debugger yourself). Very useful for debugging remote sessions over a database link or a particular remote user situation which cannot be reproduced locally.
    Have fun,
    K.

  • What is the best modem for apple tv?

    What is the best modem and router for my new apple tv?  I also have a playstation & laptop hooked up. 

    You modem is controled by your ISP.  It does not matter what kind it is.  If you already have other devices connected to it then you do not need any special hardware for ATV.

  • What is the diffrence between the revison I and the GigE Wireless "N" Router

    I have the revison I of router but what is the diffrefrenc between the one I have the the GigE that verizon sells online?

    They are in fact, one in the same. 

  • What is the difference between Routing and Call OSB regarding preformance?

    Hi excuse my ignorance, I am new to OSB and would like to know the difference in the performance of a service using the Service Callout or Routting, which would be the fastest to run?
    would like to know this to know what the best to use if I have a situation that you can use either one.
    Thanks!

    service call out .. you call a service in a SYNCHRONOUS way (pipeline blocks and waits for service response).
    we cannot use request-only wsdl operation through service callout.It is single threaded.
    route ... it's the last node of the proxy, it's not in the pipeline, you pass all processing to another (business or proxy) service . Route node can be seen as an action which defines where a Request thread stops and Response thread begins, in addition to what ever it does. It is know that by-design, request and response pipelines of a OSB proxy will be in different threads.
    route node can be (request-response and request only)
    you can have multiple ServiceCallouts but only one route node.
    Service callout under high load can cause stuck thread issues, so it has to be used wisely depending upon your requirement.
    Hope this helps.
    Regards,
    Karan

  • What is the expected behavior in a zone after adding a new static route in Solaris 10....

    Hi Folks,
    I have a few Solaris 10 production machines here at work and had a strange thing happen. I added a new route to these machines using the "add route <destination > <gateway>" command and my web applications running in the zones on these machines froze up. The applications that froze had no reason to do this as they have never talked to the machine in this new route. Here is the old route table via netstat -rn:
    Routing Table: IPv4
    Destination Gateway Flags Ref Use Interface
    172.23.248.0 172.23.248.24 U 1 6794 bge1
    204.154.126.0 172.23.248.100 UG 1 0
    204.154.127.0 172.23.248.100 UG 1 0
    204.154.134.0 172.23.248.100 UG 1 0
    204.154.135.0 172.23.248.100 UG 1 0
    204.154.132.0 172.23.248.100 UG 1 0
    204.154.133.0 172.23.248.100 UG 1 0
    204.154.130.0 172.23.248.100 UG 1 0
    204.154.131.0 172.23.248.100 UG 1 0
    204.154.128.0 172.23.248.100 UG 1 0
    204.154.129.0 204.154.129.74 U 1 7980 bge0
    204.68.180.0 172.23.248.100 UG 1 0
    204.68.181.0 172.23.248.100 UG 1 0
    192.168.0.0 172.23.248.100 UG 1 34
    172.28.0.0 172.23.248.100 UG 1 9
    172.29.0.0 172.23.248.100 UG 1 0
    172.30.0.0 172.23.248.100 UG 1 0
    172.31.0.0 172.23.248.100 UG 1 0
    172.24.0.0 172.23.248.100 UG 1 0
    172.25.0.0 172.23.248.100 UG 1 44
    172.26.0.0 172.23.248.100 UG 1 934
    172.27.0.0 172.23.248.100 UG 1 0
    172.20.0.0 172.23.248.100 UG 1 0
    172.21.0.0 172.23.248.100 UG 1 0
    172.22.0.0 172.23.248.100 UG 1 82
    172.23.0.0 172.23.248.100 UG 1 161975
    172.16.0.0 172.23.248.100 UG 1 0
    172.17.0.0 172.23.248.100 UG 1 0
    172.18.0.0 172.23.248.100 UG 1 0
    172.19.0.0 172.23.248.100 UG 1 82
    10.0.0.0 172.23.248.100 UG 1 1766
    224.0.0.0 204.154.129.74 U 1 0 bge0
    default 204.154.129.1 UG 1 8411
    127.0.0.1 127.0.0.1 UH 225713773 lo0
    Here is the route table after I added my one additional route (note the top line):
    Routing Table: IPv4
    Destination Gateway Flags Ref Use Interface
    204.154.129.209 172.23.248.100 UGH 1 5
    172.23.248.0 172.23.248.28 U 1 5757 bge1:3
    204.154.126.0 172.23.248.100 UG 1 0
    204.154.127.0 172.23.248.100 UG 1 0
    204.154.134.0 172.23.248.100 UG 1 0
    204.154.135.0 172.23.248.100 UG 1 0
    204.154.132.0 172.23.248.100 UG 1 0
    204.154.133.0 172.23.248.100 UG 1 0
    204.154.130.0 172.23.248.100 UG 1 0
    204.154.131.0 172.23.248.100 UG 1 0
    204.154.128.0 172.23.248.100 UG 1 0
    204.154.129.0 204.154.129.86 U 1 484 bge0:2
    204.68.180.0 172.23.248.100 UG 1 0
    204.68.181.0 172.23.248.100 UG 1 0
    192.168.0.0 172.23.248.100 UG 1 114
    172.28.0.0 172.23.248.100 UG 1 2
    172.29.0.0 172.23.248.100 UG 1 0
    172.30.0.0 172.23.248.100 UG 1 0
    172.31.0.0 172.23.248.100 UG 1 0
    172.24.0.0 172.23.248.100 UG 1 0
    172.25.0.0 172.23.248.100 UG 1 43
    172.26.0.0 172.23.248.100 UG 1 944
    172.27.0.0 172.23.248.100 UG 1 0
    172.20.0.0 172.23.248.100 UG 1 0
    172.21.0.0 172.23.248.100 UG 1 0
    172.22.0.0 172.23.248.100 UG 1 77
    172.23.0.0 172.23.248.100 UG 1 160823
    172.16.0.0 172.23.248.100 UG 1 0
    172.17.0.0 172.23.248.100 UG 1 0
    172.18.0.0 172.23.248.100 UG 1 0
    172.19.0.0 172.23.248.100 UG 1 0
    10.0.0.0 172.23.248.100 UG 1 183
    224.0.0.0 204.154.129.86 U 1 0 bge0:2
    default 204.154.129.1 UG 1 1850
    127.0.0.1 127.0.0.1 UH 58 225894 lo0:3
    My question is why did my application die and need to be restarted after this one simple route was added? What is the expected behavior in a zone after adding a new route to the root zone?
    Thanks,
    Tom Gellert
    tom.gellert&#64;tellabs.com

    Greg- Dreamweaver CS4/Live View integration with BrowserLab allows you to send interactive states of a local site/design directly to BrowserLab (dynamic data, rollover/mouse interaction states, widget states, et al), and from behind firewalls no less- definitely a scenario we covered.  You can get the extensions (which require Dreamweaver CS4, of course) up on the Labs site: http://labs.adobe.com/technologies/browserlab/

  • What is the correlation of Logger Private network to Router Private Network.

    What is the correlation of Logger Private network to Router Private Network.
    You have to define them in Websetup for the Router and Logger but what is communicating on the Private network path between the Logger and Router?    I thought that was over the Public network.  is it only Recovery from the Loggers talking over the Private network?

    Hi,
    you can read about the types of messages exchanged over various links in the SRND.
    G.

  • What are the ideal settings for my TC with Shaw's Cisco DPC3825 router/modem

    My old 2nd Gen Time Capsule crapped out and now I am about to set up my new 2TB TC.  My provider is Shaw Communications and they have upgraded me to their Broadband 100 Mbps service.  This came with a new modem/router combo made by Cisco and is model DPC3825.  Before I start plugging things in I'd like to know what are the ideal settings such that I gain the most of the BB100 service.  Should I be setting the Cisco modem/router to Bridge mode or the TC?  Should I have Shaw disable anything?  Any insight is greatly appreciated.

    Unless you specifically need to have the TC perform as the main router for your network, life will be a lot easier if you configure the TC to operate in Bridge Mode.
    Whether the Cisco modem/router or gateway can possibly be configured to act as a simple bridge mode modem is one thing to consider.
    Whether Shaw would support you in the event of any connection difficulties with the modem/router configured this way is another.
    Since Shaw is your provider, it might make sense to talk with them about best practice as far as configuring the DPC3825.
    Once you have that information in hand, you will then have the answer on how best to configure the TC.....which will very likely be in Bridge Mode.

  • HT4260 I have an AirPort Extreme and a linksys router. What is the best way to get 6-8 wired connections with ac speeds?

    I have an AirPort Extreme and a linksys router. What is the best way to get 6-8 wired connections with ac speeds?
    Appreciate your assistance.

    The answer depends on whether the AirPort Extreme is your main router....or...the Linksys device is your main router.
    In either case, the two devices must connect together using a wired Ethernet connection using CAT5e or CAT6 cabling.
    Any version of the AirPort Extreme produced within the last 5 years will have Gigabit Ethernet ports. In order to get the same speeds from the Linksys router, it will also need to be Gigabit Ethernet capable.
    If you need additional Ethernet ports, you will need to add a Gigabit Ethernet switch to either the AirPort Extreme or Linksys router.
    It is assumed that your other devices connecting to the AirPort Extreme and Linksys router will also have Gigabit Ethernet capability.
    Not sure why you mention "ac" speeds in your post. This relates to wireless connections, not wired.

  • What is the easiest way to access the router from an iPad?  I have no problems accessing it from my laptop.  Even though I am entering the address in the left address box, not the Google search box, it wants to do either a search, or add www beginning of

    What is the easiest way to access the router from an iPad?  I have no problems accessing it from my laptop.  Even though I am entering the address in the left address box, not the Google search box, it wants to do either a search, or add www to the beginning of the IP address.
    I usually can eventually access the router, but it is way to much trouble.  However, I use my iPad hundreds of times more often than my laptop.

    Are you typing http:// in front of the IP address of the router? e.g. http://192.168.0.1
    Sometimes, and with some routers it doesn't seem to like it if you miss off the http://

Maybe you are looking for