Routing

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How to identify whether the destination is reachable via the next hop ?

  • 1.  How to identify whether the destination is reachable via the next hop ?

    Posted 9 days ago
    Assume the next hop is ISP and I receive very limited information, say only the peering IP.  How can I identify the destination is reachable via that ISP?
    I did  the following and sometimes I got destination unreachable message.
    root@Branch1# run traceroute 192.168.0.2 no-resolve
    traceroute to 192.168.0.2 (192.168.0.2), 30 hops max, 40 byte packets
     1  172.16.1.6  5.590 ms !N  2.315 ms !N  1.855 ms !N
    
    root@Branch1# run ping 192.168.0.2
    PING 192.168.0.2 (192.168.0.2): 56 data bytes
    36 bytes from 172.16.1.6: Destination Net Unreachable
    Vr HL TOS  Len   ID Flg  off TTL Pro  cks      Src      Dst
     4  5  00 0054 716b   0 0000  40  01 9b7e 172.16.1.5  192.168.0.2
    
    
    ​

    But this might be turned off by ISP.  Any other way can I get the similar message. So I can conclude the destination is not reachable via the ISP.

    thanks in advance !!



  • 2.  RE: How to identify whether the destination is reachable via the next hop ?

    Posted 8 days ago
    it's possible that 192.168.0.2 is truly not reachable ever or at least at the moment...

    is 192.168.0.2 usually reachable?

    what do you get with....

    show route 192.168.0.2

    or 

    show route 192.168.0.2/24





    ------------------------------
    Aaron Gould
    Senior Network Engineer
    aaron@gvtc.com
    https://www.linkedin.com/in/agould123/
    ------------------------------



  • 3.  RE: How to identify whether the destination is reachable via the next hop ?

    Posted 8 days ago
    Aaron:
            thanks so much for taking a look at it.
            I need to build a GRE tunnel to the remote site,  I lack the information from ISP about the remote loopback IP reachability via ISP. I like to know any way I can tell the remote loopback IP is reachable or not via ISP. 
           In my lab, I saw destination unreachable from the next hop, so I can tell the next hop does not have any route to reach the remote IP. But if the next-hop does not return anything, what conclusion can I make ?



  • 4.  RE: How to identify whether the destination is reachable via the next hop ?

     
    Posted 8 days ago
    The address 192.168.0.2 is NOT allowed to route on the internet.  This is part of one of the prefixes reserved in rfc 1918 for internal private networks only.
    192.168.0.0/16
    10.0.0.0/8
    172.16.0.0/12

    These address ranges are private ip addresses usable only inside non-internet routed networks.

    If you are transiting an internet ISP you will need to build the tunnel via public addressed interfaces on the router pair.


    ------------------------------
    Steve Puluka BSEET - Juniper Ambassador
    IP Architect - DQE Communications Pittsburgh, PA (Metro Ethernet & ISP)
    http://puluka.com/home
    ------------------------------



  • 5.  RE: How to identify whether the destination is reachable via the next hop ?

    Posted 7 days ago
    Sorry for my confusing,  192.168.0.2 is not a real IP.  I simulated my situation in my lab with the same topology, but the IPs I used are different.

    Local site -----------ISP -------------- Remote site

    For some reason, I could not get any information from ISP, except the ISP IP.  Both sites use static routes to point to each other via ISP.

    thanks !!


  • 6.  RE: How to identify whether the destination is reachable via the next hop ?

    Posted 6 days ago

    Hi Gongyayu,
    As Aaron suggested, please provide output for the following:

    show route 192.168.0.2

    this could be due to a number of things, either ISP is not allowing traffic, remote end does not respond to ICMP or simply discards your packet and so many other possibilities. I'm not aware of a way to verify reachability of destination from your next-hops perspective. however, I have been in this situation before and this is what I did.

    Typically your ISP facing interface has a public ip that is part of your ISP's ASN, meaning its most probably aggregated part of a larger /24 at minimum or even larger prefixes.  you can source your traceroute/ ping from that ip, this will test reachability of the remote site from the /30 IP space owned by ISP (lets assume thats what you get from your ISP) and will not account for anything internal to you (the subscriber/ end user).

    run traceroute 192.168.0.2 no-resolve source <ISP facing public-ip>






  • 7.  RE: How to identify whether the destination is reachable via the next hop ?

    Posted 6 days ago
    thanks a lot !!


  • 8.  RE: How to identify whether the destination is reachable via the next hop ?

    Posted 6 days ago
    another suggestion -  if your carrier has a looking glass you can verify your ISP's routing table towards specific destinations/ prefixes from there too!
    larger T1 providers usually have a LG in place, but thats not always the case for T2 or smaller carriers.