Routing

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

    Posted 05-01-2021 19:44
    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 05-02-2021 17:02
    Edited by aaron.gould 05-02-2021 17:02
    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 05-02-2021 17:27
    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 05-03-2021 05:50
    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 05-03-2021 09:29
    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 05-04-2021 10:21

    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 05-04-2021 10:32
    thanks a lot !!


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

    Posted 05-04-2021 11:17
    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.