ICMP Destination Unreachable replies on cisco routers

Greenmet29Greenmet29 Posts: 240Member
The book i'm reading says that if you have a unreachable destination you will recieve a "U", network/subnet an "N" and can't fragment a "U". I can't seem to get any of these in a reply. Am I doing something wrong? I don't think i've ever seen them in a reply on a router, but I have seen destination unreachable messages on windows/linux boxes.

Comments

  • phoeneousphoeneous Go ping yourself... Posts: 2,333Member ■■■■■■■□□□
    What have you tried? Typically you will get 'U' replies if the router knows how to get to the destination but is unreachable. Either host doesnt exist or other filtering is in place. See example below of north router trying to ping 22.22.22.21 on west router but that host doesnt exist.



    NORTH#sh ip route
    Codes: C - connected, S - static, 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
           i - IS-IS, su - IS-IS summary, 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 10.100.0.1 to network 0.0.0.0
         33.0.0.0/32 is subnetted, 1 subnets
    C       33.33.33.33 is directly connected, Loopback33
         55.0.0.0/32 is subnetted, 1 subnets
    D       55.55.55.55 [90/2809856] via 10.100.0.1, 00:01:53, Serial0/0
         172.16.0.0/30 is subnetted, 1 subnets
    D       172.16.80.0 [90/2195456] via 10.100.0.1, 00:01:50, Serial0/0
         22.0.0.0/32 is subnetted, 1 subnets
    D       22.22.22.22 [90/2323456] via 10.100.0.1, 00:01:43, Serial0/0
         10.0.0.0/30 is subnetted, 3 subnets
    D       10.100.0.8 [90/2681856] via 10.100.0.1, 00:01:55, Serial0/0
    D       10.100.0.4 [90/2681856] via 10.100.0.1, 00:01:55, Serial0/0
    C       10.100.0.0 is directly connected, Serial0/0
         11.0.0.0/32 is subnetted, 1 subnets
    D       11.11.11.11 [90/2297856] via 10.100.0.1, 00:01:57, Serial0/0
         44.0.0.0/32 is subnetted, 1 subnets
    D       44.44.44.44 [90/2809856] via 10.100.0.1, 00:01:48, Serial0/0
    S*   0.0.0.0/0 [1/0] via 10.100.0.1
    NORTH#ping 22.22.22.21
    Type escape sequence to abort.
    Sending 5, 100-byte ICMP Echos to 22.22.22.21, timeout is 2 seconds:
    U.U.U
    Success rate is 0 percent (0/5)
    
  • JockVSJockJockVSJock Posts: 1,118Member
    For the U, it is my understanding that the local router has the destination, however a downstream router doesn't.
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