ICMP Destination Unreachable replies on cisco routers

Greenmet29

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.

phoeneous

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)

JockVSJock

For the U, it is my understanding that the local router has the destination, however a downstream router doesn't.