Network Connectivity Tools
Network connectivity tools enable you to check device connectivity by running traceroutes and pinging devices on the network:
Ping
The ping command is a common method for troubleshooting the accessibility of devices. It uses two Internet Control Message Protocol (ICMP) query messages, ICMP echo requests, and ICMP echo replies to determine whether a remote host is active. The ping command also measures the amount of time it takes to receive the echo reply.
The ping command first sends an echo request packet to an address, and then it waits for a reply. The ping is successful only if the echo request gets to the destination, and the destination is able to get an echo reply (hostname is alive) back to the source of the ping within a predefined time interval.
The bulk option has been introduced to check reachability to multiple destinations. The destinations are directly input through the CLI. This option is supported for ipv4 destinations only.
Checking Network Connectivity
As an aid to diagnosing basic network connectivity, many network protocols support an echo protocol. The protocol involves sending a special datagram to the destination host, then waiting for a reply datagram from that host. Results from this echo protocol can help in evaluating the path-to-host reliability, delays over the path, and whether the host can be reached or is functioning.
Configuration for Checking Network Connectivity
The following configuration shows an extended ping command sourced from the Router A interface and destined for the Router B interface. If this ping succeeds, it is an indication that there is no routing problem. Router A knows how to get to the interface of Router B, and Router B knows how to get to the interface of Router A. Also, both hosts have their default gateways set correctly.
If the extended ping command from Router A fails, it means that there is a routing problem. There could be a routing problem on any of the three routers: Router A could be missing a route to the subnet of Router B's interface, or to the subnet between Router C and Router B; Router B could be missing a route to the subnet of Router A's subnet, or to the subnet between Router C and Router A; and Router C could be missing a route to the subnet of Router A's or Router B's Ethernet segments. You should correct any routing problems, and then Host 1 should try to ping Host 2. If Host 1 still cannot ping Host 2, then both hosts' default gateways should be checked. The connectivity between the interface of Router A and the interface of Router B is checked with the extended ping command.
With a normal ping from Router A to Router B's interface, the source address of the ping packet would be the address of the outgoing interface; that is the address of the interface, (10.0.0.2). When Router B replies to the ping packet, it replies to the source address (that is, 10.0.0.2). This way, only the connectivity between the interface of Router A (10.0.0.2) and the 10gige interface of Router B (10.0.0.1) is tested.
To test the connectivity between Router A's interface (10.0.0.2) and Router B's interface (10.0.0.1 .), we use the extended ping command. With extended ping, we get the option to specify the source address of the ping packet.
Configuration Example
In this use case, the extended ping command verifies the IP connectivity between the two IP addresses Router A (10.0.0.2) and Router B (10.0.0.1) .
Router# ping 10.0.0.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.0.0.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5)
Router#!!!!!
*/If you do not enter a hostname or an IP address on the same line as the ping command,
the system prompts you to specify the target IP address and several other command parameters.
After specifying the target IP address, you can specify alternate values for the
remaining parameters or accept the displayed default for each parameter /*
Router# ping
Protocol [ipv4]:
Target IP address: 10.0.0.1
Repeat count [5]: 5
Datagram size [100]: 1000
Timeout in seconds [2]: 1
Interval in milliseconds [10]: 1
Extended commands? [no]: no
Sweep range of sizes? [no]:
Type escape sequence to abort.
Sending 5, 1000-byte ICMP Echos to 10.0.0.1, timeout is 1 seconds:
!!!!!
Success rate is 100 percent (5/5)
Router#!!!!!Associated Commands
Checking Network Connectivity for Multiple Destinations
The bulk option enables you to check reachability to multiple destinations. The destinations are directly input through the CLI. This option is supported for ipv4 destinations only.
Configuration Example
-
1: 1.1.1.1
-
2: 2.2.2.2
-
3: 3.3.3.3
Router# ping bulk ipv4 input cli batch
*/You must hit the Enter button and then specify one destination address per line*/
Please enter input via CLI with one destination per line and when done Ctrl-D/(exit) to initiate pings:
1: 1.1.1.1
2: 2.2.2.2
3: 3.3.3.3
4:
Starting pings...
Target IP address: 1.1.1.1
Repeat count [5]: 5
Datagram size [100]: 1
% A decimal number between 36 and 18024.
Datagram size [100]: 1
% A decimal number between 36 and 18024.
Datagram size [100]: 1000
Timeout in seconds [2]: 1
Interval in milliseconds [10]: 10
Extended commands? [no]: no
Sweep range of sizes? [no]: q
% Please answer 'yes' or 'no'.
Sweep range of sizes? [no]: q
% Please answer 'yes' or 'no'.
Sweep range of sizes? [no]:
Type escape sequence to abort.
Sending 5, 1000-byte ICMP Echos to 1.1.1.1, vrf is default, timeout is 1 seconds:
!!!!!
Success rate is 100 percent (5/5),
Target IP address: 2.2.2.2
Repeat count [5]:
Datagram size [100]: q
% A decimal number between 36 and 18024.
Datagram size [100]:
Timeout in seconds [2]:
Interval in milliseconds [10]:
Extended commands? [no]:
Sweep range of sizes? [no]:
Sending 5, 100-byte ICMP Echos to 1.1.1.1, vrf is default, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5),
Target IP address: 3.3.3.3
Repeat count [5]: 4
Datagram size [100]: 100
Timeout in seconds [2]: 1
Interval in milliseconds [10]: 10
Extended commands? [no]: no
Sweep range of sizes? [no]: no
Sending 4, 100-byte ICMP Echos to 1.1.1.1, vrf is default, timeout is 1 seconds:
!!!!!
Success rate is 100 percent (4/5),Associated Commands
Traceroute
Where the ping command can be used to verify connectivity between devices, the traceroute command can be used to discover the paths packets take to a remote destination and where routing breaks down.
The traceroute command records the source of each ICMP "time-exceeded" message to provide a trace of the path that the packet took to reach the destination. You can use the IP traceroute command to identify the path that packets take through the network on a hop-by-hop basis. The command output displays all network layer (Layer 3) devices, such as routers, that the traffic passes through on the way to the destination.
The traceroute command uses the Time To Live (TTL) field in the IP header to cause routers and servers to generate specific return messages. The traceroute command sends a User Datagram Protocol (UDP) datagram to the destination host with the TTL field set to 1. If a router finds a TTL value of 1 or 0, it drops the datagram and sends back an ICMP time-exceeded message to the sender. The traceroute facility determines the address of the first hop by examining the source address field of the ICMP time-exceeded message.
To identify the next hop, the traceroute command sends a UDP packet with a TTL value of 2. The first router decrements the TTL field by 1 and sends the datagram to the next router. The second router sees a TTL value of 1, discards the datagram, and returns the time-exceeded message to the source. This process continues until the TTL increments to a value large enough for the datagram to reach the destination host (or until the maximum TTL is reached).
To determine when a datagram reaches its destination, the traceroute command sets the UDP destination port in the datagram to a very large value that the destination host is unlikely to be using. When a host receives a datagram with an unrecognized port number, it sends an ICMP port unreachable error to the source. This message indicates to the traceroute facility that it has reached the destination.
Checking Packet Routes
The traceroute command allows you to trace the routes that packets actually take when traveling to their destinations.
Configuration Example
Trace the route from 10.0.0.2 to 20.1.1.1:
Router# traceroute 20.1.1.1
Type escape sequence to abort.
Tracing the route to 20.1.1.1
1 10.0.0.1 39 msec * 3 msec
*/If you do not enter a hostname or an IP address on the same line as the traceroute
command, the system prompts you to specify the target IP address and several other
command parameters. After specifying the target IP address, you can specify alternate
values for the remaining parameters or accept the displayed default for each
parameter/*
Router #traceroute
Protocol [ipv4]:
Target IP address: 20.1.1.1
Source address: 10.0.0.2
Numeric display? [no]:
Timeout in seconds [3]:
Probe count [3]:
Minimum Time to Live [1]:
Maximum Time to Live [30]:
Port Number [33434]:
Loose, Strict, Record, Timestamp, Verbose[none]:
Type escape sequence to abort.
Tracing the route to 20.1.1.1
1 10.0.0.1 3 msec * 3 msec
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