Verifying Connectivity- Ciscco Wired Host Networking

Once network parameters are configured, you must verify the configuration is correct.  ping and traceroute are the two most widely used utilities. The network in Figure 4-16 will be used throughout this section to illustrate tools commonly used to verify connectivity and their use.

Figure 4-16 Verifying Connectivity

Ping

The first application to use when verifying connectivity is ping.

The ping application sends a series of ICMP packets, called the ICMP echo request, to the indicated IP address. The host should respond with an ICMP echo reply packet if it receives these packets.

Note

This example assumes C is the default gateway for A and B, while D is E’s.

For instance, you could verify connectivity from host A in the network illustrated in Figure 4-16 by

• Pinging B’s interface address, verifying local connectivity on the segment is working correctly.

• Pinging the interface address of C1, verifying connectivity to the default gateway is working correctly.

• Pinging the interface address of C2, verifying connectivity to the default gateway and the default gateway is configured correctly to route packets back to A.

• Pinging the interface address of E to verify connectivity is working across the entire network (or Internet) to the destination host.

You can also ping local broadcast and multicast addresses. For instance, pinging 203.0.113.255 (the network broadcast address) should elicit a response from every device connected to the segment. If you ping 203.0.113.255 from A, you should receive replies from B and C. Pinging a broadcast or multicast address can be especially useful if you want to clear and rebuild a host’s IPv4 ARP cache and IPv6 neighbor table.

Ping is a versatile application with many options. Options for the ping application on Ubuntu Linux include

• -4: Use IPv4 packets only.

• -6: Use IPv6 packets only.

• -b: Allow pinging to a broadcast address (as described previously).

• -c: Send a specific number of ICMP echo packets; the default usually is 5 or 10 for most ping applications.

• -f: Flood ping, which provides a quick way to determine how many packets are being dropped between the source and destination.

• -i: Use a specific source address or interface.

• -p: “Pad” the ICMP echo packets with a specific pattern. This can be good for catching problems with the network transmitting packets of all 1s, all 0s, or some other specific content.

• -s: Pad the ICMP packets transmitted to a specific size; this can be good for determining if larger or smaller packets can be carried through the network.

• -t: Set the TTL to a specific number.

The source interface can be handy when verifying local connectivity if you do not have access to a specific host. If you have access to router C but not to host B, you can ping B from C twice, using

• A source address of interface C1

• A source address of interface C2

If host B answers the ping sourced from C1 but not from C2, the likely problem is B’s default gateway configuration.