Chapter 18 - Troubleshooting IPv4 Routing Flashcards
True or False. ICMP does not rely on TCP or UDP as transport layer protocols.
True. It is not associated with any transport layer protocol.
If you were to ping a device why would the first ICMP echo request fail?
A device along the route that the ping traverses is missing ARP for the device being pinged. That device then needs to wait for an ARP reply which will likely take longer than the ICMP echo request’s Timeout value.
What is an extended ping?
An extended ping allows you to customise the parameters of a ping for more in depth testing of networking and reachability. For example, with a regular ping from a router the outgoing interface will just be used as the source address for the ping. However with an extended ping you can specify the source address, number of packets (repeat count), ping packet size (datagram size (bytes)) etc.
A reason for using an extended ping over a regular ping would be to test connectivity directly from a LAN subnet if you didn’t have access to a device on that subnet. This would not only ensure that the local router could reach the destination, but that the destination also had a route back to the LAN subnet (rather than just the local router’s WAN interface).
How do you begin an extended ping from a router?
Enter the ‘ping’ command and then fill in the displayed fields. Set the ‘Extended commands’ field to ‘y’ if you want to display extra arguments (e.g. Source Address).
True or False. You can send extended pings from any address in any connected subnet on the router.
False. You can only send pings from addresses that are configured locally on the router (i.e. Even if you have a connected route for a LAN subnet, you could only ping from an address in that subnet that is configured on that router’s interface).
What is a traceroute?
- It is a troubleshooting tool used to map the path of a connection from source to destination.
- Each traceroute packet has a TTL which decreases by 1 for every hop it takes. Once the TTL reaches 0, the router that it is currently at will drop the packet and respond to the source with an ICMP TTL Exceeded message and the router’s own IP.
- Each consecutive packet has a 1 higher TTL than the last.
True or False. Traceroute will show the next hop as well as the src IP of the outgoing interface.
False. Trace routes only show the next hop, not the IP of the interface that the packet travelled out of to get to that hop.
How does a traceroute work?
Traceroute sends packets (normally 3) with low TTL values to make routers send back an ICMP TTL Exceeded message. This is then sent back to the host performing the traceroute along with the IP of the router that sent the message which identifies each hop the packet took. The first set of packets that are sent will have a TTL of 1, the next set will have a TTL of 2 and so on. This is so that each next set of packets is forwarded by some routers and responded to with the TTL Exceeded message by others.
What is TTL?
Time To Live is a metric that aims to remove looping packets. Each router a packet reaches decrements the packets TTL value by 1. Once this reaches 0 the packet is discarded by the router that decremented it to 0 and notifies the sending host of this by sending an ICMP TTL Exceeded message.
True or False. There is no such thing as an Extended Traceroute.
False. You can start one by entering the ‘traceroute’ command.
What protocol would a traceroute use from a Cisco router?
IP packets with UDP transport layer headers. Not ICMP like a lot of host devices would.
