1. Basic Router Troubleshooting
Module 9
Basic Router Troubleshooting

2. The Routing Table and show ip route command
Routers build routing tables to:
list available routes for forwarding data
select the best path to a destination
show ip route displays routing table information about all known networks and subnetworks

3. Example routing table
When RTA receives a packet destined for , it looks for the prefix /24 in its table.
RTA then forwards the packet out an interface (Ethernet0) based on the routing table entry. If RTA receives a packet destined for , it sends that packet out Serial 0/0.

4. Example routing table
4 routes – directly connected
The router will drop any packet destined for a network not listed in the routing table
So what do you do with those packets that this router doesn’t have a route to?

5. Static routing Dynamic routing Add routes needed by:
administrator types them in, so they are:
more secure
low processor overhead
have precise control of path selection
Dynamic routing
router learns route from other routers, dynamically

6. Gateway of last resort (default static route)
2 ways to statically configure default routes:
ip default network [network number]
ip route [next-hop interface or exit-interface]

7. ip default-network

8. ip route

9. Routing processes
Remember that the IP source and destination do not change in the packet, but the MAC addresses do change.
The packet is forwarded based on the MAC address of the next hop.
Layer 3 address is always used in routing, but the MAC address has to change because the data is framed as it moves out on the data link layer.
Source to destination --- we’re always talking about network addressing

10. The metric is a value that measures the desirability of a route.
Route Metrics
Routing protocols use metrics to determine the best route to a destination.
The metric is a value that measures the desirability of a route.
The smaller the metric number, the better the path.

11. RIP uses Hop count ONLY M E T R I C S

12. Route Metrics
The more factors that make up a metric, the greater the flexibility to tailor network operations to meet specific needs.
By default, IGRP uses the static factors bandwidth and delay to calculate a metric value (so bandwidth and delay can actually be entered by the administrator).
IGRP may also be configured to include the dynamic factors, load and reliability, in the metric calculation.
Remember, IGRP uses the metric
Metric = bandwidth + delay
Be sure to do the Interactive Media Activity in 9.1.6

13. Troubleshooting

14. Testing by OSI Layers Layer 1 – check obvious first (lights on a NIC)
Broken cables
Disconnected cables
Cables connected to the wrong ports
Intermittent cable connection
Wrong cables used for the task at hand (must use rollovers, crossover cables, and straight-through cables correctly)
Transceiver problems
DCE cable problems
DTE cable problems
Devices turned off

15. Testing by OSI Layers Layer 2 errors:
Improperly configured serial interfaces
Improperly configured Ethernet interfaces
Improper encapsulation set (HDLC is default for serial interfaces)
Improper clockrate settings on serial interfaces
Network interface card (NIC) problems

16. Layer 3 errors: Testing by OSI Layers Routing protocol not enabled
Wrong routing protocol enabled
Incorrect IP addresses
Incorrect subnet masks

17. ping utility (Layer 3 testing)
Maybe the host isn’t available

18. Telnet application (Layer 7 testing)
Actually verifies all seven layers of the OSI
If telnet is not working, then that means that there are upper-layer application layer problems.

19. Show interfaces serial 9.3.1 (error in graphic 1)
Interpreting the Show Interfaces Serial Command
One of the most important elements of the show interfaces serial command output is the display of the line and data link protocol status.
The line status is triggered by a Carrier Detect signal, and refers to the physical layer status. However, the line protocol, triggered by keepalive frames, refers to the data link framing.
Know all of these!

20. Show interfaces Used to discover Layer 1 & Layer 2 problems w/ router
When the line is down, the protocol is always down, because there is no useable media for the Layer 2 protocol.
If the interface is up and the line protocol is down, a Layer 2 problem exists (no keepalives, no clock rate, encap different)
You would never see serial1 is down, line protocol is up

21. Show ip protocols Shows sources of routing updates
The networks being advertised
Update timer values
Ex. If a network is not being included in the routing updates and should be, an administrator could issue this command to see which networks ARE being advertised

22. debug Be Careful!!! Debug will give you live, changing information.
Therefore it will use processor time that may disrupt normal router operation
Especially should not use “debug all”, but be more specific, like
debug ip rip (only live info on rip routing ip)
To turn off debug, use “undebug all” , or u all