1. Introduction to Networking
Router Troubleshooting

2. Routing Troubleshooting
When troubleshooting a network you believe to have router issues, one of the firsts steps will be to define the scope of the problem, just like when you troubleshoot switches
This is especially important when dealing with routers since the problem may exist outside your network somewhere you can’t fix
Two great tools to find out where the problem exists are already familiar to you - ping and traceroute
Ping can be used from a router just as easily as a PC, and can tell us what our router can see, as well as other things
Traceroute will be useful to follow packets as they travel from router to router

3. Hosts Can’t Access Hosts Outside the Local Subnet
This may not actually be a routing issue. The first thing we should check is the default gateway setting on the hosts
If one host can’t connect outside the local network, check its default gateway setting. If a large number of hosts can’t communicate outside their network, but can communicate with each other, it may be a DHCP error setting the wrong default gateway
If the default gateway is correct, then the next thing to check is to see if the router is actually connected to the network. You can test this by connecting to the console port and using the route command. This command will show you the router’s routing table
You should check for the local network, to see if the router is connected, and the routes to other local routers to see if that link is down

4. ND
If the connection to a connected router is solid, but there is still a problem routing to it, the problem may lie with the Neighbor Discovery (ND) protocol
Routers on the same link use ND to advertise their existence to neighboring routers and to learn about the existence of their neighbors. They will periodically send small “hello” packets to each other to make sure they are still functional
Issues with ND can occur however if we use a very large subnet for our point-to-point protocol (PPP)
PPP is the protocol that allows for two devices (such as routers) to be directly connected to each other

5. PPP and IPv6
Issues can occur when using a large subnet for PPP links between routers, especially when using IPv6
When we use a subnet with a large number of possible hosts, ND has to perform address resolution on all of them to check to see if they exist. This can take a long time and delay a newly connected router from working properly
A PPP connection only requires two IP addresses, so the recommended best practice is to use a subnet with very little space for hosts

6. Hosts Can’t Access Hosts Inside a Specific Subnet
If your hosts can access hosts on most networks, except for one specific one, one of the first things to check is to see if the default gateway router has an entry for the problem network in the routing table
If an entry doesn’t exist, you can either check the routing protocol on the router or statically add the route so your router can properly send information
If this doesn’t resolve the issue, then the issue may be further down the path towards the network
We can use traceroute to see where the problem lies, but it may not be a router you administrate

7. Traceroute
The traceroute command will show you the path that is used to reach that destination network
If there's a problem, the traceroute command may return a partial path, and return an error saying that the destination network cannot be reached
This will tell you the last router reached on the path to your destination, which will allow you to focus your troubleshooting there
We can also check here for a routing loop
A routing loop is when one router routes to a previous router on our path
This can be found by a repeating pattern on a traceroute, or routing table entries that oscillate between two or more routers

8. MTU Black Holes
A black hole router is a router that is dropping packets because the packet size exceeds the Maximum Transmission Unit (MTU). It is called a black hole because it does not send back an error message. The packet just vanishes
You can use ping to locate a black hole router by setting the following parameters along with the IP address of the remote host:
-f sets the IP "Do not Fragment" (DF) bit
-l sets the payload size of the packet. Specify this size by typing a number after the -l parameter

9. How to Read a Ping Response
The response to this ping will tell us a lot
If the MTU of every segment of a routed connection is at least the MTU size, the packet is successfully returned
If there are intermediate routers that have smaller MTUs, and the routers are set to return ICMP "destination unreachable" packets, the ping utility will display the message, "Packet needs to be fragmented but DF set."
If there are intermediate routers that have smaller MTUs, and the routers are set to return ICMP "destination unreachable" packets, the ping utility displays the message, "Request timed out."

10. Can’t Access The Internet
If the hosts can reach all internal networks but not the internet, check the routing table of your default gateway to make sure that there is an entry for a default network (Network address and subnet mask of ) pointing to the public facing interface
Most of the time a boundary router on the edge of a private network won’t have routing information for the internet beyond a default route pointing to an ISP’s router. If the default route is correct, the problem is most likely on the ISP’s end

11. Remote Client Can’t Access Network Resources
There are two scenarios to look for when trying to fix the situation where a client can’t use network resources while connected to a remote access server
If the server is configured to assign the client an IP address on the network
Proxy ARP should be set up on the LAN interface of the remote access server
Proxy ARP makes it appear as if the remote clients are connected to the same network segment by setting up the server as a local proxy (or stand-in) for the client
If the server is configured to assign the client an IP address outside the network
Make sure that the server is set to route packets between the local subnet and the client’s subnet