Ch. 9 – Basic Router Troubleshooting CCNA 2 version 3.0 Rick Graziani Cabrillo College

Rick Graziani Overview Students completing this module should be able to: Use the show ip route command to gather detailed information about the routes installed on the router Configure a default route or default network Understand how a router uses both Layer 2 and Layer 3 addressing to move data through the network Use the ping command to perform basic network connectivity tests Use the telnet command to verify the application layer software between source and destination stations Troubleshoot by sequential testing of OSI layers Use the show interfaces command to confirm Layer 1 and Layer 2 problems Use the show ip route and show ip protocol commands to identify routing issues Use the show cdp command to verify Layer 2 connectivity Use the traceroute command to identify the path packets take between networks Use the show controllers serial command to ensure the proper cable is attached Use basic debug commands to monitor router activity

Rick Graziani 9.1 Examining the Routing Table We have covered these and others in more depth in previous modules and the presentation on the Structure and Lookup Process of the Routing Table The show ip route Command Determining the gateway of last resort Determining route source and destination Determining L2 and L3 addresses Determining the route administrative distance Determining the route metric Determining the route next hop Determining the last routing update Observing multiple paths to destination

Rick Graziani Static Routing

Rick Graziani Dynamic Routing

Rick Graziani Default Routes There a couple of items of misinformation in this section that we need to address.

Rick Graziani Default Routes – ip default-network command The ip default-network command: Must be used with IGRP Can be used with EIGRP and RIP, but not recommended (use ip route ) On router that uses ip default-network command, it must either have a specific route to that network or a /0 default route!

Rick Graziani Default Routes - IGRP With IGRP: Use ip default-network Need specific or default route, so once packets arrive at Cisco A it can forward those packets toward public network. ip route s0 router igrp 10 network network ip default-network

Rick Graziani Default Routes - RIP With RIP: Use /0 static route Use default-information originate (IOS 12.0 and later) ip route s0 router rip network network default-information originate

Rick Graziani Determining route source and destination

Rick Graziani Path Switching Host X has a packet(s) to send to Host Y A router generally relays a packet from one data link to another, using two basic functions: 1. a path determination function - Routing 2. a switching function – Packet Forwarding Let’s go through all of the stages these routers use to route and switch this packet. See if you can identify these two functions at each router. Note: Data link addresses have been abbreviated. Path Switching and Packet Forwarding X Y Data Link Header IP (Network layer) Packet Data Link Frame = Data Link Header + IP Packet

Rick Graziani From Host X to Router RTA Host X begins by encapsulating the IP packet into a data link frame (in this case Ethernet) with RTA’s Ethernet 0 interface’s MAC address as the data link destination address. How does Host X know to forward to packet to RTA and not directly to Host Y? How does Host X know or get RTA’s Ethernet address? –Remember, it looks at the packet’s destination ip address does an AND operation and compares it to its own ip address and subnet mask. –It determines if the two ip addresses are on the same subnet or not. –If the are on the same subnet, it looks for the destination ip address of the packet in its ARP cache. – sending out an ARP request if it is not there. –If they are on different subnets, it looks for the ip address of the default gateway in its ARP cache – sending out an ARP request if it is not there. If you do not remember, be sure to review our previous presentation, “ARP – The Process and the Protocol” A

Rick Graziani RTA to RTB 1. RTA looks up the IP destination address in its routing table /24 has next-hop-ip address of and an exit-interface of e1. Since the exit interface is on an Ethernet network, RTA must resolve the next- hop-ip address with a destination MAC address. 2. RTA looks up the next-hop-ip address of in its ARP cache. If the entry was not in the ARP cache, the RTA would need to send an ARP request out e1. RTB would send back an ARP reply, so RTA can update its ARP cache with an entry for B

Rick Graziani RTA to RTB (continued) 3. Data link destination address and frame encapsulation After finding the entry for the next-hop-ip address in its ARP cache, RTA uses the MAC address for the destination MAC address in the re- encapsulated Ethernet frame. The frame is now forwarded out Ethernet 1 (as specified in RTA’s routing table. Notice, that the IP Addresses did not change. Also notice that the Routing table was used to find the next-hop ip address, used for the data link address and exit interface, to forward the packet in a new data link frame. 0B

Rick Graziani FFFF RTB to RTC 1. RTB looks up the IP destination address in its routing table /24 has next-hop-ip address of and an exit-interface of s0 (serial 0). Since the exit interface not on an Ethernet network, RTA does not need to resolve the next-hop-ip address with a destination MAC address. Remember, serial interfaces do not have MAC addresses. 2

Rick Graziani FFFF RTB to RTC 2. Data link destination address and frame encapsulation. When the interface is a point-to-point serial connection, the Routing Table process does not even look at the next-hop IP address. Remember, a serial link is like a pipe - only one way in and only one way out. RTA now encapsulates the IP packet into the proper data link frame, using the proper serial encapsulation (HDLC, PPP, etc.). The data link destination address is set to a broadcast, since there is only one other end of the pipe and the frame is now forwarded out serial 0. 2

Rick Graziani 0B-20 0C RTC to Host Y 1. RTC looks up the IP destination address in its routing table /24 is a directly connected network with an exit-interface of e0. RTC realizes that this destination ip address is on the same network as one of its interfaces and it can sent the packet directly to the destination and not another router. Since the exit interface is on an directly connected Ethernet network, RTC must resolve the destination ip address with a destination MAC address. 2. RTC looks up the destination ip address of in its ARP cache. If the entry was not in the ARP cache, the RTC would need to send an ARP request out e0. Host Y would send back an ARP reply, so RTC can update its ARP cache with an entry for

Rick Graziani 0B-20 0C RTC to Host Y (continued) 3. Data link destination address and frame encapsulation After finding the entry for the destination ip address in its ARP cache, RTC uses the MAC address for the destination MAC address in the re- encapsulated Ethernet frame. The frame is now forwarded out Ethernet 0 (as specified in RTA’s routing table

Rick Graziani Determining the route administrative distance Not the best path, but the best source of routing information. “The administrative distance of the route is the key information that the router uses in deciding (which is the best path to a particular destination) –> what is the best source of routing information to a particular destination.”

Rick Graziani Routing Metrics - Corrections MTU is not and has never been used as a routing metric with RIP, IGRP, EIGRP, OSPF, IS-IS, or BGP.

Rick Graziani Observing multiple paths to destination Cisco routers will choose up to six equal cost paths to the same destination network, four by default. –Router(config-router)#maximum-paths 6 –Fast Switching vs. Process Switching (see presentation: Ch. 7 – Distance Vector Routing Protocols, Part 1 of 2: Distance Vector Routing and RIP) –This assumes the same routing protocols or the use of static routes, as you cannot compare RIP metrics with IGRP metrics. Administrative distance will always choose one routing source over another, static routes over dynamic, IGRP over RIP, etc. The variance command and IGRP/EIGRP is never explained in this curriculum. –For more information about the variance command see: –How Does Unequal Cost Path Load Balancing (Variance) Work in IGRP and EIGRP? – ote09186a d.shtml

Network Testing

Rick Graziani Network Testing and Troubleshooting You most likely do troubleshooting already: –Cars, cooking, computer, etc. Approach might vary slightly depending upon the scenario: –Lab –New implementation –Existing network Change made No changes made Use all possible resources: –Support contracts –Web sites and newsgroups –Books –Friends and other people –Management

Rick Graziani Different Models

Rick Graziani Testing using the OSI Model Layer 1 errors can include: 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

Rick Graziani Testing using the OSI Model Layer 2 errors can include: 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

Rick Graziani Testing using the OSI Model Layer 3 errors can include: Routing protocol not enabled Wrong routing protocol enabled Incorrect IP addresses Incorrect subnet masks

Rick Graziani Various commands These commands show various levels of connectivity or lack of connectivity: –Ping –Traceroute –Telnet –Show interfaces –Show cdp neighbors –Show ip protocols –Debug –Show running-config What do these commands tell you?

Ch. 9 – Basic Router Troubleshooting CCNA 2 version 3.0 Rick Graziani Cabrillo College