1. Determine best path to send packets.
2. Forward packets toward their destinations.
Router Serial Connections
Cisco routers support the following five serial standards.
A router has a DB-60 serial port that can support all five standards.
When working with any router interfaces there are specific commands that can help you check configurations and troubleshoot issues that arise. The following commands are very useful.
show ip routeWhich command(s) will show you the Layer 2 MAC address of the FastEthernet port on a router?
show interfaces | show interface s0/0
show ip interfaces brief
Using the IOS to determine the DCE and DTE ends can be achieved by issued
show controllers s0/0
It is important that you can read the routing table and be able to use the information for troubleshooting. The routing table generally only contains networks and not individual hosts. It also explains how it learned its routes.
Changing and IP Address
1. Enter interface
2. Shutdown interface
3. Issue no ip address
4. Enter new IP address
5. Turn on interface
Cisco Discovery Protocol
Cisco Discovery Protocol (CDP) is a Cisco-proprietary information sharing protocol that works on Cisco only devices.
CDP shares information with other Cisco CDP enabled, directly connected devices.
Gives Summary of information about neighbouring devices.
CDP operates at Layer 2 only, which makes it protocol independent at Layer 3.
Discover neighbors regardless of the layer 1 (Physical Layer) & Layer 3 (Network Layer) Protocols are Used.
show cdp neighborsBy default CDP is automatically enabled.
show cdp neighbors detail
The following information is shared through CDP: Device Identifier (host name), Address List, Port Identifier (port directly connected to on device), Capabilities List (identifies if it’s a router or switch), Platform (hardware platform).
To disable CDP there are two options. The first option is to globally disable CDP and that is performed by issuing:
Router(config)# no cdp runTo disable an interface from sending CDP information issue:
Router(config-if)# no cdp enable
CAUTION: Although CDP is necessary for some management applications, CDP should still be disabled in some instances.
Disable CDP globally if
• CDP is not required at all.
• The device is located in an insecure environment.
Show CDP commands
Global CDP information:
Sending CDP packets every 60 seconds
Sending a holdtime value of 180 seconds
Sending CDPv2 advertisements is enabled
Router#show cdp neighbors
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater
Device ID Local Intrfce Holdtme Capability Platform Port ID
lab-7206 Eth 0 157 R 7206VXR Fas 0/0/0
lab-as5300-1 Eth 0 163 R AS5300 Fas 0
lab-as5300-2 Eth 0 159 R AS5300 Eth 0
lab-3621 Eth 0 140 R S 3631-telcoFas 0/0
008024 2758E0 Eth 0 132 T CAT3000 1/2
Router#show cdp neighbors detail
Device ID: R3
IP address: 10.2.2.3
Platform: Cisco 3640, Capabilities: Router Switch IGMP
Interface: FastEthernet1/0, Port ID (outgoing port): FastEthernet0/0
Holdtime : 125 sec
Cisco IOS Software, 3600 Software (C3640-JK9S-M), Version 12.4(16), RELEASE SOFTWARE (fc1)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2007 by Cisco Systems, Inc.
Compiled Wed 20-Jun-07 11:43 by prod_rel_team
advertisement version: 2
VTP Management Domain: '
The smaller the routing table the quicker a router can make a decision. If networks are
designed logically and physically well then they utilize route summarization.
Show the summarization of 172.16.1.0/24, 172.16.2.0/24, 172.16.3.0/24 into 172.16.0.0/22.
Imagine trying to set up a routing table on a router connected to the Internet. The routing table would need to be full of every possible route if you wanted to easily connect to any website. Fortunately that is not the case and the existence of a default route helps us fix this problem. A default route is the simple concept of, if I a router receive a packet addressed a destination network I have no knowledge of then send it out this interface.
The instruction saying which interface to send an unknown destination network is the default route. To add a default route issue:
Router(config)# ip route 0.0.0.0 0.0.0.0 s0/0Special Commands
# logging synchronous
sets the system not to interrupt the line of code you’re typing when a message pops up unexpectedly
# debug ip routing
if you are having routing issues and have checked all addressing, physical, and logical errors try a debug statement to see how the router is thinking. Turn debugging on and shut down the problematic interface
and turn it back on; observe the messages that appear. To turn debugging off you can issue: no debug ip routing, undebug ip routing, or undebug all.