Introducing the Topology:
The show controllers command reveals whether the cable is DCE or DTE.
#show controllers serial 0/0/0
Interface serial0/0/0
Hardware is powerQUICC MPC860
DCE v.35, no clock
Interfaces:
By default all Serial and Ethernet interfaces are down.
The routing table will be empty.
Debugging:
Allows us to watch changes a router performs in real time.
For example, to observe routes being added and removed.
#debug ip routing
#undebug all (disables all debug commands). or
#no debug all. or
#un a. (short form)
Routing Table and CDP Protocol:
When a router only has its interfaces configured and no other routing protocols are configured, then:
The routing table contains only directly connected networks.
Only devices on directly connected networks are reachable.
Router Interfaces:
#show interfaces serial 0/0
Serial 0/0 is administratively down, Line protocol is down.
Layer 1 Layer 2
L1
Administratively down – shutdown with command (administrator).
Down – no cable, bad cable, or other end shutdown.
Up –
L2
Down – no carrier signal or clock signal (no keep alive).
Up –
Purpose of CDP.
A L2 proprietary tool used to gather information about other directly connected Cisco devices, only works at L2.
Concept of neighbours.
2 types of neighbours.
L3 neighbours.
L2 neighbours. everyone that is directly connected to the one router
CDP show commands:
show cdp neighbours Displays:
Neighbour device ID.
Local interface.
Holdtime value, in seconds.
Neighbour device capability code.
Neighbour hardware platform.
Neighbour remote port ID
show cdp neighbours detail.
Same displays as above +.
Useful in determining IP address config errors.
Reveals the IP address even if you can not ping it!.
Disabling CDP:
To disable CDP globally use:
R1(config)# no cdp run.
To disable CDP on an interface use:
R1(config-if)#no cdp enable.
Static routes with Exit Interfaces:
Purpose of a static route.
A manually configured route used when routing from a network to a stub network.
R1(config)#ip route network-addr SN-mask {ip-addr | exit-interface}
E.G. R1(config)# ip route network-address subnet-mask {ip address | exit-interface}
Zinin’s 3 routing principles:
Principle 1: Every router makes its decision alone, based on the info it has in its own routing table.
Principle 2: The fact that one router has certain info does not mean that other routers have the same info.
Principle 3: Routing info about a path from one network to another does not provide the return path.
Recursive Route Lookup:
Occurs when a router has to perform multiple lookups before forwarding a packet.
A static route to a next-hop IP uses this 2 step process:
1. Match static route’s dest IP with the Next hop address.
2. Match next hop address to an exit interface.
The Routing Table:
Inside the square brackets is the Administrative Distance (AD) and the Metric.
Lower is better for both numbers.
Static routes have an AD = 1.
Static routes configured with an exit interface appear as directly connected networks even though they are not.
Static routes with an exit interface are more efficient since they can be resolved in a single search.
If the interface on which static routes depend, goes down, the routes are removed until the interface returns.
Modifying Static routes:
Existing static routes CANNOT be modified. The old static route must be deleted by placing no in front of the ip route.
Example:
No ip route 192.168.2.0 255.255.255.0 172.16.2.2.
A new static route must be rewritten in the config.
Verifying the Static Route Configuration:
Step 1 show running-config.
Step 2 verify static route has been entered correctly.
Step 3 show ip route.
Step 4 verify route was configured in routing table.
Step 5 use ping to verify reachability.
Static Routes with Ethernet:
On serial interfaces there is generally only one next hop.
A serial exit interface knows where to go (only 1 address).
With Ethernet, multiple addresses could be attached.
An exit interface on Ethernet, does NOT know the MAC to use.
SO, do NOT configure an exit-interface route on Ethernet.
Route Summarization:
Summarizing routes reduces the size of the routing table.
Route summarization is the process of combining a number of static routes into a single static route.
Can be used if the:
Networks can be summarized into a single address, and
If static routes all use the same exit-interface or next-hop address.
Look at the binary of each address and when the bits are no longer the same then you count up the bits that are the same and that will give you the prefix
E.G. 172.16.1.0, 172.16.2.0, 172.16.3.0 = 172.16.0.0 with a Subnet mask of 255.255.2
Configuring a Summary Route:
Step 1: delete the current static route.
Step 2: configure the summary static route.
Step 3: verify the new static route
Summary and Default Route:
Default Static Route
A route that will match all packets. Stub routers that have a # of static routes all exiting the same interface are good candidates for a default route.
This reduces the size of the routing table.
Configuring a default static route.
A static route with IP address and subnet mask = zeros.
Example:
R1(config)#ip route 0.0.0.0 0.0.0.0 {exit-interface | ip-addr}.
Static routes and subnet masks.
The routing table lookup process will use the most specific match when comparing dest IP address and subnet mask.
Default static routes and subnet masks.
Since the subnet mask used on a default static route is 0.0.0.0 all packets will match.
Troubleshooting a Missing Route:
Tools for isolating routing problems include:
Ping – end to end connectivity.
Traceroute – where problems start.
show ip route – routing table and forwarding process.
show ip interface brief – status of interfaces.
show cdp neighbours detail – config info from neighbours (IP address even if you can not ping or remember it).
Tags: 802.1q, study, vlan, switch, vtp, wan
