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Chapter 1 1

Static Routing

Routing Protocols and Concepts – Chapter 2

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Topics

 Role of the router in networks

 Directly connected networks and interfaces

 CDP protocol

 Static routes with exit interfaces

 Summary and default routes

 Forwarding with static routes

 Managing and troubleshooting static routes

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WAN Serial connections

Smart serial

connector to router

V35 DTE connection to

ISP CSD/DSU device

Larger DB-60

for older routers

Four other standards possible –

choose the right cable.

http://www.csdata.com/csdonline/customer/product.php?productid=18217&cat=314&page=1

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A WAN Physical Layer connection has sides:

Data Circuit-terminating Equipment (DCE) – This is the service provider. CSU/DSU is a DCE device.

 The CSU/DSU (DCE device) is used to convert the data from the router (DTE device) into a form acceptable to the WAN service provider.

a DCE device such as a CSU/DSU will provide the clock.

Data Terminal Equipment (DTE) – Typically the router is the DTE device.

Up-to-date technology

Cisco 1-Port T1/Fractional T1 DSU/CSU

WAN Interface Card (WIC-1DSU-T1-V2=)

WAN Serial connections

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LAN Ethernet connections

 Connections of a Router for WAN

-A router has a DB-60 port that can support 5 different cabling standards

–Newer routers support the smart serial interface that allows for more data to be forwarded across fewer cable pins.

 Connections of a Router for Ethernet

-2 types of connectors can be used: Straight through and Cross-over

Straight through used to connect:

-Switch-to-Router, Switch-to-PC, Hub- to-PC, Hub-to-Server

Cross-over used to connect (pin 1 connected to pin 3, and pin 2 connected to pin 6):

-Switch-to-Switch, PC-to-PC, Switch-to- Hub, Hub-to-Hub, Router-to-Router, PC-Router

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Default Condition

 By default, interfaces have no IP addresses and are administratively shut down.

 They need to be configured.

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Configure interface

R1(config)#interface fastethernet 0/0

R1(config-if)#ip address 172.16.3.1 255.255.255.0

R1(config-if)#no shutdown

*Mar 1 01:16:08.212: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up

*Mar 1 01:16:09.214: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up

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Check interfaces

R1#show interfaces fastethernet 0/0

FastEthernet0/0 is up, line protocol is up

Hardware is AmdFE, address is 000c.3010.9260

Internet address is 172.16.3.1/24 etc.

 Note MAC address of Ethernet interface

 Interface takes part in ARP with its network and keeps an ARP table

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Check interfaces

R1#show run

interface FastEthernet0/0

ip address 172.16.3.1 255.255.255.0

(Does not say no shutdown)

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Interface not up?

 If the interface does not come up – check the cable, check link lights, check the configuration at the other end.

 If the protocol does not come up – for Ethernet check that you have the right cable (straight-through or crossover) – for serial check that the clock rate is on the right end of the cable, check that the same Layer 2 protocol is used.

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Directly connected

 Configure IP address on interface

 It appears in routing table

 Note subnetting information

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Serial interface

 R2(config)#interface serial 0/0

 R2(config-if)#ip address 172.16.2.2 255.255.255.0

 R2(config-if)#no shutdown

 This is DTE end

 DCE end needs an additional command

 R3(config-if)#clock rate 64000

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Usual show commands

 Check that interface and protocol are up

show interfaces

show ip interface brief

show running-config

 Check that network is in routing table

show ip route

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Show controllers

R1#show controllers serial 0/0

Interface Serial0/0

Hardware is PowerQUICC MPC860

DCE V.35, no clock etc.

 Shows if a cable is connected

 Shows the type of cable

 Shows if clock rate set

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debug ip routing

 R2#debug ip routing

 If you give this command before configuring the interfaces then you see a report of the networks being added to the routing table.

 R2#no debug ip routing or R2#no debug all

 Switch off debugging as soon as possible after use – it takes up resources.

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Cisco Discovery Protocol

 Works at layer 2

 Directly connected devices only

 Cisco devices only

 Devices send CDP advertisements to neighbors at regular intervals

 Use it to find out about networking devices

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Show cdp neighbors

 Does not show IP address

 You need show cdp neighbors detail for that.

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Going further

 CDP only shows immediate neighbors, but it does tell you their IP addresses.

 Telnet to the neighbor and then use CDP to find out about its neighbors and so on.

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Disable CDP

 Disable CDP for the entire device:

 Router(config)#no cdp run

 Stop CDP advertisements on one interface:

 Router(config-if)#no cdp enable

 Do this for security.

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Static routes

 Use for stub networks: only one entry.

Stub network

networks

Static route

Default route

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Configure a static route

 R1(config)#ip route 172.16.1.0 255.255.255.0 172.16.2.2

 Give the next hop address

 R1(config)#ip route 172.16.1.0 255.255.255.0 Serial 0/0

 Or give the exit interface

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Show ip route

 Routing table now includes:

 S 172.16.1.0 /24 [1/0] via 172.16.2.2

 Or S 172.16.1.0 /24 is directly connected, serial 0/0

 Administrative distance 1

 Metric 0

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Return route

 If you use static routing, you need to configure static routes in both directions.

 A route in one direction does not ensure that there is a route in the other direction.

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Find a route to 172.16.1.0/24

Recursive lookup

There it is, via

172.16.2.2

How do you get to

172.16.2.2?

There it is via serial 0/0.

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Static route giving interface

 R1(config)#ip route 192.168.2.0 255.255.255.0 serial 0/0

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Interface down

 If the interface used by a static route goes down, then the static route is removed from the routing table.

 The static route remains in the configuration.

 If the interface comes up again then the static route goes back in the routing table.

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Delete a static route

 R1(config)#no ip route 192.168.2.0 255.255.255.0 serial 0/0

 Give the same command again with no in front.

 Most commands can be reversed like this.

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Next hop or exit interface?

 For a point to point serial link, configure the static route with the exit interface.

Only one look-up, less processing.

 For an Ethernet link, configure the static route with the next hop address (or with both).

This identifies the device that should receive the packet next. The MAC address can be found and used in the frame header.

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Summarizing static routes

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Default route

 All packets from the stub network need to go on the same route to the right hand router.

 Configure a static default route.

Stub network

networks

Static route

Default route

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Configure a default route

 Router(config)#ip route 0.0.0.0 0.0.0.0 serial 0/0 Using exit interface.

 Router(config)#ip route 0.0.0.0 0.0.0.0 172.16.2.2 Using next hop address

 0.0.0.0 network address and subnet mask is called quad zero. It matches anything.

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Routing table

 S* 0.0.0.0/0 is directly connected, Serial0/0

 The default route is shown with a *

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Troubleshooting tools

 ping

 traceroute

 show ip route

 show ip interface brief

 show cdp neighbors

 show running-config

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General Role of the Router

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General Role of the Router

 Connections of a Router for WAN

-A router has a DB-60 port that can support 5 different cabling standards

 Connections of a Router for Ethernet

-2 types of connectors can be used: Straight through and Cross-over

Straight through used to connect:

-Switch-to-Router, Switch-to-PC, Router-to-Server, Hub-to-PC, Hub-to- Server

Cross-over used to connect:

-Switch-to-Switch, PC-to-PC, Switch-to-Hub, Hub-to-Hub, Router-to- Router

 Functions of a Router

Best Path Selections

Forwarding packets to destination

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Interfaces

 Examining Router Interfaces

-Show IP router command – used to view routing table

-Show Interfaces command – used to show status of an interface

-Show IP Interface brief command – used to show a portion of the interface information

-Show running-config command – used to show configuration file in RAM

 Configuring an Ethernet interface

-By default all serial and Ethernet interfaces are down

-To enable an interface use the No Shutdown command

 Verifying Ethernet interface

-Show interfaces for fastEthernet 0/0 – command used to show status of fast Ethernet port

-Show ip interface brief

-Show running-config

 Ethernet interfaces participate in ARP

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 Configuring a Serial interface

-Enter interface configuration mode

-Enter in the ip address and subnet mask

-Enter in the no shutdown command

 Example:

-R1(config)#interface serial 0/0

-R1(config-if)#ip address 172.16.2.1 255.255.255.0

-R1(config-if)#no shutdown

Interfaces

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 Examining Router Interfaces

-Physically connecting a WAN Interface.

-A WAN Physical Layer connection has sides:

Data Circuit-terminating Equipment (DCE) – This is the service provider. CSU/DSU is a DCE device.

Data Terminal Equipment (DTE) – Typically the router is DTE device.

Interfaces

 Configuring serial links in a lab environment

One side of a serial connection must be considered a DCE

This requires placing a clocking signal – use the clock rate command.

Example:

-R1(config)#interface serial 0/0

-R1(config-if)#clockrate 64000

Serial Interfaces require a clock signal to control the timing of the communications.

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Routing Table and CDP Protocol

 Purpose of the debug ip routing command

Allows viewing changes that the router performs when adding or removing routes

Example:

-R2#debug ip routing

-IP routing debugging is on

 To configure an Ethernet interface

Example:

-R2(config)#interface fastethernet 0/0

-R2(config-if)#ip address 172.16.1.1 255.255.255.0

-R2(config-if)#no shutdown

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Routing Table and CDP Protocol

 When a router only has its interfaces configured & no other routing protocols are configured then:

-The routing table contains only the directly connected networks

-Only devices on the directly connected networks are reachable

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 Checking each route in turn

The ping command is used to check end to end connectivity

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Routing Table and CDP Protocol

 Purpose of CDP

A layer 2 cisco proprietary tool used to gather information about other directly connected Cisco devices.

 Concept of neighbors

-2 types of neighbors

Layer 3 neighbors

Layer 2 neighbors

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Routing Table and CDP Protocol

 CDP show commands

Show cdp neighbors command

-Displays the following information:

Neighbor device ID

Local interface

Holdtime value, in seconds

Neighbor device capability code

Neighbor hardware platform

Neighbor remote port ID

Show cdp neighbors detail command

-Useful in determining if an IP address configuration error

 Disabling CDP

To disable CDP globally use the following command

Router(config)#no cdp run

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Static Routes with Exit Interfaces

 Purpose of a static route

A manually configured route used when routing from a net to a stub net

 IP route command

To configure a static route use the following command: ip route

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Static Routes with Exit Interfaces

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 Dissecting static route syntax

ip route - Static route command

172.16.1.0 – Destination network address

255.255.255.0 - Subnet mask of destination network

172.16.2.2 - Serial 0/0/0 interface IP address on R2, which is the "next- hop" to this network

Static Routes with Exit Interfaces

 Configuring routes to 2 or more remote networks

Use the following commands for R1

-R1(config)#ip route 192.168.1.0 255.255.255.0 172.16.2.2

-R1(config)#ip route 192.168.2.0 255.255.255.0 172.16.2.2

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Static Routes with Exit Interfaces

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Static Routes with Exit Interfaces

Zinin’s 3 routing principles

Principle 1: "Every router makes its decision alone, based on the information it has in its own routing table.“

Principle 2: "The fact that one router has certain information in its routing table does not mean that other routers have the same information.“

Principle 3: "Routing information about a path from one network to another does not provide routing information about the reverse, or return path."

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Static Routes with Exit Interfaces

 Using Zinin’s 3 routing principles, how would you answer the following?

-Would packets from PC1 reach their destination?

Yes, packets destined for 172.16.1.0/24 and 192.168.1.0/24 networks would reach their destination.

-Does this mean that any packets from these networks destined for 172.16.3.0/24 network will reach their destination?

No, because neither R2 nor R3 router has a route to the 172.16.3.0/24 network.

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Static Routes with Exit Interfaces

 Resolving to an Exit Interface

-Recursive route lookup - Occurs when the router has to perform multiple lookups in the routing table before forwarding a packet. A static route that forwards all packets to the next-hop IP address goes through the following process (reclusive route lookup)

The router first must match static route’s destination IP address with the Next hop address

The next hop address is then matched to an exit interface

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 Configuring a Static route with an Exit Interface

-Static routes configured with an exit interface are more efficient because the routing

–The routing table can resolve the exit interface in a single search instead of 2 searches

-Example of syntax require to configure a static route with an exit interface

Static Routes with Exit Interfaces

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 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 configuration

Static Routes with Exit Interfaces

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 Verifying the Static Route Configuration

-Use the following commands

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 issue ping command to verify packets can reach destination and that Return path is working

Static Routes with Exit Interfaces

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 Ethernet interfaces and ARP.

– If a static route is configured on an Ethernet link

-If the packet is sent to the next-hop router then…

the destination MAC address will be the address of the next hop’s Ethernet interface

This is found by the router consulting the ARP table.

If an entry isn’t found then an ARP request will be sent out

Static Routes with Exit Interfaces

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Summary and Default Route

 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.

 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

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Summary and Default Route

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Summary and Default Route  Default Static Route

This is a route that will match all packets. Stub routers that have a number of static routes all exiting the same interface are good candidates for a default route.

-Like route summarization this will help reduce the size of routing table

 Configuring a default static route

Similar to configuring a static route. Except that destination IP address and subnet mask are all zeros

Example:

-Router(config)#ip route 0.0.0.0 0.0.0.0 [exit-interface | ip-address ]

 Static routes and subnet masks

The routing table lookup process will use the most specific match when comparing destination 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.

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Static Routes and Packet Forwarding

 Packet forwarding with static routes. (recall Zinin’s 3 routing principles)

 Router 1

Packet arrives on R1’s Fastethernet 0/0 interface

R1 does not have a route to the destination network, 192.168.2.0/24

R1 uses the default

static route.

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 Packet forwarding with static routes. (Zinin’s 3 routing principles)

 Router 2

The packet arrives on the Serial 0/0/0 interface on R2.

R2 has a static route to 192.168.2.0/24 out Serial0/0/1.

Static Routes and Packet Forwarding

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 Packet forwarding with static routes. (Zinin’s 3 routing principles)

 Router 3

The packet arrives on the Serial0/0/1 interface on R3.

R3 has a connected route to 192.168.2.0/24 out Fastethernet 0/1.

Static Routes and Packet Forwarding

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 Troubleshooting a Missing Route

 Tools that can be used to isolate routing problems include:

-Ping– tests end to end connectivity

-Traceroute– used to discover all of the hops (routers) along the path between 2 points

-Show IP route– used to display routing table & ascertain forwarding process

-Show ip interface brief- used to show status of router interfaces

-Show cdp neighbors detail– used to gather configuration information about directly connected neighbors

Static Routes and Packet Forwarding

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 Solving a Missing Route

 Finding a missing or mis-configured route requires methodically using the correct tools

-Start with PING. If ping fails then use traceroute to determine where packets are failing to arrive

 Issue: show ip route to examine routing table.

-If there is a problem with a mis-configured static route remove the static route then reconfigure the new static route

Static Routes and Packet Forwarding

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 Solving a Missing Route

Static Routes and Packet Forwarding

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Static Routes and Packet Forwarding  Solving a Missing Route

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Summary  Routers

-Operate at layer 3

-Functions include best path selection & forwarding packets

 Connecting Networks

WANs

Serial cables are connected to router serial ports.

In the lab environment clock rates must be configured for DCE

LANs

Straight through cables or cross over cables are used to connect to fastethernet port. (The type of cable used depends on what devices are being connected)

 Cisco Discovery Protocol

A layer 2 proprietary protocol

Used to discover information about directly connected Cisco devices

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Summary  Static Routes

-This is a manually configured path that specifies how the router will get to a certain point using a certain path.

 Summary static routes

-This is several static routes that have been condensed into a single static route.

 Default route

-It is the route packets use if there is no other possible match for their destination in the routing table.

 Forwarding of packets when static route is used

-Zinin’s 3 routing principles describe how packets are forwarded

 Troubleshooting static routes may require some of the following commands

-Ping

-Traceroute

-Show IP route

-Show ip interface brief

-Show cdp neighbors detail

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