Configuring Static IP Routing

Table Of Contents

Configuring IP Unicast Routing

Understanding IP Routing

Types of Routing

IP Routing and Switch Stacks

Steps for Configuring Routing

Enabling IP Unicast Routing

Assigning IP Addresses to SVIs

Configuring Static Unicast Routes

Monitoring and Maintaining the IP Network

show platform ip unicast

Configuring IP Unicast Routing

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This chapter describes how to configure IP Version 4 (IPv4) static IP unicast routing on the Catalyst 2960-S and 2960 switch. Static routing is supported only on switched virtual interfaces (SVIs) and not on physical interfaces. The switch does not support routing protocols.

Unless otherwise noted, the term switch refers to a standalone switch and a switch stack. A switch stack operates and appears as a single router to the rest of the routers in the network.

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Note Stacking is supported only on Catalyst 2960-S switches.

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For complete syntax and usage information for the commands used in this chapter, see the Cisco IOS IP Command Reference, Volume 1 of 3: Addressing and Services, Release 12.2

This chapter consists of these sections:

•Understanding IP Routing

•Steps for Configuring Routing

•Enabling IP Unicast Routing

•Configuring Static Unicast Routes

•Monitoring and Maintaining the IP Network

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Note When configuring routing parameters on the switch and to allocate system resources to maximize the number of unicast routes allowed, use the sdm prefer lanbase-routing global configuration command to set the Switch Database Management (sdm) feature to the routing template. For more information on the SDM templates, see Chapter 8, "Configuring SDM Templates" or see the sdm prefer command in the command reference for this release.

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Understanding IP Routing

In some network environments, VLANs are associated with individual networks or subnetworks. In an IP network, each subnetwork is mapped to an individual VLAN. Configuring VLANs helps control the size of the broadcast domain and keeps local traffic local. However, network devices in different VLANs cannot communicate with one another without a Layer 3 device to route traffic between the VLAN, referred to as inter-VLAN routing. You configure one or more routers to route traffic to the appropriate destination VLAN.

Figure 38-1 shows a basic routing topology. Switch A is in VLAN 10, and Switch B is in VLAN 20. The router has an interface in each VLAN.

Figure 38-1 Routing Topology Example

When Host A in VLAN 10 needs to communicate with Host B in VLAN 10, it sends a packet addressed to that host. Switch A forwards the packet directly to Host B, without sending it to the router.

When Host A sends a packet to Host C in VLAN 20, Switch A forwards the packet to the router, which receives the traffic on the VLAN 10 interface. The router checks the routing table, finds the correct outgoing interface, and forwards the packet on the VLAN 20 interface to Switch B. Switch B receives the packet and forwards it to Host C.

Types of Routing

Routers and Layer 3 switches can route packets in three different ways:

•By using default routing to send traffic with a destination unknown to the router to a default outlet or destination.

•By using static routes to forward packets from predetermined ports through a single path into and out of a network.

•By dynamically calculating routes by using a routing protocol.

The switch supports static routes; it does not support routing protocols.

IP Routing and Switch Stacks

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Note Stacking is supported only on Catalyst 2960-S switches.

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A switch stack appears to the network as a single switch, regardless of which switch in the stack is connected to a peer. For additional information about switch stack operation, see Chapter 5, "Managing Switch Stacks."

The stack master performs these functions:

•The MAC address of the stack master is used as the router MAC address for the whole stack, and all outside devices use this address to send IP packets to the stack.

•All IP packets that require software forwarding or processing go through the CPU of the stack master.

Stack members perform these functions:

•They act as routing standby switches, ready to take over in case they are elected as the new stack master if the stack master fails.

•They program the routes into hardware.

If a stack master fails, the stack detects that the stack master is down and elects one of the stack members to be the new stack master. During this period, except for a momentary interruption, the hardware continues to forward packets.

At election, the new stack master performs these functions:

•It starts generating, receiving, and processing routing updates.

•It builds routing tables and distributes it to stack members.

•It uses its MAC address as the router MAC address. To notify its network peers of the new MAC address, it periodically (every few seconds for 5 minutes) sends a gratuitous ARP reply with the new router MAC address.

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Note If you configure the persistent MAC address feature on the stack and the stack master changes, the stack MAC address does not change for the configured time period. If the previous stack master rejoins the stack as a member switch during that time period, the stack MAC address remains the MAC address of the previous stack master. See the "Enabling Persistent MAC Address" section on page 5-18.

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Steps for Configuring Routing

By default, IP routing is disabled on the switch, and you must enable it before routing can take place. For detailed IP routing configuration information, see the Cisco IOS IP Configuration Guide, Release 12.2 from the Cisco.com page under Documentation > Cisco IOS Software > 12.2 Mainline > Configuration Guides.

In the following procedures, the specified interface must be a switch virtual interface (SVI)—a VLAN interface created by using the interface vlan vlan_id global configuration command and by default a Layer 3 interface. All Layer 3 interfaces on which routing will occur must have IP addresses assigned to them. See the "Assigning IP Addresses to SVIs" section on page 38-4.

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Note The switch supports 16 user-configured static routes in addition to any directly connected routes and default routes for the management interface. switch can have an IP address assigned to each SVI. The number of routed ports and SVIs that you can configure is not limited by software. However, the interrelationship between this number and the number and volume of features being implemented might have an impact on CPU utilization because of hardware limitations. Before enabling routing, enter the sdm prefer lanbase-routing global configuration command and reload the switch.

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Configuring routing consists of several procedures:

•To support VLAN interfaces, create and configure VLANs on the switch, and assign VLAN membership to Layer 2 interfaces. For more information, see Chapter 14, "Configuring VLANs."

•Configure Layer 3 interfaces (SVIs).

•Enable IP routing on the switch.

•Assign IP addresses to the Layer 3 interfaces.

•Configure static routes

Enabling IP Unicast Routing

By default, the switch is in Layer 2 switching mode and IP routing is disabled. To use the Layer 3 capabilities of the switch, you must enable IP routing.

Beginning in privileged EXEC mode, follow these steps to enable IP routing:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	ip routing	Enable IP routing.
Step 3	end	Return to privileged EXEC mode.
Step 4	show running-config	Verify your entries.
Step 5	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Use the no ip routing global configuration command to disable routing.

This example shows how to enable IP routing on a switch:

Switch# configure terminal 

Enter configuration commands, one per line. End with CNTL/Z.

Switch(config)# ip routing 

Switch(config-router)# network 10.0.0.0 

Switch(config-router)# end 

Assigning IP Addresses to SVIs

A required task for configuring IP routing is to assign IP addresses to Layer 3 network interfaces to enable the interfaces and allow communication with the hosts on those interfaces that use IP. IP routing is disabled by default and no IP addresses are assigned to SVIs.

An IP address identifies a location to which IP packets can be sent. Some IP addresses are reserved for special uses and cannot be used for host, subnet, or network addresses. RFC 1166, "Internet Numbers," contains the official description of IP addresses.

An interface can have one primary IP address. A mask identifies the bits that denote the network number in an IP address. When you use the mask to subnet a network, the mask is referred to as a subnet mask. To receive an assigned network number, contact your Internet service provider.

Beginning in privileged EXEC mode, follow these steps to assign an IP address and a network mask to an SVI

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	interface vlan vlan_id	Enter interface configuration mode, and specify the Layer 3 VLAN to configure.
Step 3	ip address ip-address subnet-mask	Configure the IP address and IP subnet mask.
Step 4	end	Return to privileged EXEC mode.
Step 5	show interfaces [interface-id] show ip interface [interface-id] show running-config interface [interface-id]	Verify your entries.
Step 6	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Configuring Static Unicast Routes

Static unicast routes are user-defined routes that cause packets moving between a source and a destination to take a specified path. Static routes can be important if the router cannot build a route to a particular destination and are useful for specifying a gateway of last resort to which all unroutable packets are sent.

Beginning in privileged EXEC mode, follow these steps to configure a static route:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	ip route prefix mask {address | interface} [distance]	Establish a static route.
Step 3	end	Return to privileged EXEC mode.
Step 4	show ip route	Display the current state of the routing table to verify the configuration.
Step 5	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Use the no ip route prefix mask {address | interface} global configuration command to remove a static route. The switch retains static routes until you remove them.

When an interface goes down, all static routes through that interface are removed from the IP routing table. When the software can no longer find a valid next hop for the address specified as the forwarding router's address in a static route, the static route is also removed from the IP routing table.

Monitoring and Maintaining the IP Network

You can specific statistics for the routing table or database. Use the privileged EXEC commands in Table 38-1 to clear display status:

Table 38-1 Commands to Clear IP Routes or Display Route Status

Command	Purpose
show ip route [address [mask] [longer-prefixes]] |	Display the current state of the routing table.
show ip route summary	Display the current state of the routing table in summary form.
show platform ip unicast	Display platform-dependent IP unicast information.

show platform ip unicast

Use the show platform ip unicast privileged EXEC command to display platform-dependent IP unicast routing information.

show platform ip unicast {adjacency | cef-idb | counts | dhcp | failed {adjacency | arp [A.B.C.D] | route} | loadbalance | mpaths | proxy | route | standby | statistics | table | trace} [ | {begin | exclude | include} expression]

Syntax Description

adjacency	Display the platform adjacency database.
cef-idb	Display platform information corresponding to Cisco Express Forwarding (CEF) interface descriptor block.
counts	Display the current counts for the Layer 3 unicast databases.
dhcp	Display the DHCP system dynamic addresses.
failed {adjacency | arp [A.B.C.D] | route}	Display the hardware resource failures. The keywords have these meanings: •adjacency—Display the adjacency entries that failed to be programmed in hardware. •arp—Display the Address Resolution Protocol (ARP) deletions because of failure and because of retries. •A.B.C.D—(Optional) Prefix of the ARP entries to display. •route—Display the route entries that failed to be programmed in hardware.
loadbalance	Display the platform loadbalance database.
mpaths	Display the Layer 3 unicast routing multipath adjacency database.
proxy	Display the platform proxy ARP database.
route	Display the platform route database.
standby	Display the platform standby information.
statistics	Display the Layer 3 unicast routing accumulated statistics.
table	Display the platform IP version 4 (IPv4) information.
trace	Display the platform event trace logs.
| begin	(Optional) Display begins with the line that matches the expression.
| exclude	(Optional) Display excludes lines that match the expression.
| include	(Optional) Display includes lines that match the specified expression.
expression	Expression in the output to use as a reference point.

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Note Though visible in the command-line help strings, the proxy and table keywords are not supported.

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Command Modes

Privileged EXEC

Command History

Release	Modification
12.2(55)SE	This command was introduced.

Usage Guidelines

You should use this command only when you are working directly with a technical support representative while troubleshooting a problem. Do not use this command unless a technical support representative asks you to do so.

Expressions are case sensitive. For example, if you enter | exclude output, the lines that contain output do not appear, but the lines that contain Output appear.

Note: other enabled commands are IOS PI and we do not include in our docs.