Isl 101 Assignment Definition

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Configuring Routing Between VLANs with Inter-Switch Link Encapsulation


This chapter describes the Inter-Switch Link (ISL) protocol and provides guidelines for configuring ISL and Token Ring ISL (TRISL) features.

For a complete description of the commands in this chapter, refer to the the Cisco IOSSwitching Services Command Reference. To locate documentation of other commands that appear in this chapter, use the command reference master index or search online.

To identify the hardware platform or software image information associated with a feature, use the Feature Navigator on Cisco.com to search for information about the feature or refer to the software release notes for a specific release. For more information, see the section "Identifying Supported Platforms" in the chapter "Using Cisco IOS Software."

Overview of the ISL Protocol

ISL is a Cisco protocol for interconnecting multiple switches and maintaining VLAN information as traffic goes between switches. ISL provides VLAN capabilities while maintaining full wire speed performance on Fast Ethernet links in full- or half-duplex mode. ISL operates in a point-to-point environment and will support up to 1000 VLANs. You can define virtually as many logical networks as are necessary for your environment.

This chapter describes how to configure routing between VLANs using ISL encapsulation.

Frame Tagging in ISL

With ISL, an Ethernet frame is encapsulated with a header that transports VLAN IDs between switches and routers. A 26-byte header that contains a 10-bit VLAN ID is prepended to the Ethernet frame.

A VLAN ID is added to the frame only when the frame is destined for a nonlocal network. Figure 77 shows VLAN packets traversing the shared backbone. Each VLAN packet carries the VLAN ID within the packet header.

Figure 77 VLAN Packets Traversing the Shared Backbone

ISL Encapsulation Configuration Task List

You can configure routing between any number of VLANs in your network. This section documents the configuration tasks for each protocol supported with ISL encapsulation. The basic process is the same, regardless of the protocol being routed. It involves the following tasks:

Enabling the protocol on the router

Enabling the protocol on the interface

Defining the encapsulation format as ISL or TRISL

Customizing the protocol according to the requirements for your environment

To configure routing between any number of VLANs in your network, perform the tasks described in the following sections particular to your network:

Configuring AppleTalk Routing over ISL

Configuring Banyan VINES Routing over ISL

Configuring DECnet Routing over ISL

Configuring the Hot Standby Router Protocol over ISL

Configuring IP Routing over TRISL

Configuring IPX Routing over TRISL

Configuring VIP Distributed Switching over ISL

Configuring XNS Routing over ISL

Configuring CLNS Routing over ISL

Configuring IS-IS Routing over ISL

Monitoring and Maintaining VLAN Subinterfaces

Refer to the "ISL Encapsulation Configuration Examples" section at the end of this chapter for sample configurations.

Configuring AppleTalk Routing over ISL

AppleTalk can be routed over VLAN subinterfaces using the ISL and IEEE 802.10 VLAN encapsulation protocols. The AppleTalk Routing over ISL and IEEE 802.10 Virtual LANs feature provides full-feature Cisco IOS software AppleTalk support on a per-VLAN basis, allowing standard AppleTalk capabilities to be configured on VLANs.

To route AppleTalk over ISL or IEEE 802.10 between VLANs, you need to customize the subinterface to create the environment in which it will be used. Perform the tasks described in the following sections in the order in which they appear:

Enabling AppleTalk Routing

Defining the VLAN Encapsulation Format

Configuring AppleTalk on the Subinterface

Enabling AppleTalk Routing

To enable AppleTalk routing on either ISL or 802.10 interfaces, use the following command in global configuration mode:

Command Purpose

Router(config)# appletalk routing [eigrprouter-number]

Enables AppleTalk routing globally.


Defining the VLAN Encapsulation Format

To define the VLAN encapsulation format as either ISL or 802.10, use the following commands in interface configuration mode:

  Command Purpose

Step 1 

Router(config-if)# interfacetype slot/port.subinterface-number

Specifies the subinterface the VLAN will use.

Step 2 

Router(config-if)# encapsulation islvlan-identifier


or

Router(config-if)# encapsulation sde said

Defines the encapsulation format as either ISL (isl) or IEEE 802.10 (sde), and specifies the VLAN identifier or security association identifier, respectively.

Configuring AppleTalk on the Subinterface

After you enable AppleTalk globally and define the encapsulation format, you need to enable it on the subinterface by specifying the cable range and naming the AppleTalk zone for each interface. To enable the AppleTalk protocol on the subinterface, use the following commands in interface configuration mode:

Command Purpose

Router(config-if)# appletalk cable-range cable-range [network.node]

Assigns the AppleTalk cable range and zone for the subinterface.

Router(config-if)# appletalk zone zone-name

Assigns the AppleTalk zone for the subinterface.


Configuring Banyan VINES Routing over ISL

Banyan VINES can be routed over VLAN subinterfaces using the ISL encapsulation protocol. The Banyan VINES Routing over ISL Virtual LANs feature provides full-feature Cisco IOS software Banyan VINES support on a per-VLAN basis, allowing standard Banyan VINES capabilities to be configured on VLANs.

To route Banyan VINES over ISL between VLANs, you need to configure ISL encapsulation on the subinterface. Perform the tasks described in the following sections in the order in which they appear:

Enabling Banyan VINES Routing

Defining the VLAN Encapsulation Format

Configuring Banyan VINES on the Subinterface

Enabling Banyan VINES Routing

To begin the VINES routing configuration, use the following command in global configuration mode:

Command Purpose

Router(config)# vines routing [address]

Enables Banyan VINES routing globally.


Defining the VLAN Encapsulation Format

To define the VINES routing encapsulation format, use the following commands in interface configuration mode:

  Command Purpose

Step 1 

Router(config-if)# interfacetype slot/port.subinterface-number

Specifies the subinterface on which ISL will be used.

Step 2 

Router(config-if)# encapsulation islvlan-identifier

Defines the encapsulation format as ISL (isl), and specifies the VLAN identifier.

Configuring Banyan VINES on the Subinterface

After you enable Banyan VINES globally and define the encapsulation format, you need to enable VINES on the subinterface by specifying the VINES routing metric. To enable the Banyan VINES protocol on the subinterface, use the following command in interface configuration mode:

Command Purpose

Router(config-if)# vines metric [whole [fractional]]

Enables VINES routing on an interface.


Configuring DECnet Routing over ISL

DECnet can be routed over VLAN subinterfaces using the ISL VLAN encapsulation protocols. The DECnet Routing over ISL Virtual LANs feature provides full-feature Cisco IOS software DECnet support on a per-VLAN basis, allowing standard DECnet capabilities to be configured on VLANs.

To route DECnet over ISL VLANs, you need to configure ISL encapsulation on the subinterface. Perform the tasks described in the following sections in the order in which they appear.

Enabling DECnet Routing

Defining the VLAN Encapsulation Format

Configuring DECnet on the Subinterface

Enabling DECnet Routing

To begin the DECnet routing configuration, use the following command in global configuration mode:

Command Purpose

Router(config)# decnet[network-number] routing [decnet-address]

Enables DECnet on the router.


Defining the VLAN Encapsulation Format

To define the encapsulation format, use the following commands in interface configuration mode:

  Command Purpose

Step 1 

Router(config-if)# interfacetype slot/port.subinterface-number

Specifies the subinterface on which ISL will be used.

Step 2 

Router(config-if)# encapsulation islvlan-identifier

Defines the encapsulation format as ISL (isl), and specifies the VLAN identifier.

Configuring DECnet on the Subinterface

To configure DECnet routing on the subinterface, use the following command in interface configuration mode:

Command Purpose

Router(config-if)# decnet cost [cost-value]

Enables DECnet routing on an interface.


Configuring the Hot Standby Router Protocol over ISL

The Hot Standby Router Protocol (HSRP) provides fault tolerance and enhanced routing performance for IP networks. HSRP allows Cisco IOS routers to monitor each other's operational status and very quickly assume packet forwarding responsibility in the event the current forwarding device in the HSRP group fails or is taken down for maintenance. The standby mechanism remains transparent to the attached hosts and can be deployed on any LAN type. With multiple Hot Standby groups, routers can simultaneously provide redundant backup and perform loadsharing across different IP subnets.

Figure 78 illustrates HSRP in use with ISL providing routing between several VLANs.

Figure 78 Hot Standby Router Protocol in VLAN Configurations

A separate HSRP group is configured for each VLAN subnet so that Cisco IOS router A can be the primary and forwarding router for VLANs 10 and 20. At the same time, it acts as backup for VLANs 30 and 40. Conversely, Router B acts as the primary and forwarding router for ISL VLANs 30 and 40, as well as the secondary and backup router for distributed VLAN subnets 10 and 20.

Running HSRP over ISL allows users to configure redundancy between multiple routers that are configured as front ends for VLAN IP subnets. By configuring HSRP over ISLs, users can eliminate situations in which a single point of failure causes traffic interruptions. This feature inherently provides some improvement in overall networking resilience by providing load balancing and redundancy capabilities between subnets and VLANs.

To configure HSRP over ISLs between VLANs, you need to create the environment in which it will be used. Perform the tasks described in the following sections in the order in which they appear.

Defining the Encapsulation Format

Defining the IP Address

Enabling HSRP

Defining the Encapsulation Format

To define the encapsulation format as ISL, use the following commands in interface configuration mode:

  Command Purpose

Step 1 

Router(config-if)# interfacetype slot/port.subinterface-number

Specifies the subinterface on which ISL will be used.

Step 2 

Router(config-if)# encapsulation islvlan-identifier

Defines the encapsulation format, and specifies the VLAN identifier.

Defining the IP Address

After you have specified the encapsulation format, to define the IP address over which HSRP will be routed, use the following command in interface configuration mode:

Command Purpose

Router(config-if)# ip address ip-address mask [secondary]

Specifies the IP address for the subnet on which ISL will be used.


Enabling HSRP

To enable HSRP on an interface, enable the protocol, then customize it for the interface. Use the following command in interface configuration mode:

Command Purpose

Router(config-if)# standby [group-number] ip [ip-address [secondary]]

Enables HSRP.



Note For more information on HSRP, see the "Configuring IP Services" chapter in the Cisco IOS IP Configuration Guide.


To customize Hot Standby group attributes, use the following commands in interface configuration mode, as needed:

Command Purpose

Router(config-if)# standby [group-number] timershellotime holdtime

Configures the time between hello packets and the hold time before other routers declare the active router to be down.

Router(config-if)# standby [group-number] prioritypriority

Sets the Hot Standby priority used to choose the active router.

Router(config-if)# standby [group-number] preempt

Specifies that if the local router has priority over the current active router, the local router should attempt to take its place as the active router.

Router(config-if)# standby [group-number] tracktype-number [interface-priority]

Configures the interface to track other interfaces, so that if one of the other interfaces goes down, the Hot Standby priority for the device is lowered.

Router(config-if)# standby [group-number] authenticationstring

Selects an authentication string to be carried in all HSRP messages.


Configuring IP Routing over TRISL

The IP routing over TRISL VLANs feature extends IP routing capabilities to include support for routing IP frame types in VLAN configurations.

Enabling IP Routing

IP routing is automatically enabled in the Cisco IOS software for routers. To reenable IP routing if it has been disabled, use the following command in global configuration mode: 

Command Purpose

Router(config)# ip routing

Enables IP routing on the router.


Once you have IP routing enabled on the router, you can customize the characteristics to suit your environment. If necessary, refer to the IP configuration chapters in the Cisco IOS IP Routing Configuration Guide for guidelines on configuring IP.

Defining the VLAN Encapsulation Format

To define the encapsulation format as TRISL, use the following commands in interface configuration mode:

  Command Purpose

Step 1 

Router(config-if)# interfacetype slot/port.subinterface-number

Specifies the subinterface on which TRISL will be used.

Step 2 

Router(config-if)# encapsulation tr-isl trbrf-vlan vlanid bridge-num bridge-number

Defines the encapsulation for TRISL.

The DRiP database is automatically enabled when TRISL encapsulation is configured, and at least one TrBRF is defined, and the interface is configured for SRB or for routing with RIF.

Assigning IP Address to Network Interface

An interface can have one primary IP address. To assign a primary IP address and a network mask to a network interface, use the following command in interface configuration mode:

Command Purpose

Router(config-if)# ip address ip-address mask

Sets a primary IP address for an interface.


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 then referred to as a subnet mask.


Note TRISL encapsulation must be specified for a subinterface before an IP address can be assigned to that subinterface.


Configuring IPX Routing on 802.10 VLANs over ISL

The IPX Encapsulation for 802.10 VLAN feature provides configurable IPX (Novell-FDDI, SAP, SNAP) encapsulation over 802.10 VLAN on router FDDI interfaces to connect the Catalyst 5000 VLAN switch. This feature extends Novell NetWare routing capabilities to include support for routing all standard IPX encapsulations for Ethernet frame types in VLAN configurations. Users with Novell NetWare environments can now configure any one of the three IPX Ethernet encapsulations to be routed using Secure Data Exchange (SDE) encapsulation across VLAN boundaries. IPX encapsulation options now supported for VLAN traffic include the following:

Novell-FDDI (IPX FDDI RAW to 802.10 on FDDI)

SAP (IEEE 802.2 SAP to 802.10 on FDDI)

SNAP (IEEE 802.2 SNAP to 802.10 on FDDI)

NetWare users can now configure consolidated VLAN routing over a single VLAN trunking FDDI interface. Not all IPX encapsulations are currently supported for SDE VLAN. The IPX interior encapsulation support can be achieved by messaging the IPX header before encapsulating in the SDE format. Fast switching will also support all IPX interior encapsulations on non-MCI platforms (for example non-AGS+ and non-7000). With configurable Ethernet encapsulation protocols, users have the flexibility of using VLANs regardless of their NetWare Ethernet encapsulation. Configuring Novell IPX encapsulations on a per-VLAN basis facilitates migration between versions of Netware. NetWare traffic can now be routed across VLAN boundaries with standard encapsulation options (arpa, sap, and snap) previously unavailable. Encapsulation types and corresponding framing types are described in the "Configuring Novell IPX" chapter of the Cisco IOS AppleTalk and Novell IPX Configuration Guide.


Note Only one type of IPX encapsulation can be configured per VLAN (subinterface). The IPX encapsulation used must be the same within any particular subnet; a single encapsulation must be used by all NetWare systems that belong to the same VLAN.


To configure Cisco IOS software on a router with connected VLANs to exchange different IPX framing protocols, perform the tasks described in the following sections in the order in which they are appear:

Enabling NetWare Routing

Defining the VLAN Encapsulation Format

Configuring NetWare on the Subinterface

Enabling NetWare Routing

To enable IPX routing on SDE interfaces, use the following command in global configuration mode:

Command Purpose

Router(config)# ipx routing [node]

Enables IPX routing globally.


Defining the VLAN Encapsulation Format

To define the encapsulation format as SDE, use the following commands in interface configuration mode:

  Command Purpose

Step 1 

Router(config)# interface fddi slot/port.subinterface-number

Specifies the subinterface on which SDE will be used.

Step 2 

Router(config-if)# encapsulation sde vlan-identifier

Defines the encapsulation format and specifies the VLAN identifier.

Configuring NetWare on the Subinterface

After you enable NetWare globally and define the VLAN encapsulation format, to enable the subinterface by specifying the NetWare network number (if necessary) and the encapsulation type, use the following command in interface configuration mode:

Command Purpose

Router(config-if)# ipx network network encapsulation encapsulation-type

Specifies the IPX encapsulation among Novell-FDDI, SAP, or SNAP.


Configuring IPX Routing over TRISL

The IPX Routing over ISL VLANs feature extends Novell NetWare routing capabilities to include support for routing all standard IPX encapsulations for Ethernet frame types in VLAN configurations. Users with Novell NetWare environments can configure either SAP or SNAP encapsulations to be routed using the TRISL encapsulation across VLAN boundaries. The SAP (Novell Ethernet_802.2) IPX encapsulation is supported for VLAN traffic.

NetWare users can now configure consolidated VLAN routing over a single VLAN trunking interface. With configurable Ethernet encapsulation protocols, users have the flexibility of using VLANs regardless of their NetWare Ethernet encapsulation. Configuring Novell IPX encapsulations on a per-VLAN basis facilitates migration between versions of Netware. NetWare traffic can now be routed across VLAN boundaries with standard encapsulation options (sap and snap) previously unavailable. Encapsulation types and corresponding framing types are described in the "Configuring Novell IPX" chapter of the Cisco IOS AppleTalk and Novell IPX Configuration Guide.


Note Only one type of IPX encapsulation can be configured per VLAN (subinterface). The IPX encapsulation used must be the same within any particular subnet: A single encapsulation must be used by all NetWare systems that belong to the same LANs.


To configure Cisco IOS software to exchange different IPX framing protocols on a router with connected VLANs, perform the tasks described in the following sections in the order in which they are appear:

Enabling NetWare Routing

Defining the VLAN Encapsulation Format

Configuring NetWare on the Subinterface

Enabling NetWare Routing

To enable IPX routing on TRISL interfaces, use the following command in global configuration mode:

Command Purpose

Router(config)# ipx routing [node]

Enables IPX routing globally.


Defining the VLAN Encapsulation Format

To define the encapsulation format as TRISL, use the following commands in interface configuration mode:

  Command Purpose

Step 1 

Router(config-if)# interfacetype slot/port.subinterface-number

Specifies the subinterface on which TRISL will be used.

Step 2 

Router(config-if)# encapsulation tr-isl trbrf-vlan trbrf-vlan bridge-num bridge-num

Defines the encapsulation for TRISL.

Configuring NetWare on the Subinterface

After you enable NetWare globally and define the VLAN encapsulation format, to enable the subinterface by specifying the NetWare network number (if necessary) and the encapsulation type, use the following command in interface configuration mode:

Command Purpose

Router(config-if)# ipx network network encapsulation encapsulation-type

Specifies the IPX encapsulation.



Note The default IPX encapsulation format for Cisco IOS routers is "novell-ether" (Novell Ethernet_802.3). If you are running Novell Netware 3.12 or 4.0, the new Novell default encapsulation format is Novell Ethernet_802.2 and you should configure the Cisco router with the IPX encapsulation format "sap."


Configuring VIP Distributed Switching over ISL

With the introduction of the VIP distributed ISL feature, ISL encapsulated IP packets can be switched on Versatile Interface Processor (VIP) controllers installed on Cisco 7500 series routers.

The second generation VIP2 provides distributed switching of IP encapsulated in ISL in VLAN configurations. Where an aggregation route performs inter-VLAN routing for multiple VLANs, traffic can be switched autonomously on-card or between cards rather than through the central Route Switch Processor (RSP). Figure 79 shows the VIP distributed architecture of the Cisco 7500 series router.

Figure 79 Cisco 7500 Distributed Architecture

This distributed architecture allows incremental capacity increases by installation of additional VIP cards. Using VIP cards for switching the majority of IP VLAN traffic in multiprotocol environments substantially increases routing performance for the other protocols because the RSP offloads IP and can then be dedicated to switching the non-IP protocols.

VIP distributed switching offloads switching of ISL VLAN IP traffic to the VIP card, removing involvement from the main CPU. Offloading ISL traffic to the VIP card substantially improves networking performance. Because you can install multiple VIP cards in a router, VLAN routing capacity is increased linearly according to the number of VIP cards installed in the router.

To configure distributed switching on the VIP, you must first configure the router for IP routing. Perform the tasks described in the following sections in the order in which they appear:

Enabling IP Routing

Enabling VIP Distributed Switching

Configuring ISL Encapsulation on the Subinterface

Enabling IP Routing

To enable IP routing, use the following command in global configuration mode:

Command Purpose

Router(config)# ip routing

Enables IP routing on the router.


Once you have IP routing enabled on the router, you can customize the characteristics to suit your environment. Refer to the IP configuration chapters in the Cisco IOS IP Routing Configuration Guide for guidelines on configuring IP.

Enabling VIP Distributed Switching

To enable VIP distributed switching, use the following commands beginning in interface configuration mode:

  Command Purpose

Step 1 

Router(config-if)# interfacetype slot/port-adapter/port

Specifies the interface and interface configuration mode.

Step 2 

Router(config-if)# ip route-cache distributed

Enables VIP distributed switching of IP packets on the interface.

Configuring ISL Encapsulation on the Subinterface

To configure ISL encapsulation on the subinterface, use the following commands in interface configuration mode:

  Command Purpose

Step 1 

Router(config-if)# interfacetype slot/port-adapter/port

Specifies the interface, and enters interface configuration mode.

Step 2 

Router(config-if)# encapsulation isl vlan-identifier

Defines the encapsulation format as ISL, and specifies the VLAN identifier.

Configuring XNS Routing over ISL

XNS can be routed over VLAN subinterfaces using the ISL VLAN encapsulation protocol. The XNS Routing over ISL Virtual LANs feature provides full-feature Cisco IOS software XNS support on a per-VLAN basis, allowing standard XNS capabilities to be configured on VLANs.

To route XNS over ISL VLANs, you need to configure ISL encapsulation on the subinterface. Perform the tasks described in the following sections in the order in which they appear:

Enabling XNS Routing

Defining the VLAN Encapsulation Format

Configuring XNS on the Subinterface

Enabling XNS Routing

To configure XNS routing, use the following command in global configuration mode:

Command Purpose

Router(config)# xns routing [address]

Enables XNS routing globally.


Defining the VLAN Encapsulation Format

To define the VLAN encapsulation format, use the following commands in interface configuration mode:

  Command Purpose

Step 1 

Router(config-if)# interfacetype slot/port.subinterface-number

Specifies the subinterface on which ISL will be used.

LIM homeobox genes have a prominent role in the regulation of neuronal subtype identity and distinguish motor neuron subclasses in the embryonic spinal cord. We have investigated the role of Isl-class LIM homeodomain proteins in motor neuron diversification using mouse genetic methods. All spinal motor neuron subtypes initially express both Isl1 and Isl2, but Isl2 is rapidly downregulated by visceral motor neurons. Mouse embryos lacking Isl2 function exhibit defects in the migration and axonal projections of thoracic level motor neurons that appear to reflect a cell-autonomous switch from visceral to somatic motor neuron character. Additional genetic mutations that reduce or eliminate both Isl1 and Isl2 activity result in more pronounced defects in visceral motor neuron generation and erode somatic motor neuron character. Thus, an early phase of high Isl expression and activity in newly generated motor neurons permits the diversification of visceral and somatic motor neuron subtypes in the developing spinal cord.

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