Connecting VLAN Trunks with ISL
Problem
You want to connect an InterSwitch Link (ISL) VLAN trunk to your router.
Solution
The following set of commands allows you to connect an ISL trunk to your router:
Router1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router1(config)#interface FastEthernet0/0 Router1(config-if)#no ip address Router1(config-if)#speed 100 Router1(config-if)#full-duplex Router1(config-if)#exit Router1(config)#interface FastEthernet0/0.1 Router1(config-subif)#encapsulation isl 1 Router1(config-subif)#ip address 172.25.1.5 255.255.255.0 Router1(config-subif)#exit Router1(config)#interface FastEthernet0/0.2 Router1(config-subif)#encapsulation isl 2 Router1(config-subif)#ip address 172.16.2.1 255.255.255.0 Router1(config-subif)#exit Router1(config)#interface FastEthernet0/0.3 Router1(config-subif)#encapsulation isl 574 Router1(config-subif)#ip address 10.22.1.2 255.255.255.0 Router1(config-subif)#exit Router1(config)#end Router1#
Discussion
A trunk is a point-to-point link containing one ore more Virtual LANs (VLANs). The main purpose of a trunk is to save physical interfaces. Without VLANs, if you wanted to connect two LAN segments into a router, you would need two Ethernet interfaces. Unfortunately, this does not scale well. It is relatively common for a switch to support many VLANs. So there is a clear advantage to using trunks to bundle the VLANs together into a single link.
While trunks carry traffic for many different VLANs, they are able to keep them separate by tagging each frame with the unique identification number for the appropriate VLAN. This allows traffic from multiple LAN segments to share the same physical link without any danger of frames leaking onto the wrong segment. When a network device receives a tagged frame from a trunk link, it removes the tag, and then forwards the frame to the appropriate LAN segment as a normal frame.
When you connect a router to a trunk, it can route Layer 3 packets between the various VLANs on the trunk. Because of the VLAN tagging scheme, Layer 2 frames cannot pass from one VLAN to another. So, without a router device of some kind, there is no way to interconnect the VLANs. The configuration where a router is connected to a trunk, to allow routing between the different VLANs, is often called a "router on a stick" because the router routes its packets back out onto the same physical interface that it received them through.
Cisco routers support two main trunking protocols, ISL and 802.1Q. ISL is a Cisco proprietary protocol, so you can only use it between Cisco devices. Conversely, 802.1Q is an IEEE open standard that is supported by most manufacturers of network hardware. Recipe 16.13 shows how to configure an 802.1Q trunk interface on a router.
Unfortunately, the 802.1Q open standard reached the market some time after the initial demand for trunking protocols. So most manufacturers of Layer 2 switching equipment developed their own proprietary standards to fill the void. Cisco developed ISL. All newer Cisco equipment now supports 802.1Q, but there are still many older Catalyst switches that cannot support the open standard, so ISL is your only choice with this equipment. In any case, Cisco's ISL support is generally more mature and stable than its 802.1Q implementations. So while we generally recommend working with open standards where possible, ISL is still clearly the more viable option in some networks.
The first step when configuring a trunk on a router is to select a physical LAN interface to connect the trunk to. In general, we don't recommend using anything slower than a Fast Ethernet interface for this purpose:
Router1(config)#interface FastEthernet0/0 Router1(config-if)#no ip address Router1(config-if)#speed 100 Router1(config-if)#full-duplex
As you can see, there is no special configuration necessary on the physical interface.
Then you need to create one subinterface on this physical interface for each different VLAN. Because each VLAN represents a different Layer 3 network, you need to give each of the subinterfaces IP addresses from the corresponding IP subnets:
Router1(config)#interface FastEthernet0/0.1 Router1(config-subif)#encapsulation isl 1 Router1(config-subif)#ip address 172.25.1.5 255.255.255.0
The encapsulation command associates this subinterface with a particular ISL VLAN number. ISL VLAN numbers can have any value between 1 and 1,000. With this subinterface configured, the router is now able to route packets for any devices on this VLAN, exactly as if it were directly connected to the physical LAN segment.
The show vlans command displays information about all of the VLANs configured on the router:
Router1#show vlans Virtual LAN ID: 1 (Inter Switch Link Encapsulation) vLAN Trunk Interface: FastEthernet0/0.1 Protocols Configured: Address: Received: Transmitted: IP 172.25.1.5 203626 342261 Virtual LAN ID: 2 (Inter Switch Link Encapsulation) vLAN Trunk Interface: FastEthernet0/0.2 Protocols Configured: Address: Received: Transmitted: IP 172.16.2.1 0 153807 Virtual LAN ID: 574 (Inter Switch Link Encapsulation) vLAN Trunk Interface: FastEthernet0/0.3 Protocols Configured: Address: Received: Transmitted: IP 10.22.1.2 0 6 Router1#
We have configured this router to support three different VLANs, each with its own subinterface and its own IP address. The subinterface number does not necessarily need to correspond to the VLAN ID, as we have assigned VLAN number 574 to subinterface FastEthernet0/0.3. But if you make it a general rule to always keep the subinterface number the same as the VLAN number, it will make maintenance and troubleshooting considerably simpler in a large network.
It is useful to remember that you don't need to create a distinct subinterface for every VLAN on the switch. There may be some VLANs on this switch that you don't wish to terminate on the router. In this case, the router will simply ignore any frames that are tagged with VLAN numbers that it doesn't support.
You can use the show interfaces command to see information about the trunking configuration of a particular subinterface:
Router1#show interfaces FastEthernet0/0.3 FastEthernet0/0.3 is up, line protocol is up Hardware is AmdFE, address is 0001.9670.b780 (bia 0001.9670.b780) Internet address is 10.22.1.2/24 MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation ISL Virtual LAN, Color 574. ARP type: ARPA, ARP Timeout 04:00:00 Router1#
This shows the encapsulation type (ISL) and the VLAN number (574), along with the interface's IP address information.
Cisco added support for IP unnumbered addressing of VLAN subinterfaces in Version 12.2(4)T:
Router1(config)#interface FastEthernet0/0.1 Router1(config-if)#ip unnumbered Loopback0
See Also
Recipe 16.13
Router Configuration and File Management
- Configuring the Router via TFTP
- Saving Router Configuration to Server
- Booting the Router Using a Remote Configuration File
- Storing Configuration Files Larger Than NVRAM
- Clearing the Startup Configuration
- Loading a New IOS Image
- Booting a Different IOS Image
- Booting over the Network
- Copying an IOS Image to a Server
- Copying an IOS Image Through the Console
- Deleting Files from Flash
- Partitioning Flash
- Using the Router as a TFTP Server
- Using FTP from the Router
- Generating Large Numbers of Router Configurations
- Changing the Configurations of Many Routers at Once
- Extracting Hardware Inventory Information
- Backing Up Router Configurations
- Warm Reload
- Warm Upgrade
- Configuration Archiving
- Locking Configuration Access
Router Management
- Creating Command Aliases
- Managing the Routers ARP Cache
- Tuning Router Buffers
- Auto Tuning Buffers
- Using the Cisco Discovery Protocol
- Disabling the Cisco Discovery Protocol
- Using the Small Servers
- Enabling HTTP Access to a Router
- Enabling Secure HTTP (HTTPS) Access to a Router
- Using Static Hostname Tables
- Enabling Domain Name Services
- Disabling Domain Name Lookups
- Specifying a Router Reload Time
- Scheduling of Router Commands
- Displaying Historical CPU Values
- Creating Exception Dump Files
- Generating a Report of Interface Information
- Generating a Report of Routing Table Information
- Generating a Report of ARP Table Information
- Generating a Server Host Table File
User Access and Privilege Levels
- Setting Up User IDs
- Encrypting Passwords
- Using Better Password-Encryption Techniques
- Removing Passwords from a Router Configuration File
- Deciphering Ciscos Weak Password Encryption
- Displaying Active Users
- Sending Messages to Other Users
- Changing the Number of VTYs
- Changing VTY Timeouts
- Restricting VTY Access by Protocol
- Enabling Absolute Timeouts on VTY Lines
- Implementing Banners
- Disabling Banners on a Port
- Disabling Router Lines
- Reserving a VTY Port for Administrative Access
- Restricting Inbound Telnet Access
- Logging Telnet Access
- Setting the Source Address for Telnet
- Automating the Login Sequence
- Using SSH for Secure Access
- Changing Privilege Level of IOS Commands
- Defining Per User Privileges
- Defining Per Port Privileges
TACACS+
- Authenticating Login IDs from a Central System
- Restricting Command Access
- Losing Access to the TACACS+ Server
- Disabling TACACS+ Authentication on a Particular Line
- Capturing User Keystrokes
- Logging System Events
- Setting the IP Source Address for TACACS+ Messages
- Sample Server Configuration Files
IP Routing
- Finding an IP Route
- Finding Types of IP Routes
- Converting Different Mask Formats
- Using Static Routing
- Floating Static Routes
- Using Policy-Based Routing to Route Based on Source Address
- Using Policy-Based Routing to Route Based on Application Type
- Examining Policy-Based Routing
- Changing Administrative Distances
- Routing Over Multiple Paths with Equal Costs
- Static Routes That Track Interfaces or Other Routes
- Keeping Statistics on Routing Table Changes
RIP
- Configuring RIP Version 1
- Filtering Routes with RIP
- Redistributing Static Routes into RIP
- Redistributing Routes Using Route Maps
- Creating a Default Route in RIP
- Disabling RIP on an Interface
- Default Passive Interface
- Unicast Updates for RIP
- Applying Offsets to Routes
- Adjusting Timers
- Configuring Interpacket Delay
- Enabling Nonperiodic Updates
- Increasing the RIP Input Queue
- Configuring RIP Version 2
- Enabling RIP Authentication
- RIP Route Summarization
- Route Tagging
EIGRP
- Configuring EIGRP
- Filtering Routes with EIGRP
- Redistributing Routes into EIGRP
- Redistributing Routes into EIGRP Using Route Maps
- Disabling EIGRP on an Interface
- Adjusting EIGRP Metrics
- Adjusting Timers
- Enabling EIGRP Authentication
- EIGRP Route Summarization
- Logging EIGRP Neighbor State Changes
- Limiting EIGRPs Bandwidth Utilization
- EIGRP Stub Routing
- Route Tagging
- Viewing EIGRP Status
OSPF
- Configuring OSPF
- Filtering Routes in OSPF
- Adjusting OSPF Costs
- Creating a Default Route in OSPF
- Redistributing Static Routes into OSPF
- Redistributing External Routes into OSPF
- Manipulating DR Selection
- Setting the OSPF RID
- Enabling OSPF Authentication
- Selecting the Appropriate Area Types
- Using OSPF on Dial Interfaces
- Summarizing Routes in OSPF
- Disabling OSPF on Certain Interfaces
- Changing the Network Type on an Interface
- OSPF Route Tagging
- Logging OSPF Adjacency Changes
- Adjusting OSPF Timers
- Reducing OSPF Traffic in Stable Networks
- OSPF Virtual Links
- Viewing OSPF Status with Domain Names
- Debugging OSPF
BGP
- Configuring BGP
- Using eBGP Multihop
- Adjusting the Next-Hop Attribute
- Connecting to Two ISPs
- Connecting to Two ISPs with Redundant Routers
- Restricting Networks Advertised to a BGP Peer
- Adjusting Local Preference Values
- Load-Balancing
- Removing Private ASNs from the AS Path
- Filtering BGP Routes Based on AS Paths
- Reducing the Size of the Received Routing Table
- Summarizing Outbound Routing Information
- Prepending ASNs to the AS Path
- Redistributing Routes with BGP
- Using Peer Groups
- Authenticating BGP Peers
- Using BGP Communities
- Using BGP Route Reflectors
- Putting It All Together
Frame Relay
- Setting Up Frame Relay with Point-to-Point Subinterfaces
- Adjusting LMI Options
- Setting Up Frame Relay with Map Statements
- Using Multipoint Subinterfaces
- Configuring Frame Relay SVCs
- Simulating a Frame Relay Cloud
- Compressing Frame Relay Data on a Subinterface
- Compressing Frame Relay Data with Maps
- PPP over Frame Relay
- Viewing Frame Relay Status Information
Handling Queuing and Congestion
- Fast Switching and CEF
- Setting the DSCP or TOS Field
- Using Priority Queuing
- Using Custom Queuing
- Using Custom Queues with Priority Queues
- Using Weighted Fair Queuing
- Using Class-Based Weighted Fair Queuing
- Using NBAR Classification
- Controlling Congestion with WRED
- Using RSVP
- Manual RSVP Reservations
- Aggregating RSVP Reservations
- Using Generic Traffic Shaping
- Using Frame-Relay Traffic Shaping
- Using Committed Access Rate
- Implementing Standards-Based Per-Hop Behavior
- AutoQoS
- Viewing Queue Parameters
Tunnels and VPNs
- Creating a Tunnel
- Tunneling Foreign Protocols in IP
- Tunneling with Dynamic Routing Protocols
- Viewing Tunnel Status
- Creating an Encrypted Router-to-Router VPN in a GRE Tunnel
- Creating an Encrypted VPN Between the LAN Interfaces of Two Routers
- Generating RSA Keys
- Creating a Router-to-Router VPN with RSA Keys
- Creating a VPN Between a Workstation and a Router
- Creating an SSL VPN
- Checking IPSec Protocol Status
Dial Backup
- Automating Dial Backup
- Using Dialer Interfaces
- Using an Async Modem on the AUX Port
- Using Backup Interfaces
- Using Dialer Watch
- Using Virtual Templates
- Ensuring Proper Disconnection
- View Dial Backup Status
- Debugging Dial Backup
NTP and Time
- Time-Stamping Router Logs
- Setting the Time
- Setting the Time Zone
- Adjusting for Daylight Saving Time
- Synchronizing the Time on All Routers (NTP)
- Configuring NTP Redundancy
- Setting the Router As the NTP Master for the Network
- Changing NTP Synchronization Periods
- Using NTP to Send Periodic Broadcast Time Updates
- Using NTP to Send Periodic Multicast Time Updates
- Enabling and Disabling NTP Per Interface
- NTP Authentication
- Limiting the Number of Peers
- Restricting Peers
- Setting the Clock Period
- Checking the NTP Status
- Debugging NTP
- NTP Logging
- Extended Daylight Saving Time
- NTP Server Configuration
DLSw
- Simple Bridging
- Configuring DLSw
- Using DLSw to Bridge Between Ethernet and Token Ring
- Converting Ethernet and Token Ring MAC Addresses
- Configuring SDLC
- Configuring SDLC for Multidrop Connections
- Using STUN
- Using BSTUN
- Controlling DLSw Packet Fragmentation
- Tagging DLSw Packets for QoS
- Supporting SNA Priorities
- DLSw+ Redundancy and Fault Tolerance
- Viewing DLSw Status Information
- Viewing SDLC Status Information
- Debugging DSLw
Router Interfaces and Media
- Viewing Interface Status
- Configuring Serial Interfaces
- Using an Internal T1 CSU/DSU
- Using an Internal ISDN PRI Module
- Using an Internal 56 Kbps CSU/DSU
- Configuring an Async Serial Interface
- Configuring ATM Subinterfaces
- Setting Payload Scrambling on an ATM Circuit
- Classical IP Over ATM
- Configuring Ethernet Interface Features
- Configuring Token Ring Interface Features
- Connecting VLAN Trunks with ISL
- Connecting VLAN Trunks with 802.1Q
- LPD Printer Support
Simple Network Management Protocol
- Configuring SNMP
- Extracting Router Information via SNMP Tools
- Recording Important Router Information for SNMP Access
- Using SNMP to Extract Inventory Information from a List of Routers
- Using Access Lists to Protect SNMP Access
- Logging Unauthorized SNMP Attempts
- Limiting MIB Access
- Using SNMP to Modify a Routers Running Configuration
- Using SNMP to Copy a New IOS Image
- Using SNMP to Perform Mass Configuration Changes
- Preventing Unauthorized Configuration Modifications
- Making Interface Table Numbers Permanent
- Enabling SNMP Traps and Informs
- Sending Syslog Messages As SNMP Traps and Informs
- Setting SNMP Packet Size
- Setting SNMP Queue Size
- Setting SNMP Timeout Values
- Disabling Link Up/Down Traps per Interface
- Setting the IP Source Address for SNMP Traps
- Using RMON to Send Traps
- Enabling SNMPv3
- Strong SNMPv3 Encryption
- Using SAA
Logging
- Enabling Local Router Logging
- Setting the Log Size
- Clearing the Routers Log
- Sending Log Messages to Your Screen
- Using a Remote Log Server
- Enabling Syslog on a Unix Server
- Changing the Default Log Facility
- Restricting What Log Messages Are Sent to the Server
- Setting the IP Source Address for Syslog Messages
- Logging Router Syslog Messages in Different Files
- Maintaining Syslog Files on the Server
- Testing the Syslog Sever Configuration
- Preventing the Most Common Messages from Being Logged
- Rate-Limiting Syslog Traffic
- Enabling Error Log Counting
- XML-Formatted Log Messages
- Modifying Log Messages
Access-Lists
- Filtering by Source or Destination IP Address
- Adding a Comment to an ACL
- Filtering by Application
- Filtering Based on TCP Header Flags
- Restricting TCP Session Direction
- Filtering Multiport Applications
- Filtering Based on DSCP and TOS
- Logging When an Access-List Is Used
- Logging TCP Sessions
- Analyzing ACL Log Entries
- Using Named and Reflexive Access-Lists
- Dealing with Passive Mode FTP
- Using Time-Based Access-Lists
- Filtering Based on Noncontiguous Ports
- Advanced Access-List Editing
- Filtering IPv6
DHCP
- Using IP Helper Addresses for DHCP
- Limiting the Impact of IP Helper Addresses
- Using DHCP to Dynamically Configure Router IP Addresses
- Dynamically Allocating Client IP Addresses via DHCP
- Defining DHCP Configuration Options
- Defining DHCP Lease Periods
- Allocating Static IP Addresses with DHCP
- Configuring a DHCP Database Client
- Configuring Multiple DHCP Servers per Subnet
- DHCP Static Mapping
- DHCP-Secured IP Address Assignment
- Showing DHCP Status
- Debugging DHCP
NAT
- Configuring Basic NAT Functionality
- Allocating External Addresses Dynamically
- Allocating External Addresses Statically
- Translating Some Addresses Statically and Others Dynamically
- Using Route Maps to Refine Static Translation Rules
- Translating in Both Directions Simultaneously
- Rewriting the Network Prefix
- Using NAT for Server Load Distribution
- Stateful NAT Failover
- Adjusting NAT Timers
- Changing TCP Ports for FTP
- Checking NAT Status
- Debugging NAT
First Hop Redundancy Protocols
- Configuring Basic HSRP Functionality
- Using HSRP Preempt
- Making HSRP React to Problems on Other Interfaces
- Load-Balancing with HSRP
- Redirecting ICMP with HSRP
- Manipulating HSRP Timers
- Using HSRP on Token Ring
- HSRP SNMP Support
- Increasing HSRP Security
- Showing HSRP State Information
- Debugging HSRP
- HSRP Version 2
- VRRP
- Gateway Load-Balancing Protocol
IP Multicast
- Configuring Basic Multicast Functionality with PIM-DM
- Routing Multicast Traffic with PIM-SM and BSR
- Routing Multicast Traffic with PIM-SM and Auto-RP
- Filtering PIM Neighbors
- Configuring Routing for a Low-Frequency Multicast Application
- Multicast over Frame Relay or ATM WANs
- Configuring CGMP
- Using IGMP Version 3
- Static Multicast Routes and Group Memberships
- Routing Multicast Traffic with MOSPF
- Routing Multicast Traffic with DVMRP
- DVMRP Tunnels
- Configuring Bidirectional PIM
- Controlling Multicast Scope with TTL
- Controlling Multicast Scope with Administratively Scoped Addressing
- Exchanging Multicast Routing Information with MBGP
- Using MSDP to Discover External Sources
- Configuring Anycast RP
- Converting Broadcasts to Multicasts
- Showing Multicast Status
- Debugging Multicast Routing
IP Mobility
- Local Area Mobility
- Home Agent Configuration
- Foreign Agent Configuration
- Making a Router a Mobile Node
- Reverse-Tunnel Forwarding
- Using HSRP for Home Agent Redundancy
IPv6
- Automatically Generating IPv6 Addresses for an Interface
- Manually Configuring IPv6 Addresses on an Interface
- Configuring DHCP for IPv6
- Dynamic Routing with RIP
- Modifying the Default RIP Parameters
- IPv6 Route Filtering and Metric Manipulation in RIP
- Using OSPF for IPv6
- IPv6 Route Filtering and Metric Manipulation in OSPF
- Route Redistribution
- Dynamic Routing with MBGP
- Tunneling IPv6 Through an Existing IPv4 Network
- Translating Between IPv6 and IPv4
MPLS
- Configuring a Basic MPLS P Router
- Configuring a Basic MPLS PE Router
- Configuring Basic MPLS CE Routers
- Configuring MPLS over ATM
- PE-CE Communication via RIP
- PE-CE Communication via OSPF
- PE-CE Communication via EIGRP
- PE-CE Communication via BGP
- QoS over MPLS
- MPLS Traffic Engineering with Autoroute
- Multicast Over MPLS
- Your Service Provider Doesnt Do What You Want
Security
- Using AutoSecure
- Using Context-Based Access-Lists
- Transparent Cisco IOS Firewall
- Stopping Denial of Service Attacks
- Inspecting Applications on Different Port Numbers
- Intrusion Detection and Prevention
- Login Password Retry Lockout
- Authentication Proxy
Appendix 1. External Software Packages
Appendix 2. IP Precedence, TOS, and DSCP Classifications
- IP Precedence, TOS, and DSCP Classifications
- Queueing Algorithms
- Dropping Packets and Congestion Avoidance
Index
EAN: 2147483647
Pages: 505
