Troubleshooting STP

Some common things to look for when troubleshooting Spanning Tree Protocol include:

• Duplex mismatch—When one side of a link is half-duplex and the other is full-duplex. This causes late collisions and FCS errors.

• Unidirectional link failure—The link is up but data flows only in one direction. It can cause loops.

• Frame corruption—Physical errors on the line cause BPDUs to be lost, and the port incorrectly begins forwarding. This is caused by duplex mismatch, bad cable, or cable too long.

• Resource errors—STP is implemented in software, so a switch with an overloaded CPU or memory might neglect some STP duties.

• Port Fast configuration errors—Connecting a switch to two ports that have Port Fast enabled. This can cause a loop.

• STP tuning errors—Max age or forward delay set too short can cause a loop. A network diameter that is set too low causes BPDUs to be discarded and affects STP convergence.

Identifying a Bridging Loop

Suspect a loop if you see the following:

• You capture traffic on a link, and see the same frames multiple times.

• All users in a bridging domain have connectivity problems at the same time.

• There is abnormally high port utilization.

To remedy a loop quickly, shut redundant ports and then enable them one at a time. Some switches allow debugging of STP (not 3550/2950) to help in diagnosing problems.


What to Use Where

Confused by all the acronyms and STP features? Figure 3-3 shows the STP features you might use in your network and where you might use them.

Additional Spanning Tree Features

Some additional features available to help you tune Spanning Tree include:

• BPDU Guard

• BPDU Filtering

• Root Guard

• UDLD

• Loop Guard

BPDU Guard

BPDU Guard is used to prevent loops if another switch is attached to a Portfast port. When BPDU Guard is enabled on an interface, it is put into an error-disabled state (basically, shut down) if a BPDU is received on the interface. It can be enabled at either global config mode—in which case it affects all Portfast interfaces, or at interface mode. Portfast does not have to be enabled for it to be configured at a specific interface. The following configuration example shows BPDU guard being enabled.


BPDU Filtering

BPDU filtering is another way of preventing loops in the network. It also can be enabled either globally or at the interface, and functions differently at each. In global config, if a Portfast interface receives any BPDUs, it is taken out of Portfast status. At interface config mode, it prevents the port from sending or receiving BPDUs. The commands are:

• (config)# spanning-tree portfast bpdufilter default

• (config-if)# spanning-tree bpdufilter enable

Root Guard

Root Guard is meant to prevent the wrong switch from becoming the Spanning Tree root. It is enabled on ports other than the root port and on switches other than the root. If a Root Guard port receives a BPDU that might cause it to become a root port, then the port is put into “rootinconsistent” state and does not pass traffic through it. If the port stops receiving these BPDUs, it automatically re-enables itself.

(config-if)# spanning-tree guard root


Unidirectional Link Detection (UDLD)

A switch notices when a physical connection is broken by the absence of Layer 1 electrical keepalives (Ethernet calls this a link beat). However, sometimes a cable is intact enough to maintain keepalives, but not to pass data in both directions. This is a Unidirectional Link.
Unidirectional Link Detection (UDLD) detects a unidirectional link by sending periodic hellos out to the interface. It also uses probes, which must be acknowledged by the device on the other end of the link. UDLD operates at Layer 2. The port is shut down if a unidirectional link is found.

To enable UDLD on all fiber-optic interfaces, use the following command:
(config)# udld enable

Although this command is given at global config mode, it applies only to fiber ports.

To enable UDLD on non-fiber ports, give the same command at interface config mode.

To disable UDLD on a specific fiber port, use the following command:
(config-if)# udld disable


To disable UDLD on a specific non-fiber port, use the following command:
(config-if)#no udld enable


To re-enable all interfaces shut by UDLD, use the following:
#udld reset

To verify UDLD status, use the following:
#show udld interface


Loop Guard

Loop Guard prevents loops that might develop if a port that should be blocking inadvertently transitions to the forwarding state. This can happen if the port stops receiving BPDUs (perhaps because of a unidirectional link or a software/configuration problem in its neighbor switch). When one of the ports in a physically redundant topology stops receiving BPDUs, the STP conceives the topology as loop-free. Eventually, the blocking port becomes designated and moves to forwarding state, thus creating a loop. With Loop Guard enabled, an
additional check is made.

If no BPDUs are received on a blocked port for a specific length of time. Loop Guard puts that port into “loop inconsistent” blocking state, rather than transitioning to forwarding state. Loop Guard should be enabled on all switch ports that have a chance of becoming root or designated ports. It is most effective when enabled in the entire switched network in conjunction with UDLD.

To enable Loop Guard for all point-to-point links on the switch, use the following command:

(config)# spanning-tree loopguard default


To enable Loop Guard on a specific interface, use the following:

(config-if)# spanning-tree guard loop

Loop Guard automatically re-enables the port if it starts receiving BPDUs again.

EtherChannels

EtherChannel is a way of combining several physical links between switches into one logical connection. Normally, Spanning Tree blocks redundant links; EtherChannel gets around that and allows load balancing across those links. Load is balancing on the basis of such things as source or destination MAC address or IP address. The Etherchannel load-balancing method is configured at global configuration mode.

(config)#port-channel load-balance type

A logical interface—the Port Channel interface—is created. Configuration can be applied to both the logical and physical interfaces.

Some guidelines for EtherChannels are as follows:

• Interfaces in the channel do not have to be physically next to each other or on the same module.

• All ports must be the same speed and duplex.

• All ports in the bundle should be enabled.

• None of the bundle ports can be a SPAN port.

• Assign an IP address to the logical Port Channel interface, not the physical ones.

• Put all bundle ports in the same VLAN, or make them all trunks.

• If they are trunks, they must all carry the same VLANs and use the same trunking mode.

• Configuration you apply to the Port Channel interface affects the entire EtherChannel. Configuration you apply to a physical interface only affects that interface.

Configuring an EtherChannel

Basically, for a Layer 3 EtherChannel, you should configure the logical interface and then put the physical interfaces into the channel group:

(config)#interface port-channel number
(config-if)#no switchport
(config-if)#ip address address mask

Then, at each port that is part of the EtherChannel, use the following:

(config)#interface { number | range interface – interface}
(config-if)#channel-group number mode {auto | desirable | on}

Putting the IP address on the Port Channel interface creates a Layer 3 EtherChannel. Simply putting interfaces into a channel group creates a Layer 2 EtherChannel, and the logical interface is automatically created.

The Cisco proprietary Port Aggregation Protocol (PAgP) dynamically negotiates the formation of a channel. There are three PAgP modes:

• On—The port channels without using PAgP negotiation. The port on the other side must also be set to On.

• Auto—Responds to PAgP messages but does not initiate them. Port channels if the port on the other end is set to Desirable. This is the default mode.

• Desirable—Port actively negotiates channeling status with the interface on the other end of the link. Port channels if the other side is Auto or Desirable.

There is also a non-proprietary protocol called Link Aggregation Control Protocol (LACP), IEEE 802.3ad, which does the same thing. LACP has two modes:

• Active—Port actively negotiates channeling with the port on the other end of the link. A channel forms if the other side is Passive or Active.

• Passive—Responds to LACP messages but does not initiate them. A channel forms if the other end is set to Active.

If you want to use LACP, specify it under the interface and put the interface in either active or passive mode:

(config-if)#channel-protocol lacp


Verifying an EtherChannel

Some typical commands for verifying include:

• #show running-config interface number

• #show interfaces number etherchannel

• #show etherchannel number port-channel

• #show etherchannel summary

Spanning Tree and PVST

With PVST (Per Vlan STP), there is a different instance of STP for each VLAN. To derive the VLAN BID, the switch picks a different MAC address from its base pool for each VLAN. Each VLAN has its own root bridge, root port, and so on. You can configure these so that data flow is optimized, and traffic load is balanced among the switches. Spanning Tree is enabled by default on every VLAN.


Configuring Spanning Tree

To change the STP priority value, use the following:

Switch (config)#spanning-tree vlan vlan_no. priority value


To configure a switch as root without manually changing priority values, use the following:

Switch (config)# spanning-tree vlan vlan_no. root {primary | secondary}


To change the STP port cost for an access port, use the following:

Switch(config-if)# spanning-tree cost value


To change the STP port cost for a VLAN on a trunk port, use the following:

Switch(config-if)# spanning-tree vlan vlan_no. cost value


To display STP information for a VLAN, use the following:

Switch# show spanning-tree vlan vlan_no.


To display the STP information for an interface, use the following:

Switch # show spanning-tree interface interface_no. [detail]


To verify STP timers, use the following:

Switch #show spanning-tree bridge brief


Spanning Tree Enhancements

Cisco has some proprietary enhancements to Spanning Tree that help speed up network convergence. They include:

• PortFast

• UplinkFast

• BackboneFast

Portfast

Portfast is for access (user) ports only. It causes the port to bypass the STP listening and learning states and transition directly to forwarding. Connecting a switch to a Portfast port can cause loops to develop.

(config-if)#spanning-tree portfast


UplinkFast

UplinkFast is for speeding convergence when a direct link to an upstream switch fails. The switch identifies backup ports for the root port (these are called an uplink group). If the root port fails, then one of the ports in the uplink group is unblocked and transitions immediately to forwarding—it bypasses the listening and learning stages. It should be used in wiring closet switches with at least one blocked port. The command to enable uplinkfast is shown below. Please note that uplinkfast is enabled globally, so the command affects all ports and all VLANs.

(config)# spanning-tree uplinkfast


BackboneFast

BackboneFast is used for speeding convergence when a link fails that is not directly connected to the switch. It helps the switch detect indirect failures. If a switch running BackboneFast receives an inferior BPDU from its designated bridge, it knows a link on the path to the root has failed. (An inferior BPDU is one that lists the same switch for root
bridge and designated bridge.)

The switch then tries to find an alternate path to the root by sending a Root Link Query (RLQ) frame out all alternate ports. The root then responds with an RLQ response, and the port receiving this response can transition to forwarding. Alternate ports are determined in this way:

• If the inferior BPDU was received on a blocked port, then the root port and any other blocked ports are considered alternates.

• If the inferior BPDU was received on the root port, then all blocked ports are considered alternates.

• If the inferior BPDU was received on the root port and there are no blocked ports, the switch assumes it has lost connectivity with the root and advertises itself as root.

Configure this command on all switches in the network:

(config)#spanning-tree backbonefast


Rapid Spanning Tree (RSTP)

Rapid Spanning Tree (RSTP) 802.1w is a standards-based, non-proprietary way of speeding STP convergence. Switch ports exchange an explicit handshake when they transition to forwarding. RSTP describes different port states than regular STP, as shown in the Table 3-3.



RSTP Port Roles

RSTP also defines different Spanning Tree roles for ports:

• Root port—The best path to the root (same as STP).

• Designated port—Same role as with STP.

• Alternate port—A backup to the root port.

• Backup port—A backup to the designated port.

• Disabled port—One not used in the Spanning Tree.

• Edge port—One connected only to an end user.

BPDU Differences in RSTP

In regular STP, BPDUs are originated by the root and relayed by each switch. In RSTP, each switch originates BPDUs, whether or not it receives a BPDU on its root port. All eight bits of the BPDU type field are used by RSTP. The TC and TC Ack bits are still used. The other six bits specify the port’s role and its RSTP state, and are used in the port handshake. The RSTP BPDU is set to Type 2, Version 2. PVST is done by Rapid PVST+ on Catalyst switches.


RSTP Fast Convergence

The Rapid Spanning tree process understands and incorporates topology changes much quicker than the previous version.

• RSTP uses a mechanism similar to BackboneFast—When an inferior BPDU is received, the switch accepts it. If the switch has another path the root, it uses that and informs its downstream switch of the alternate path.

• Edge ports work the same as Portfast ports—They automatically transition directly to forwarding.

• Link type—If you connect two switches through a point-to-point link and the local port becomes a designated port, it exchanges a handshake with the other port to quickly transition to forwarding. Full-duplex links are assumed to be point-to-point, half-duplex links are assumed to be shared.
  

• Backup and alternate ports—Ports that can transition to forwarding when no BPDUs are received from a neighbor switch (similar to UplinkFast).

If an RSTP switch detects a topology change, it sets a TC timer to twice the hello time and sets the TC bit on all BPDUs sent out to its designated and root ports until the timer expires. It also clears the MAC addresses learned on these ports.

If an RSTP switch receives a TC BPDU, it clears the MAC addresses on that port and sets the TC bit on all BPDUs sent out its designated and root ports until the TC timer expires.


Multiple Spanning Tree (MST)

With Multiple Spanning Tree (MST), you can group VLANs and run one instance of Spanning Tree for a group of VLANs. This cuts down on the number of root bridges, root ports, designated ports, and BPDUs in your network. Switches in the same MST Region share the same configuration and VLAN mappings. Configure MST with these commands:


To be compatible with 802.1Q trunking, which has one common Spanning Tree (CST) for all VLANs, MST runs one instance of an Internal Spanning Tree (IST). The IST appears as one bridge to a CST area and is MST instance number 0. The original MST Spanning Trees (called M-Trees) are active only in the region—they combine at the edge of the CST area to form one.

Spanning Tree

Ethernet network design balances two separate imperatives. First, Ethernet has no capacity for detecting circular paths. If such paths exist, traffic loops around and accumulates until new traffic is shut out (this is called a broadcast storm). Second, having secondary paths is good preparation for inevitable link failure.

Spanning Tree is a protocol that prevents loop formation by detecting redundant links and disabling them until needed. Designers can therefore build redundant links and the protocol will allow one to pass traffic and keep the other in reserve. When the active link fails, the secondary link is enabled quickly.


Understanding the Spanning Tree Protocol

Switches either forward or filter Layer 2 frames. The way they make the forwarding/filtering decision can lead to loops in a network with redundant links. Spanning Tree is a protocol that detects potential loops and breaks them.

A Layer 2 switch is functionally the same thing as a transparent bridge. Transparent bridges:

• Learn MAC (Media Access Control) addresses by looking at the source address of incoming frames. They build a table mapping MAC address to port number.

• Forward broadcasts and multicasts out all ports except the on which they came. (This is called flooding.)

• Forward unknown unicasts out all ports except the one on which they came. An unknown unicast is a message bound for a unicast MAC address that is not in the switch’s table of addresses and ports.

• Do not make any changes to the frames as they forward them.

Spanning Tree Protocol (STP) works by selecting a root bridge, then selecting one loop-free path from the root bridge to every other switch. (STP uses the term bridge because it was written before there were switches.) Consider the following switched network (see Figure 3-1).


Spanning Tree must select:

• One root bridge

• One root port per nonroot bridge

• One designated port per network segment

Spanning Tree Election Criteria

Spanning Tree builds paths out from a central point along the fastest available links. It selects path according to the following criteria:

1. Lowest root bridge ID (BID)
2. Lowest path cost to the root
3. Lowest sender bridge ID
4. Lowest sender port ID (PID)

When reading the path selection criteria, remember the following:

• Bridge ID—Bridge priority: Bridge MAC address.

• Bridge priority—2-btye value, 0–65,535 (0–0xFFFF).

• Default priority is 32,768 (0x8000).

• Port ID—Port priority: port number.

• Port priority—A 6-bit value, 0–63, default is 32.

• Path cost—This is the cumulative value of the cost of each link between the bridge and the root. Cost values were updated in 2000 and you should see only new cost values, but both are given in the following table (see Table 3-1). Old and new switches work together.

The STP Election

Spanning Tree builds paths out from a starting point, the “root” of the tree. The first step in selecting paths is to identify this root device. Then, each device selects its best path back to the root, according to the criteria laid out in the previous sections (lowest root BID, lowest cost, lowest advertising BID, lowest port).


Root Bridge Election

Looking at Figure 3-1, first select the root bridge. Assume each switch uses the default priority.

• Switch A BID = 80-00-00-0c-11-11-00-11

• Switch B BID = 80-00-00-0c-26-78-10-10

• Switch C BID = 80-00-00-0c-32-1a-bc-de

• Switch D BID = 80-00-00-0c-81-81-11-22

• Switch E BID = 80-00-00-0c-26-79-22-22

Switch A has the lowest BID, so it is the root. Each nonroot switch must now select a root port.


Root Port Election

The root port is the port that leads back to the root. Continuing with Figure 3-1, once A is acknowledged as the root, the remaining bridges sort out their lowest cost path back to the A.

• Switch B—Uses the link to A with a cost of 19 (link speed of 100 Mbps).

• Switch C—The connected link has a cost of 100 (Ethernet), the link through B has a path cost of 38 (two 100 Mbps links), and so B is chosen.

• Switch D—The link through B has a path cost of 119, the path cost through C to A is 119, the path through C then B is 57, so C is chosen.

• Switch E—The lowest path cost is the same for both ports (76 through D to C to B to A). Next check sender BID—sender for both ports is D, so that it does not break the tie. Next check sender Port ID. Assuming default port priority, the PID for 0/1 is lower than the PID for 0/2, so the port on the left is the root port.

Designated Port Election

Designated ports are ports that lead away from the root. Obviously, all ports on the root bridge are designated ports (A-B and A-C in Figure 3-1).

• Segment B-D—B has the lowest path cost to root (19 vs 119), so it is designated for this segment.

• Segment C-D—C has the lowest path cost to the root (100 vs 119), so it is designated for this segment.

• Segment B-C—B has the lowest path cost to the root (19 vs 100), so it is designated for this segment.

• Both segments D-E—D has the lowest cost to the root (57 vs 76), so it is designated for both segments.

Now the looped topology has been turned into a tree with A at the root. Notice that there are no more redundant links.


Bridge Protocol Data Units (BPDUs)

Switches exchange BPDUs. There are two types of BPDUs:
Configuration and Topology Change (TCN).

Configuration BPDUs are sent every two seconds from the root toward the downstream switches. They:

• Are used during an election.

• Maintain connectivity between switches.

• Send timer information from the root.

TCN BPDUs are sent toward the root when:

• There is a link failure.

• A port starts forwarding, and there is already a designated port.

• The switch receives a TCN from a neighbor.

When a switch receives a TCN BPDU, it acknowledges that with a configuration BPDU that has the TCN Acknowledgment bit set.

When the root bridge receives a TCN, it starts sending configuration BPDUs with the TCN bit set for a period of time equal to max age plus forward delay. Switches that receive this change their MAC table aging time to the Forward Delay time, causing MAC addresses to age faster. The topology change also causes an election of the root bridge, root ports, and designated ports.


BPDU Fields

Some of the fields in the BPDU include:

• Root bridge ID—The BID of the current root.

• Sender’s root path cost—The cost to the root.

• Sender’s bridge ID—Sender’s priority concatenated to MAC.

• Sender’s port ID—The port number, transmitted as final tiebreaker.

• Hello time—Two seconds by default.

• Forward Delay—15 seconds by default.

• Max Age—20 seconds by default.

Spanning Tree Port States

When a port is first activated, it transitions through the following stages shown in Table 3-2.




Designing for Spanning Tree

To optimize data flow in the network, design and configure switches for the following STP roles:

• Primary and secondary root bridges (set priority values)

• Designated and root ports (set port priorities/path cost)

• Enable STP enhancements, such as Root Guard

802.1Q Tunnels

Tunneling is a way to send 802.1Q-tagged frames across a foreign network (such as a Service Provider’s network) and still preserve the original 802.1Q tag. The SP configures their end of the trunk link as a tunnel port and assigns a VLAN to carry your traffic within their network. The SP switch then adds a second 802.1Q tag to each frame that came in the tunnel port. Other switches in the SP network see only this second tag, and do not read the original tag. When the frame exits the SP network, the extra tag is removed, leaving the original 802.1Q tag to be read by the receiving switch in your network.


Layer 2 Protocol Tunneling (GBPT)

If a Service Provider separates sections of your network, you can use Layer 2 protocol tunneling to tunnel CDP, Spanning Tree Protocol (STP), and VLAN Trunking Protocol (VTP) frames across the SP’s cloud. This is called Generic Bridge PDU Tunneling (GBPT). Frames from the above control protocols are encapsulated as they enter the SP’s network on a tunnel port, and de-encapsulated when they exit that network.


Troubleshooting Trunking

Troubleshooting trunking links happens mostly at the physical and datalink layers. Start with the most basic assumptions and work your way “up” the OSI model. It is important to show that physical layer connectivity is present, before moving on to, for instance before trying to troubleshoot IP problems.

• Are both sides of the link in the correct trunking mode?

• Is the same trunk encapsulation on both sides?

• If 802.1Q, is the same native VLAN on both sides?

• Are the same VLANs permitted on both sides?

VLAN Trunking Protocol (VTP)

VTP is a protocol that runs over trunk links and synchronizes the VLAN databases of all switches in the VTP domain. A VTP domain is an administrative group—all switches within that group must have the same VTP domain name configured or they do not synchronize databases.
VTP works by using Configuration Revision numbers and VTP advertisements:

• All switches send out VTP advertisements every five minutes, or when there is a change to the VLAN database (when a VLAN is created, deleted, or renamed).

• VTP advertisements contain a Configuration Revision number. This number is increased by one for every VLAN change.

• When a switch receives a VTP advertisement, it compares the Configuration Revision number against the one in its VLAN database.

• If the new number is higher, the switch overwrites its database with the new VLAN information, and forwards the information to its neighbor switches.

• If the number is the same, the switch ignores the advertisement.

• If the new number is lower, the switch replies with the more up-to-date information contained in its own database.

VTP Switch Roles

A switch can be a VTP:

• Server—The default VTP role. Servers can create, delete, and rename VLANs. They originate both periodic and triggered VTP advertisements and synchronize their databases with other switches in the domain.
  

• Client—Clients cannot make VLAN changes. They originate periodic VTP advertisements and synchronize their databases with other switches in the domain.

• Transparent—It can create, delete, and rename VLANs, but its VLANs are only local. It does not originate advertisements or synchronize its database with any other switches. It forwards VTP advertisements out its trunk links, however.

VTP Pruning

By default, switches flood broadcasts, multicasts, and unknown unicasts across trunk links. Suppose a host in VLAN 10 on Switch B sends a broadcast. Hosts in VLAN 10 on Switch C need to see that broadcast, but Switch A has no ports in VLAN 10, so it doesn’t need to receive the broadcast traffic.

Enabling VTP pruning causes the switch to keep track of VLAN port assignments in its downstream switches. The switch then sends flooded traffic only on trunks toward switches that have ports assigned to the VLAN originating the traffic. It prunes flooded traffic from all other trunks. VTP pruning increases the available bandwidth by preventing unnecessary traffic on trunk links.

There are two versions of VTP: Version 1 and Version 2. To use Version 2, all switches in the domain must be capable of using it. Configure one server for Version 2, and the information is propagated through VTP.
Version 2 has the following added features:

• It supports Token Ring VLANs.

• Transparent switches pass along messages from both versions of VTP.

• Consistency checks are performed only when changes are configured through the CLI or SNMP.

Configuring VTP

VTP configuration is done at the global config mode. To configure the switch’s VTP mode:
(config)#vtp {server | client |transparent}

To configure the VTP domain name:
(config)#vtp domain name

To configure a VTP password (all switches in the domain must use the same password):
(config)#vtp password password

To configure the switch to use VTP Version 2:
(config)#vtp version 2

To enable pruning:
vtp pruning

To specify which VLANs are to be pruned:
(config-if)#switchport trunk pruning vlan {add | except | none | remove} vlan-list [,vlan[,vlan[,,,]]


Verifying and Monitoring VTP

To get basic information about the VTP configuration, use show vtp status. The example shows the default settings:

Troubleshooting VTP

The following are some common things to check when troubleshooting problems with VTP:

• Make sure you are trunking between the switches. VTP is sent only over trunk links.

• Make sure the domain name matches on both switches (name is case sensitive).

• If the switch is not updating its database, make sure it is not in transparent mode.

• If using passwords, make sure they all match. To remove a password, use no vtp password.

Adding a New Switch to a VTP Domain

Adding a new switch in client mode does not prevent it from propagating its incorrect VLAN information. A server synchronizes to a client if the client has the higher configuration revision number. You must reset the revision number back to 0 on the new switch. The easiest way to do this is to change the domain name. Then change it back to the correct one, and attach the switch to the network.

VLAN Trunking

A trunk is a link that carries traffic for more than one VLAN. Trunks multiplex traffic from multiple VLANs. Trunks connect switches and allow ports on multiple switches to be assigned to the same VLAN.

Two methods of identifying VLANs over trunk links are:

• Inter-Switch Link (ISL)—A Cisco proprietary method that encapsulates the original frame in a header, which contains VLAN information. It is protocol-independent and can identify Cisco Discovery Protocol (CDP) and bridge protocol data unit (BPDU) frames.
  

• 802.1Q—Standards-based, tags the frames (inserts a field into the original frame immediately after the source MAC address field), and supports Ethernet and Token Ring networks.

When a frame comes into a switch port, the frame is tagged internally within the switch with the VLAN number of the port. When it reaches the outgoing port, the internal tag is removed. If the exit port is a trunk port, then its VLAN is identified in either the ISL encapsulation or the 802.1Q tag. The switch on the other end of the trunk removes the ISL or 802.1Q information, checks the VLAN of the frame, and adds the internal tag. If the exit port is a user port, then the original frame is sent out unchanged, making the use of VLANs transparent to the user.

If a nontrunking port receives an ISL-encapsulated frame, the frame is dropped. If the ISL header and footer cause the MTU size to be exceeded, it might be counted as an error.

If a nontrunking port receives an 802.1Q frame, the source and destination MAC addresses are read, the tag field is ignored, and the frame is switched normally at Layer 2.


Configuring a Trunk Link

Ports can become trunk ports either by static configuration or dynamic negotiation using Dynamic Trunking Protocol (DTP). A switch port can be in one of five DTP modes:

• Access—The port is a user port in a single VLAN.
• Trunk—The port negotiates trunking with the port on the other end of the link.
• Non-negotiate—The port is a trunk and does not do DTP negotiation with the other side of the link.
• Dynamic Desirable—Actively negotiates trunking with the other side of the link. It becomes a trunk if the port on the other switch is set to trunk, dynamic desirable, or dynamic auto mode.

Configure a port for trunking at the interface configuration mode:

(config-if)#switchport mode {dynamic {auto | desirable} | trunk}

If dynamic mode is used, DTP negotiates the trunking state and encapsulation. If trunk mode is used, you must specify encapsulation:

(config-if)#switchport trunk encapsulation {isl | dot1q | negotiate}


Native VLAN with 802.1Q

If you are using 802.1Q, specify a native VLAN for the trunk link with the command:

(config-if)#switchport trunk native vlan vlan no

Frames from the native VLAN are sent over the trunk link untagged. Native VLAN is the VLAN the port would be in if it were not a trunk, and it must match on both sides of the trunk link. VLAN 1 is the default native VLAN for all ports.


VLAN Mapping

ISL trunking recognizes only VLANs numbered 1–1001, but 802.1Q can use VLANs 0–4094. If you are using both ISL and 802.1Q in your network and have VLANs numbered above 1001, you have to map the 802.1Q VLANS to ISL numbers. Some rules about mapping VLANs include:

• You can configure only eight mappings.
• Mappings are local to the switch; the same mappings must be configured on all switches in the network.
• You can map only to Ethernet ISL VLANs.
• The 802.1Q VLANs with the same number as mapped ISL VLANs are blocked. (For example, you map 802.1Q VLAN 1500 to ISL VLAN 150, then 802.1Q VLAN 150 is blocked on that switch.)
• You should not map the 802.1Q native VLAN.

VLANs Allowed on the Trunk

By default, a trunk carries traffic for all VLANs. You can change that behavior for a particular trunk link by giving the following command at the interface config mode:

switchport trunk allowed vlan vlans

Make sure that both sides of a trunk link allow the same VLANs.


Verifying a Trunk Link

Two commands you can use to verify your trunk configuration are:

#show running-config
#show interfaces [interface no.] switchport | trunk


Using the trunk keyword with the show interfaces command gives information about the trunk link: