Catalyst 6500 Release 12.2SY Software Configuration Guide

EVC Overview

Ethernet virtual circuits (EVCs) define a Layer 2 bridging architecture that supports Ethernet services. An EVC is defined by the Metro-Ethernet Forum (MEF) as an association between two or more user network interfaces that identifies a point-to-point or multipoint-to-multipoint path within the service provider network. An EVC is a conceptual service pipe within the service provider network. A bridge domain is a local broadcast domain that exists separately from VLANs.

Ethernet Flow Points

An Ethernet flow point (EFP) service instance is a logical interface that connects a bridge domain to a physical port or to an EtherChannel. Configuring a service instance on a Layer 2 port creates a pseudoport or EFP on which you configure EVC features. Each service instance has a unique number per interface, but you can use the same number on different interfaces because service instances on different ports are not related.

An EFP classifies frames from the same physical port to one of the multiple service instances associated with that port, based on user-defined criteria. Each EFP can be associated with different forwarding actions and behavior.

The three major characteristics (or parameters) of an EFP are

• Encapsulation
• Rewrite Information
• Forwarding instance or method (bridge-domain or xconnect)

An EVC broadcast domain is determined by a bridge domain and the EFPs that are attached to it. An incoming frame is matched against EFP matching criteria on the interface, learned on the matching EFP, and forwarded to one or more EFPs in the bridge domain. If there are no matching EFPs, the frame is dropped.

You can use EFPs to configure VLAN translation. For example, if there are two EFPs egressing the same interface, each EFP can have a different VLAN rewrite operation, which is more flexible than the traditional switch port VLAN translation model.

When an EFP is created, the initial state is UP. The state changes to DOWN under the following circumstances:

• The EFP is explicitly shut down by a user.
• The main interface to which the EFP is associated is down or removed.
• If the EFP belongs to a bridge domain, the bridge domain is down.
• The EFP is forced down as an error-prevention measure of certain features.

Service Instances and EFPs

Configuring a service instance on a Layer 2 port or EtherChannel creates a pseudoport or Ethernet flow point (EFP) on which you configure EVC features. Each service instance has a unique number per interface, but you can use the same number on different interfaces because service instances on different ports are not related.

If you have defined an EVC by entering the ethernet evc evc-id global configuration command, you can associate the EVC with the service instance (optional). There is no default behavior for a service instance. You can configure a service instance only on trunk ports with no allowed VLANs. Any other configuration is not allowed. After you have configured a service instance on an interface, switchport commands are not allowed on the interface. You can also configure a service instance on an EtherChannel group.

Use the service instance number ethernet [ name ] interface configuration command to create an EFP on a Layer 2 interface or EtherChannel and to enter service instance configuration mode. You use service instance configuration mode to configure all management and control date plane attributes and parameters that apply to the service instance on a per-interface basis.

• The service instance number is the EFP identifier, an integer from 1 to 4000.
• The optional ethernet name is the name of a previously configured EVC. You do not need to enter an EVC name, but you must enter ethernet. Different EFPs can share the same name when they correspond to the same EVC. EFPs are tied to a global EVC through the common name.

When you enter service instance configuration mode, you can configure these options:

• default —Sets a command to its defaults
• description —Adds a service instance specific description
• encapsulation —Configures Ethernet frame match criteria
• errdisable —Configures error disable
• ethernet —Configures Ethernet-lmi parameters
• exit —Exits from service instance configuration mode
• l2protocol —Configures Layer 2 control protocol processing
• mac —Commands for MAC address-based features
• no —Negates a command or sets its defaults
• service-policy —Attaches a policy-map to an EFP
• shutdown —Takes the service instance out of service

Enter the [ no ] shutdown service-instance configuration mode to shut down or bring up a service instance.

On a Layer 2 port with no service instance configured, multiple switchport commands are available ( access, backup, block, host, mode, and trunk). When one or more service instances are configured on a Layer 2 port, no switchport commands are accepted on that interface.

Encapsulation (Flexible Service Mapping)

Encapsulation defines the matching criteria that map any of these in any combination to a service instance:

• A VLAN
• A range of VLANs
• The class of service (CoS) bits
• The Ethertype

VLAN tags and CoS can be a single value, a range, or a list. Ethertype can be a single type or a list of types. These are the encapsulation types:

• default
• dot1q
• priority-tagged
• untagged

Priority-tagged frames are always single-tagged. All Ethernet traffic is supported. The encapsulation classification options are:

• inner tag CoS
• inner tag VLAN

When you configure an encapsulation method, you enable flexible service mapping, which allows you to map an incoming packet to an EFP based on the configured encapsulation.

The default behavior for flexible service mapping based on the outer 802.1q VLAN tag value is nonexact, meaning that when the EFP encapsulation configuration does not explicitly specify an inner (second) VLAN tag matching criterion, the software maps both single-tagged and double-tagged frames to the EFP as long as the frames fulfill the criteria of outer VLAN tag values. The command-line interface (CLI) does allow you to specify exact mapping with the exact keyword. If this keyword is specified, the EFP is designated as single-tagged-frame-only and double-tagged frames are not classified to that EFP.

Using the CLI encapsulation command in service-instance configuration mode, you can set encapsulation criteria. You must configure one encapsulation command per EFP (service instance). After you have configured an encapsulation method, these commands are available in service instance configuration mode:

• bridge-domain —Configures a bridge domain.
• rewrite —Configures Ethernet rewrite criteria.

If a packet entering a port does not match any of the encapsulations on that port, the packet is dropped, resulting in filtering of the packet. The encapsulation must match the packet on the wire to determine filtering criteria. On the wire refers to packets ingressing the switch before any rewrites and to packets egressing the switch after all rewrites.

EFPs and MSTP

EFP bridge domains are supported by the Multiple Spanning Tree Protocol (MSTP). These restrictions apply when running STP with bridge domains.

• All incoming VLANs (outer-most or single) mapped to a bridge domain must belong to the same MST instance or loops could occur.
• For all EFPs that are mapped to the same MST instance, you must configure backup EFPs on every redundant path to prevent loss of connectivity due to STP blocking a port.
• When STP mode is PVST+ or PVRST, EFP information is not passed to the protocol. EVC only supports only MSTP.
• Changing STP mode from MST to PVST+ or PVRST for a multicast port is not allowed.

Bridge Domains

• Bridge Domain Overview
• Ethernet MAC Address Learning
• Flooding of Layer 2 Frames for Unknown MAC and Broadcast Addresses
• Layer 2 Destination MAC Address-Based Forwarding
• MAC Address Aging
• MAC Address Table

Bridge Domain Overview

A bridge domain defines a broadcast domain internal to a platform and allows the decoupling of a broadcast domain from a VLAN. This decoupling enables per-port VLAN significance, thus removing the scalability limitations associated with a single per-device VLAN ID space. Frames received from one of the EFPs participating in a bridge domain matches are bridged.

A service instance must be attached to a bridge domain. Flooding and communication behavior of a bridge domain is similar to that of a VLAN domain. Bridge-domain membership is determined by which service instances have joined it (based on encapsulation criteria), while VLAN domain membership is determined by the VLAN tag in the packet.

Note You must configure encapsulation before you can configure the bridge domain.

IGMP snooping is enabled by default on the switch and on all VLANs but is automatically disabled on a VLAN when you configure a bridge domain under 4094. The switches support up to 124 bridge domains.

Ethernet MAC Address Learning

MAC address learning is always enabled and cannot be disabled.

Flooding of Layer 2 Frames for Unknown MAC and Broadcast Addresses

A Layer 2 frame with an unknown unicast or broadcast destination MAC address is flooded to all the EFPs in the bridge domain except to the originating EFP.

Replication of frames involves recirculating the frame several times. Recirculation negatively affect forwarding performance and reduce the packet forwarding rate for all features.

Layer 2 Destination MAC Address-Based Forwarding

When bridging is configured, a unicast frame received from an EFP is forwarded based on the destination Layer 2 MAC address. If the destination address is known, the frame is forwarded only to the EFP/NNI associated with the destination address.

Because the bridge and EFP configurations are interrelated, bridging is supported only on EFPs. To support multiple bridge domains, MAC address entries are associated with the bridge domain of the EFP. Only unicast MAC addresses need to be dynamically learned.

The EVC infrastructure does not modify frame contents.

MAC Address Aging

The dynamically learned MAC address entries in the MAC table are periodically aged out and entries that are inactive for longer than the configured time period are removed from the table. The supported range of aging-time values, in seconds, is 5 through 1000000, with a granularity of 1. The default is 8 minutes. The aging-time parameter can be configured per bridge domain and is a relative value. The value is the aging time relative to the time a frame was received with that MAC address.

MAC Address Table

The MAC address table is used to forward frames based on Layer 2 destination MAC addresses. The table consists of static MAC addresses downloaded from the route processor (RP) and the MAC addresses dynamically learned by the data path.

While the MAC Learning feature is enabled, an entry is added to the MAC table when a new unique MAC address is learned on the data path and an entry is deleted from the table when it is aged out.

Rewrite Operations

The rewrite command pushes the 802.1ad tag onto ingress packets to forward the packet on the 802.1ad cloud.

Enter the rewrite ingress tag push dot1ad vlan-id symmetric service-instance configuration mode command to specify the encapsulation of additional dot1ad tag on the frame ingress to the EFP.

Note The symmetric keyword is required to complete rewrite to configuration.

When you enter the symmetric keyword, the egress counterpart performs the inverse action and pushes (adds) the encapsulation VLAN.

Layer 3 and Layer 4 ACL Support

Configuring an ACL on an EFP is the same as configuring an ACL on other types of interfaces.

Note ACLs are not supported for packets prefixed with a Multiprotocol Label Switching (MPLS) header, including when an MPLS packet contains either Layer 3 or Layer 4 headers of supported protocols.

Advanced Frame Manipulation

The Advanced Frame Manipulation feature supports a PUSH operation that adds one VLAN tag to both the incoming and outgoing frames of an EFP.

When a VLAN tag exists and a new one is added, the CoS field of the new tag is set to the same value as the CoS field of the existing VLAN tag; otherwise, the CoS field is set to a default of 0. Using QoS marking configuration commands, you can change the CoS marking.

Egress Frame Filtering

Egress frame filtering is performed to ensure that frames exiting an EFP contain a Layer 2 header that matches the encapsulation characteristics associated with the EFP. This filtering is done primarily to prevent unintended frame leaks and is always enabled on EFPs.

Two Service Instances Joining the Same Bridge Domain

In this example, service instance 1 on interfaces Gigabit Ethernet 1/1 and 1/2 can bridge between each other.

Bridge Domains and VLAN Encapsulation

Use the VLAN ID configured with the rewrite ingress tag push dot1ad command as the bridge-domain number, rather than the VLAN ID configured with the encapsulation dot1q command, which can be the same or a different value.

Traffic cannot be forwarded if the the VLAN ID configured with the encapsulation dot1q commands do not match in a bridge domain. In this example, the service instances on Gigabit Ethernet 1/1 and 1/2 cannot forward between each other, because the encapsulation VLAN IDs do not match (filtering criteria). You can use the rewrite command to allow communication between these two.

Rewrite

In this example, the VLAN ID configured in the rewrite ingress tag push dot1ad command (4000 in the example) is pushed onto packets that match the VLAN ID configured in the encapsulation dot1q command (10 in the example). The symmetric keyword enables the inverse action on packets in the reverse direction: packets that egress from this service instance with VLAN ID 4000 are deencapsulated, resulting in VLAN ID 10 with cos 1.