References

This section provides additional information on understanding and implementing Layer 2 VPNs.

Gigabit Ethernet Protocol Standards

The 10-Gigabit Ethernet architecture and features deliver network scalability and performance, while enabling service providers to offer high-density, high-bandwidth networking solutions designed to interconnect the router with other systems in the point-of-presence (POP), including core and edge routers and L2 and Layer 3 (L3) switches.

The Gigabit Ethernet interfaces in Cisco NCS 5500 Series Routers support these standards:

  • Protocol standards:

    • IEEE 802.3 Physical Ethernet Infrastructure

    • IEEE 802.3ae 10 Gbps Ethernet

  • Ethernet standards

    • Ethernet II framing also known as DIX

    • IEEE 802.3 framing also includes LLC and LLC/SNAP protocol frame formats

    • IEEE 802.1q VLAN tagging

    • IEEE 802.1ad Provider Bridges

For more information, see Carrier Ethernet Model References.

Carrier Ethernet Model References

This topic covers the references for Gigabit Ethernet Protocol Standards.

IEEE 802.3 Physical Ethernet Infrastructure

The IEEE 802.3 protocol standards define the physical layer and MAC sublayer of the data link layer of wired Ethernet. IEEE 802.3 uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD) access at a variety of speeds over a variety of physical media. The IEEE 802.3 standard covers 10 Mbps Ethernet. Extensions to the IEEE 802.3 standard specify implementations for Gigabit Ethernet, 10-Gigabit Ethernet, and Fast Ethernet.

IEEE 802.3ae 10 Gbps Ethernet

Under the International Standards Organization’s Open Systems Interconnection (OSI) model, Ethernet is fundamentally a L2 protocol. 10-Gigabit Ethernet uses the IEEE 802.3 Ethernet MAC protocol, the IEEE 802.3 Ethernet frame format, and the minimum and maximum IEEE 802.3 frame size. 10 Gbps Ethernet conforms to the IEEE 802.3ae protocol standards.

Just as 1000BASE-X and 1000BASE-T (Gigabit Ethernet) remained true to the Ethernet model, 10-Gigabit Ethernet continues the natural evolution of Ethernet in speed and distance. Because it is a full-duplex only and fiber-only technology, it does not need the carrier-sensing multiple-access with the CSMA/CD protocol that defines slower, half-duplex Ethernet technologies. In every other respect, 10-Gigabit Ethernet remains true to the original Ethernet model.

General Ethernet Standards

  • IEEE 802.1q VLAN tagging—This standard defines VLAN tagging, and also the traditional VLAN trunking between switches. Cisco NCS 5500 Series Routers do NOT support ISL.

  • IEEE 802.1ad Provider Bridges—This standard is a subset of 802.1q and is often referred to as 802.1ad. Cisco NCS 5500 Series Routers do not adhere to the entire standard, but large portions of the standard's functionality are supported.

Ethernet MTU

The Ethernet Maximum Transmission Unit (MTU) is the size of the largest frame, minus the 4-byte Frame Check Sequence (FCS), that can be transmitted on the Ethernet network. Every physical network along the destination of a packet can have a different MTU.

Cisco NCS 5500 Series Routers support two types of frame forwarding processes:

  • Fragmentation for IPV4 packets—In this process, IPv4 packets are fragmented as necessary to fit within the MTU of the next-hop physical network.


    Note
    IPv6 does not support fragmentation.

    Note
    To allow hasing for L3 header only when traffic is fragmented, use hw-module profile load-balance algorithm L3-Only command.

  • MTU discovery process determines largest packet size—This process is available for all IPV6 devices, and for originating IPv4 devices. In this process, the originating IP device determines the size of the largest IPv6 or IPV4 packet that can be sent without being fragmented. The largest packet is equal to the smallest MTU of any network between the IP source and the IP destination devices. If a packet is larger than the smallest MTU of all the networks in its path, that packet will be fragmented as necessary. This process ensures that the originating device does not send an IP packet that is too large.

Jumbo frame support is automatically enable for frames that exceed the standard frame size. The default value is 1514 for standard frames and 1518 for 802.1Q tagged frames. These numbers exclude the 4-byte FCS.

Flow Control on Ethernet Interfaces

The flow control used on 10-Gigabit Ethernet interfaces consists of periodically sending flow control pause frames. It is fundamentally different from the usual full- and half-duplex flow control used on standard management interfaces. By default, both ingress and egress flow control are off on Cisco NCS 5500 Series Routers.

Default Configuration Values for Gigabit Ethernet and 10-Gigabit Ethernet

The below table describes the default interface configuration parameters that are present when an interface is enabled on a Gigabit Ethernet or 10-Gigabit Ethernet modular services card and its associated PLIM.


Note
You must use the shutdown command to bring an interface administratively down. The interface default is no shutdown. When a modular services card is first inserted into the router, if there is no established preconfiguration for it, the configuration manager adds a shutdown item to its configuration. This shutdown can be removed only be entering the no shutdown command.
Table 1. Gigabit Ethernet and 10-Gigabit Ethernet Modular Services Card Default Configuration Values
Parameter Configuration File Entry Default Value Restrictions

Flow control

flow-control

egress on ingress off

none

MTU

mtu

1514 bytes for normal frames

1518 bytes for 802.1Q tagged frames

1522 bytes for QinQ frames

none

MAC address

mac address

Hardware burned-in address (BIA2)

L3 only

L2 port

l2transport

off/L3

L2 subinterfaces must have L3 main parent interface

Egress filtering

Ethernet egress-filter

off

none

Link negotiation

negotiation

off

physical main interfaces only

Tunneling Ethertype

tunneling ethertype

0X8100

configured on main interface only; applied to subinterfaces only

VLAN tag matching

encapsulation

all frames for main interface; only ones specified for subinterfaces

encapsulation command only subinterfaces

  1. The restrictions are applicable to L2 main interface, L2 subinterface, L3 main interface, interflex L2 interface etc.

  2. burned-in address

References for Configuring Link Bundles

This section provides references to configuring link bundles. For an overview of link bundles and configurations, see Configure Link Bundles for Layer 2 VPNs.

Characteristics of Link Bundles

  • Any type of Ethernet interfaces can be bundled, with or without the use of LACP (Link Aggregation Control Protocol).

  • Physical layer and link layer configuration are performed on individual member links of a bundle.

  • Configuration of network layer protocols and higher layer applications is performed on the bundle itself.

  • A bundle can be administratively enabled or disabled.

  • Each individual link within a bundle can be administratively enabled or disabled.

  • Ethernet link bundles are created in the same way as Etherokinet channels, where the user enters the same configuration on both end systems.

  • The MAC address that is set on the bundle becomes the MAC address of the links within that bundle.

  • When LACP configured, each link within a bundle can be configured to allow different keepalive periods on different members.

  • Load balancing is done by flow instead of by packet. Data is distributed to a link in proportion to the bandwidth of the link in relation to its bundle.

  • QoS is supported and is applied proportionally on each bundle member.

  • Link layer protocols, such as CDP, work independently on each link within a bundle.

  • Upper layer protocols, such as routing updates and hello messages, are sent over any member link of an interface bundle.

  • Bundled interfaces are point to point.

  • A link must be in the UP state before it can be in distributing state in a bundle.

  • Access Control List (ACL) configuration on link bundles is identical to ACL configuration on regular interfaces.

  • Multicast traffic is load balanced over the members of a bundle. For a given flow, internal processes select the member link and all traffic for that flow is sent over that member.

Methods of Forming Bundles of Ethernet Interfaces

Cisco IOS-XR software supports the following methods of forming bundles of Ethernet interfaces:

  • IEEE 802.3ad—Standard technology that employs a Link Aggregation Control Protocol (LACP) to ensure that all the member links in a bundle are compatible. Links that are incompatible or have failed are automatically removed from a bundle.

    For each link configured as bundle member, information is exchanged between the systems that host each end of the link bundle:

    • A globally unique local system identifier

    • An identifier (operational key) for the bundle of which the link is a member

    • An identifier (port ID) for the link

    • The current aggregation status of the link

    This information is used to form the link aggregation group identifier (LAG ID). Links that share a common LAG ID can be aggregated. Individual links have unique LAG IDs.

    The system identifier distinguishes one router from another, and its uniqueness is guaranteed through the use of a MAC address from the system. The bundle and link identifiers have significance only to the router assigning them, which must guarantee that no two links have the same identifier, and that no two bundles have the same identifier.

    The information from the peer system is combined with the information from the local system to determine the compatibility of the links configured to be members of a bundle.

    Bundle MAC addresses in the routers come from a set of reserved MAC addresses in the backplane. This MAC address stays with the bundle as long as the bundle interface exists. The bundle uses this MAC address until the user configures a different MAC address. The bundle MAC address is used by all member links when passing bundle traffic. Any unicast or multicast addresses set on the bundle are also set on all the member links.


    Note
    It is recommended that you avoid modifying the MAC address, because changes in the MAC address can affect packet forwarding.

  • EtherChannel—Cisco proprietary technology that allows the user to configure links to join a bundle, but has no mechanisms to check whether the links in a bundle are compatible.

Link Aggregation Through LACP

The optional Link Aggregation Control Protocol (LACP) is defined in the IEEE 802 standard. LACP communicates between two directly connected systems (or peers) to verify the compatibility of bundle members. For a router, the peer can be either another router or a switch. LACP monitors the operational state of link bundles to ensure these:

  • All links terminate on the same two systems.

  • Both systems consider the links to be part of the same bundle.

  • All links have the appropriate settings on the peer.

LACP transmits frames containing the local port state and the local view of the partner system’s state. These frames are analyzed to ensure both systems are in agreement.