L2 protocols

LACP(Link Aggregation Control protocols)

Link aggregation increases bandwidth, provides graceful degradation as failure occurs, and increases availability. It provides network redundancy by load-balancing traffic across all available links. If one of the links fails, the system automatically load-balances traffic across all remaining links.

When LACP is not enabled, a local LAG might attempt to transmit packets to a remote single interface, which causes the communication to fail. When LACP is enabled, a local LAG cannot transmit packets unless a LAG with LACP is also configured on the remote end of the link.

A typical LAG deployment includes aggregate trunk links between an access switch and a distribution switch or customer edge (CE) device.

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LLDP(Link Layer Discovery Protocol)
Link Layer Discovery Protocol (LLDP) is a vendor independent link layer protocol used by network devices for advertising their identity, capabilities to neighbors on a LAN segment.

In LLDP information’s are sent by devices from each of their interfaces at a fixed interval, in the form of an Ethernet frame. Each frame contains one LLDP Data Unit (LLDPDU). Each LLDPDU is a sequence of type-length-value (TLV) structures.

• EtherType field is set to 0x88cc.
• Each LLDP frame starts with the following mandatory TLVs and must intact in LLDP implementations.
  • Chassis ID
  • Port ID, and
  • Time-to-Live (TTL).
• Any numbers of optional TLVs are followed by mandatory TLVs.
• The frame ends with a special TLV named end of LLDPDU in which both the type and length fields are 0.
• The LLDP specification allows for various organizations to define and encode their own TLVs. These are called Organizationally Specific TLVs. All Organizationally Specific TLVs start with an LLDP TLV Type value of 127.

 

 

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Spanning-tree protocols such as Spanning Tree Protocol (STP), Rapid Spanning Tree Protocol (RSTP), and Multiple Spanning Tree Protocol (MSTP) generate their own BPDUs. These peer STP applications use their BPDUs to communicate, and ultimately, the exchange of BPDUs determines which interfaces block traffic and which interfaces become root ports and forward traffic.

• RSTP. Rapid Spanning Tree Protocol  (IEEE 802.1w). An evolution of the spanning tree protocol with faster convergence time, relying one some more advanced switch capabilities.

• PVSTP. Per-VLAN Spanning Tree Protocol . Cisco proprietary protocol that allows each VLAN in a network to run an independent spanning tree with an independent root rather than forcing a single topology for all VLANs.

• MST. Multiple Spanning Tree Protocol   (IEEE 802.1s). A standarized and evolved form of PVSTP.

The problem of STP is the convergence time. RSTP can reduce the convergence time to ~1 second with enhancements like Alternative Port and Backup Port, etc

STP (802.1d)	Rapid STP (802.1w)
In stable topology only the root sends BPDU and relayed by others.	In stable topology all bridges generate BPDU every Hello (2 sec) : used as “keepalives” mechanism.
Port states
Disabled Blocking Listening Learning Forwarding	Discarding(replace disabled, blocking and listening) Learning Forwarding
Port roles
Root (Forwarding) Designated (Forwarding) Non-Designated (Blocking)	Root (Forwarding) Designated (Forwarding) Alternate (Discarding) Backup (Discarding)

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TRILL(TRansport interconnection lot of links)
TRILL applies network layer routing protocols to the link layer.


TRILL, which is undergoing IETF’s standardization process, is meant to replace the spanning tree protocol (STP). STP, which was created to prevent bridge loops, only allows one path between network switches or ports. When a network segment goes down, an alternate path is chosen and this process can cause unacceptable delays in a data center network. TRILL is designed to address this problem by applying the Intermediate System-to-Intermediate System protocol (IS-IS) Layer 3 routing protocol to Layer 2 devices. This essentially allows Layer 2 devices to route Ethernet frames.