CCIE R&S Written Overview: Frame Relay

Frame Relay Overview

• Non-Broadcast Multi Access (NBMA)

– Address resolution issues implied

• Data Link Connection Identifier (DLCI)

– Layer 2 addressing

– DLCI number only locally significant

• Local Management Interface (LMI)

– DTE / DCE (router / switch) communication

– Reports virtual circuit (VC) status

Frame Relay LMI

• Enabled automatically with Frame Relay

– encapsulation frame-relay

• LMI Types

– Automatically detected

– frame-relay lmi-type [cisco | ansi |q933a]
– show frame-relay lmi

• LMI Advertises VC Status

– show frame-relay pvc

• Status can be…

– Active

– Inactive

– Deleted

– Static

Full Mesh

• Topology where all devices have a direct layer 2 circuit to each other

• More closely emulates a LAN

• More expensive to provision


CCIE R&S Written Overview: Etherchannel


• Used to aggregate bandwidth of physical links

– Same logic as PPP Multilink

• Consists of two parts

– Port-Channel interface

• Logical interface representing the link bundle

– Members interfaces

• Physical links part of a link bundle

• Channel can be any type of interface

– i.e. layer 2 access, trunk, tunnel, or layer 3 routed

EtherChannel Negotiation

• channel-group [number] mode [mode]

• Mode determines how negotiation occurs

– On

• No negotiation

– Desirable & Auto

• Initiate or listen for PAgP

– Active & Passive

• Initiate or listen for LACP

• PAgP vs. LACP

– Like ISL vs. 802.1q

EtherChannel Mode Compatibility

• On –On

• Desirable – Desirable

• Desirable – Auto

• Active – Active

• Active – Passive

EtherChannel Load balancing

• Load balancing between member interface based on…

– Source MAC address

– Destination MAC address

– Source IP Address

– Destination IP Address

– Combinations of the four

• Modified with…

– port-channel load-balance

CCIE R&S Written Overview: STP


  • 802.1d (STP/CST)
  • 1 STP calculation for all VLANs in Layer 2 Network
  • Cisco Propertiary Per VLAN STP
  • 802.1w (RSTP)
  • 802.1s (MST)


  • Elect one Root Bridge
  • Elect one Root Port per bridge
  • Elect Designated Ports

Root Bridge Election

  • Lowest Bridge ID
  • Bridge Priority (Inc of 4096) 0-61440
  • System ID Extension (VLAN NUMBER) Added to Bridge ID
  • MAC Address
  • 32768 = Default Priority
  • Once RB is elected, BPDUs flow down from the root of the tree to the ‘leaves’.
  • On newer versions of STP, BPDUs are sent from all switches, not just the RB.

Root Port Election

  • RP is upstream facing towards RB
  • Elected based on lowesr Root Path Cost
  • Cost of all links to get to the root
  • Higher bandwidth = lower cost
  • If tie in cost, then break tie with lowest Bridge ID or lowest Port ID.

Designated Port Election

  • DPs are downstream ports facing away from the RB.
  • All ports on RB have to be DPs.
  • Lowest Root Path Cost
  • Lowest BID
  • Lowest Port Identifer
  • All other ports will BLK. Will still receive BPDUs and will not/can not send traffic.
  • No MACs will be associated with a BLK port.

802.1d convergence

  • CST convergence based on timers set on RB
  • Hello – How often BPDUs are sent – 2 seconds
  • Forward Delay – Each stages of LISTEN/LEARN – 15 seconds
  • Max Age – 20 seconds – Time period to wait when no BPDU received? (CHECK CHRIS BRYANT DOC)
  • TCN BPDUS used to notify the root bridge of changes
  • Flag is set in TCN BPDU
  • Flows up to the root, root replies with ACK
  • CAM aging time set to Forward Delay to flush out MAC addresses in table to relearn new topology.




– One instance of Legacy STP per VLAN

– Cisco ISL support


– One instance of Legacy STP per VLAN

– Cisco ISL and 802.1Q support

– Provides interoperability between CST and PVST

– Default mode on most Catalyst platforms

– Allows root bridge/port placement per VLAN

Cisco’s STP Enhancements

• PortFast

– Edge ports shouldn’t be subject to Forwarding Delay or generate TCNs

• UplinkFast

– Direct Root Port failure should re-converge immediately if Alternate Port available

• BackboneFast

– Indirect failures should start recalculating immediately. (Discard the Max Age time)

Other Cisco STP Features

• BPDU Filter

– Filter BPDUs in and out

• BPDU Guard

– If BPDU is received shut port down

• Root Guard

– If superior BPDU is received shut port down

• Loop Guard & UDLD

– Prevent unidirectional links

802.1w STP Rapid Spanning-Tree Protocol

– Rapid convergence based on sync process (Proposal process)

– Allows for faster initial convergence

• Simplifies port states

– Discarding, Learning, & Forwarding

All bridges generate BPDUs

– Send out all every Hello interval

– If three Hellos are missed neighbor is declared down and CAM is immediately flushed

– Allows for faster re-convergence

• Runs backwards compatibility with 802.1D

802.1s STP Multiple Spanning-Tree Protocol

– User defined instances are separate from VLANs

– PVST+ uses one instance per VLAN

• Uses 802.1w for rapid convergence

• Highly scalable

– Switches with same instances, configuration revision number, and name form a “region”

– Different regions see each other as virtual bridges

CCIE R&S Written Overview: VLANs/Trunking/VTP


VLAN Numbering

• VLAN membership defined by number

• 12-bit field (0-4095)

– 0 & 4095 reserved per 802.1Q standard

• Normal VLANs 1-1005

– 1 – Default Ethernet VLAN

– 1002/1004 – Default FDDI VLANs

– 1003/1005 – Default Token Ring VLANs

• Extended VLANs 1006-4094

VLAN Trunks

• Traffic sent over a trunk link receives special trunking encapsulation

– Normal Ethernet header does not have a field for VLAN number

– ISL or 802.1Q headers are added to include this information

Trunking Encapsulations

• Both ISL and 802.1Q accomplish the same goal of encoding VLAN number in frame header to separate traffic

• The key differences are…


• Cisco proprietary

• 30-byte encapsulation for all frames

– 26-byte header

– 4-byte trailer (FCS)

• Does not modify original frame


• IEEE standard

• 4-byte tag except for “native” VLAN

• Modifies original frame

– See Inter-Switch Link and IEEE 802.1Q Frame Format for more info

DTP – Dynamic Trunking Protocol

• Dynamic switchports automatically choose whether to run in access or trunking mode

• Runs Dynamic Trunking Protocol (DTP) to negotiate, in order…

– ISL trunk

– 802.1Q trunk

– Access port

• Configured as switchport mode dynamic [auto|desirable]

• Disabled with switchport nonegotiate or switchport mode access

Auto vs Desirable

  • Auto will not initiate but will accept
  • Desirable will initiate a negotiation and also accept one

VTP Overview

• VLAN Trunk Protocol

• Cisco proprietary

• Used to dynamically…

– Advertise addition, removal, modification of VLAN properties

• Number, name, etc.

– Negotiate trunking allowed lists

• “VTP Pruning”

• Does not affect actual VLAN assignments

– Still manually needed with switchport access vlan [vlan]

How VTP Works

• VTP Domain

– To exchange information, switches must belong to the same


• VTP Mode

– Controls who can advertise new/modified information

– Modes are…

• Server

• Client

• Transparent

• VTP Revision Number

– Sequence number to ensure consistent databases

– Higher revision indicates newer database

VTP Domains

• VTP domain name controls which devices can exchange VTP advertisements

• VTP domain does not define broadcast domain

– Switches in different VTP domains that share same VLAN numbers hosts’ are still in the same broadcast domain

• Configured as vtp domain [name]

• Defaults to null value

– Switch inherits VTP domain name of first advertisement it hears

VTP Pruning

• Broadcasts and unknown unicast/multicast frame are flooded everywhere in the broadcast domain

– Includes trunk links

• Editing allowed list limits this flooding, but large administrative overhead

• VTP pruning automates this procedure

– Switches advertise what VLANs they need

– All other VLANs are pruned (removed) off the trunk link

• Does not work for transparent mode