CCIE RS – Written – L2 – Implement and Troubleshoot VLANs

CCIE RS – Written – L2 – Implement and Troubleshoot VLANs

VLAN – Virtual LAN

  • Administratively defined subset of switch ports that are in the same broadcast domain
  • Broadcast domain – devices that can receive broadcast sent by another device
  • Best Practice – 1 to 1 relationship between IP subnet and VLAN

Configuration

  • Vlan [id]
  • Under interface > switchport access vlan [id]
  • Modify VLAN operational state (L2 only)
    • Can be suspended globaly for entire VTP or to local switch
    • Vlan [id] > state suspend (global) | shutdown (local)

Access Ports

Belongs and carries traffic for only 1 VLAN 

Configuration

interface > switchport access vlan [#]


VLAN Database

  • Vlan 0 – Reserved, not available for use
  • Vlan 1 – Default vlan for all access ports
    • Cannot be deleted or pruned
  • Vlan 4095 – Reserved, not available for use

Switch#sh vlan

VLAN Name                             Status    Ports
—- ——————————– ——— ——————————-
1    default                          active    Gi0/0, Gi0/1, Gi0/2
1002 fddi-default                     act/unsup
1003 token-ring-default               act/unsup
1004 fddinet-default                  act/unsup
1005 trnet-default                    act/unsup 

VLAN Type  SAID       MTU   Parent RingNo BridgeNo Stp  BrdgMode Trans1 Trans2
—- —– ———- —– —— —— ——– —- ——– —— ——
1    enet  100001     1500  –      –      –        –    –        0      0   
1002 fddi  101002     1500  –      –      –        –    –        0      0   
1003 tr    101003     1500  –      –      –        –    –        0      0   
1004 fdnet 101004     1500  –      –      –        ieee –        0      0   
1005 trnet 101005     1500  –      –      –        ibm  –        0      0   

Primary Secondary Type              Ports
——- ——— —————– ——————————————

Switch#


Normal VLAN

1 – 1001

  • Can be advertised with VTPv1 and 2
  • Configured in vlan database, global config
  • Details stored in vlan.dat
  • Can be pruned

1002 – 1005 Special uses

  • Cannot be pruned
  • 1002 fddi-default
  • 1003 token-ring-default
  • 1004 fddinet-default
  • 1005 trnet-default

Extended VLAN

1006 – 4094

VTPv1 and 2 must be in transparent mode


Voice VLAN

Enables access ports to carry traffic from an IP Phone

  • Portfast is automatically enabled when voice vlan is configured
    • Not disabled if voice vlan is removed

Configure how the Cisco IP Phone carries voice traffic:

  • vlan-id —Configure the phone to forward all voice traffic through the specified VLAN. By default, the Cisco IP Phone forwards the voice traffic with an IEEE 802.1Q priority of 5. Valid VLAN IDs are 1 to 4094.
  • dot1p —Configure the phone to use IEEE802.1p priority tagging for voice traffic and to use the default native VLAN (VLAN 0) to carry all traffic. By default, the Cisco IP Phone forwards the voice traffic with an IEEE 802.1p priority of 5.
  • none —Allow the phone to use its own configuration to send untagged voice traffic.
  • untagged —Configure the phone to send untagged voice traffic.

CCIE RS – Written – L2 – Implement and Troubleshoot Trunking

Implement and Troubleshoot Trunking


Trunking

  • Allows devices to send traffic for multiple vlans across a single link
  • Trunking protocols must match on both ends

ISL

  • Cisco Proprietary
  • Supports normal and extended vlans
  • Encapsulates original frame
    • 26 byte header + trailer for FCS
  • No concept of native vlan

802.1Q

  • Open Standard (IEEE)
  • Supports normal and extended vlans
  • Inserts tag into original frame
    • 4 byte header
    • Frame tagging
  • Concept of native vlan
    • Traffic in native vlan is untagged
    • All received untagged frames are sent over native vlan
    • Must match on both ends of trunk
    • Detection for mismatch
      • Cisco Switches – Proprietary extension in PVST+ and Rapid PVST+
      • CDP Neighbor can detect and report (syslog)
    • Default native vlan is 1
      • Best practice is to change to prevent vlan hopping attacks

DTP – Dynamic Trunking Protocol

  • Dynamically learn if device on other end wants to perform trunking and negotiate what protocol to use
  • Modes
    • Dynamic Auto – negotiate automatically
      • Prefers to be access port
    • Dynamic Desirable
      • Prefers to be a trunk port
      • Highest priority
      • Will choose ISL is both sides support
    • Different switch models have different default behaviors
    • DTP and VTP are independent
      • DTP carries the VTP domain name
  • Reduce number of vlans on trunk
    • Vlans must be configured on switch before being considered active
    • Administratively configure or use VTP pruning
      • Switchport trunk allowed [add | remove ]
  • Config
    • Switchport mode [trunk | dynanmic desirable / auto | access
    • Switchport nonegotiate
      • Port no longer sends DTP messages
    • Switchport trunk encapsulation [isl | dot1q]

Q in Q Tunneling

  • VLANs traditionally do not extend past WAN boundary
  • Standard 802.1ad – Provider Bridges
  • Allow SP to preserve 802.1q vlan tag across WAN service
  • CDP and VTP can be configured to pass transparently over Q-in-Q
  • New tagging concept
    • C-Tag – Original customer tagged frame
    • S-Tag – Additional tag added to C-Tag frame at ingress of SP
      • Tag is removed at egress of SP, preserving original C-Tag
      • Different customers get different S-Tags
  • Configuration
    • Change MTU to 1504 for extra header size

Manual VLAN Pruning

Manually specify which vlans are allowed across a trunk link


VTP – VLAN Trunking Protocol

  • Advertises vlan config information to neighboring switches
  • Allows vlan config to be done from 1 switch in the domain (dynamic)
  • Advertises VLAN ID, name and type
  • Does not forward which ports are in the vlan
  • Sends updates out all active trunk links by default
    • Without domain name switches do not send any VTP updates
  • VTP starts working when there is an active trunk link and domain name
  • Configuration
    • Vtp domain [name]
    • Show vtp status
    • Options
      • Domain – sets domain name
        • Switch can only belogn to one domain
      • Password – sets password to prevent unauthorized switches from joining domain
      • Mode [server | client | transparent]
      • Version [1 | 2 | 3]
        • Can configure on server switch and will sync with other switches
        • V3 must be manually configured and have domain name configured
      • Pruning
        • Prevent flooding per-VLAN to switches that do not have the VLAN configured
        • Only applied to normal VLAN range
      • Interface
        • Sets interface whos IP is used to ID the switch
        • Default is to use IP of lowest numbered SVI

VTPv1

Version 1 is default

  • Supports only normal vlan range

Message Types (v1 and v2)

  • Summary advertise
    • Originated by VTP server and client every 5 minutes and after each modification to vlan database
    • Carries domain name, revision number, ID of updater, timestamp, MD5, password & number of subset advertise
    • Does not carry vlan database contents
  • Subset Advertise
    • Originated by server and client after modification of vlan database
    • Carries full vlan database contents
  • Advertise Request
    • Request neighbors complete vlan database or part of it
    • Request sent when switch enters client mode or client switch is restarted
  • Join
    • Every 6 seconds if VTP pruning is active

Update Process (v1 and v2)

  • Begins when a modification is made on a vlan
  • VTP server will increment revision number by 1 and advertise entire vlan database with new revision number
  • Revision number allow switches to know when vlan database changes have occurred
    • Switches replace their vlan database when a larger revision shows up

VTP Modes

  • Server –
    • Default with no domain name
    • Updates to not occur until a domain name is configured
  • Client
    • Will assume first received domain name
    • Default domain is NULL
  • Transparent / Off
    • Prevents switches from listening to other switches VTP updates
  • Protect VTP domain – VTP password
    • Summary adv – carries MD5 hash computed over the vlan database contents and VTP password if configured

VTPv2

Enhanced version of v1

  • Supports token ring concentrator relay function and bridge relay function
    • Not used in ethernet based networks
  • Supports unknown TLV (Type Length Value)
    • V1 would drop unrecognized TLV’s which would stop propagation to other switches
  • Optimized vlan database consistency checking
    • Implementation optimization
    • Skips consistency check if change was received by a VTP message

VTPv3

Introduced to IOS release 12.2(52)SE

  • New server roles – Addresses problem of inadvertent rewrites to the vlan database
    • Primary – Only 1 at a time
      • Can modify VTP domain contents
      • Only switch who’s vlan database is propagated through domain
      • Database will be shared if switch agrees on domain name and ID of primary server (MAC address)
    • Secondary –
      • Cannot make changes,
      • but can be promoted
      • All other server switches in domain are secondary
    • Primary role is a runtime state. Does not get saved into configuration
    • Server role helps prevent unwanted changes to VTP domain
  • Password storage and usage improved
    • Encrypted and cannot be displayed in plaintext
    • Promotion of secondary required entering password in plaintext
    • String is carried encrypted to different switches
  • Capable of distributing full range of vlans and private vlans
    • No longer need to be in transparent mode when using extended vlan range and private vlans
    • Pruning only applied to normal vlan range
  • Supports VTP off
    • Switch does not participate in VTP and drops all VTP messages
  • Distributes content of MST region config
  • Client switches cannot make changes to domain or be promoted
  • Both Secondary and Client store copy of primaries vlan database and share with neighboring server and client if they agree on the primaries ID
  • Secondary or client servers with higher revision number can overwrite vlan database, but must match on domain name, primary ID and password
  • Conflicts
    • Server or client in domain having different primary server IDs
    • Conflciting switches do not sync the vlan database even if all other parameters match
  • Promotion to primary sever done in exec mode
    • Vtp primary
  • Cannot reset revision number to 0 by switching mode to transparent
    • Only will be reset by modifying the domain name or configuring password
  • Switches that cannot run v3, the port will revert to v2
    • V3 to v1 is not supported

VTP Pruning

Reduces unnecessary flooded traffic such as broadcasts, multicast and unicast packet

  • Disabled by default
  • Enabling on server enables for entire domain
  • Vlans 2 – 1000 are eligible for pruning
  • Cmd: vtp pruning

CCIE RS – Written – Network Principles – Use IOS Troubleshooting Tools

Use IOS Troubleshooting Tools

Multiple troubleshooting tools are built into IOS.

  • show – monitor normal behavior and isolate problems
    • version – system hardware, software version, uptime, boot image
    • running-config – current configuration
    • startup-config – config stored in NVRAM
    • interface – interface statistics, bandwidth, errors
  • debug – assist in isolating a protocol and configuration problem
  • ping – determine connectivity
  • trace – show the path packets are taking

debug, conditional debug

Debugs must be turned on using the debug command. To show running debugs – show debug

Debugs are sent to console by default (no logging console – to turn off). User terminal monitor if you are remotely connected into the device. 

Turn off debug – R1#undebug all

Conditional debug – add parameters around what debugs you want displayed to the console.

Stacking multiple debug conditions will generate output if at least 1 condition is met.

R1#debug condition ?
called called number
callingcalling
cplCisco Provisioning Language debugging
glbp interface group
interfaceinterface
ip IP address
mac-addressMAC address
match-list apply the match-list
profileMedia Services Profile
standbyinterface group
username username
vcid VC ID
vrfVirtual Routing and Forwarding
xconnect Xconnect conditional debugging on segment pair

R1#debug condition ip 1.1.1.1 
Condition 1 set
R1#debug condition interface gi0/0
Condition 2 set
R1#
R1#
R1#sho debug

Condition 1: ip 1.1.1.1 (0 flags triggered)
Condition 2: interface Gi0/0 (1 flags triggered)
Flags: Gi0/0

R1#

Above condition will generate debugs messages for anything containing the ip 1.1.1.1 or interface gi0/0

ping, traceroute with extended options

Ping – common method for troubleshooting accessibility to a device

  • Uses ICMP echo
    • Tells if host is active / inactive
    • RTD to host
    • Packet Loss

Issues if cannot ping

  • Routing issue
  • Interface down
  • ACL
  • ARP issue
  • Delay
  • Source Address
  • High Input Queue drops

Traceroute – Discover the routers a packet takes to a destination

  • Sequence of UDP datagrams on an invalid port
  • 3 datagrams sent with TTL of 1
  • TTL of 1 causes datagram to timeout and first hop responds with ICMP “Time Exceeded Message (TEM)
  • Process continues increasing TTL by 1 each step until packets reach the destination
  • Destination responds with ICMP Port Unreachable message, indicates traceroute is finished
Traceroute Text Characters

Traceroute Text Characters

Embedded packet capture

  • Onboard packet capture facility
  • Consumes CPU and memory resources during its operation
  • Export captures via TFTP, FTP and local disk
  • Define a buffer size and type (circular or linear) and max number of bytes of each packet capture
  • Capture can be throttled using admin controls
    • Filter packets with ACL
    • Specify max packet capture rate or specify sampling interval
  • Benefits
    • Ability to capture IPv4 and IPv6 in CEF path
    • Flexible method to specify capture buffer parameters
    • Filter captured packets
    • Method to decode data packets
    • Facilty to export capture (PCAP)
    • Extensible infrastructure for enabling capture points

Performance monitor

Enables to be able to monitor the flow of traffic in the network. Similar to netflow.

Pre-req for configuration

  • IPv4
    • routing and CEF must be configured / enabled
  • IPv6
    • ipv6 cef must be enabled

Can monitor a long list of traffic – https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/media_monitoring/configuration/15-mt/mm-15-mt-book/mm-pasv-mon.html

Configuration Components

  1. Interface – attach performance monitor to interface – service-policy type performance-monitor
  2. Policy – Associate with flow monitor – policy-map type performance-monitor
  3. Class – filtering criteria – class-map
  4. Flow Monitor – Associated with flow record and optional flow monitor – flow monitor performance-monitor
  5. Flow Record – Specify match and collect – flow record type performance-monitor
  6. Flow Exporter – Specify the destination for exporting traffic

show performance monitor status

 

Router#show policy-map type performance-monitor 
Policy Map type performance-monitor PM_FLOW_MONITOR
Class CM_FLOW_MONITOR
flow monitor FLOW_MONITOR
react status: inactive
Router#
Router#sh run | s flow
flow record type performance-monitor FLOW_RECORD
 match ipv4 destination address
 match transport destination-port
 collect application media event
 collect counter bytes long
 collect ipv4 dscp
 collect monitor event
 collect routing forwarding-status
 collect timestamp interval
 collect transport packets expected counter
 collect flow direction
flow exporter FLOW_EXPORT
 description *** Export Flows ***
 destination 2.2.2.2
 source GigabitEthernet0/0
 output-features
 dscp 46
 transport udp 650
flow monitor type performance-monitor FLOW_MONITOR
 description *** FLOW MONITOR ***
 exporter FLOW_EXPORT
flow monitor FLOW_MONITOR
Router#
Router#sh run | s class
class-map match-all CM_FLOW_MONITOR
 match any 
 class CM_FLOW_MONITOR
flow monitor FLOW_MONITOR
Router#
Router#
Router#sh run | s policy
policy-map type performance-monitor PM_FLOW_MONITOR
 class CM_FLOW_MONITOR
flow monitor FLOW_MONITOR
Router#

Apply troubleshooting methodologies

Diagnose the root cause of networking issue (analyze symptoms, identify and describe root cause)

Design and implement valid solutions according to constraints

Verify and monitor resolution


Interpret packet capture

Using Wireshark trace analyzer

https://www.wireshark.org/docs/wsug_html/

Personal writing from experience – you need to understand traffic flows. Knowing protocol basics and using different filters in wireshark to remove the noise in the capture. Would love feedback for this section on your experience with wireshark.

Using IOS embedded packet capture

Router#monitor capture buffer CAPTURE size 256 max-size 100 circular 
Router#monitor capture point ip cef CAPTURE_POINT gi0/0 ?
bothcapture ingress and egress
incapture on ingress
out capture on egress
removeremove capture point
Router#monitor capture point ip cef CAPTURE_POINT gi0/0 both
Router#monitor capture point associate CAPTURE_POINT CAPTURE
*Aug 18 04:12:11.060: %BUFCAP-6-CREATE: Capture Point CAPTURE_POINT created.
Router#monitor capture point associate CAPTURE_POINT CAPTURE
Router#
Router#monitor capture point start CAPTURE_POINT
Router#
Router#
Router#
Router#
*Aug 18 04:12:29.789: %BUFCAP-6-ENABLE: Capture Point CAPTURE_POINT enabled.
Router#show monitor capture buffer CAPTURE dump

Router#monitor capture point stop all 
Router#
*Aug 18 04:15:23.960: %BUFCAP-6-DISABLE: Capture Point CAPTURE_POINT disabled.
Router#

Packet capture can be exported to TFTP server to look at the capture in wireshark

Router#monitor capture buffer CAPTURE export ?
  flash0:  Location to dump buffer
  flash1:  Location to dump buffer
  flash2:  Location to dump buffer
  flash3:  Location to dump buffer
  flash:   Location to dump buffer
  ftp:     Location to dump buffer
  http:    Location to dump buffer
  https:   Location to dump buffer
  pram:    Location to dump buffer
  rcp:     Location to dump buffer
  scp:     Location to dump buffer
  snmp:    Location to dump buffer
  tftp:    Location to dump buffer

Router#monitor capture buffer CAPTURE export 

CCIE RS – Written – L2 – Implement and Troubleshoot Layer 2 Protocols

Implement and Troubleshoot Layer 2 Protocols


CDP – Cisco Discovery Protocol

  • Propriety, runs on all Cisco equipment
  • Used to obtain protocol address of neighboring devices and discover the platform of those devices
  • Media and protocol-independent
  • Sends periodic messages – advertisements, every 60 seconds
  • Multicast Address: 01-00-0C-CC-CC-CC
  • CDPv2 is latest version

TLV – Type Length Value: Embedded in CDP advertisements

  • Device-ID TLV: Identifies the device name in the form of a character string.
  • Address TLV: Contains a list of network addresses of both receiving and sending devices.
  • Port-ID TLV: Identifies the port on which the CDP packet is sent.
  • Capabilities TLV: Describes the functional capability for the device in the form of a device type, for example, a switch.
  • Version TLV: Contains information about the software release version on which the device is running.
  • Platform TLV: Describes the hardware platform name of the device, for example, Cisco 4500.
  • IP Network Prefix TLV: Contains a list of network prefixes to which the sending device can forward IP packets. This information is in the form of the interface protocol and port number, for example, Eth 1/0.
  • VTP Management Domain TLV: Advertises the system’s configured VTP management domain name-string. Used by network operators to verify VTP domain configuration in adjacent network nodes.
  • Native VLAN TLV: Indicates, per interface, the assumed VLAN for untagged packets on the interface. CDP learns the native VLAN for an interface. This feature is implemented only for interfaces that support the IEEE 802.1Q protocol.
  • Full/Half Duplex TLV: Indicates status (duplex configuration) of CDP broadcast interface. Used by network operators to diagnose connectivity problems between adjacent network elements.

http://www.cisco.com/c/en/us/td/docs/ios/12_2/configfun/configuration/guide/ffun_c/fcf015.html


LLDP – Link Layer Discovery Protocol

  • Open Standard – IEEE 802.1AB
  • Switch supports basic management TLV’s
    • Port description TLV
    • System name TLV
    • System description TLV
    • System capabilities TLV
    •  Management address TLV
    • These organizationally specific LLDP TLVs are also advertised to support LLDP-MED.
      • Port VLAN ID TLV ((IEEE 802.1 organizationally specific TLVs)
      • MAC/PHY configuration/status TLV(IEEE 802.3 organizationally specific TLVs)

Link Layer Discovery Protocol – Media Endpoint Discovery (LLDP-MED)

  • Extension of LLDP that operates between endpoint devices (IP Phones)
  • TLVs
    • Capabilities
    • Network Policy
    • Power Management
    • Inventory
    • Location

UDLD – UniDirectional Link Detection

  • Cisco Proprietary
  • Allows devices connected through fiber or copper to monitor the physical configuration of the cables and detect when a unidirectional link exists
  • Layer 2 protocol that works with layer 1 protocol to determine physical status of a link
  • RFC 5171

Aggressive Mode

  • Disabled by default
  • Configure only on p2p links
  • Bidirectional lnik send UDLD messages, if stops receiving, UDLD tries to reestablish connection. After 8 retriesthe port is disabled
    • Error Disables

Normal Mode

  • Default
  • Does not disable the port when unidirectional link is detected

Configuration Defaults

  • UDLD global enable state — Globally disabled
  • UDLD aggressive mode — Disabled
  • UDLD per-port enable state for fiber-optic media — Enabled on all Ethernet fiber-optic LAN ports
  • UDLD per-port enable state for twisted-pair (copper) media — Disabled on all Ethernet 10/100 and 1000BASE-TX LAN ports

CCIE RS – Written – Network Principles – Evaluate Proposed Changes to a network

Evaluate Proposed Changes to a network

Before making changes to the network you’ll want to know what the current state is. Capture appropriate show output depending on the type of change being made. Ensure network diagrams are up to date (physical and logical). Backup configurations. 

If possible test the changes in a lab environment that directly mimics the production network. Write out a plan for the flow of changes and validations after the change is made to ensure the expected result occurred.


Changes to routing protocol parameters

Each routing protocol has a set of parameters can be changed depending on a technical or business requirement. The parameters for each routing protocol will be gone into in depth in future blog posts.

Before making the changes, check the routing table (show route), and check specifics about the routing protocol your about to change.


Migrate parts of a network to IPv6

IPv4 and IPv6 can exist in the network at the same time (as long as the device supports IPv6). IPv6 has it’s own routing table and if a device has both IPv4 and IPv6 addressing, the IPv6 address will be preferred.

  • Dual Stack
    • Requires infrastructure to support IPv4 and IPv6
    • Applications choose between IPv4 and IPv6 based on response to DNS requests
  • Tunneling
    • Encapsulates IPv6 traffic within an IPv4 packet
    • Used between IPv6 sites over an IPv4 backbone
    • Tunneling Techniques
      • ISATAP
      • Teredo
      • 6PE
      • 6VPE
      • mGRE v6 over v4
    • Manual or automatically configured
  • Translation
    • NAT between v4 to v6, v6 to v4

Details about configurations will be in future blog posts 


Routing protocol migration

Moving between routing protocols can be done in a couple ways

  1. Modify administrative distance
  2. Redistribution 

Multiple routing protocols can be running at the same time, but only 1 route to the destination will be put into the routing table (disregarding multiple paths). Administrative distance is one of the criteria that says which routing protocol is more trustworthy than another.

Redistribution will be explained in future blog posts


Adding multicast support

To add IPv4 multicast support to a router you’ll need to configure multicast routing and enable PIM on interfaces that will be participating in the multicast routing path. 

 

Configuration of multicast will be covered in future blog posts


Migrate spanning tree protocol

Specifics of the different STP types will be explained in future blog posts.

PVST+ to Rapid-PVST+

  • Rapid uses the same BPDU format as PVST+
  • BackboneFast and UplinkFast get disabled when Rapid-PVST+ is configured
  • Start at the access switches and work your way up to the core
  • Clean up configuration
  • Do these changes during a maintenance window because a disruption will occur
  • Verify changes and make sure STP is behaving the way you intended

Rapid-PVST+ to MST

  • Identify p2p and edge ports (portfast)
  • Map vlans to instances
  • Place as many switches as possible into a single region
  • Start at core and work your way to the access

Evaluate impact of new traffic on existing QoS design

Before adding more traffic to an existing QOS queue you’ll want to know the behavior of the traffic and how it needs to be treated. Is the traffic UDP or TCP based? Is the existing queue policed or shaped? Is bandwidth over utilized? 

CCIE RS – Written – Network Principles – Explain General Network Challenges

Explain General Network Challenges


Unicast Flooding

  • Occurs when destination MAC address of the packet is not in the L2 forwarding table of the switch
  • Packet is forwarded out all forwarding ports in that VLAN except the originating port packet was received on
  • Reasons MAC address many not be known
    • Asymmetric routing
    • STP Topology Changes
    • Forwarding Table Overflow

How to Detect

  • No special commands used to detect flooding
  • Unicast Flood Protection – allows switch to monitor amount of unicast flooding per VLAN and take a specified action if flooding exceeds specified amount
  • Actions
    • Syslog
      • %UNICAST_FLOOD-4-DETECTED: Host 0000.0000.2100 on vlan 1 is flooding
        to an unknown unicast destination at a rate greater than/equal to 1 Kfps
    • Limit or shutdown VLAN
  • Show mac-address-table unicast-flood
  • Captures during time of slowdown / outage
    • See packets that are not destined for the workstation

Asymmetric Routing

  • Packets follow different paths depending on the traffic direction
  • Approaches to limit flooding
    • Bring routers ARP timeout and switches forwarding table aging time close to each other
      • Scenario – L3 switch with HSRP configured to LB different vlans
  • Traffic does not stop flooding even after the destination replies

How to detect

  • Follow the routing path
  • Traceroutes

STP Topology changes

TCN – Topology Change Notification

  • Designed to correct the forwarding table after forwarding table changes
  • Used to avoid connectivity outages. If a port goes down and the destination is available over a different port.
  • TCN operates by shortening the forwarding table aging time and if the MAC address is not relearned flooding will occur
  • TCNs are triggered by a port transitioning to or from the forwarding state
  • Issues arise when TCNs are occurring repeatedly with short intervals
    • Switch is constantly fast-aging forwarding table so flooding will be nearly constant
  • Limit TCN’s with the use of portfast

Forwarding Table Overflow

  • New MAC addresses cannot be learned and packets destined to such MAC are flooded until space becomes available
  • Can be caused by an attack on the network where a host starts generating frames sourced with different MAC addresses
  • Detected by examining the switches forwarding table
    • MAC addresses will point to a single port
  • Prevent by limiting the number of MAC addresses that can be learned on an untrusted port
    • See Switchport Security
  • High amount of packets or normal packets with high number of different source MAC addresses

Out of Order Packets

Using per-packet load balancing to share the traffic load across available paths to a given destination can lead to out-of-order packets for a given data flow.


Impact of Microbursts

  • Patterns or spikes of traffic causing interfaces to ne temporally oversubscribed and drop traffic
  • Typically buffers can handle bursts of traffic. This is in excess the buffers can handle
  • Detect by looking at Total Output Drops under show interface
    • Drops increment but utilization stays the same
  • Burst of traffic occur in microseconds
  • Show up as ignores and/or overruns
    • Input errors on show interface
  • Detect by capturing traffic over a long period of time

CCIE RS – Written – L2 – Implement and Troubleshoot Switch Administration

Implement and Troubleshoot Switch Administration

Managing MAC Address Table

The MAC address table contains address information that a switch uses to forward traffic between ports

  • MACs are associated with 1 or more ports
  • Dynamic address
    • Switch learned addresses that will age out when not in use
  • Static Addresses
    • Manually entered, does not age and not lost during switch reset

Disabling MAC Address Learning on Interface or VLAN

  • Causes flooding on network
  • Disabling on SVI, all IP packets are flooded in L2 domain
  • No mac address-table learning [interface]
  • Show vlan internal usage
  • Show mac address-table learning [interface]
  • Default mac address-table learning (Global config)

Errdisable Recovery

Software on switch that detects an error situation and diables the port

  • Port is effectivly shutdown
  • LED is set to amber
  • Show interface
cat6knative#show interfaces gigabitethernet 4/1 status

PortName Status Vlan DuplexSpeed Type
Gi4/1err-disabled 100full 1000 1000BaseSX
  • Syslog Message example (BPDU Guard)
    • %SPANTREE-SP-2-BLOCK_BPDUGUARD:
         Received BPDU on port GigabitEthernet4/1 with BPDU Guard enabled. Disabling port.
      %PM-SP-4-ERR_DISABLE:
         bpduguard error detected on Gi4/1, putting Gi4/1 in err-disable state
  • Causes of error disable
    • Duplex mismatch
    • Port channel misconfiguration
    • BPDU guard violation
    • UniDirectional Link Detection (UDLD) condition
    • Late-collision detection
    • Link-flap detection
    • Security violation
    • Port Aggregation Protocol (PAgP) flap
    • Layer 2 Tunneling Protocol (L2TP) guard
    • DHCP snooping rate-limit
    • Incorrect GBIC / Small Form-Factor Pluggable (SFP) module or cable
    • Address Resolution Protocol (ARP) inspection
    • Inline power
  • Error disable detection is enabled by default
    • Disable – no errdisable detect cause
    • Show errdisable detect
cat6knative#show errdisable recovery
ErrDisable ReasonTimer Status
-------------------------------
udld Enabled
bpduguardEnabled
security-violatioEnabled
channel-misconfigEnabled
pagp-flapEnabled
dtp-flap Enabled
link-flapEnabled
l2ptguardEnabled
psecure-violationEnabled
gbic-invalid Enabled
dhcp-rate-limitEnabled
mac-limitEnabled
unicast-floodEnabled
arp-inspection Enabled

Timer interval: 300 seconds

Interfaces that will be enabled at the next timeout:

InterfaceErrdisable reasonTime left(sec)
--------------------------------------------
Fa2/4bpduguard273
  • Recover port from error disable
    • Identify root cause of errdisable
    • Correct the problem
    • Re enable the port
      • Shutdown , no shutdown on the interface
      • Cmd: Errdisable recovery
cat6knative#errdisable recovery cause ?
all Enable timer to recover from all causes
arp-inspectionEnable timer to recover from arp inspection error disable
state
bpduguard Enable timer to recover from BPDU Guard error disable
state
channel-misconfig Enable timer to recover from channel misconfig disable
state
dhcp-rate-limit Enable timer to recover from dhcp-rate-limit error
disable state
dtp-flapEnable timer to recover from dtp-flap error disable state
gbic-invalidEnable timer to recover from invalid GBIC error disable
state
l2ptguard Enable timer to recover from l2protocol-tunnel error
disable state
link-flap Enable timer to recover from link-flap error disable
state
mac-limit Enable timer to recover from mac limit disable state
pagp-flap Enable timer to recover from pagp-flap error disable
state
psecure-violation Enable timer to recover from psecure violation disable
state
security-violationEnable timer to recover from 802.1x violation disable
state
udldEnable timer to recover from udld error disable state
unicast-flood Enable timer to recover from unicast flood disable state

L2 MTU

The MTU on a switch may need to be adjusted based on protocols being used. Example: 802.1q tunnel requires extra header space, adjusting the MTU from 1500 to 1504 would prevent fragmentation.