I was hoping to more than halfway done at this point, but there is a lot to learn and the labs are taking a little longer than I was hoping, but this is a good thing as the more I use the commands the better off I will be for the test and plus this will all be a reference for future review when I can’t remember commands 🙂

Section 20: OSPF – Open Shortest Path First

  • OSPF Adjacencies
    • Focus on the first three steps in the OSPF Operations
      • OSPF Operations Review
        • Discover neighbors
        • Form adjacencies
        • Flood LSDB (Link State Database)
        • Compute the shortest path
        • Install best routes in the routing table
        • Respond to network changes
      • OSPF Packet Types Review
        • Hello – Used to find adjacent routers
        • DBD (DataBase Description) packets – Used for adjacent routers to tell each other the networks they know about.
        • LSR Link State Request – Used to populated missing info in the received DBD
        • LSA (Link State Advertisement) – Route Update
        • LSU (Link State Update) – List of LSA’s which should be updated, used during flooding
        • LSAck – LSA acknowledgment
    • Hello Packets
      • OSPF routers discover each other and form adjacencies via Hello packets
      • Hello packets our sent out non-passive OSPF enabled interfaces.
      • Multicast on 224.0.0.5 every 10 seconds
      • Hello Packet Contents
        • Router ID
        • Hello Interval – Default 10 seconds
        • Dead Interval – Default 4x Hello Interval
        • Neighbors – list of adjacent OSPF routers that it has received Hello packets from
        • Area ID – 
        • Router Priority – 8 bit number used to select DR (Designated Router) and BDR (Backup Designated Router)
        • DR and BDR IPv4 Address – If known
        • Authentication Flag – Auth details
        • Stub Area Flag – uses ABR to connect two OSPF areas
      • Routers must match for adjacency
        • Must be each other’s Neighbor list
        • Following must match
          • Hello and Dead Intervals
          • Area IDs
          • IP Subnet 
          • Auth Flag
          • Stub Area Flag
    • OSPF DR and BDR Designated Routers
      • When there are multiple routers in a multiaccess segment/subnet it is not effective for the routers to create a 1 to 1 link between each router to pass information.
      • DR (Designated Router)
        • Will control all replication for all routers on the segment
      • BDR (Backup Designated Router)
        • In place, if the DR goes down
      • The DR and BDR are elected
        • Router with the highest priority (0-255) becomes DR and the second-highest priority becomes BDR with the highest Router ID breaking a tie.  Priority is set manually if not left at default which is 1.
          • Set Priority to zero if you wish the router to not become a DR or BDR
        • Ethernet interfaces will be considered a multiaccess segment and a DR/BDR will be designated, but serial connections are point to point and no DR/BDR will be advertised/elected.
      • OSPF Priority Command
        • (config#interface [interface]
        • (config-if)#ip ospf priority [0-255]
          • OSPF restart on the interface for change to take place
            • Restart router
            • Disable/Enable interface
            • Clear OSPF
              • command: #clear ip ospf process
              • This does not cause a full election and the BDR will remain the same and the command would need to be restarted on the old BDR or the old DR for the BDR to change.
      • In the case of four routers on a segment, the DR and BDR will be in a full state communication with all the routers to pass information, the two routers that are not a DR or BDR, they will reside in a 2-way state to ensure each other are up and running but will not passing routing information.
        • If there is a link-state change an LSU packet is sent multicast over 224.0.0.6 to all designated routers
        • DR will multicast the update on 224.0.0.5 to all OSPF routers
    • OSPF Areas
      • Potential issues
        • Too many routes can use up to much memory
        • Network changes on large networks can take a long time to reconverge and use a lot of CPU resources
      • Resolution for large networks
        • OSPF supports a hierarchical design, large networks into smaller areas
          • Transit area (backbone/area 0), generally does not contain users
          • Regular areas connect end-users to the transit area to connect to other areas
        • Routers maintains full info about its own area and summary info about other areas
      • Router types
        • Backbone contains all routes for area 0
        • ABR (Area Border Router) and contains multiple areas
          • Separates LSA flooding zones
          • Summarizes area address
          • Source for default routes
          • Maintains the LSDB for each area that is it connected to
          • Recommended that it is only connected to two areas
          • Summarisation is not automatic and needs to be configured manually
            • (config-router)#area [area id] range [network address] [subnet]
      • Summary routes will show in the ‘#show ip route’ as ‘O IA’ which is OSPF inter area
      • ASBR (Autonomous System Boundry Router)
        • it is running OSPF but it is providing routes from another source
          • Example: RIP, EIGRP or static routes being distributed from another source
          • From “#show ip route’ it will show up as O*E1 or O*E2 route

 

 

Section 21: VLANs Virtual Local Area Networks

  • VLAN
    • Operate at Layer 3
    • Separate IP subnets and need a router to be able to communicate.
    • Provide performance and security by splitting networks into smaller domains
    • Layer 2 Switches broadcast traffic everywhere including between different IP subnets
  • VLAN Access Ports
    • Where end hosts are plugged in.
    • Have one VLAN
    • end host is not VLAN aware
  • VLAN commands
    • Create VLAN
      • (config)#vlan [vlan#]
      • (config-vlan)#name [text]
    • Assign interface to VLAN
      • (config)#interface [interface] – Single interface
        • For multiple interfaces: (config)#interface range [interface range]
      • (config-if)#switchport mode access
      • (config-if)#switchport access vlan [vlan#]
    • Trunk Ports
      • Dot1Q Trunk Port
        • Tags layer 2 Dot1Q header with correct VLAN
        • Receiving switch only forwards traffic out to requested VLAN ports
        • Dot1Q tad is removed from frame when it sends to the end host
      • Trunk Commands
        • (config)#interface [interface]
        • (config-interface)#description [text]
        • (config-interface)#switchport trunk encapsulation dot1q
        • (config-interface)#switchport mode trunk
      • Access Trunk Port
        • (config)#interface [interface]
        • (config-interface)#description [text]
        • (config-interface)#switchport mode access
        • (config-interface)#switchport access vlan 10
        • (config-interface)#switchport voice vlan 20
      • Native VLAN
        • Switch needs to know which VLAN to assign to any traffic which is untagged on a trunk port
        • Default VLAN is VLAN 1
          • It is recommended to change to a different VLAN then VLAN 1 for security concerns.
        • native VLAN must match on both sides of a trunk for it to come up
        • Native VLAN Configuration Command
          • Create VLAN
            • (config)#vlan [vlan#]
            • (config-vlan)#name [test description: Native]
          • Configure Trunk Port
            • (config)#interface [interface]
            • (config-interface)#description [text]
            • (config-interface)#switchport trunk encapsulation dot1q
            • (config-interface)#switchport mode trunk
            • (config-interface)#switchport trunk native vlan 199
        • Allow VLAN over Trunk
          • (config)# interface [interface]
          • (config-interface)#switchport trunk allowed vlan [vlan#s separated by a comma]
      • DTP Dynamic Trunking Protocol
        • It is recommended not to use DTP and to manually configure the access and trunk ports
        • DTP configuration commands
          • #Switchport mode dynamic auto
            • Will form a trunk if the neighbor switch port is set to trunk or desirable.
          • #Switchport mode dynamic desirable
            • Will form a trunk if the neighbor switch port is set to trunk, desirable or auto
          • #Switchport nonegotiate
            • disables DTP
      • VTP VLAN Trunking Protocol
        • Allows you to add, edit or delete VLANs on switches configured as VTP server
        • Switches configured as VTP clients synchronize their VLAN database with VTP servers
        • Good for large campus
        • If switch with a hight VLAN database revision number in the domain it can wipe out all your production VLANs
        • VTP domain name has to match on neighbor switches for DTP trunks to be formed.
        • VTP Modes
          • VTP Server
            • Can add, edit or delete VLANs
            • Sync VLAN database from another server with a higher revision number
          • VTP Client
            • Cannot add, edit or delete VLANs
            • Sync VLAN database from the server with the highest revision number
          • VTP Transparent
            • Does not participate in the VTP domain
            • Can add, edit or delete VLANs in its own local VLAN database
        • VTP Commands
          • Create a VTP domain
            • (config)#vtp domain [Domain name]
          • Assign device VTP mode
            • Server
              • (config)#vtp mode server
            • Client
              • (config#vtp mode client
            • Transpaent
              • (config)#vtp mode transparent
          • Verification
            • #show vtp status

 

 

Section 22: Inter-VLAN Routing

  • Router with separate Interfaces
    • Config like it is a regular setup, no special setup.
  • Router on a Stick
    • Create a sub interface on the router and use the same setting as a physical interface
    • Commands
      • Enable router the interface that will be used
        • (config)#interface [interface: example: f0/1]
        • (config-interface)#no ip address
        • (config-interface)#no shutdown
      • Create VLAN interface
        • (config)#interface [interface: example: f0/1.10]
        • (config)#encapsulation dot1q 10
        • (config-interface)#ip address [ip address] [subnet]
        • (config)#interface [interface: example: f0/1.10]
        • (config)#encapsulation dot1q 20
        • (config-interface)#ip address [ip address] [subnet]
      • Set switch trunk
        • (config)#interface [interface]
        • (config-if)#switchport mode trunk
  • Layer 3 Switch
    • Use SVI (Switched Virtual Interfaces) for the gateways interfaces
    • Inter-VLAN Routing Configuration Commands
      • (config)#ip routing
      • (config)#interface vlan [vlan1#]
      • (config-if)#ip address [gateway ip address] [gateway subnet]
      • (config)#interface vlan [vlan2#]
      • (config-if)#ip address [gateway ip address] [gateway subnet]
    • WAN Routing Configuration
      • Switch Example
        • (config)#interface F0/1
        • (config-if)#no switchport
        • (config-if#ip address 10.10.100.1 255.255.255.0
        • (config)#ip route 0.0.0.0 0.0.0.0 10.10.100.2
      • Router
        • (config)#interface f0/1
        • (config-interface)#ip address 10.10.100.2 255.255.255.0
        • (config)#interface f0/2
        • (config-interface)#ip address 203.0.113.1 255.255.255.0
        • (config)#ip route 0.0.0.0 0.0.0.0 203.0.113.2
        • (config)#ip route 10.10.0.0 255.255.0.0.10.10.100.1