Plugga CCNP Route--OSPF

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Main Page CCNP Route plugg

Contents

OSPF

States

  • Down -- No hello recieved, but sent
  • (attempt) only neghbor command on nmbma
  • Init -- Hello recieved, but my Router-ID was not in the neighbor list
  • 2-way -- Go here if HELLO or DBD recieved (Dflt hello timer=10 on broadcast & p2p, 30secs in NBMA & Multipoint
    Don't come here if:
    1. Network-mask (exceptions exist). Network-number not checked ?!?
    2. Area ID
    3. Hello-timer and Dead-timer
    4. Password
    5. CCIE: Stub flag of network must match (usually also "link-type", but not always )
    at the end of this stage: do DR/BDR election
    Here: Reset the Dead-timer (Default is = 4xHello-timer)
  • EXstart -- Exchange DBD (a summary = All LSA headers)
    The router with the higher router ID becomes the master and starts the LSU exchange (includes LSA headers only), and as such, is the only router that can increment the sequence number.
    If Priority is high and Router-ID is low, the DR might be the slave !!
  • EXchange
    Slave sends LSR to fill in the gaps of the LSU(to populate the LSDB with full LSA's). Master sends LSU with full LSA's. THEN Master->Slave
    LSU's are ACKnowledged with LSACK-packet
  • Full state -- Normal state (2-way is normal with two non-DR/BDR are neighbors)

Good explanation: cisco.com-theory

Commands 

 interfa XX 0/0
    ip ospf cost <1-65535>
    ip ospf authentication-key KALLE (remember router conf)
    ip ospf network point-to-point
    ip ospf priority 255  (0=do not participate)
 
 Router ospf PID
   router-id 1.1.1.1
   ospf auto-cost reference-bandwidth <1-65535>
   area x authentication    (remember intf conf)
   redistribute static subnets metric 200 metric-type 2
   network 10.2.2.1 0.0.0.0 area 0             
   network 192.168.1.0 0.0.0.255 area 10       
   area 10 range 192.168.0.0 255.255.0  (ABR only)
   summary-address 172.16.0.0 255.255.0.0  (summerize IN-redistributed routes)
 -------
 ip ospf message-digest-key 1 md5 kal  
 area 0 authentication message-digest

OSPF Multicast

224.0.0.5 (FF02::5) to all others 224.0.0.6 (FF02::6) to DR and BDR

Modes (chaos in writing)


1. Non-broadcast (NBMA) -- RFC Standard -- neighbors manually configured, DR req'rs full connectivity
2. Point.to.multipoint -- RFC Standard -- no manual neighbors BUT pseudo broadcasts req'rd (f-r map 103 1.2.3.4 broadcast)
3. point-to-point -- cisco proprietary -- PPP
4. broadcast -- cisco proprietary --- ETHERNET
5. point-to-multipoint, non-broadcast -- cisco propr. --no pseudo broadcasts req'rd BUT manual neighbors

  • interface serial 0/0.1 point-to-point defaults OSPF network typ to POINT_TO_POINT
  • interface serial 0/0.2 multipoint defaults OSPF network typ to NON_BROADCAST

The page ipexpert.com mentions:

  • ip ospf network point-to-multipoint non-broadcast


1. use frame-relay map ip 1.2.3.4 102 broadcast and neighbor command
2. interface serial 0/0.1 point-to-multipoint
  frame-relay map ip 1.2.3.4 102 broadcast
3. interface serial 0/0.1 point-to-point

different types form neighbours 

 Hub-router# interface serial 0/1.1 multipoint
               ip ospf network point-to-multipoint            show ip ospf interface reveals POINT_TO_MULTIPOINT, Hello 30, Dead 120
  
 Spoke5# interface serial 0/1.1 point-to-point                sho ip ospf int initially reveals POINT_TO_POINT, Hello 10, Dead 40
         ! Now change the timers
         !

Hub-router# interface serial 0/1.1 multipoint 

               ip ospf hello-interval 10                      show ip ospf interf now reveals POINT_TO_MULTIPOINT, Hello 10, Dead 40
                                                              AND the neighbors form (even though different network types)

Virtual-link

On Abr between area1 and area2 (other end (2.2.2.2) connects to area0 

R3# router ospf 1
   router-id 3.3.3.3
   area 1 <through area> virtual-link 2.2.2.2 
R3# router ospf 1
   router-id 2.2.2.2
   area 1 <through area> virtual-link 3.3.3.3

NEW: Show ip ospf BORDER-ROUTERS 

 Router# show ip ospf border-routers
 
 OSPF Process 109 internal Routing Table
 
 Codes:  i - Intra-area route, I - Inter-area route
 i 192.168.97.53   [10] via 172.16.1.53,   Serial0, ABR,  Area 0.0.0.3, SPF 3
 i 192.168.103.51  [10] via 192.168.96.51, Serial0, ABR,  Area 0.0.0.3, SPF 3
 I 192.168.103.52  [22] via 192.168.96.51, Serial0, ASBR, Area 0.0.0.3, SPF 3
 I 192.168.103.52  [22] via 172.16.1.53,   Serial0, ASBR, Area 0.0.0.3, SPF 3

Hard Questions/Answers

  1. How to form neighbors on Frame-Relay
    A) ip ospf network broadcast
    B) ip ospf network point-to-point
    C) neighbor 10.255.255.1 (IP, not ID) of neighbor closest interface)
  2. Which three restrictions apply to stub areas
    Stub area can not be Backbone
    No ASBR are allowed
    "No virtual links are allowed"
  3. In which state does DR and BDR establish adjecency with each OSPF router in the network?
    Init State
  4. The maximum number of routers per OSPF area typically depends on which three factors?
    The kind of OSPF areas being implemented
    the number of external LSAs in the network
    how well the areas can be summarized
  5. Which three are advantages to create multiple areas in OSPF
    Less frequent SPF calculations (Rob: changes create recalc's)
    smaller routing tables (Rob: ABR can summarize)
    reduced LSU overhead (Rob: ABR can summarize)
  6. How to create a stib-area of area 3
    on ABR, area 3 stub no-summary
    (Rob: it is better to answer one correct then three half-bad, but working ie "area 3 stub no-summary" in all routers in area 3
  7. What to check first when problem with OSPF virtual links
    OSPF Router ID (Becouse "area <area> virtual-link <router-id>
  8. R2 stuck in INIT -state ??
    R2 has not recieved the OSPF hello from R1 router
    (: INI-state == Sent HELLO, not (yet) got any HELLO back)
  9. When learning a new route, if a LSA recieved is not found in the topology database, what will an internal OSPF router do?
    The LSA is flooded immediately out of all the OSPF,interfaces, except the interface from which the LSA was recieved.
  10. Exhibit
    O IA 6.0.0.0/8 [110/65] via 5.0.0.2, 00:00:18 Serial 2/1/0
    O*N2 0.0.0.0/0 [110/1] via 5.0.0.2, 00:00:18 Serial 2/1/0
    "CISCO" (;-) ANSWER: OSPF Router 5.0.0.2 is an ABR
  11. LSA flapping from source "172.16.20.2" of LSA for 172.16.10.0
    LSA Content seen from "show ip ospf database" changes repeatedly, everey few seconds
    ANSWER: Two routers have the same OSPF router ID
  12. What are two IOS commands that can be used to view neighbor adjacencieds
    show ip ospf neighbors
    show ip ospf interfaces (Example: ... Adjacent neighbor count is 1)
  13. Exhibit! Fixing OSPF without fixing links
    D1:area 16 virtual-link 8.187.175.82 (Loopback of D2)
  14. Highest Priority/RouterID/Loopback/IP wins DR/BDR Election
  15. ip ospf network nonbroadcast
    True DR/BDR election will occur (due to NMBMA)
    "neighbor" command is required (No multicast(broadcast) HELLO's exists)
  16. show ...current state of OSPF database loading process.
    show ip ospf neighbors
  17. External routes are not showing up. What to check first?
    subnet-keyword in redistribution?
    Are the ASBRs trying to redistribute the external routes into a totally stubby area?

NBMA Table

  • non-broadcast (RFC-compliant)
    • One IP subnet.
    • Neighbors must be manually configured.
    • DR and BDR are elected.
    • DR and BDR need to have full connectivity with all other routers.
    • Typically used in a full- or partial-mesh topology.
  • point-to-multipoint (RFC-compliant)
    • One IP subnet.
    • Uses a multicast OSPF hello packet to automatically discover the neighbors.
    • DR and BDR are not required. The router sends additional LSAs with more information about neighboring routers.
    • Typically used in a partial-mesh or star topology.
  • point-to-multipoint nonbroadcast (Cisco proprietary)
    • If multicast and broadcast are not enabled on the VCs, the RFC-compliant point-to-multipoint mode cannot be used, because the router cannot dynamically discover its neighboring routers using the hello multicast packets; this Cisco mode should be used instead.
    • Neighbors must be manually configured.
    • DR and BDR election is not required.
  • broadcast (Cisco proprietary)
    • Makes the WAN interface appear to be a LAN.
    • One IP subnet.
    • Uses a multicast OSPF hello packet to automatically discover the neighbors.
    • DR and BDR are elected.
    • Full- or partial-mesh topology.
  • point-to-point (Cisco proprietary)
    • Different IP subnet on each subinterface.
    • No DR or BDR election.
    • Used when only two routers need to form an adjacency on a pair of interfaces.
    • Interfaces can be either LAN or WAN.
    • Rob says discover
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