Friday, January 14, 2011

CCIE Study Guide: OSPF Notes

This is the second title in our CCIE Study Guide Series, which shall provide some useful guidelines that help to learn about OSPF.

•    Two tier hierarchical design
•    Dijkstra’s algorithm – shortest path first

Used to calculate best paths to destinations
Based on link state database
Each router in OSPF runs SPF itself

•    OSPF neighbor table – Adjacency Database
•    OSPF topology table – OSPF topology DB – LSDB
•    Routing table – Forwarding DB
•    Two area types

Transit – backbone – area 0
Regular – user access – all other areas
All regular areas must connect to area 0
Areas minimize routing tables
Localize impact of topology changes
Stop detailed LSA floods at area boundaries

•    ABR – area backbone router
Functions as default route or default path out of area
Separates LSA flood zones
Connects multiple areas

•    DR/BDR – designated router / backup designated router
Multicast address for DR/BDR
All other routers on network form adjacency with DR/BDR
LSA’s are only exchanged with these two
All other routers called DROTHERs
Elected by highest priority (default priority is 1)

  •  Priority zero cannot be elected DR/BDR
•    Ip ospf priority {#} interface command
  • Changing DR/BDR priority to zero takes effect immediately
  • Changing priority on DROTHER to something else has no effect until re-election
  • If tie in priority, highest router-ID (active IP interface) is used to break tie

Any new routers brought onto the network will form full adjacency with dr/bdr and two way state with other neighbors.
hanging priority to zero on a DR/BDR will take effect immediately.

•    ASBR
Connects the OSPF AS to an outside AS
Redistribution point

•    Five OSPF Packet types
Hello – discover and building adjacency
Hello packets are sent every 10 seconds on multi access networks
Dead interval is 4 times hello interval
Database description – DBD/DDP – checks for db synchronization
Link-state request – requests specific link-state records
Link-state update – response to LSR
Link-state ack – acknowledges other packet types

•    Operational States
Init state – routers multicast initial hello (
Two way – routers send unicast hellos listing neighbors
Exstart – master/slave relationship established with DR/BDR
Exchange – DDPs are exchanged
Loading – LSRs for specific networks
Full – all LSDBs are synchronized with DR/BDR.  Routers are able to route traffic

•    Communication multicast to everyone multicast to DR/BDR
LSAcks are sent unicast
LSDB summaries are multicast every 30 minutes

  •     Entries have a max life of 60 minutes
32 bit sequence numbers are used for link-state advertisements

  • The sequence number can be seen with the “show ip ospf database” command

Loopback interfaces are recommended for stability and RID

  •  RID can be manually set using router-id router configuration command
•    If RID was set with loopback, a router reboot is required for the router-id command to take effect
•    If RID was set with router-id  command, it can be changed with “clear ip ospf process” command
•    Network Types
  •     Point to Point

Multicast to discover neighbors
ip unnumbered is possible over point to point links
10/40 hello/dead interval
  • Broadcast

DR/BDR are the central point of contact in the network

  • Non broadcast multi-access

By default, OSPF cannot form neighbor adjacencies
DR/BDR elections become crucial

•    Hub/spoke, not all spoke sites can communicate directly

Three topologies

•    Full mesh – costly, requires separate VC’s for connectivity between each site
•    Partial mesh
•    Star – hub/spoke
  •     Modes of operation
•    Ip ospf network {mode} interface command
•    Broadcast – Cisco Proprietary

WAN links are treated like LAN interfaces
Multicast hello for discover
Full/partial mesh

•    Non broadcast – RFC Compliant

One IP subnet for all spokes
Neighbors are manually configured
DR/BDR is also manually rigged/configured to insure connectivity to DR

•    Point to multi-point – RFC Compliant

Multicast hello for discover
No DR/BDR (requires additional LSAs)

•    point to multi-point non broadcast – Cisco Proprietary

Used in place of P2MP where broadcasts and multicast are disabled
Neighbors are manually configured

•    point to point – Cisco Proprietary

Different IP subnet on each interface
No DR/BDR elected or needed
LAN or WAN interface

•    Default Modes

Point to point FR – point to point mode
Multipoint FR (subinterfaces) – non broadcast
main FR interface – non broadcast
•    NB mode neighbor configuration

Neighbor {ip} priority {# default 0} poll-interval {#} cost {1-65535} database-filter all

mode prefer topo subnethello timer adjacency RFCexample
bcastFull/partialSame10 secAuto DRCiscoLAN
non bcastFull/partialSame30 secManual DRRFCFR
p2mpPartial/starSame30 sec Auto no DR RFC FR bcast
p2mp nbPartial/starSame30 sec ManualCiscoFR nbcast
p2pPartial/starDiff10 secAuto no DRCiscoserial/sub

•    LSA Types
  • Router LSA – type 1

IntraArea LSA generated by every router in the area.  Advertises link states.  The LSID = RID of originator
  • Network LSA – type 2

Network LSA generated for Multiaccess networks
Generated by DR – LSID = RID of DR
  • Summary LSA – type 3

Summary advertisements generated by ABR
Summarizes type 1 LSAs from one area to another
Describes routes to area’s networks (aggregate routes)
LSID = destination network #
Not flooded to stubby, totally stubby, or not so stubby areas
Routes are NOT automatically summarized

  • Summary LSA – type 4

Generated by ABR to advertise the presence of an ASBR.  ASBR sends type 1 with e-bit set to ID itself
Routes to ASBR
LSID = RID of describe ASBR
Not flooded to stubby, totally stubby, or not so stubby areas
Routes are NOT automatically summarized

  • AS External LSA – type 5

Generated by ASBRs to advertise external networks and autonomous systems.  LSID = external network #
Contains all routes separately, unless manually summarized
ABRs pass type 5 LSAs on to the rest of the AS

  • Multicast OSPF LSA – type 6
  • NSSA External LSA – type 7

ASBR in a stubby area that needs to pass external routes back into the AS.  Stubby area needs to be reclassified as NSSA to allow those routes.

  • ABR receives type 7 LSAs and forwards them as type 5 to the rest of the AS
  • External LSA for BGP – type 8
•    Cost/Metric
  •   100mbps/link speed = cost

Example  100mbps interface has cost 1
Unfortunately, 1000mbps interface also has cost 1

•    Auto-cost reference-bandwidth {ref bw} interface command to change this behavior

bandwidth {value} interface command to define actual bandwidth
ip ospf cost {value} interface command

•    Route summarization

Occurs at ABRs, relies on contiguous IP design
Area # range {addr | mask} advertise/not-advertise cost {#}
Router will create a summarized route to null 0
ASBR Summarization

Summary-address {addr | mask } tag router config command
  • Default route

Default-information originate always metric {value}
Per Cisco, default metric of 10. 

OSPF Area Types
Standard Area

Accepts link updates, route summaries, and external routes

  • Stub Area

Blocks type 5 LSAs.  No routes external to the AS.  If stubs need to connect to external AS, they use default routes.

  • Totally Stubby Area

Blocks type 3, 4, and 5 LSAs.  No external AS, no summary routes.  Uses default route for everything outside of the local area.

  • Not So Stubby Area

Acts like a stub/tsa but allows ASBR connected to it
ASBR in an NSSA generates type 7 LSA
ABR to NSSA translates Type 7 to Type 5 before passing along to the rest of the ASRoutes from Type 7 LSA show in routing table as O N1 or O N2 (type 2 is default)

  • Stub/TSA/NSSA configuration

Area # stub router configuration command on all routers in area
Area # stub no-summary router configuration command on ABR for TSA
 area # nssa no-summary

  • Virtual Links
Allow extension of Area 0 through another area
Area # virtual-link RID where RID is the RID of the neighbor on the other side of the VL
sho ip ospf virtual-links

  •  Helpful commands

Sho ip ospf neighbor
Sho ip ospf database
Sho ip ospf adj  this is ADJ, not adjacency!!!!!!

Previous posts in our CCIE Study Guide Series are:

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