Monday, March 2, 2020

Check SFP Module Optical Signal Strength?

SFP optical transceiver is a hot-swappable, compact component that provides fiber connectivity for optical networking. They support various applications like Fibre Channel (FC) switches, SONET/SDH network, Gigabit Ethernet, high-speed computer links, and CWDM and DWDM interfaces. When connected to switches, the optical signal strength of SFP modules is a critical parameter to ensure the normal working of the whole connections. This article will introduce the method of SFP module signals measurement and how to check SFP module optical signal strength.

Understanding SFP Module Optical Signal Strength and Its Importance

Generally, the signal strength of the SFP module includes two parts: Tx power and Rx power. The former one stands for the transmit power signal, and the latter stands for the receive power signal. For a normal SFP transceiver, the value of the Tx and Rx power lies in a specific range, in which the SFP transceiver can work normally. Take Cisco GLC-SX-MM 1000BASE-SX SFP as an example, its transmit power range is -3 to -9.5dBm, and the receiver power range is 0 to -17dBm. If either the Tx or Rx power is in the -30dBm or lower range, it means there is no actual signal being transmitted or received.
cisco sfp

1000BASE SFP Transceiver Module

The strength of optical signals directly determines whether the network connections can work normally or not. If the Rx power is not strong enough, there will be no signals in the optical links. That’s why a long-reach transceiver or an optical amplifier is needed in long-haul transmission. And if the Rx power is too strong, the SFP module will be damaged. Therefore, a quality SFP transceiver is the basic guarantee for a smooth connection.

Measurement of SFP Module Optical Signal Strength

Generally speaking, there are two common methods to measure optical power strength: milliwatt (mW) and dBm which is short for decibel of the measured power referenced to one milliwatt. The former one measures optical signal strength by power, while the latter describes signal strength with absolute power value. Different vendors may adopt one of them to describe signal power. For instance, Cisco switches are used to adopt dBm to measure the power, while other switches get accustomed to using mW. Since the optical power is small, sometimes microwatt (µW) is also used in some switch vendors. Therefore, there are conversions between these methods.
dBm=10*lgP (P indicates optical power, in mW.) For example, 1 mW can be converted into 0 dBm.
1mW = 1000µW
Here are some figures recommended by EMC.
microwattmilliwattdBmDescription
1.00.0010-30.00Loss of Signal
10.00.0100-20
25.10.0251-162Gbps minimum accept signal
31.60.0316-154Gbps minimum accept signal
50.00.0500-13.01
100.00.1000-10.002Gbps minimum send signal
125.90.1259-9.004Gbps minimum send signal
150.00.1500-8.24
200.00.2000-6.99Normal range of optical signal strength
250.00.2500-6.02
300.00.3000-5.23
350.00.3500-4.26
400.00.4000-3.98
Notes: Optical signals get attenuated during transmission. To ensure the transmission quality, network operators also need to pay attention to the attenuation caused by transceivers modules. There is the acceptable light attenuation range of some common modules rate.
8Gbps maximum acceptable signal attenuation: -13.8dBm
4Gbps maximum acceptable signal attenuation: -15.4dBm
2Gbps maximum acceptable signal attenuation: -18.2dBm

How to View SFP Module Optical Signal Strength?

To determine if a SFP module (transmitter and receiver pair) is operating at the appropriate signal levels, the data sheets of SFP transceiver should be referenced. It often provides critical information such as link reach, fiber type (single-mode or multimode), transmitter output power range and optical receive power range, etc., which is helpful.
In addition, some switches like Cisco and Brocade SAN switches will offer CLI (Command-Line Interface) reference for users to view SFP modules details which include SFP rate, serial number, Part Number, accept/send direction optical signal strength. The following pictures show the results of SFP module details in Cisco and Brocade switches. Of course, the optical signal strength is included.
cisco

Cisco CLI -- Show Interface Transceiver Details

brocade

Brocade CLI -- sfpshow

From the result above, we can see that the method that Cisco and Brocade marking the signal strength is different. But both of them offer the current signal strength and the range of effective optical signal strength of the SFP modules. As long as the SFP signal strength lies in the valid range, the SFP module can be considered to work normally.

Summary

Optical signal strength is an important element affecting the whole optical links. This post gives a simple introduction to it and how to view SFP module signal strength in Cisco and Brocade switches. Hope it would help you.
Related Article: SFP Module: What’s It and How to Choose It? Related Article: How Many Types of SFP Transceivers Do You Know

Tuesday, February 25, 2020

Port Channels

Fast EtherChannel (FEC)
Gigabit EtherChannel (GEC)

Configuration


config-if# channel-group 1 mode on

Load Balancing


config# port-channel load-balance <type>

Discovery


Port Aggregation Protocol (PAgP)


config-if# channel-group 1 mode [auto| desirable]

Link Aggregation Control Protocol (LACP)


config-if# channel-group 1 mode [active|passive]

Requirements


Same speed and Duplex
If not trunking then same access VLAN
If trunking, same trunk type, same allowed VLANs and same native VLAN
Same STP cost per VLAN on all links in channel
No SPAN & RSPAN

IP Subnetting

Subnetting
2^7 |2^6|2^5|2^4|2^3|2^2|2^1|2
128 |64 |32|16|8|4|2|1|
 
 
Network= 2^N where N=Network bits
Hosts = 2^H - 2 Where H is the host bits
 
Classfull Networks
A 0-191.0.0.0 255.0.0.0
B 192-223.0.0.0 255.255.0.0
C 224-
D
E
subnetting is taking a large network and dividing it into smaller network
Network: all host bits set to Zero
Broadcast: All host bits set to one
150.50.0.0
150.50.255.255
If you have a class c network 192.168.3.0/24 and you need 4 networks
class bits = 8 bits 2^8=256 so each group will be 256/4 = 64
Network = 2^N =4
N=2 so you need 2 bits
255.255.255.11000000
Host bits
2^6 -2 = 64-2 =62 Hosts
Networks
0-63
64-127
128-191
192-255



Subnetting

2^7 |2^6|2^5|2^4|2^3|2^2|2^1|2
128 |64  |32|16|8|4|2|1|



Network= 2^N  where N=Network bits
Hosts = 2^H  - 2 Where H is the host bits


Classfull Networks

A       0-191.0.0.0  255.0.0.0
B   192-223.0.0.0  255.255.0.0
C  224-
D
E

subnetting is taking a large network and dividing it into smaller network
Network: all host bits set to Zero
Broadcast: All host bits set to one
150.50.0.0
150.50.255.255

If you have a class c network  192.168.3.0/24  and you need 4 networks
class bits = 8 bits 2^8=256 so each group will be 256/4 = 64

Network = 2^N =4
N=2 so you need 2 bits
255.255.255.11000000

Host bits
2^6 -2 = 64-2 =62 Hosts

Networks
0-63
64-127
128-191
192-255



Friday, February 14, 2020

Understand OSPF


OSPF - Link State 

WHY OSPF  IS BETTER THAN RIP 

The following is a list of reasons OSPF is considered a better routing protocol than RIP:
  • OSPF has no hop count limitations. (RIP has 15 hops only.)
  • OSPF understands variable-length subnet masks (VLSMs) and allows for summarization.
  • OSPF uses multicasts (not broadcasts) to send updates.
  • OSPF converges much faster than RIP, because OSPF propagates changes immediately.
  • OSPF allows for load balancing with up to six equal-cost paths.
  • OSPF has authentication available. (RIPv2 does also, but RIPv1 does not.)
  • OSPF allows for tagging of external routes injected by other autonomous systems.
  • OSPF configuration, monitoring, and troubleshooting have a far greater IOS tool base than RIP.

OSPF  OPEN SHORTEST PATH FIRST 
  • Most popular link state routing protocol. 
  • An open standard so it can be run on routers produced by different vendors
  • Contrast to EIGRP, a Cisco proprietary protocol that can be run only on Cisco devices.
  • It is classless, supports VLSM, Manual route summarization, incremental updates, equal cost load balancing, etc.
  • It uses a single parameter – the interface cost as metric.
  • It uses multicast addresses of 224.0.0.5 and 224.0.0.6 are for the communication between OSPF-enabled routers. 
  • Default administrative distance for OSFP routes is 110.
OSPF OVERVIEW
  • OSPF need to establish the neighbor relationship before exchanging routing updates.
  • OSPF is a link state routing protocol, neighbors don’t exchange eouting tables; instead, they exchange information about network topology.
  • Each OSFP router runs the SFP algorithm to calculate the best routes and adds those to the routing table.
  • Each router knows the entire topology of a network, a chance for a routing loop to occur is minimal.
  • OSPF routers stores routing and topology information in three tables:
    • Neighbor Table - keeps information about OSPF neighbors.
    • Topology Table - keeps the topology structure of the network.
    • Routing Table  -   keeps the best routes.
OSPF Neighbor Discovery  - HELLO PACKET
  • Routers running OSPF need to establish a neighbor relationship before exchanging routing updates. 
  • OSPF neighbors are dynamically discovered by sending Hello packets out each OSPF-enabled interface on a router. 
  • Hello packets are sent to the multicast IP address of 224.0.0.5. If the two neighbors have compatible OSPF parameters listed in the Hello packets, the neighbor relationship will be formed.
  • By default, Hello packets are sent out every 10 second on an Ethernet network (this interval is known as the Hello interval). 
  • A Dead interval is four times the value of the Hello interval, so if a router on an Ethernet network doesn’t receive at least one Hello packet from an OSFP neighbor for 40 seconds, the routers will declares that neighbor to be down.
  • Routers first exchange hellos and become neighbors. 
  • Then they decide to form adjacencies. 
  • An adjacency is a state where two routers agree to exchange LSAs (link state advertisements).
  • The LSA exchange between any two routers will populate their link state databases. 
  • At this point both routers will have the same copy of the link state database for the particular area.
  • Then the routers will individually run SPF (Dijkstra Shortest Path First algorithm) against the recently populated link state database  to determine the shortest path between the calculating router and all other routers in the network. You can think of the link state database as your input to the Dijkstra SPF algorithm (program).
  • Because all routers run the same calculation on the same data (same link state database), every router has the same picture of the network, and packets are routed consistently at every hop. 
  • In summary, database is you input to SFP. 
  • LSAs for  to a missing neighbor will be removed and SFP will run again against your new database without the old LSAs (possibly with some new replacement LSAs)
  • Yes ip ospf process will trigger a new calculation.
  • Fields in the OSPF Hello packets must be agreed on the following parameters (the same on both routers in order for routers to become neighbors):
    • Same Subnet   (must be able to ping both router interface where OSPF will be established)
    • Area ID
    • Hello Interval and Dead Interval timers
    • Authentication (if used)
    • Area stub flag (Area Type Stub, NSSA)
    • Router ID must be unique
    • MTU
OSPF  Multicast Address 
  • 224.0.0.5 - All OSPF Routers
  • 224.0.0.6 - OSPF DRs


OSPF AREA
  • OSPF uses areas to simplify administration: optimize traffic and resource utilization. 
  • An area is a logical grouping of contiguous networks and routers. 
  • All routers in the same area have the same topology table and don’t know about routers in the other areas. 
  • The main benefits of using areas in an OSPF network are:
    • The routing tables on the routers are reduced.
    • less time is required to run the SFP algorithm, since routers need to recalculate their link-state database only when there’s a topology change within their own area.
    • routing updates are reduced.
  • Each area in an OSPF network must be connected to the backbone area (area 0). 
  • All routers inside an area must have the same area ID in order to become OSPF neighbors. 
  • A router that has interfaces in more than one area (area 0 and area 1, for example) is known as an Area Border Router (ABR). 
  • A router that connects an OSPF network to other routing domains (to an EIGRP network, for example) is called an Autonomous System Border Routers (ASBR).
OSPF   Router ID   (RID)
  • The router is known to OSPF by the router ID number
  • The Router ID is used in the LSDBs to differentiate one router from the next
  • OSPF requires at least one active interface with an IP address
  • By Default, the Router ID is :
  • The highest IP address on an active interface at the moment of OSPF process startup
  • If a loopback interface exists, the highest IP address on any active loopback interface, A loopback interface overrides the OSPF Router ID
  • The OSPF Router-ID command can be used to override the default OSPF router ID selection process
  • Using a loopback interface or Router-ID command is recommended for stability
  • The process ID is the ID of the OSPF process to which the interface belongs.
  • The process ID is local to the router, and two OSPF neighboring routers can have different OSPF process IDs.
  • Router Boot process ( POST, Load IOS, Bring up Interfaces, OSPF Process
  • OSPF can use Physical Interface IP as the Process ID, Loopback IP or Router ID 
    • Physical Interface can be unreliable because it can be down
    • Highest IP used as a tie breaker in the DR election
    • Loopback Address Logical Interface, more reliable then physical Interface, 
    • Router ID – hard code process, appears in log messages. 
  • use "config-router# router-id" command
  • use highest up/up loopback address
  • use highest up/up non-loopback address

OSPF   LSA - Link State Advertisements
  • The LSAs (Link-State Advertisements) are used by routers running OSPF to exchange topology information. 
  • An LSA contains routing and topology information that describe a part of an OSPF network. 
  • Routers exchange LSAs and learn the complete topology of the network until all routers have the exact same topology database.
  • When two neighbors decide to exchange routes, they send each other a list of all LSAa in their respective topology database. 
  • Each router then checks its topology database and sends a Link State Request (LSR) requesting all LSAs not found in its topology table. 
  • The other router responds with the Link State Update (LSU) that contains all LSAs requested by the neighbor.
OSPF   LSA - Link State Advertisements  TYPES
  • Type 1 LSA  aka Router Link Advertisement RLA
    • Type 1 LSA is sent by every router to other routers in its area. 
    • It contains the router ID RID, interfaces, IP information, and current interface state. 
    • Note that Type 1 LSAs are flooded only across their own area.
  • Type 2 LSA  aka Network Link Advertisement NLA 
    • Type 2 LSA is generated by designated routers DRs to send out information about the state of other routers that are part of the same network. 
    • Type 2 LSAs are flooded across their own area only.
  • Type 3 LSA  aka Summary Link Advertisement SLA
    • Type 3 LSA is generated by area border routers (ABRs) and sent toward the area external to the one where they were generated. 
    • It contains the IP information and RID of the ABR that is advertising an LSA Type 3.
  • Type 4 LSA  informs the rest of the OSPF domain how to get to the ASBR. 
    • The link-state ID includes the router ID of the described ASBR.
  • Type 5 LSA aka AS External Link Advertisements, 
    • A Type 5 LSA is sent by autonomous system boundary routers (ASBRs) to advertise routes that are external to the OSPF autonomous system and are flooded everywhere
LSA Types
  1. Router - one per router; listing RID and all interface ip addresses; also represents stub networks
  2. Network - one per transit network; created by DR on subnet; represents the subnet and router interfaces
  3. Net Summary - created by ABRs; represent area's type 1 & 2 LSAs into another area
  4. ASBR summary - like type 3; advertises host route to reach each ASBR
  5. AS external - created by ASBR; for externals routes injected into OSPF
  6. Group membership - defined for MOSPF; not supported by Cisco
  7. Not-so-stubby-area external - created by ASBRs inside NSSA area instead of type 5
  8. External attributes - not implemented in Cisco routers
  9. -11. Opaque - used as generic LSAs for future expansion

MESSAGES 
  1. Hello - discover neighbors; heartbeat
  2. Database Description (DD) - exchange brief LSA headers
  3. Link-state Request (LSR) - request full details of LSAs
  4. Link-state Update (LSU) - contains fully detailed LSAs
  5. Link-state Acknowledgement (LSAck) - confirm LSU

OSPF   DR / BDR  - Designated Router and Back up Designated Router
  • OSPF router can elect one router to be a designated router (DR) and one router to be a backup designated router (BDR). 
  • On multiaccess broadcast networks (such as LANs) routers defaults to elect a DR and BDR. DR and BDR are elected to minimize the number of adjacencies formed and to serve as the central point for exchanging OSPF routing information. 
  • On point-to-point links, the DR and BDR are not elected since only two routers are directly connected.
  • Each non-DR or non-BDR router will exchange routing information only with the DR and BDR, instead of exchanging updates with every router on the network segment. 
  • DR will then distribute topology information to every other router inside the same area. The backup designated router (BDR) serves as a hot standby for the DR. 
  • It receives all routing updates from OSPF adjacent routers, but it will not disperse LSA updates.
  • To send routing information to a DR or BDR, the multicast address of 224.0.0.6 is used. 
  • A DR sends routing updates to the multicast address of 224.0.0.5. If the DR fails, the BDR will take its role of redistributing routing information. 
    OSPF  AREA TYPE
      1. Backbone - Area 0
      2. Stub - Blocks external routes (no LSA type 5; ABR is default gateway)
      3. area <area-id> stub
      4. Totally Stubby - Blocks external routes and routes of other areas (no LSA type 3,5; ABR is default gateway)
      5. area <area-id> nssa no-summary
      6. Not-so-stubby (NSSA) - no LSA type 5; can create type 7
      7. area <area-id> nssa
      8. Totally NSSA - no LSA type 3, 5; can create type 7
      9. area <area-id> stub no-summary.

      OSPF   Requesting, Getting and Acknowledging LSA
      1. After [Database Description (DD)] - exchange brief LSA headers exchange of LSA headers, router will request full copies of LSAs that it needs
        • Compares sequence numbers in local LSADB with headers in DD
        • Sequence numbers start with 0x80000001, wrap around to 0x7FFFFFFF and re-flood at 0x80000000
      Acknowledgements
      1. Sends the same packet back
      2. LSAck Message - list of LSA headers that be acknowledged

      DR Election
      • Occurs after routers have become neighbors, before DD packets
      • If Hello says DR is 0.0.0.0 (means no DR has been elected yet)
        • Then routers wait a little longer for others to initialize (OSPF wait time; same value as dead timer)
      • Rules
        • If priority is set, routers put their own RID in their Hello messages
        • Others examine Hellos, look at the priority
        • If a router receives better priority, it replaces the RID in it's Hello messages with the better one
        • Highest priority is considered better
        • If a router doesn't want to be a DR but still have a high priority, then it will become a BDR
        • Late routers don't count
        • If a DR fails, its replaces by the BDR and a election for a new BDR starts
      Network Types

      • Broadcast - DR & BDR; hello 10; 3+ hosts
      • Point-to-Point - No DR or BDR, only 224.0.0.5; hello 10
      • NonBroadcast-MultiAccess (NBMA) - Neighbors configured statically (unicast); hello 30; 3+ hosts
      • Point-to-Multipoint - No DR or BDR (multicast & broadcast); hello 30; 3+ hosts
      • Point-to-point Nonbroadcast - hello 30; neighbor command; 3+ ho

      • OSPF  - propagetes LSA rather than Routing table updates
      • LSA   -  (Link State Advertisements)  Floods All OSPF routers in the Area
      • OSPF Link State database is pieced together LSA generated by by the OSPF routers
      • SPF algorithm to calculate the shortest path to destination based by
      • LINK = router interface
      • STATE = description of an interface and its relationship to neighboring routers
      • OSPF Hierarchical routing consist of AREAs and Autonomous systems AS
      • it minimized routing update traffic
      • SPF Shortest Path First algorithm places each router at the root of a tree and calculate the shortest path to each destinations based on cumulative cost  Cost = 10^8/Bandwidth (bps)

      Advertised Loopback interfaces is in routing table, can ping and uses address spaceRouter-ID number by which router is known by OSPFDefault the highest IP address on the active interface at the moment of OSPF process startupoverwritten by loopback interface: highest IP address of any active loopback interface.
      Configuring Single Area OSPFRouter(config)#router ospf process-idRouter(config-router)#network address mas area area-ID
      Defines OSPF as the IP routing protocolsAssigns networks to a specific OSPF area
      Key CharacteristicsType: Link StateAlgorithm: Dijkstra’s (Shortest Path First) AlgorithmStandard: RFC 2328Administrative Distance: 110Metric: CostProtocol/Protocol Number: IP/89Authentication: Yes (MD5 and Plain Text)Supports VLSM and Route SummarizationSupport for IPv6 (RFC 2740)Fast Convergence
      Router ID (RID)Router ID must be configured before an OSPF process could be started.Cisco Routers uses the following criteria to select a router ID:1. RID configured with “router-id” command2. If manual RID not configured, select the highest number IP address on any loopback interface in “up/up” state3. If loopback interfaces not configured, select the highest number IP address on any non-loopback interface in “up/up” state
      Metric CalculationCost = 100 Mbps / Link SpeedOSPF cost can be modified in three ways:1. (config-if)#ip ospf cost2. (config-if)#bandwidth3. (config-router)#auto-costreference-bandwidth


      Router Types

      1. Internal Router: whose (all) interfaces resides within the same area
      2. Backbone Router: A router that resides in the backbone area
      3. Area Border Router: an ABR connect two or more Areas
      4. ASBR: Autonomous System Boundary Router or an
      5. ASBR connects an external routing domain to an OSPF routing domain

      OSPF Neighbor States

      1. Down: Previously known neighbor has failed
      2. Init: an interim state in which Hello has been heard from the neighbor but that Hello does not list the local router’s RID
      3. Two-way: the neighbor has sent a Hello that lists the local router’s RID in the list of seen routers
      4. Full: Both routers complete the database exchange process and have identical LSDB. Fully adjacent

      Route Types and Preference

      1. Intra-Area Routes: A route to a network in the same
      2. area as the router. Denoted by “O” in the routing table.
      3. Inter-Area Routes: A route to a network in another area as the router. Denoted by “O IA” in the routing table
      4. External Route: A route to network that is external to the OSPF routing domain. Denoted by ‘E1’ or ‘E2’ in therouting table.

      Routes Preference:

      1. Intra-Area (O) > Inter-Area (O IA)2. Inter-Area (O IA) > External Type-1 (E1)3. External Type-1 (E1) > External Type-2 (E2)


      AREA 

      OSPF runs SPF algorithm and requires a lot of processing power and memory. If the size of network is too large this could cause slower convergence and can lead to following problems:
      1. More memory is required to maintain the link state database
      2. More processing power is required to process the link state database
      3. The links state database grows exponentially with the size of OSPF domain
      4. A single change in network topology (for example: link up/down) would trigger all routers to re-run the SPF (again) to calculate the shortest path

      To cope with these problems, areas are configured. There are two basic types:
      1. Backbone Area or Area 0: All other area must be connected to area 0
      2. Non-backbone Area: any other area with area-id other than zero


      Timers
      • Hellos are sent to multicast address: 224.0.0.5 (ALLSPFRouters)
      • Broadcast Multi-access = 10 seconds
      • Point-to-Point & NBMA = 30 seconds
      • Dead Timer = Four Times the hello interval
      • Broadcast = 40 seconds
      • NBMA and P2P = 120 seconds
      • To change hello and dead intervals use the command 
      ‘config-if)# ip ospf hello-interval seconds’ and
      ‘config-if)# ip ospf dead-interval seconds’


      Designated Router (DR) / Backup DR (BDR) Election

      There are two problems with multi-access networks:
      1. For “N” routers, it requires “N(N-1)/2” adjacencies
      2. Flooding of this excess LSAs would be chaotic itself for the network. DR/BDR addresses the challenge of adjacency creation and LSA flooding on multi-access networks only No election on P2P and P2MP network type.
      The following criteria is used for DR/BDR election:

      1. Router with highest interface priority is elected as DR
      2. Any other router with second highest priority is elected as BDR
      3. If priority is equal, highest RID is used as tie-breaker
      4. The DR/BDR election is held between two or more neighbors who reach the TWO-WAY state

      The priority ranges from 0-to-255 and default value is 1

      1. Priority of 0 means that router will not take part in DR and BDR election
      2. DR is never preempted even if a router with better priority is present. Manual reset is required for preemption If a router becomes active and it checks for an active DR and BDR on the network. 
      3. If there already is an active DR and BDR on the segment, the new router simply accepts them. 
      4. If there is not, then an election is held for DR/BDR selection After the DR/BDR have been elected, the other router known as DROthers establish adjacencies with DR and BDR only Neighbors are still tracked on multicast address: 224.0.0.5 but DROthers multicast updates to AllDRRouters address: 224.0.0.6.
      5. Only DR and BDR listen to this address and DR in-turn flood updates to DROthers on 224.0.0.5
      6. DR/BDR is property of a router’s interface not the router itself

      Virtual LinksIt is link through non-backbone area to backbone area. Used to connect:
      1. An area to backbone area through non-backbone area
      2. A partitioned backbone area through non-backbone area

      Rules:1. A virtual link can only be configured between ABRs

      2. The transit area must have full routing information and it cannot be stub

      Configuration
      Basic





      config# router ospf 1
      config-router# log-adjacency-changes detail
      config-router# ip ospf priority 255
      config-router# router-id 1.1.1.1
      config-router# network 172.16.2.0 0.0.0.255 area 0 (any interface that matches the ip address will run ospf)
      config-router# area 0 range 172.16.0.0 255.255.0.0 (ABR injects internal router summaries)
      config-router# summary-address 172.30.0.0 255.255.0.0 (configured an ABR for external router summaries)
      config-router# area 2 stub (configures an ABR and Internal Routers for a stub area)
      config-router# area 3 stub no-summary (configures an ABR in a totally stubby area, internal routers should be configured as stub)

      Alternative to Network Command

      config-if# ip ospf 1 area 3 (ospf AS and area number on each interface)

      RIP Redistribute

      config# router ospf 1
      config-router# network 172.30.0.0
      config-router# redistribute rip metric 1000 metric-type [1|2] subnets (1= metric increments; 2 = no increment)
      config# router rip
      config-router# redistribute ospf 1 metric 10

      NonBroadcast MultiAccess Mode

      config-subif# ip ospf priority 0...255 (0 = not DR/BDR; 255 = DR/BDR)
      config-router# neighbor 170.100.100.2 (need to configure only one direction; used when nonbroadcast)
      config-if# frame-relay map ip 170.100.100.3 201 broadcast (need to configure on both ABRs to communicate even though routes have been learned)

      Point-to-Multipoint

      config-subif# ip ospf network point-to-multipoint (need to configure on both directions)

      Timer Configuration

      config-subif# ip ospf hello-interval 30
      config-subif# ip ospf dead-interval 30
      config-subif# ip ospf dead-timer minimal hello-multiplier 4
      config-subif# ip ospf retransmit-interval

      Virtual Links

      • for areas not directly connected to backbone
      • middle router becomes ABR with full copy of area 0's LSDB

      config-router# area 1 virtual-link 1.1.1.1 (use router id; links two ABRs)

      Redistribution Using Tags and ACLs

      config# route-map eigrp2ospf
      config-route-map# match ip address 20 (ACL 20 to allow)
      config-route-map# set tag 10 (sets a tag of 10 to whatever matches this route map)
      config# router ospf 1
      config-router# redistribute eigrp 10 metric 100 subnets route-map eigrp2ospf
      config# router eigrp 10
      config-router# redistribute ospf 1 metric 1500 0 255 1 1500 route-map ospf2eigrp
      config# route-map ospf2eigrp
      config-route-map# match tag 5
      config# route map ospf2eigrp 20 (sequence number 20; start is 10)
      config-route-map# match route-type internal

      Inject Default Route into OSPF Domain

      config-router# default-information originate always (configure this router as the gateway of last resort)

      Authentication and VirtualLinks

      config-router# area 0 authentication message-digest
      config-subif# ip ospf message-digest-key 1 md5 WORD
      config-router# area 1 virtual-link 6.6.6.6 message-digest-key 1 md5 WORD

      View/Debug Commands

      show ip ospf interface
      show ip ospf database
      show ip ospf database network (lsa type 2) 
      show ip ospf database router (lsa type 1) 
      show ip ospf database summary (lsa type 3)
      show ip ospf database asbr-summary (lsa type 4)
      show ip ospf database external (lsa type 5)
      show ip ospf database nssa-external (lsa type 7)
      show ip ospf virtual-links
      show ip ospf border-routers
      show ip ospf statistics
      debug ip ospf hello
      debug ip ospf adj




      Configuration Example



      Single Area
      Router R1:
      interface loopback 0
      ip address 10.1.1.1 255.255.255.255
      !
      interface serial 0/0
      ip address 192.168.12.1 255.255.255.0
      !
      router ospf 100
      router-id 1.1.1.1
      network 192.168.12.0 0.0.0.255 area 0
      network 10.1.1.1 0.0.0.0 area 0


      Router R2:
      interface loopback 0
      ip address 10.2.2.2 255.255.255.255
      !
      interface serial 0/0
      ip address 192.168.12.2 255.255.255.0
      !
      router ospf 100
      router-id 2.2.2.2
      network 192.168.12.0 0.0.0.255 area 0
      network 10.2.2.2 0.0.0.0 area 0


      R2# show ip route | begin Gateway
      Gateway of last resort is not set
      C 192.168.12.0/24 is directly connected, Serial0/0
      10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
      C 10.2.2.0/24 is directly connected, Loopback0
      O 10.1.1.1/32 [110/65] via 192.168.12.1, 00:00:02, Serial0/0


      Example 2





      Multi-Area
      Router R1:
      interface loopback 0
      ip address 10.1.1.1 255.255.255.255
      !
      interface serial 0/0
      ip address 192.168.12.1 255.255.255.0
      !
      interface serial 0/1
      ip address 192.168.13.1 255.255.255.0
      !
      router ospf 100
      router-id 1.1.1.1
      network 192.168.12.1 0.0.0.0 area 0
      network 192.168.13.1 0.0.0.0 area 1
      network 10.1.1.1 0.0.0.0 area 0



      Router R2:
      interface loopback 0
      ip address 10.2.2.2 255.255.255.255
      !
      interface serial 0/0
      ip address 192.168.12.2 255.255.255.0
      !
      router ospf 100
      router-id 2.2.2.2
      network 192.168.12.2 0.0.0.0 area 0
      network 10.2.2.2 0.0.0.0 area 0


      Router R3:
      interface serial 0/0
      ip address 192.168.13.3 255.255.255.0
      !
      interface loopback 0
      ip address 10.3.3.3 255.255.255.255
      !
      router ospf 100
      router-id 3.3.3.3
      network 192.168.13.3 0.0.0.0 area 1
      network 10.1.1.3 0.0.0.0 area 1


      R2# show ip route | begin Gateway
      Gateway of last resort is not set
      C 192.168.12.0/24 is directly connected, Serial0/0
      O IA 192.168.13.0/24 [110/128] via 192.168.12.1, 00:00:03, Serial0/0
      10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
      O IA 10.3.3.3/32 [110/129] via 192.168.12.1, 00:00:12, Serial0/0
      C 10.2.2.0/24 is directly connected, Loopback0
      O 10.1.1.1/32 [110/65] via 192.168.12.1, 00:00:12, Serial0/0
      OSPF Troubleshooting Command
      1. show ip protocols
      2. show ip ospf [<process-id>]
      3. show ip route [ospf]
      4. show ip ospf interface [brief | <interface-id>]
      5. show ip ospf neighbor
      6. show ip ospf database
      7. debug ip ospf [hello | adjacency | events]






      Verify and changing the OSFP RID (RID - Router ID)

      Topology
      R1- Hub
      R2 - spoke
      R3 - spoke

      R1# show ip osfp neighbor

      Nighbor ID Pri State Dead Time Address
      3.3.3.3 0 Full/Drother 00:01:41 172.12.123.3
      200.200.200.2 0 Full/Drother 00:01:30 172.12.123.2
      R1#

      R2# show ip osfp
         Routing Process "ospf 1" with ID 200.200.200.1


      R2# show ip osfp neighbor
      Nighbor ID Pri State Dead Time Address
      172.12.123.1 1 Full/DR 00:01:47 172.12.123.1


      To change the "address 172.12.123.1" the router ID (RID)

      R1#
      R1#conf t
      R1(config)#router ospf 1
      R1(config-router)#router-id 1.1.1.1

      Reload or use "clear ip ospf process" command for this to take effect


      Note


      RULE 1
      If there are no loopback on a router, then
      the highest ip address on any interface on the router
      will be used as the OSPF RID even if the address is asssigned to an interface
      that is not OSPF enabled

      RULE 2
      if there is a loopback interface on a router, if there is a single one
      then that IP is going to be used as the OSPF RID by default.

      If you have multiple loopbacks, the highest IP address assigned to the loopback will be
      be the OSFP RID. but again, as we see in router 2,
      The interface IP address be used as the OSFP RID does not have to be OSFP enabled.

      If we have a loopback address as in R3

      R2# show run
       interface Loopback0
        ip address 3.3.3.3 255.255.255.0