Link State Routing Protocol

  • Maintains Neighbor table, topology table, Routing table.
  • The difference when compared to EIGRP in regards to these tables is that Topology table where EIGRP will have the list of just a list of what the neighbors passed on, thats why it is alway referred as Routing by rumor(Distance vector routing protocol).
  • EIGRP topology table doesn’t have the roadmap of the network whereas OSPF does.

Dijkstra’s SPF algorithm

  • SPF algorithm runs on the Link state database
  • Finds the best path to the destination and generates the routing table.
  • Processor intensive than DUAL algorithm.


  • Why?  when SPF alogrithm run too often, updates become too numerous  because the network has grown too large. So by splitting into multiple areas we make the topology database smaller for routers within that area. So that SPF algorithm can be run less often, the reason being summarization can be done at the borders(ABR and ASBR).
  • Area 0 – Backbone Area /** All areas must connect to area 0**/
  • All routers within an area will have same database and roadmap.
  • ABR – Area Border Router – Router connecting two area in which one interface connects to backbone area and another interface connects to non-backbone area within an autonomous system.
  • ASBR – Router connects two networks that part of different autonomous system.
  • Summarization can be done only on ABR and ASBR.(Area boundaries). Summarization will generate LSA’s.

Neighbor Relationship

R1 ——- R2

  • Once the OSPF process is started, router will determine its own Router-ID.(Router-id will be the highest active IP  address, Preference – Loopback interface > Interface address.
  • Adds interfaces to the link state database (dictated by the network command)
  • Sends Hello message on chosen interfaces – “DOWN” state
  • Router-ID, Hello & Dead timers , Network mask, Area-id, Neighbors, Router Priority, DR/BDR IP address, Authentication Password, Stub flag (options field).
  • Bold are mandatory parameters that should be matched between the neighbors to form a successful neighborship.
  • Receive Hello (Check Hello/Dead Interval, Net-mask, Area-id, Authentication passwords) – “INIT” State
  • Send Reply Hello – After the verification is successful, enters into “2WAy” state. R1 and R2 both look at each other’s hello packets and checks if am I listed as neighbor in your hello packet ?(If yes, Reset the DEAD timer; If No, Add as a new neighbor). Continue in case only if the neighbor is new…….
  • MASTER – SLAVE relationship  – Exstart (Exchange start) State, exchanges the link state database between the neighbors. Master-slave determination determines who sends the information first.
  • Determined by Priority, Router-id break tie (higher router-id)Master send DBD packetSlave sends its DBD packet.
  • DBD’s are acknowledged and reviewed – “Loading” state (loading into the memory).
  • If any information is missing in the DBD’s that were exchanged, Master/Slave requests the information , sends LSR.
  • Master/Slave sends the update (LSU).
  • The information will be exchanged until the LSDB is synchronized between the neighbors. At this point, neighbors have same database and enters into “FULL” state.

SPF algorithm will be run on the top of the LSDB and forms the routing table which will have only the best paths to the destination vs LSDB contains all the possible paths to the destination.


OSPF metric is Cost, Cost = 100/BW-IN-Mbps.

DR and BDR (NBMA & Ethernet)

  • OSPF DR/BDR uses multicast address to send and receive/Listens on
  • DR and BDR are elected for every shared segment.
  • On point to point links, the only address used is
  • In an ethernet/shared network world, it doesn’t matter which router will become DR/BDR, but it is important in the frame-relay network.
  • DR Election doesn’t have the capability of “pre-empt”.
  • Inside the hello packet (two fields Router Priority and router-id (neighbor))
  • Router Priority – By default set to: “1”(Cisco); 128(Juniper)
  • Highest router priority wins
  • Higher router-id breaks tie.
  • In a shared network, Non-DR/BDR routers will stay in “2Way state” as there is no need for these routers to synchronize the LSDB. That’s the basic idea behind electing DR/BDR.

Virtual-Links (Breaking Rules)


Area 0 is the main office in USA and Area 1 in UK of company A. Company A want to merge with company B. In this scenario, temporarily we need to merge Company A and B so lets say Company B is in Area 2. In order to provide connectivity to area 0 (main office) we need to create a virtual links. This solution is used during the transition period.

=>Virtual-link is like a tunnel interface. Virtual-link is a tunnel that looks like it is directly connected to the device in area 0.  We can accomplish the same by configuring the tunnel interface.

LSA Types (Advertise routes,

  • LSA Type 1 : Router LSA (Advertise routes)
  • LSA Type 2 : Network LSA (Generated where DR has been elected)
  • LSA Type 3 : Summary LSA (Provide summary info about Type 1 and Type 2 LSA’s for other areas).
  • LSA Type 4 : Summary LSA
  • LSA Type 5 : External LSA (ASBR Summary)
  • LSA Type 7 : NSSA (OSPF Not So Stubby Area (NSSA) (area x nssa) is like a Stub area in that it does not allow Type 5 LSA’s.  However, the NSSA is allowed to have an ASBR originating External Routes as Type 7 LSA’s)
  • Type 4 LSA’s (Summary ) are always generated by an ABR when an ASBR is present in an area it touches.  OSPF routers in a different area than the ASBR’s area will look at the Type 5 and Type 4 LSA’s to calculate the path and metric to reach the  external route.  OSPF routers in the same area as the ASBR can look at just the Type 5 LSA to calculate the path. However, the Type 5 LSA by itself does not have enough information for OSPF routers outside the area*, hence the need for the Type 4 LSA.* The Type 5 LSA remains unmodified as it passes from area to area.  One of the key pieces of information that remains unmodified is the “Advertising Router”.  Routers outside of the ASBR’s  area do not have the Type 1 LSA  describing the ASBR (because its a different area).  The Type 4 LSA has information about the ASBR that matches the “Advertising Router” information in the Type 5 LSA.  Furthermore, the Type 4 LSA’s “Advertising Router” field changes to that of the ABR as it passes into a new area.  All of this information helps the OSPF router in other areas calculate the metric and next hop to reach the external routes


  • Stub Area – Blocks Type 5 LSA’s from entering.(Default route will be generated by ABR as type-3 LSA and will be installed in the stub router and next-hop as ABR)
  • Totally stubby (Cisco Proprietary) : Blocks Type 3, 4, 5 LSA from entering(Default route will be generated by ABR as Type3 LSA and will be installed in the totally stub router).
  • Not-So-Stubby-Area Passes external routes through Type 7 LSA, these convert back to Type 5 once they reach the Backbone by ABR.


  • Redistribution is a bad idea,
  • Why do we redistribute?
  • Politics, Merging companies, Vendors, Application/Business Requirements (Credit card companies).

Redistribution issues

  • Don’t do two way FULL redistribution. (Use Static or default for one way and one way do FULL table).
  • Looping issues with Two way full redistribution. (Filtering is one solution).


  • Passive Interfaces
  • Administrative distance modification (External routes)
  • Distribute-list/ Prefix-list
  • Route Maps / Route Tagging
  • Seed Metric
  • By default Redistribute into RIP and EIGRP will have metric of Infinity; into OSPF will have 20, into BGP will keeps the what ever the other protocol have.
  • Subnets Keyword : By default, when you redistribute into OSPF, it will summarize. Subnet command will redistribute the individual subnets.

Advance Redistribution

  • Route Tag : Tagging routes will give flexibility to use it for later purposes like filtering.
  • Under the route-map, When you match tag like match tag 10 20 30 (This is an OR operation) where as match 10, match 20, match 30 (This is an AND operation).( Specific to Cisco IOS)
  • Administrative Distance is local to the router and it is not advertised.


Additional Adjacency State

ATTEMPT This state is only valid for manually configured neighbors in an NBMAenvironment. In Attempt state, the router sends unicast hello packets every poll interval to the neighbor, from which hellos have not been received within the dead interval

Broadcast, Multi-access Networks

  • Eg: Ethernet, Token Ring
  • Single mode of operation
  • DR/BDR election, 10 sec Hello’s, DUAL multicast Address

Point-to-Point Networks

  • Eg: T1, ISDN BRI/PRI
  • single mode of operation
  • No DR/BDR, 10 sec Hello’s, Single Multicast Address. (

Non-Broadcast Multi-access Network (NBMA)

  • Eg: Frame-relay, ATM, MPLS, X.25 (WAN technologies)
  • Five modes of operation

Five modes of operation

  1. NBMA- RFC Standard
  • NBMA networks by default deny broadcast and multicast.
  • Broadcast keyword in the frame-relay map doesn’t allow broadcast, it creates “pseudo broadcasting” (its a directed broadcast).
  • Neighbor statically configured and must be on subnet.
  • Acts like a LAN environment.
  • DR/BDR elected.(Must have full connectivity)

2.      Point-to-Multipoint – RFC Standard

  • Treat the cloud like a series of point to point networks
  • Fixes issues with NBMA networks, requires single subnet.
  • No DR/BDR elected
  • Neighbors automatically formed.
  • Partial Mesh (No full connectivity, spokes cannot ping each other)
  • Full Mesh (full connectivity)

3.Point-to-Point – Cisco Proprietary

  • Uses separate sub-interfaces and requires different subnets.
  • Consumes lots of public ip address.

4.Broadcast – Cisco Proprietary

  • Works like broadcast in ethernet world.

5.Point-to-multipoint, Non-Broadcast – Cisco Proprietary

  • Works like Point to Multipoint except for neighbors do not form automatically, need static configuration

Protocol Independent Routing – Junos Specific

  1. Static Routes
  2. Aggregate Routes
  3. Generated Routes

Static Routing

Next-Hop Options

  • Directly Connected IP Address
  • Remote IP address – Add “resolve” keyword.
  • reject –
    • Configured NULL value-Drop& Sends an “ICMP message of Destination Host Unreachable”
  • Discard- Silently drops the packet.
  • Qualified-Next-hop
  • LSP

Floating Static Routes – Use qualified next-hop with preference values

Static Route’s “static-options

Static-options can be configured under two hierarchies

  • Under static defaults
  • Under specific Static route.

static-options” Junos Defaults

The oppposite functions are available and can be configured by prepending the keywords with “no” (like no-install)

  • active – Remove routes from the routing table  if next-hop is not available.
  • install – Places usable static routes into the forwarding table.
  • readvertise –
    • Allows static route to be exported from routing table and redistributed into another routing protocol.
  • no-retain –
    • If the routing protocol process shuts down, static routes are removed from the forwarding table.

Aggregate Routes

  • Promote Route Summarization
  • Once aggregate routes become active in inet.0 table, aggregate routes will remain until you manually remove them from the configuration.

Contributing Routes

  • Key to making an aggregate route active in the routing table is the presence of one or more contributing routes.
  • A contributing route is a more specific route to the aggregate route.

Next-Hop Options

  • Reject – (Default)
    • Configured NULL value. Route with reject next-hop are dropped and an ICMP message with destination host unreachable” returned ti the source of the IP packet.

Generated Routes

Generated routes are almost like aggregate routes and in fact the routing table views them as one protocol, protocol aggregate.

  • The difference between the aggregate and generated routes is that the next-hop attribute, where
    • aggregate route’s default next-hop is “reject” and option is discard.
    • Generated route’s default next-hop is “IP address” and option is discard.

Selection of IP next-hop of Generated Route

  • The next-hop IP address need not be configured for the generated route. The next-hop will be selected automatically by Junos software.
  • One of the contributing route will be selected as the next-hop for the generated route. This route is called “Primary contributing route”.
  • When you have multiple contributing routes, the primary contributing route is selected based on the route having “numerically smallest prefix
  • The automatic selection mechanism can be changed using Routing policy by allowing the specific contributing route.

Martian Routes

  1. and more specific routes
  2. and more specific routes
  3. and more specific routes
  4. and more specific routes
  5. and more specific routes
  6. and more specific routes
  7. and more specific routes

RFC 1918 Private IP address space of,, are not included in the above list. This is due to Junos treats martian routes specially.

Junos Implementation

Martian route is not allowed to be placed into a routing table. The local router can never forward packets to those destinations.

Defaults can be modified at routing-options and martians hierarchy.

JUNOS Software Routing Tables

  • inet.0
    • Store IPV4 unicast routes.
    • Route Status- “+” – Active; “-” – Last active route; “No Icon”- In Active, “*”- Both current and last active routes.
      • Only Active routes (*) are copied into the forwarding table.
    • Protocol Name – Used by routing policies to advertise & filter routes.
    • Protocol Preference
    • Next Hop – When multiple nexthop exists, the routing table selects a single next-hop to be placed in the forwarding table. This selected next-hop is marked as “>”.
  • inet.1
    • Stores IPV4 Multicast routes. Also called “Multicast forwarding cache”
    • Each (S,G) pair in the network is placed in this table.
  • inet.2
    • Stores IPV4 unicast routes but usage is different than inet.0
    • Used by multicast routing tables to prevent loops using RPF checks.
  • inet.3
    • Contains the Egress IP address of a MPLS LSP.
  • inet.4
    • Stores information learned using MSDP(Multcast. Src.. Disc… Proto).
  • inet6.0
    • Stores IPV6 unicast routes.
  • mpls.0
    • This is a switching table which stores MPLS label values.
  • bgp.l3vpn.0
    • Stores routing information in a layer 3 VPN.
  • bgpl2vpn.0
    • Stores routing information in a layer 2 VPN.

Route Preference

The reason for using the preference system is that each protocol have separate method for determining the best path to destination. (Eg: RIP-Hop Count, OSPF-Cost). These parameters are not comparable hence Route-Preference.

  1. Direct – 0
  2. Local – 0
  3. Static – 5
  4. RSVP – 7
  5. LDP – 9
  6. OSPF Internal – 10
  7. IS-IS Level 1 Internal – 15
  8. IS-IS Level 2 Internal – 18
  9. RIP 100
  10. PIM 105
  11. Aggregate 130
  12. OSPF External 150
  13. IS-IS Level 1 External 160
  14. IS-IS Level 2 External 165
  15. BGP 170
  16. MSDP 175

Load Balancing

Junos Defaults



Show Commands

  • show route protocol static
  • show route protocol aggregate
  • show route protocol aggregate detail (contributing routes)
  • show route hidden
  • show route martians

Junos BGP Configuration

Junos BGP Configuration


  1. Autonomous System Number.
  2. Peer Group
  3. neighbor <nei if-ip>
  4. Peer-AS
  5. Type External.


  1. Autonomous System Number
  2. Peer Group
  3. neighbor <Loopback>
  4. Type Internal
  5. local-address <loopback of local router>

Show commands

  • show bgp summary
    • Field “State” – 4/4/0  : Active/Received/Damped. The local router received four routers from its peer and all of them are active. None of the routes are damped.
  • show bgp group (Remote TCP Port #)
  • show bgp neighbor <ip>
  • show route receive-protocol bgp <peer-ip>
    • Database pointers representing the Adjacency-RIB-In
  • show route advertising protocol bgp <peer-ip>
    • Database pointers representing the Adjacency-RIB-Out
  • show route protocol bgp
    • Representation of BGP Local-RIB dataabase.
  • show route <dest-ip> detail
    • Displays all the attributes.
  • show route hidden extensive
    • Hidden routes are unusable as the next-hop is not resolvable.

BGP – Vendor Defaults


After BGP Peer Establishment

  • Each peer advertise their entire routing knowledge to each other. (By default only ACTIVE BGP routes learned from EBGP are advertised into IBGP)
  • After the initial exchange, Updates occur only on as-needed basis. Updates are only sent when new information is learned or existing information is withdrawn.

“BGP is a Routing Protocol ? Or Application Protocol?”


  • Default – EBGP connection is between directly connected and IBGP routers are not directly connected.
  • TTL of EBGP = 1
  • TTL of IBGP = 64.

BGP message header consists of “Marker, length and Type field”

  • Type field (1 octet) represents the type of BGP message
  • 1 – Open; 2- Update; 3- Notification; 4- Keep alive.
  •  Open Message consists of Version, Local AS, Hold Time, BGP ID,   Optional Params lenght and Optional Params
  • Update message consist of advertised routes and withdrawn routes. The fields of Update message are Unfeasible routes length, WithDrawn Routes, Total Path Attributes Length, Path Attributes and NLRI.
  • If the length value in Total Path attributes is “0”, it means that No Routes are being advertised with this message.
  • Each attribute in the Path Attribute filed is encoded as TLV (Type, Length and Value).
  • Notification message consist of “Error Code, Error subcode, Data” fields.
    • Error Codes 1- Message Header error
    • 2- Open Message error
    • 3-Update message error
    • 4-Hold time expired.
    • 5- Finite State machine error
    • 6-cease. (When you manually clear a BGP connection or change configuration of BGP parameters, you generate a cease).
  • KeepAlive Message are exchanged at 1/3rd the negotiated hold-time value for the session.
  • Keepalive is sent in the absence of other messages for a particular session.
  • If the local router don’t receive keepalive or update message within the hold-time period, Notification message of Hold Time Expired is generated and session is torn down.

JUNOS Default Values

  1. Hold Time = 90 seconds
  2. Keep alive = 30 seconds

BGP Internal Routing Tables (Junos Specific)

  • BGP router establishes memory locations in which to store routing knowledge. These are collectively known as Routing Information Base(RIB).
  • BGP peer maintains three categories of RIB
    1. Adjaceny-RIB-In
    2. Local-RIB
    3. Adjacency-RIB-Out


  • This table is created on the local router for each established BGP peer.
  • All routes received from the peer are placed in the appropriate memory table.
  • Adjacency-RIB-In table will be examined for the routes received(Update Message) from the peer only after the local router implements any applied import routing policies.
  • The purpose of examining this table is to identify the best path advertisement to each unique destination.


  • The best path to each destination is stored in the Local-RIB table.

Adjacency -RIB-Out

  • This table is created on the local router for each established BGP peer for Outbound route advertisements.
  • Only routes currently located in the Local-RIB are eligible to be placed in this outbound database.
  • By default, All Loca-RIB routes are placed in each Adjacency -RIB-Out table. Using export policies, the behavior can be changed.

Path Attributes

Path Attributes are encoded in the Update Message using TLV triple. The type portion is a 2 octet field.

  • Optional Bit (Bit 0) – Well Known  (“0”) or Optional (“1”).
  • Transitive Bit (Bit 1) – Transitive (1) Non Transitive (0).
  • Partial Bit (Bit 2) –
    • Only optional transitive attributes use this bit. “0” means BGP router along the path recognized this attribute. “1” means at least one of the BGP router didn’t recognize this attribute.
  • Extended Length Bit (Bit 3) – Size of the TLV Length.
  • Unused (Bits 4-7) – Unused and so set to “0”
  • Type Code (8-15) – Specific kind of attribute is encoded in this 1 octet field.
    • Type Code 3 – Next Hop
    • Type Code 5 – Local Preference
    • Type Code 1 – Origin
    • Type Code 4  – MED
    • Type Code 8 – Community.



Deep Dive into Packets and Bits


  • OSPF Common Packet Header  – 24-Octet header
  • Version(1), Type,(1)Packet Length(2), Router-id(4), Area-id(4), Checksum(2), Authentication Type(2), Authentication(8)
  • Checksum ( Doesn’t perform checksum to authentication data )
  • Type ( Hello, Data Base Description, LSR, LSU, LSA)

Hello Packet (Type1) (ALLSPF Routers Multicast

  • Network Mask(4), Hello Interval(2),  options(1), Router Priority(1), Router Dead interval(4), DR(4), BDR(4), Neighbor(4)
  • Network Mask : Unnumbered Pt-toPt interfaces and Virtual links set this value to
  • Options(1):
  • Bit 7-DN bit used for loop prevention in VPN environment;
    Bit 6 – O bit – Opaque LSA support;
    Bit 5 : DC bit – Demand Circuits support;
    Bit 4- EA External attributes LSA (BGP into OSPF);
    Bit 3 N/P Not-so-stubby LSA’s;
    Bit 2 MC – Multicast support;
    Bit 1 E bit-External LSA’s;
    Bit 0 – T bit TOS routing(Junos doesn’t support)
  • Router Priority – Default Value – 128, Range (0-255)
  • Router Dead Interval Default Value (40 sec); Range  (1-65535)
  • DR and BDR is used when they are not elected. (Interface address will be displayed by default)
  • Neighbor – Router-id will be displayed

DataBaseDescription Packet(Type 2)

  • Interface MTU(2); (MTU of outgoing interface)
  • Options(1); (Bits same as hello packet)
  • Flag(1); – Gives capability to exchange multiple DD packets.
  • Bit 3 – Bit 7 – Undefined, must be set to “0”
    Bit 2  – “I” (Initial bit) – This DD packet is first in the series of packets. First packet has a value of “1” while others are “0”
    Bit 1 – “M” (More bit) to let know the remote router if the DD packet is the last in a series. Last packet has a value of “0” while others are “1”.
    Bit 0 – “MS” (Master/Slave) Master – “1” and Slave “0”
  • DD Sequence number(4)
  • Each packet is incremented by “1” to the packet Master initiates during LSDB synchronization.
  • LSA headers (20 Octets)  Unique identifier of LSA.

Link State Request (Type 3) 

  • Link State Type (4) ( Types of LSA’s)
  • Link State ID (4)(specific to LSA)
  • Advertising Router(4) (router-id of the Master)

Link State Update

  • Number of LSA’s(4)
  • Link state advertisements(Variable).( LSA sequence numbers starts from 0x80000001 to 0x7fffffff.
  • Contains Routing, Metric, Topology information.
  • Link State Acknowledgement
  • LSA Headers -Variable-length (provides flexibility multiple LSA’s in a single pack

LSA Types

  • Type 1-Router LSA ( Advertise networks connected, Links,metric of interfaces and OSPF capabilities)                                                                => V(Virtual Link 0x4),E(ASBR router0x2),B(ABR0x1), Both ABR and ASBR (0x3), None(0x0) bits                                                                         => Link-ID and Link data Fields for different interface types
    ***Link- ID: Router ID of the adjacent peer(Pt-Pt & Virtual link),interface address of DR(Trasit links), network# of the subnet(Stub).
    =>Link Data : IP address of the local router’s interface(Pt-Pt, V-link, Transit), Subnet mask (Stub)
  • Type 2- Network LSA (DR generates network LSA represent broadcast segment to the rest of the network.                                                               =>Network Mast and Attached Router
  • Type 3 – Network Summary(Advertise local routing info(router and network lsa)  in both directions across the boundary.                                                    =>Network Mask, Metric(SPF cost + Cost carried in LSA).
  • Type 4 – ASBR Summary (Required to reach the ASBR in a remote OSPF area, ABR generates ASBR summary LSA for Type 5 LSA to reach ASBR).=>Network Mask , Metric , Router-id of the ASBR.
  • Type 5 – ASBR External (External routes into OSPF domain)                     =>Network Mask, Ebit (1-default (Type 2(External Metric); 2-Type 1(internal+external metric); Forwarding address- ( means ASBR itself- default); External Route Tag(Junos don’t use)


  • When multiple ABRs are present in the NSSA, only the ABR with the highest-router ID performs the 5-to-7 LSA translation.
  • Ebit – 1 (default) – Type 2 NSSA – External Metric
  • Ebit -2 – Type 1 NSSA – Internal (to ASBR) + External Metric
  • ABR that translates the type 7 into type 5 changes the Advertising router id field to itself thus by looking like an ASBR to the internal routers.(Router LSA Ebit will be changed to 0x3). NSSA LSA opt “0x8” – setting the P Bit.

Opaque LSA

Link-Local (Type 9 -Graceful restart), Area-scope(Type 10 -MPLS-TE), AS-wide scope(Type11).

Cli Commands

  • show ospf database router area <id> extensive ( Display Router LSA)
  • show ospf database network area <id> extensive
  • show ospf database netsummary area <id>
  • show ospf database area <id> lsa-id <id>
  • show ospf database extern extensive lsa-id <id>
  • show ospf database asbrsummary lsa-id <id>
  • show ospf database nssa
  • show ospf database extern advertising-router

Vendor Specific Defaults – OSPF

Hello Interval


Hello – 10; Dead interval – 40


Hello – 30; Dead interval – 120


OSPF metric is Cost, Cost = 100/BW-IN-Mbps.

This formula will not work for gigabit and higher, as anything higher than fast-ethernet will be having metric of  “1”.

Timers ((Applies to both Cisco and Junos)

  • OSPF wait timer : The wait time for electing the first DR on the segment is WaitTimer (40 sec – Router dead interval)  –
  • LSA Max-age : 3600 seconds
  • LSA Refresh timer : 1800 secs

Auto-cost Reference-bandwidth

auto-cost reference-bandwidth <bandwidth>

Seed Metric

By default Redistribute into RIP and EIGRP will have metric of Infinity; into OSPF will have 20, into BGP will keeps the what ever the other protocol have.

Subnets Keyword : By default, when you redistribute into OSPF, it will summarize. Subnet command will redistribute the individual subnets.

DR/BDR Election

Router Priority – By default set to “1″

Highest router priority wins

Higher router-id breaks tie.

OSPF External Routes

E1 – Increments the metric (Adds internal and external metric)

E2 – Doesn’t increase the metric ( only external metric, internal metric will not be added)

Default – E2  (metric-type 2)

Note: Router-id should be configured manually before forming the neighbor relationship, otherwise it picks up the router-id automatically. Changing the router-id needs “clearing the OSPF process or reload of router to take affect.


“Broadcast”  – In frame-relay configuration, Broadcast keyword will not allow the broadcast but it does “pseudo broadcast means directed broadcast”

Default mode – Non-broadcast (sh ip ospf int)

Loopback by default will be advertised as /32, in order to advertise as a normal network which ever it is configured for,

configure loopback as point-to-point under the interface.

NBMA-RFC :   Default – Priority is “0” for the neighbor.

OSPF path selection Criteria (Regardless of Metric and AD) (IOS & JUNOS)

Intra-Area (O)
Inter-Area (O IA)
External Type 1 (E1)
External Type 2 (E2)
NSSA Type 1 (N1)
NSSA Type 2 (N2)

  • Junos – Each LSA in the link-state database originated by the local router is noted with an asterisk (*).
Vendor Specific Defaults – OSPF