OSPF Notes

OSPF Overview

==========

Classless link-state protocol

-uses Dijkstra SPF algorithm

-maintains active adjacencies

-supports VLSM

-supports both topology and NLRI summarization

Enabling OSPF

===============

Enable the global process

-router ospf [process-id]

.process-id is locally significant

-Must be an up/up interface running IP

.used for OSPF Router-ID

Enable the interface process

-network [address] [wildcard] area [area]

-ip ospf [process-id] area [area]

Verifying OSPF

===============

Verify OSPF is enabled

-show ip ospf

-show ip ospf interface [brief]

Verify OSPF adjacency's

-show ip ospf neighbor

-show ip ospf adj

Verify ospf database

-show ip ospf database [router | network | summary]

Neighbor and Topology Discovery

==========================

Like EIGRP, OSPF use hello packets to discover neighbors

-transport via IP protocol 89 (OSPF)

-sent as multicast to 224..0.0.5 or 224.0.0.6 or unicast

Hello packets contain attributes that neighbors must agree on to form adjacency

Once adjacency is negotiated, LSDB is exchanged.

Negotiating OSPF Adjacencies

========================

Neighbors must agree on attributes to form adjacency

-Not all ospf neighbors actually form adj

-most ospf configuration problems happen at this stage

Unique attributes include…

-local Router-id

-local interface ip address

Negotiating OSPF adjacencies

========================

Common attributes include…

-interface area-id

-hello interval and dead interval

-interface network address

-interface MTU

-network Type

-Authentication

-stub flags

-other optional capabilities

OSPF Media Dependencies

=======================

OSPF  behavior changes based on media

-eg Ethernet vs FR vs PPP

Different media uses different “network types “ to control..

-how updates are sent

-who forms adjacency

-how next-hop is calculated

OSPF Network Types

=================

Broadcast

Non-broadcast

Point-to-point

Point-to-multipoint

Point-to-multipoint nonbroadcast

Loopback

OSPF Network Broadcast

====================

Ip ospf network broadcast

Default on multi-access broadcast medias

-ethernet, token ring, & FDDI

Sends hellos and updates as multicast

-224.0.0.5 (AllSPFRouters)

-224.0.0.6 (All DR Routers)

Performs DR and BDR election

DR/BDR Overview

====================

Designated Router (DR)

-used on broadcast links to

.minimize adjacencies

.minimize LSA replication

Backup DR

-used for redundancy of DR

DROthers

-All other routers on link

-form full adjacency with DR & BDR

-stop at 2way adj with each other

DR/BDR choosen through election process

DR/BDR election

===================

Election based on the below field present in hello packets

-priority

0-255

Higher better

0 = never

-Router-id

.Highest loopback/interface Ip

.can be statically set

.higher better

.No preemption unlike IS-IS’s DIS

OSPF Network Non-Broadcast

======================

Ip ospf network non-broadcast

Default on multipoint NBMA medias

-FR & ATM

Sends hellos as unicast

-manually defined addresses with neighbor command

-performs DR/BDR election

OSPF Network Point-to-multipoint

==========================

Ip ospf network point-to-multipoint

Treats network as a collection of point-to-point links

Sends hellos as multicast

-224.0.0.5

No DR/BDR election

Special next-hop processing

Usually the best design option for partial mesh NBMA networks

OSPF network Point-to-point

======================

ip ospf network point-to-point

Default on point-to-point medias

-HDLC/PPP

Sends hellos as multicast

-224.0.0.5

No DR/BDR election

Supports only two neighbors on the link

Point to multipoint Non-broadcast

===========================

Ip ospf network point-to-multipoint non-broadcast

Same as point-to-multipoint, but sends hellos as unicast

-manually defined addresses with neighbor command

-allows for per-VC OSPF cost over NBMA
No DR/BDR election

Special next-hop processing

OSPF Network Loopback

===================

Special case for loopback and looped-back interfaces

Advertises link as /32 stub host route

Ip ospf network point-to-point (used to disable loopback)

OSPF PATH Selection

=================

Once databases are synchronized, path selection begins

Each router’s LSA include a “cost” attribute for each described link

Best path to that link is lowest end-to-end cost

Cisco’s implementation uses bandwidth based cost, but per RFC it is arbitrary

-default cisco cost = 100 Mbps/Link BW

-reference bandwidth can be modified to accommodate higher speed links (eg GigE)

OSPfF path selection order

============================

Per RFC, ospf path selection state machine prefers..

-Intra area routes (O)

-Inter area routes (O IA)

-External Type 1 (E1)

-External Type 2 (E2)

-NSSA Type 1 (N1)

-NSSA Type 2 (N2)

Cannot be modified with metric or distance.

Modifying OSPF path selection

=============================

Ospf uses bandwidth based cost

-COST = ref BW/Interface_BW

Cost can be modified with

-interface bandwidth

-interface ip ospf cost

-process auto-cost

-process neighbor [address] cost

OSPF convergence Timers

======================

Convergence based on hello and dead timer

-supports sub-second timers

Different timers for different network types

-show ip ospf interface

Changing hello time automatically adjusts dead time

-ip ospf hello-interval

-ip ospf dead-interval

Note: bidirectional forwarding detection is used for L2 link failures in ospf when we use switches in between routers.

OSPF Authentication

====================

OSPF supports 3 types of authentication

-0 = Null

-1 = clear test

-2 = MD5

Can be enabled

-on all links in the area (ie configured on a process)

-on a per link basis

Key is always applied at link level

-virtual-links are area 0 interfaces

OSPF Summarization

======================

All devices within the area must have the same LSDB

Implies summarization can only occur

-between areas

.area [source area] range [address] [mask]

-during redistribution

.summary-address [address] [mask]

Automatically generates discard route

-disabled with no discard route [internal | external]

Can be used for TE via longest match routing

OSPF Filtering Overview

====================

OSpf is a link-state routing protocol

-to calculate identical SPTs everyone must have the same input to SPF(the LSDB)

-Implies that filtering cannot be configured within an area

Inter-area filtering through

-stub areas

-LSA 3 filter

OSPF Stub Areas

=================

Stub areas used to limit type of LSAs allowed to enter an area

-Intra Area routes (O)

.LSA 1 & 2

Inter area routes (O IA)

.LSA 3 & 4

-External routes (E1 & E2)

.LSA 5

-NSSA external routes (N1 & N2)

.LSA 7

All routers in an area must agree on the stub flag

OSPF Stub Areas

===========

Stub Area

-removes external routes (LSA 5)

-removes ASBR advertisement (LSA 4)

-ABR originates inter-area default route (LSA 3)

Enabled on all routers in an area

-area [area id] stub

OSPF Stub Areas

============

Totally stub Area

-removes external routes (LSA 5)

-removes ASBR advertisement (LSA 4)

-removes inter-area routes (LSA 3)

-ABR orginates inter-area default route (LSA 3)

Stub enabled on all routers in the area

-area [area] stub

Totally stubby enabled on ABR(s) of the area

-area [area] stub no-summary

OSPF Stub Areas

=============

Not-so-stubby area (NSSA)

-allows NSSA external generation (LSA 7)

-removes external routes (LSA 5)

-removes ASBR advertisement (LSA 4)

All routers must agree on NSSA

-area [area] nssa

ABR does not originate default automatically

-can be configured to generate LSA 7 default

-area [area] nssa default-information-originate

OSPF stub Areas

===============

Not-so-totally-stubby area

-allows NSSA external generation (LSA 7)

-removes external routes (LSA 5)

-removes ASBR advertisement (LSA 4)

-removes inter-area routes (LSA 3)

-ABR originates inter-area default route (LSA 3)

NSSA enabled on all routers in the area

-area [area] nssa

Totally stubby enabled on ABR(s) of the area

-area [area] nssa no-summary

Controlling NSSA Redistribution

============================

Redistributed routes on an NSSA router are originated as LSA 7

If NSSA ASBR is also an ABR..

-type 7 originated into NSSA

-type 5 originated into Area 0

Type 7 origination can be suppressed

-area [area] nssa no-redistribution

-send type 5 to area 0 but not type 7 to NSSA

NSSA Translator Election

================

NSSA ABR translates Type 7 LSAs into Type 5 for Area 0 advertisement

If multiple ABRs, only on of them performs translation

-NSSA translator election chooses ABR with the higher router-id

IF forwarding address is non-zero, ABR need not be in the transit path

LSA 3 Filter

==============

Stub areas can only filter on LSA type

ABRs can filter which summary LSAs (LSA 3) they generate between areas

Applied to process level of ABR

-area [area] filter-list prefix [prefix-list] [in | out]

-In/out allows for control of ABRs with more than 2 areas