OSPF - Operation

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How it all works

• Internet model

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AS X AS Y AS Z

iBGP iBGP iBGP

IGP IGP IGP

eBGP eBGP

Barry Greene & Philip Smith “Cisco ISP Essentials”

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Recap• In ISP networks, IGPs– Allow routers within an AS to learn about each other– Carry next-hop reachability info• Carries infrastructure info (loopbacks & ptp)• NOT customer routes!

– scalability and fast convergence• Hence, minimise the number of prefixes carried in IGP!

• BGP– Carries customer prefixes– Exchanges network info with other networks• Carries internet route across the AS

OSPF - Intro• Link-state protocol– SPF algorithm– Protocol number (89) – runs on top of IP

– Only sends triggered updates– Supports hierarchical routing (multi-area) – scalability

• OSPFv2 (RFC2328), OSPFv3 (RFC5340), OSPFv3 AF (RFC5838)

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OSPF MessageOSPF HeaderIP Header

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LSAsR2 R1 LSDB (Topology)LSAs

SPF Tree

Best Paths

Routing Table

Dijkstra’s S

PF

Link State Operation

R2

R3

R1

R4

Link State Operation• Each link-state router learns about its links and

connected networks– builds a link state packet – LSP (LSAs for each link)

• Sends out Hellos for neighbor discovery– To establish adjacency

• Floods LSP to all its neighbors– Stores all LSPs learned from its neighbors in a LSDB, and floods

to other neighbors – Eventually all routers receive all LSPs• same view of the network!

• Computes the best path to each destination – using the SPF algorithm (SPF tree)

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Shortest Path First (SPF) Tree

• Every router in an OSPF network maintains an identical topology database

• Router places itself at the root of SPF tree when calculating the best path

Best path selection • Lowest cumulative cost = best path

• Load balances over equal cost paths

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FE

FE

FE

GE1

1010

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FE

FE

GE

GE1

1010

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OSPF Metric• Uses link/path cost as metric

• Generally, inversely proportional to the link BW– Higher the BW lower the cost• configurable

– The reference BW is generally 100Mbps (FE)• interfaces bigger than a FE would have a cost of 1

– For more granularity/accuracy of cost calculation• change reference BW for bigger links (all OSPF routers)

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Cost = &'('&')*' +,-).'&(/*' +, (+12)

IOS:router ospf/v3 <process-id>auto-cost reference-bandwidth <Mbps>

Junos:set protocols ospf/3 reference-bandwidth <Gbps>

Router ID• Uniquely identifies a link-state router– 4-byte Router ID

• Either:

– Explicitly configured =>

– Else, the highest/lowest IPv4 address of any active loopback interface

– If no loopbacks, the highest/lowest IPv4 address of any active physical interface

– ** Loopbacks preferred!• Why??

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IOS:router-id <4-byte>

Junos:set routing-options router-id <4-byte>

OSPF Packets

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Version Type Length

Router-ID

Area-ID

Checksum AuType

Authentication

Authentication

Type Description

1 Hello

2 Database Description

3 Link State Request

4 Link State Update

5Link State Acknowledgement

OSPF Header OSPF Message

OSPF Packets• Hello packet

– Initially exchanged for neighbor discovery, and

– Later periodically to maintain adjacency• Hello/Dead interval

– Hello packets contain:• Router ID (sender’s)• Area ID *• Hello and Dead interval *• Neighbors (list/router-id of neighbors - valid Hellos received)• Network mask *• DR/BDR identity (IP)• Authentication (if enabled) *

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OSPF Packets• DBD packet

– To sync LSDBs

– Summary of local LSDB• List of LSAs (headers) with sequence number

– DBD exchange uses a poll-response paradigm• Master sends DBD (polls); Slave acks with its own DBD (responds)

– The router with higher Router ID - Master

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OSPF Packets• LSR packet– During the sync process, if the local router finds its LSDB is

out of date (older LSAs than those received in the DBD –seq#)

– Sends a LSR for the missing/newer LSAs• Each LSA requested is identified by LS type, link state ID, and the router

that advertised the LSA

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OSPF Packets• LSU packet– Link state info is propagated through LSAs

– LSAs are advertised in LSU packet to neighbors• Flooded reliably (ack) throughout the nw

• Initially (after adjacency is built), all LSAs in local LSDB• Later, LSUs are only sent during a topology change• Also in response to LSRs

– LSUs contain• # of LSAs• List of LSAs (single or multiple)

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OSPF Packets• LSAck packet– OSPF routers need to acknowledge receipt of each LSA• LSAck

– Dataless packet

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Neighbor States

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+----+|Down|+----+

+----+|Init|+----+

+-----+|2-way|+-----+

Hello received (no local Router-ID)

2-way received (local Router-ID seen)

DR/BDR selected from the “neighbor” list

+-------+|ExStart|+-------+

Master-Slave decision + DBD Seq#(higher router ID)

+--------+|Exchange|+--------+

Exchange DBD (local LSDB)

+--------+|Loading |+--------+

+----+|Full|+----+

LSRs sent to neighbor Adjacency estb

(inc Router LSAs & Network LSAs)

Y - more recent LSAs req?N

Scaling - Hierarchy• With single area OSPF, as network grows:– Larger LSDB and routing table– Frequent LSA flooding and SPF compute

• Hence, two-level hierarchy– LSA flooding contained within areas– Only summary routes exchanged between areas• Through the backbone

Area 0

Area 1Area 3

10.10.10.0/26

10.10.10.64/26

10.10.10.128/2610.10.10.192/26

2001:db8::/64

2001:db8:0:1::/64

2001:db8:0:2::/642001:db8:0:3::/64

Area 2

ABRABR ABR

Virtual links• OSPF requires regular areas to be connected to the

backbone (Area-0)– Inter-area routes propagated through the backbone

• Virtual links allow regular areas to connect (logically) to the backbone– Physically not feasible

Area 0Area 1

Area 3ABR ABRVirtual link

OSPF Network Types• Point-to-point– Can ONLY have one neighbor

connected on the link– LSUs sent to ”all OSPF routers”

multicast• 224.0.0.5/FF02::5

• Broadcast (multi-access) – Could have more than one

neighbor connected on the link• Ethernet links

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Broadcast NW issues• Number of adjacencies– # of Adj = n(n-1)/2;

• Extensive LSA flooding– Initially, the whole LSDB• LSAck too

– Periodic hellos for adjacencies– Triggered updates• During topology changes, each router will

send LSUs to neighbors - contains the same info

• LSAck too

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LSALSALSA LSA

LSA

LSALSALSALSA

DR/BDR• Hence, OSPF elects a Designated and Backup

Designated router for broadcast networks– Adjacencies only formed with DR and BDR

– LSAs sent only to DR (BDR listens)• 224.0.0.6/FF02::6

– DR floods to others• 224.0.0.5/FF02::5

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DR BDR

DR/BDR Election• Uses the Hello protocol (Rtr Priority)– highest OSPF interface priority – DR• Next highest priority – BDR

– Configurable:

– Else, highest router ID – DR• Next highest - BDR

– Recommended:• configure higher priority for routers meant to be DR and BDR!

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IOS:(config-if)#ip/ipv6 ospf priority <0-255>

Junos:set protocols ospf/3 area <area-id> interface <id> priority <0-255>

LSA Types• LSA Header

– Age: time since LSA was generated– Link state ID: identifies what the LSA is

carrying– Advertising Router: Router ID of the router

originating the LSA– Seq#: indicates newness of the LSA– Checksum: of the LSA content except age– Length: of LSA (includes 20 byte header)

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Type Link-state ID

1Originating router’s router ID

2 Interface IP of DR

3 Network address

4 ASBR’s router ID

5The external network address

LS age Options LS TypeLink State ID

Advertising RouterLS Sequence#

Checksum Length

Type Description

1 Router LSA

2 Network LSA

3 Summary LSA (ABR)

4 Summary LSA (ASBR)

5 AS-external LSA

LSA Types• Type-1 (Router LSA)– router’s connected (active) links/interfaces and metrics– flooded within the area (does not cross ABR)– Identified by router ID of originating router

• Type-2 (Network LSA)– broadcast/multi-access networks• generated by DR

– describes routers connected to the broadcast segment• Adjacent to the DR, including itself

– flooded within the area– Identified by DR’s interface IP

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LSA Types• Type-3 (Summary LSA - ABR)– Inter-area routes• Allows condensation at the are borders

– Originated by the ABR• ABRs store LSAs from each area in a separate LSDB• Generates a Type 3 for each subnet in the area

– Floods to the backbone• Propagated to other areas

– Identified by the subnets carried in the LSA

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LSA Types

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Type 1

Type 1

Type

3

Type 3

Type

3

Type 3

Area10

Area0Backbone

Area20

LSA Types• Type-4 (Summary LSA - ASBR)– To identify the ASBR• Route/path to the ASBR, to forward traffic destined for nws outside the

domain

– Originated by the ABR• When a ABR receives Type-1 LSA with the E-bit set, generates a Type-4

LSA

– Floods to the backbone• Propagated to other areas

– Identified by router ID of the ASBR

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LSA Types• Type-5 (AS-external LSA)– To advertise external routes into OSPF• Destinations outside the domain

– Originated by the ASBR• Generates a Type-5 for each external route

– Floods to the area it belongs• Propagated unaltered to other areas via backbone by ABR of the area

– Identified by the external subnet carried in the LSA

• Note: DO NOT redistribute external routes into OSPF!– Floods unaltered throughout the network – convergence??

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LSA Types

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Type5

Type

5

Type 5

Type5Type

4

Type4

Area10

Area0Backbone

Area20

Type4

Type

4

Type

5

Type

5

Non-OSPF

LSA Types• Opaque LSAs– To advertise optional router capabilities• Protocol extensibility• RFC5250

– Ex: Router Information Opaque LSA (RFC7770)• SR capabilities

– Flooding scope depends on the type• 9- link • 10 – area• 11 - domain

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Best path compute• Intra-area routes– Lowest cost to the each nw within the area

• Inter-area routes– Type3 (summary LSA-ABR) includes cost to each network– Best path = lowest (cost to ABR + cost in Type3 LSA)

• External routes (E-bit defines the metric type – E2>E1)– If not set (0), Type-1 ext metric (E1): cumulative as the route

gets propagated through the domain (more than one ASBR)• Seed metric + cost to ASBR

– If set (1), Type-2 ext metric (E2): same throughout the domain (only one ASBR)• Seed metric

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OSPFv3 – RFC5340• To carry IPv6 in OSPF– Router ID, Area ID and link state ID still 32-bit

– OSPFv3 uses link-local address for adjacency• All OSPFv3 packets are sourced using link-local

– LSUs are sent to the IPv6 multicast• FF02::5 (all OSPF routers) and FF02::6 (DR/BDR)

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OSPFv3 – New LSAs• Type-8 (Link LSA)– Advertise local router’s link-local to other routers on the link,

and list of v6 prefixes associated with link– Not flooded beyond the link (link-scope)– Link state ID is the interface ID on the link

• Type-9 (Intra-Area-Prefix LSA)– Either advertises v6 prefixes associated• With a router (directly connected interfaces) – references Router/Type-1

LSA, OR• With a broadcast segment (Network/Type-2 LSA)

– Flooded within the area (area-scope)

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OSPFv3 – Renamed LSAs• Type-3 (Inter-Area-Prefix LSA)– Equivalent to Type-3 summary LSA in OSPFv2• Describes routes to v6 prefixes within an area

– Originated by ABR• Generates a Type-3 for each IPv6 prefix

– Flood to the backbone for propagation to other areas

• Type-4 Summary (Inter-Area-Router LSA)– Equivalent to Type-4 summary (ASBR) in OSPFv2• Route(s) to ASBR

– Originated by ABR

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Passive interface• When OSPF is configured/enabled on an interface

(or for a subnet)– router will try to discover neighbors on that interface/within

the subnet– We can disable sending OSPF packets on those interfaces

where we know there will be no neighbors• While still advertising the network in OSPF

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IOS:ipv6 router ospf <process-id>router ospf <process-id>passive-interface defaultno passive-interface <interface-id>

IOS:ipv6 router ospf <process-id>router ospf <process-id>passive-interface <interface-id>

Junos:set protocols ospf/3 area <area-id> interface <int-id> passive

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