Why OSPF paths aren’t always shortest

David Applegate

Carsten Lund

Aman Shaikh

AT&T Labs (Research)

NANOG 54February 06, 2012

NANOG 54: OSPF Routing with Areas

Introduction

OSPF is widely used for intra-AS routing

OSPF routes packets along shortest paths• In terms of weights configured on links

For scalability, OSPF divides domains into areas• Areas are widely used as well

Areas make OSPF routing complicated• Paths are no longer shortest• Other tweaks to areas have increased complexity

– Multi-Area Adjacencies (RFC 5185)– Multi-Area Routers (RFC 3509)

2

NANOG 54: OSPF Routing with Areas

Outline

Introduction to OSPF• Basics of OSPF routing

Areas• How routes are computed with areas

Tweaks to areas• Multi-area adjacency (RFC 5185)• Multi-area routers (RFC 3509)

AS Border Routers (ASBRs)

Quiz

3

NANOG 54: OSPF Routing with Areas

OSPF as a Link-State Protocol

With a link-state protocol, every router …• learns entire network topology– represents topology as a weighted graph

• computes Shortest Path Tree rooted at itself

OSPF follows this with some tweaks• Nodes are of two types:–Transit: routers, subnets–Stub: prefixes advertized by routers – Example: /32 for loopbacks

• Path is computed from a source router to a stub–Via one or more transit nodes

4

NANOG 54: OSPF Routing with Areas

access4

5050

50

50

50

50

50

100

50

100

50

1

50

192.168.0.3

50

access1

350

1

350

192.168.0.7

375

access3

375

1

50

192.168.0.2

50

back2

150

1

150

192.168.0.6

100

back4

100

1192.168.0.1back1

1192.168.0.5back3

1192.168.0.4access2

1192.168.0.8

50

Example

5

source

dest

RouterStub IP,weightLink,weights

1 151

51101

401451

451 451

NANOG 54: OSPF Routing with Areas

Areas

For scalability, OSPF domain is divided in areas• Areas are numbered 0, 1, 2, …• Conceptually a hub-and-spoke

– Area 0 is hub, non-zero areas are spokes

Each link and stub is assigned to a single area• A router can have links in multiple areas–Such a router is called an area border router (ABR)–An ABR must have a link in area 0 (RFC 2328)

With areas, every router learns …• entire topology of areas it has links to• distance of from ABRs to stubs in remote areas

6

NANOG 54: OSPF Routing with Areas

access4

5050

50

50

50

50

50

100

50

100

50

1

50

192.168.0.3

50access1

350

1

350

192.168.0.7

375

access3

375

1

50

192.168.0.2

50

back2

150

1

150

192.168.0.6

100

back4

100

1192.168.0.1back1

1192.168.0.5back3

1192.168.0.4access2

1192.168.0.8

50

Example of OSPF Areas

7

ABR

ABRABR

ABR

Color indicates area(Links and Stubs)

Black is area 0

NANOG 54: OSPF Routing with Areas

Path Calculation with Areas

A router ….• Calculates SPTs for all attached areas–Leads to intra-area paths to stubs

• Calculates paths to all remote stubs–Minimize the total distance from itself to the stub

• Total distance = dist(router, ABR) in an attached area + advertized dist(ABR, stub)

–Leads to inter-area paths to (remote) stubs

8

NANOG 54: OSPF Routing with Areas

access4

5050

50

50

50

50

50

100

50

100

50

1

50

192.168.0.3

50access1

350

1

350

192.168.0.7

375

access3

375

1

50

192.168.0.2

50

back2

150

1

150

192.168.0.6

100

back4

100

1192.168.0.1back1

1192.168.0.5back3

1192.168.0.4access2

1192.168.0.8

50

Example: Path Calculation with Areas

9

source

destABRd=426

ABRd=401

ABRd=51

ABRd=51

NANOG 54: OSPF Routing with Areas

Intra-area v/s Inter-area Paths

When a router has to choose between intra-area and inter-area paths, it always chooses intra-area path

Leads to …• Sub-optimal paths (within an area)–Packet takes a longer intra-area path over a shorter

inter-area path• Area hijacking at ABRs–Actual path (and the distance) differs from the path

(and distance) calculated by the source router

10

NANOG 54: OSPF Routing with Areas

access4

5050

50

50

50

50

50

100

50

100

50

1

50

192.168.0.3

50access1

350

1

350

192.168.0.7

375

access3

375

1

50

192.168.0.2

50

back2

150

1

150

192.168.0.6

100

back4

100

1192.168.0.1back1

1192.168.0.5back3

1192.168.0.4access2

1192.168.0.8

50

Area Hijacking Example

11

source

dest

ABRd=426

ABRd=51

ABRd=451ABRd=401

ABRd=151ABRd=51

?

NANOG 54: OSPF Routing with Areas

Multi Area Adjacencies (MADJ)

RFC 5185 allows a link to be in multiple areas• Stubs can only be in a single area• Links have a primary area for their interface stubs

Protects against some hijacking cases, but not all

12

NANOG 54: OSPF Routing with Areas

access4

5050

50

50

50

50

50

50

50

50

50

100

350

100

3501375 192.168.0.3

375

access1

1

50

192.168.0.7access3

50

1

150

192.168.0.2

150

back2

100

1

100

192.168.0.6back4

1192.168.0.1back1

1192.168.0.5back3

1192.168.0.4access2

1192.168.0.8

50

Area Hijacking Example with MADJ

13

source

destABRd=426

ABRd=51

ABRd=101

ABRd=451

NANOG 54: OSPF Routing with Areas

50

50

50

50

200

1

200

200

1

200

192.168.1.10

200

access2

200

192.168.1.9

200

access1

2001

agg1192.168.1.5 1

50

back1192.168.0.1 1 192.168.0.4

access3192.168.2.11 1

50

agg2192.168.1.6 1

back4192.168.0.2 1

50

agg3192.168.2.7 1

back2 back3192.168.0.3 1

50

50

50

50

50

1

50

50

50

192.168.2.8

50

200

50

agg4

50

200

50

50

agg4

50 50

50

50

200

50

50

200

50

200

50

200

50

200

50

200

50

1200

192.168.0.3back3 200

1

50

192.168.2.7

50

agg3

50

1

50

192.168.0.2

50

back2

50

1

10000

192.168.1.6

10000

agg2

200

1

200

192.168.2.11

50

access3

50

1192.168.0.1back1

1192.168.1.5agg1

1192.168.1.9access1

1192.168.1.10access2

1192.168.0.4back4

1192.168.2.8

50

Multi Area Routers

14

source

dest

ABR!!!

NANOG 54: OSPF Routing with Areas

Multi Area Routers (MAR)

RFC 3509 allows a router to be in multiple non-zero areas without being in area 0• Protects against dropping traffic• Specifies subtly different behavior of Cisco and

IBM routers (only)

Leads to more opportunities for area hijacking

15

NANOG 54: OSPF Routing with Areas

Area Hijacking Example with MAR

16

50

50

50

50

200

1

200

200

1

200

192.168.1.10

200

access2

200

192.168.1.9

200

access1

2001

agg1192.168.1.5 1

50

back1192.168.0.1 1 192.168.0.4

access3192.168.2.11 1

50

agg2192.168.1.6 1

back4192.168.0.2 1

50

agg3192.168.2.7 1

back2 back3192.168.0.3 1

50

50

50

50

50

1

50

50

50

192.168.2.8

50

200

50

agg4

50

200

50

50

agg4

50 50

50

50

200

50

50

200

50

200

50

200

50

200

50

200

50

1200

192.168.0.3back3 200

1

50

192.168.2.7

50

agg3

50

1

50

192.168.0.2

50

back2

50

1

10000

192.168.1.6

10000

agg2

200

1

200

192.168.2.11

50

access3

50

1192.168.0.1back1

1192.168.1.5agg1

1192.168.1.9access1

1192.168.1.10access2

1192.168.0.4back4

1192.168.2.8

50

source dest

MAR!!!

NANOG 54: OSPF Routing with Areas

1

50192.168.1.4

192.168.1.6access2

500

agg2

5001 192.168.0.2 1

350

agg1192.168.1.3 1

50

192.168.0.1back2

350

50

back1

5050

200

200

1

50

192.168.2.5

50

access1

50

50

501

More Area Hijacking(Stub Area Matters)

17

source

dest

ABR

d=351

ABR

d=251

d=501d=751?

NANOG 54: OSPF Routing with Areas

Importing External Information

External routes can be imported into OSPF• Example: static routes

Router importing external routes is called an ASBR (AS Border Router)• Route contains distance from ASBR to prefix

A router ….• Calculates SPTs for all the attached areas• Calculates paths to all remote stubs• Calculates paths to all external stubs–Calculates path (and distance) to ASBR and

combines that with dist(ASBR, stub)18

NANOG 54: OSPF Routing with Areas

Routing to an ASBR

Unusual, because ASBR is not in an area• ASBR could be reachable in multiple areas

So a router has to calculate per-area path to an ASBR, and then choose the best path

• Tie-breaking rules depend on whether RFC1583compatibility is set to disabled– RFC 1583 is the older OSPF RFC

• Ties broken by– Least cost– Highest area number of the link

19

NANOG 54: OSPF Routing with Areas

back1 200

1

200

192.168.1.4

50

192.168.0.1 1

access2

back2192.168.0.2 1

50

192.168.1.3access1

1

50 50

50

50

5050

Example of ASBR routing

20

sourcedest (ASBR)dest (stub)dest

NANOG 54: OSPF Routing with Areas

5050

50150

50

150100

100

192.168.5.0 1192.168.5.3 1

100

r1192.168.1.1 1 1

r2192.168.2.1 1

100

r5

r3192.168.3.1 1

192.168.4.0

50

100

50

r4

50

100

50

Quiz: What’s the path from source to dest?

21

source

dest

Black = area 0Red = area 1Blue = area 2Green = area 3

NANOG 54: OSPF Routing with Areas

Answer

Depends on …• What ‘dest’ refers to…

– Choices: r5:192.168.5.0, r5:192.168.5.3 or r5:ASBR• Whether routers are RFC-3509 compliant or not

– i.e., can r1, r2, and r3 act as true MARs or not• When RFC-3509 compliant:

– Whether r3 advertises routes learned in green area into red and blue areas or not• Vendor dependent

– Cisco and Junipers behave differently

22

NANOG 54: OSPF Routing with Areas

5050

50150

50

150100

100

192.168.5.0 1192.168.5.3 1

100

r1192.168.1.1 1 1

r2192.168.2.1 1

100

r5

r3192.168.3.1 1

192.168.4.0

50

100

50

r4

50

100

50

Quiz: What’s the path from source to dest?dest 192.168.5.0, r1 not RFC3509

23

source

dest

Black = area 0Red = area 1Blue = area 2Green = area 3

NANOG 54: OSPF Routing with Areas

5050

50150

50

150100

100

192.168.5.0 1192.168.5.3 1

100

r1192.168.1.1 1 1

r2192.168.2.1 1

100

r5

r3192.168.3.1 1

192.168.4.0

50

100

50

r4

50

100

50

source

dest

Black = area 0Red = area 1Blue = area 2Green = area 3

Quiz: What’s the path from source to dest?dest 192.168.5.0, RFC3509

24

NANOG 54: OSPF Routing with Areas

Quiz: What’s the path from source to dest?dest 192.168.5.3, RFC3509, Cisco

25

5050

50150

50

150100

100

192.168.5.0 1192.168.5.3 1

100

r1192.168.1.1 1 1

r2192.168.2.1 1

100

r5

r3192.168.3.1 1

192.168.4.0

50

100

50

r4

50

100

50

source

dest

Black = area 0Red = area 1Blue = area 2Green = area 3

NANOG 54: OSPF Routing with Areas

5050

50150

50

150100

100

192.168.5.0 1192.168.5.3 1

100

r1192.168.1.1 1 1

r2192.168.2.1 1

100

r5

r3192.168.3.1 1

192.168.4.0

50

100

50

r4

50

100

50

source

dest

Black = area 0Red = area 1Blue = area 2Green = area 3

Quiz: What’s the path from source to dest?dest 192.168.5.3, RFC3509, Juniper

26

NANOG 54: OSPF Routing with Areas

5050

50150

50

150100

100

192.168.5.0 1192.168.5.3 1

100

r1192.168.1.1 1 1

r2192.168.2.1 1

100

r5

r3192.168.3.1 1

192.168.4.0

50

100

50

r4

50

100

50

source

dest

Black = area 0Red = area 1Blue = area 2Green = area 3

Quiz: What’s the path from source to dest?dest r5:ASBR, RFC3509, Cisco

27

NANOG 54: OSPF Routing with Areas

Quiz: What’s the path from source to dest?dest r5:ASBR, RFC3509, Juniper

28

5050

50150

50

150100

100

192.168.5.0 1192.168.5.3 1

100

r1192.168.1.1 1 1

r2192.168.2.1 1

100

r5

r3192.168.3.1 1

192.168.4.0

50

100

50

r4

50

100

50

source

dest

Black = area 0Red = area 1Blue = area 2Green = area 3