1

Evaluating OSPFAdvanced Features of OSPF Protocol

2

Our routing study thus far - idealization

scale: with 200 million destinations:

can’t store all destinations in routing tables!

routing table exchange would swamp links!

administrative autonomy – Autonomous Systems(AS)

internet = network of networks

each network admin may want to control routing in its own network

all routers identical network “flat”… not true in practice

3

OSPF AdvanatagesNo limitation on hop count Supports classless (VLSM) routingRouting updates sent only when there is

a change or very rarelyFaster convergenceBetter load balancingLogical definition of areasAuthentication and external routes

tagging

4

Review: Evaluation Criteria for Routing Protocols

BandwidthMetric calculation Sharing and managing routing information

ScalabilityConvergence Performance

Hierarchy Scalability

Administration and Management Hardware and software resources Reliability Security

5

OSPF - Link State ProtocolLink

an interface on the routerLink state

description of the interface and the neighboring routers IP address, mask, type, routers connected to

Link state databasecollection of link state advertisement for all

routers and networks

6

OSPF Basic Configuration Example

router ospf 63network 172.16.5.3 0.0.0.0. area 1network 172.16.0.0 0.0.255.255 area 0network 192.168.10.5 0.0.0.0 area 1

Area 0

Area 1

172.16.1.1

172.16.3.2

192.168.10.5 E1

T0

E0

172.16.5.3E2

TokenRing

Router (config) #

Router (config-router) # network address wildcard-mask

area area-id

Wild card mask: inverse of subnet mask

router ospf process-id

7

Bandwidth- The Metrics in OSPFformula: cost = 108 /bandwidth in bps

56 Kbps serial link 175864 Kbps serial link 1562T1 (1.544 Mbps serial link) 65E1 (2.048 Mbps serial link) 484 Mbps token ring 25Ethernet 1016 Mbps token ring 6FDDI 1The faster the link, smaller is the number => more desirable is the route

8

OSPF Metric - OptimisingBandwidth

S0/0/0 S0/0/1DCE

R2

R1

S0/0/0DCE S0/0/1

192.168.10.8/30

10.10.10.0/24

172.16.1.32/29R3172.16.1.16/28

Fa0/0 Fa0/0

192.168.10.0/30

.17 .1

.2 .9

.10

.1

.33

S0/0/1

.5192.168.10.4/30

S0/0/0DCE .6

Fa0/0

Lo010.1.1.1

Lo010.3.3.3

Lo010.2.2.2

64kbps 128kbps

256kbps

R1(config-router) auto-cost reference-bandwidth

9

OSPF Metric - Cost

S0/0/0 S0/0/1DCE

R2

R1

S0/0/0DCE S0/0/1

192.168.10.8/30

10.10.10.0/24

172.16.1.32/29R3172.16.1.16/28

Fa0/0 Fa0/0

192.168.10.0/30

.17 .1

.2 .9

.10

.1

.33

S0/0/1

.5192.168.10.4/30

S0/0/0DCE .6

Fa0/0

Lo010.1.1.1

Lo010.3.3.3

Lo010.2.2.2

64kbps 128kbps

256kbps

10

Load Balancing and Link Cost•OSPF allows for Equal-Cost load balancing.

•R6 has two routers to R7 networksThru R5-R4Thru R4-R7

•Which path will be taken?•If you want to load-balance using both paths:

• R6 needs to believe that the path cost through R5 and R4 are the same.  

• Artificially increase the cost of the currently preferred link of R6, using

• IP ospf cost command,• Once the cost of the current

preferred link is increased (made worse) and is made the same as the other path, equal cost load balancing will automatically begin. 

 

1.5Mbps

500 Kbps

11

Bandwidth: Managing Routing information Routing information is not exchanged in form of routes

(Which protocol does that?)Each router generates link-state advertisements containing

elements of network topology routers neighbor relationshipsConnected subnets and Others

Link-state advertisements are flooded to all routers when areas are not configured: Issue : LSA flooding -> hampers performance

Link-state database is used for storing network topology information

Dijkstra’a SPF (Shortest path first) algorithm used to compute shortest path in terms of COST (OSPF metric), and result stored in RIB(routing information database)

OSPF RIB is collection of best paths to each destination, installed in Routing table

When information in link state database changes, only a partial calculation is necessary

12

Issue: Performance - Flooding LSAs

R2

R1

R3

R4

R5

R5 - LSA224.0.0.6 DR

BDR

• Multi-Access Networks:• To avoid flooding LSAs to all routers in the network, • Routers are designated:

• Election of DR (Designated Router)- Routers send LSAs to the DR using the multicast address 224.0.0.6

• BDR (Backup Designated Router) : back up for DR, if DR fails

R5 - LSA224.0.0.5

DRother

DRother

DRother

R5 - LSA224.0.0.5

R5 - LSA224.0.0.5

• The DR is responsible for forwarding the LSAs from R1 to all other routers. The DR uses the multicast address 224.0.0.5

R5 - LSA224.0.0.6

13

Hierarchical StructureIntroduced to put a boundary on the explosion of link-

state updatesEvery area is connected to the backbone area

BackboneArea #0

Area #2Area #1 Area #3

14

OSPF Areas The border area is OSPF area 0all routers belonging to the same area have

identical databaseSPF calculations are performed separately for

each areaLSA flooding is bounded by area

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OSPF: Multiple Areas

Area 2 Area 3

Area 4

Area 1

area 0BackboneASBR

Interior Router (IR)IR

to other AS

Virtual link

Two-level hierarchy: local area, also called backbone.area Link-state

advertisements only in area

each nodes has detailed area topology;

only knows direction (shortest path) to networks in other areas.

Area border routers (ABR): “summarize” distances to networks in own area, advertise to other Area Border routers.

Backbone routers: run OSPF routing limited to backbone.

Autonomous System Boundary routers: connect to other AS’s. (Autonomous Systems)

ABR: Area Border routers

ASBR: Autonomous System Border Routers

17

Scaling OSPFRule of thumb

no more than 150 routers /areaReality

no more than 500 routers/areaBackbone area is an area that glue all the other

areasalways marked as area 0

proper use of areas reduces bandwidthsummarized routes instability is limited within the area

18

OSPF Basic Configuration Example

router ospf 63network 172.16.5.3 0.0.0.0. area 1network 172.16.0.0 0.0.255.255 area 0network 192.168.10.5 0.0.0.0 area 1

Area 0

Area 1

172.16.1.1

172.16.3.2

192.168.10.5 E1

T0

E0

172.16.5.3E2

TokenRing

Router (config) #

Router (config-router) # network address wildcard-mask

area area-id

Wild card mask: inverse of subnet mask

router ospf process-id

19

Route Summarization Example

R2

R2#

router ospf 100

network 172.16.64.0 0.0.31.255 area 2

network 172.16.96.0 0.0.31.255 area 0

area 0 range 172.16.96.0 255.255.224.0

area 2 range 172.16.64.0 255.255.224.0

R1#

router ospf 100

network 172.16.32.0 0.0.31.255 area 1

network 172.16.96.0 0.0.31.255 area 0

area 0 range 172.16.96.0 255.255.224.0

area 1 range 172.16.32.0 255.255.224.0

172.16.64.0 - 172.16.95.0

255.255.255.0

Area 2

172.16.64.1

172.16.96.0 - 172.16.127.0

255.255.255.0 172.16.127.1172.16.96.1

172.16.32.0 - 172.16.63.0

255.255.255.0

Area 1

172.16.32.1

Interface Addresses(255.255.255.0 mask)

Interface Addresses(255.255.255.0 mask)

R1 R2

Area 0

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Area Link State DatabaseLink state database for every area is

differentArea database is composed of

router links advertisementsnetwork links advertisementssummary links advertisementsAS external advertisements

21

Stub Areas: Router performance OSPF allows certain areas to be configured as

stub areas. Configuring a stub area reduces the

topological database size inside an area and reduces the memory requirements of routers inside that area.

RTC# interface Ethernet 0 ip address 203.250.14.1 255.255.255.0 interface Serial1 ip address 203.250.15.1 255.255.255.252 router ospf 10 network 203.250.15.0 0.0.0.255 area 2 network 203.250.14.0 0.0.0.255 area 0 area 2 stub RTE# interface Serial1 ip address 203.250.15.2 255.255.255.252 router ospf 10 network 203.250.15.0 0.0.0.255 area 2 area 2 stub

22

Link State Advertisement (LSA)Generated periodically or in response to any

changeContains:

source identificationsequence numberlink state agelist of neighbors

23

Load Balancing by Multiple Path

N1N2

R2

R1 R3

R4

path 1

path 2

equal orproportional cost multiple paths

Unequal cost multiple paths not supported

24

Equal cost paths•Two routers are connected to each other via two p2p serial links of equal cost. R1 has Loopback 0 interface 1.1.1.1/32 and R2 has Loopback 0 interface 2.2.2.2/32. OSPF is used as the routing protocol. Hence, R1 can reach 2.2.2.2/32 via two equal-cost paths and R2 can reach 1.1.1.1/32 via two equal-cost paths.

R1 R2

1.1.1.1/32

2.2.2.2/32

R1# show ip route | begin GatewayGateway of last resort is not set

     1.0.0.0/32 is subnetted, 1 subnetsC       1.1.1.1 is directly connected, Loopback0     2.0.0.0/32 is subnetted, 1 subnetsO       2.2.2.2 [110/65] via 10.2.2.2, 00:01:44, Serial0/1                    [110/65] via 10.1.1.2, 00:01:44, Serial0/0     10.0.0.0/30 is subnetted, 2 subnetsC       10.2.2.0 is directly connected, Serial0/1C       10.1.1.0 is directly connected, Serial0

R1# show ip route 2.2.2.2Routing entry for 2.2.2.2/32  Known via "ospf 1", distance 110, metric 65, type intra area  Last update from 10.1.1.2 on Serial0/0, 00:02:10 ago  Routing Descriptor Blocks:    10.2.2.2, from 2.2.2.2, 00:02:10 ago, via Serial0/1      Route metric is 65, traffic share count is 1  * 10.1.1.2, from 2.2.2.2, 00:02:10 ago, via Serial0/0      Route metric is 65, traffic share count is 1

25

Authenticated Routing Updates

Two possibilities are definedno authentication (configured by default)authentication

simple password authentication message digest authentication

26

Simple Password AuthenticationSimple password

authentication allows a password (key) to be configured per area. Routers in the same area that want to participate in the routing domain will have to be configured with the same key.

Drawback Vulnerable to passive

attacks. Anybody with a link analyzer could easily get the password off the wire.

interface Ethernet0ip address 10.10.10.10

255.255.255.0ip ospf authentication-

key mypasswordrouter ospf 10network 10.10.0.0

0.0.255.255 area 0area 0 authentication

27

Message Digest AuthenticationCryptographic authentication A key (password) and key-id areconfigured on each router. The router uses an algorithm

based on the OSPF packet, the key, and the keyid

to generate a "message digest" that gets appended to the packet. Unlike the simple authentication, the

key is not exchanged over the wire. A non-decreasing sequence number is also included in each OSPF

packet to protect against replay attacks.

interface Ethernet0 ip address

10.10.10.10 255.255.255.0

ip ospf message-digest-key 10 md5 mypassword

router ospf 10 network 10.10.0.0

0.0.255.255 area 0 area 0 authentication

message-digest

28

Memory Issues Usually come up when too many external routes are injected in

the OSPF domain. A backbone area with 40 routers and a default route to the outside

world would have less memory issues compared with a backbone area with 4 routers and 33,000 external routes injected into OSPF.

The total memory used by OSPF is the sum of the memory used in the routing table (show ip route

summary) and the memory used in the link-state database.Example:

Each entry in the routing table will consume between approximately 200 and 280 bytes

Each LSA will consume a 100 byte overhead plus the size of the actual link state advertisement

This should be added to memory used by other processes and by the IOS itself.