multi-area ospf on ios xr

MULTI-AREA OSPFV2 ON

IOS XRBy Aladdin

CCIE Service Provider & Security

CoderGenie Technologies

AGENDA

OSPF Packet Types

OSPF LSA Types

OSFP Router and Area Types

Demonstration

OSPF OPERATION

Establishing Router Adjacencies

Electing DR and BDR

Discovering Routes

Choosing Routes

Maintaining Routing information

OSPF PACKET HEADER

OSPF PACKET TYPES

OSPF STATES

OSPF STATES

OSPF STATES

OSPF HIERARCHICAL ROUTING OSPF requires hierarchical network

design through configuring areas where

every router must belong to at least one

area and every OSPF network must have

an area 0.

Area 0 is our Transit (backbone) area.

Any other area is called regular (non

backbone) area. All regular areas must

touch area 0 except a few exceptions.

Areas minimize the impact of network

events within an area and help scale the

size of the network through various area

types (stub, nssa etc) and route

summarization.

OSPF ROUTER TYPES Internal: All interfaces within the

same area.

Backbone: At least one interface

in area 0.

Area Border (ABR): Interfaces in

different areas.

Autonomous System Boundary

Router (ASBR): one interface

connected to external network.

LSA TYPES

LSA TYPE 1: ROUTER LSA One router LSA (type 1) for

every router in an area

Includes list of directly

attached links

Each link identified by IP

prefix assigned to link and

link type

Identified by the router ID of

the originating router

Floods within its area only; does

not cross ABR

LSA TYPE 2: NETWORK LSA One network (type 2) LSA for

each transit broadcast or

NBMA network in

an area

Includes list of attached

routers on the transit link

Includes subnet mask of link

Advertised by the DR of the

broadcast network

Floods within its area only; does

not cross ABR

LSA TYPE 3: SUMMARY LSA Type 3 LSAs are used to flood

network information to areas outside the originating area (interarea)

Describes network number and mask of link.

Advertised by the ABR of originating area.

Regenerated by subsequent ABRs to flood throughout the autonomous system.

By default, routes are not summarized, and type 3 LSA is advertised for every subnet.

LSA TYPE 4: ASBR SUMMARY LSASummary (type 4) LSAs are used

to advertise an ASBR to all other areas in the autonomous system.

They are generated by the ABR of the originating area.

They are regenerated by all subsequent ABRs to flood throughout the autonomous system.

Type 4 LSAs contain the router ID of the ASBR.

LSA TYPE 5: EXTERNAL LSAExternal (type 5) LSAs are used to

advertise networks from other autonomous systems.

Type 5 LSAs are advertised and owned by the originating ASBR.

Type 5 LSAs flood throughout the entire autonomous system.

The advertising router ID (ASBR) is unchanged throughout the autonomous system.

Type 4 LSA is needed to find the ASBR.

By default, routes are not summarized.

AREA TYPESStub Area – as per Cisco Doc

“A stub area is an area that does not accept route advertisements or detailed network information external to the area. Area 0 cannot be a stub area.”

Not So Stubby Area – as per Cisco Doc

“NSSA does not flood Type 5 external LSAs from the core into the area, but can import autonomous system external routes in a limited fashion within the area. Area 0 cannot be an NSSA.”

STUB AREA ON IOS XRDefault route is advertised into stub

area by the ABR1 with a default cost of 1.

RP/0/RP0/CPU0:ABR1(config)# router ospf 1

RP/0/RP0/CPU0:ABR1(config-ospf)# area 50

RP/0/RP0/CPU0:ABR1(config-ospf-ar)# stub

RP/0/RP0/CPU0:ABR1(config-ospf-ar)#default-cost 15

no-summary keyword prevents the ABR from sending summary link-state advertisements (Type 3) in the stub area making a “totally” stubby area. default-cost specifies a cost for the default summary route sent into stub area.

NOT-SO-STUBBY AREA ON IOS XR

Default route is advertised into stub area by the ABR1 with a default cost of 1.

RP/0/RP0/CPU0:ABR1(config)# router ospf 1

RP/0/RP0/CPU0:ABR1(config-ospf)# area 50

RP/0/RP0/CPU0:ABR1(config-ospf-ar)# nssa

RP/0/RP0/CPU0:ABR1(config-ospf-ar)#default-cost 15

no-summary keyword prevents the ABR from sending summary link-state advertisements (Type 3) in the not-so-stubby area making a “totally” not-so-stubby area. default-cost specifies a cost for the default summary route sent into an NSSA.

DEMO

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