multi-area ospf on ios xr
TRANSCRIPT
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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