Regarding OSPF STUB area default route

ncnaveen_arasu · ‎07-11-2016

Hi Team,

Whenever we configure the OSPF area as stub, it allows only TYPE1,2and 3 LSA and in Totaly STUBBY only Type1 and 2 LSA's are allowed.

In that case ABR will be advertising the default route within the stub area.

So My question is

1) Will default route is created by ABR automatically to help stubarea to communicate to Area 0 or it should be created manually.

2) The default route belongs to which type of LSA and STUB area does'nt allow all types of LSA's

Please clarify .

thanks

Naveen

Mark Malone · ‎07-12-2016

Hi

1 Once the stub command is entered in the process the default route is automatically generated into the stub area, rather than filling up the area with the whole routing table just IA routes , this is useful licenses are expensive but you can have 200 dynamic routes on ipbase save you biking ipservcies , you may not require all the routes in the actual table a default will do so the stub command can be cost effective and resource effective depending what part of the network its deployed in, you can go one further with the total stub and just have the default being learnt in only

2 It would be a Type 3 LSA default route being injected in

ncnaveen_arasu · ‎07-12-2016

Hi Mark

Thanks for the reply.

You have said, default route will be auto generated and its a type 3 LSA in Stub area.

What in case of totally stubby area as it does not allow Type 3 LSA ? How the default route will be considered in TOtally stubby area?

And please also clarify, Stubby area allowed to pass only type 1,2 and 3 LSA and Totally Stubby area allow to pass only LSA type 1 ,2 , am i correct.

Please clarify.

Thanks

Naveen

Mark Malone · ‎07-13-2016

Hi

yes that's correct 123 for stub and 1,2 for t.stub but what's strange is the doc states as well that the ABR generates a summary LSA of the default route which is type 3 LSA so maybe its an exception as this is locally generated bit contradictory since it also says type 1 and 2 only allowed in :)

Doc

"In stub and totally stub areas, the ABR to the stub area generates a summary LSA with the link-state ID 0.0.0.0. This is true even if the ABR doesn't have a default route of its own"

How Does OSPF Generate Default Routes?

http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13692-21.html

e.ciollaro · ‎08-17-2016

Hi,

is Totally stubby areas, the default route is the only LSA-3 (this is an exception to stub area definition). Below a little test made using GNS3:

Topology: R3---R1---R2 (R3-R1 link is in Area 0, R1-R2 is area 1)
R1's loopbacks are in area 0 (10.1.1.1 and 10.11.11.11)
R3's loopback is redistributed in OSPF (10.3.3.3)

Here is the LSDB before making area 1 a stub area:

R2#sh ip ospf database

            OSPF Router with ID (10.2.2.2) (Process ID 1)

                Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
10.2.2.2        10.2.2.2        264         0x80000002 0x002DED 1
10.11.11.11     10.11.11.11     265         0x80000002 0x003AAA 1

                Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.12.1    10.11.11.11     265         0x80000001 0x004A13

                Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
10.1.1.1        10.11.11.11     341         0x80000001 0x00AD56
10.11.11.11     10.11.11.11     341         0x80000001 0x006283
192.168.13.0    10.11.11.11     341         0x80000001 0x006A27

                Summary ASB Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
10.3.3.3        10.11.11.11     280         0x80000001 0x00B73C

                Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
10.3.3.0        10.3.3.3        34          0x80000001 0x0086F6 0
192.168.23.0    10.3.3.3        329         0x80000001 0x009E6E 0

Here the LSDB after Area1 has been made a stub area:

R2#sh ip ospf database

            OSPF Router with ID (10.2.2.2) (Process ID 1)

                Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
10.2.2.2        10.2.2.2        3           0x80000004 0x0047D3 1
10.11.11.11     10.11.11.11     4           0x80000004 0x005490 1

                Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.12.1    10.11.11.11     4           0x80000003 0x0064F8

                Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         10.11.11.11     10          0x80000001 0x006FA3
10.1.1.1        10.11.11.11     10          0x80000002 0x00C93B
10.11.11.11     10.11.11.11     10          0x80000002 0x007E68
192.168.13.0    10.11.11.11     10          0x80000002 0x00860C

LSA 4 and 5 are no longer in LSDB and there is the default route (LSA-3).

Finally the LSDB after area 1 became a totally stub area:

R2#sh ip ospf database

            OSPF Router with ID (10.2.2.2) (Process ID 1)

                Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
10.2.2.2        10.2.2.2        106         0x80000004 0x0047D3 1
10.11.11.11     10.11.11.11     107         0x80000004 0x005490 1

                Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.12.1    10.11.11.11     107         0x80000003 0x0064F8

                Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         10.11.11.11     50          0x80000002 0x006DA4

As you can see the default route is the only LSA-3 in the Area.

By the way: remember that totally sutb area is Cisco proprietary extension (also I think that other vendor support it)

Bye,

enrico

Prashant Sheshasayee · ‎08-16-2016

STUB AREAS

External LSAS are stopped (E1 and E2 routes)
Default route is advertised into the stub area by the ABR
All routes in the stub area must be configured as stub

STUB Area Rules

It should not be area 0
No Virtual links must pass through the area
All routers in the area must be configured as stub routers
There should not be an ASBR in the area.