OSPF Area Type Queries

linkadmin

Hi All

having few queries regarding the OSPF Area types if any 1 of you can clewar the same

1) as we know in NSSA ,the area is connected to the ASBR which is the boundary router as we know NSSA is the stub area hence ASBR sends type 7 LSA to ABR for extarnal routes , how does the internal routers in the area sends the traffic to external destinition which is reachable through the ASBR in the same area.
: as in stubby areas there is defaut route in internal router towarsds teh ABR for the external routes, hence in teh internal router in NSSA traffic will be first sent to ABR and then forwarded to ASBR or there is default route in internal routers towards ASBR ???????


2) can stubby areas have more that 2 ABR's?????
: if yes , how an internal router will decide to send traffic to which ABR to reach teh extarnal destinition ?????


: does totally stuby area can have 2 ABR's

:

EdTheLad

linkadmin wrote:

1) as we know in NSSA ,the area is connected to the ASBR which is the boundary router as we know NSSA is the stub area hence ASBR sends type 7 LSA to ABR for extarnal routes , how does the internal routers in the area sends the traffic to external destinition which is reachable through the ASBR in the same area.
: as in stubby areas there is defaut route in internal router towarsds teh ABR for the external routes, hence in teh internal router in NSSA traffic will be first sent to ABR and then forwarded to ASBR or there is default route in internal routers towards ASBR ???????

You have an ASBR inside a stub area, since the area cannot propagate type 5 LSAs it must be configured as a NSSA.The NSSA will flood type 7 LSAs throughout the area and all routers in the area will receive these routes and know how to get to the flooded destinations.When the ABR receives type 7 LSAs it converts them to type 5 and floods them into the backbone.

linkadmin wrote:

2) can stubby areas have more that 2 ABR's?????
: if yes , how an internal router will decide to send traffic to which ABR to reach teh extarnal destinition ?????

: does totally stuby area can have 2 ABR's

:

Yes a stubby and totally stubby area's can have more than one ABR, the internal router will decide which path is best using the metric.By default the metric is type 2, if the internal router see's two paths with the same type 2 metric it will use the type 1 metric as the deciding factor.If the type 2 and 1 are both equal it will load-balance.

Paul Boz

Here's a useful page describing multi-homed stub networks

http://www.netkit.org/labs/slides/netkit-lab_bgp-6-multi-homed-stub.pdf

Cucumber

OK, Im going to try to answer this one too, from a diferent standpoint.

1) as we know in NSSA ,the area is connected to the ASBR which is the boundary router as we know NSSA is the stub area hence ASBR sends type 7 LSA to ABR for extarnal routes , how does the internal routers in the area sends the traffic to external destinition which is reachable through the ASBR in the same area. as in stubby areas there is defaut route in internal router towarsds teh ABR for the external routes, hence in teh internal router in NSSA traffic will be first sent to ABR and then forwarded to ASBR or there is default route in internal routers towards ASBR ???????

Internal routers use the ROUTING TABLE to know when to send packets to the ASBR and when to the ABR. For instance, lets suppose you have the following scenario:

(External Net)   (--NSSA Area--)
10.0.0.0/8------ASBR---R1----ABR---BACKBONE

The ASBR injects an external route 10.0.0.0/8 into the area (using the LSA Type 7) whereas the ABR injects a default route into the area (Using an LSA Type 3 default route). Then, R1 will build its routing table like this:

R1>show ip route
O IA * 0.0.0.0/0 ABR <

---

Default route injected by the ABR(LSA Type 3 default)
O N2 10.0.0.0/8 ASBR <

---

External route injected by the ASBR (LSA Type 7)

So when R1 wants to send a packet to, say 10.1.1.1, it will look up its routing table, and will find that network 10.0.0.0 is reachable through the ASBR, so in that case it is not necessary to use the default route because a more specific route exists in the routing table.

Notice LSA Type 7 appear in the routing table as O N2 or O N1. Whereas LSA Type 3 appear as O IA.

2) can stubby areas have more that 2 ABR's?????

yes, each ABR injects a default route into the area, it is recomendable to use different metrics for the default routes, so internal routers will pick only one ABR as the "default gateway" for the area (see the command "area n default-cost n").

if yes , how an internal router will decide to send traffic to which ABR to reach teh extarnal destinition ?????

Using the default route.
External routes are not injected into STubby areas (only intra-area routes), therefore internal routers will forward packets to unknown destinations to the ABR, because the ABR is advertising a default route. If you have two ABRs, the ABR advertising the default route having lowest metric is used.

does totally stuby area can have 2 ABR's

Yes, again, each ABR should use a different metric when advertising the default route, using the command "area n default-cost n"