As we already discussed basic and some OSPF advanced features in Part-1 and Part-2. But still, it has lots to discuss and explore. It has a concept called Area Type and Router type which we discuss in this part, which helps to segregate the traffic and divide it into different boundaries and areas.
OSPF Area Router Type
It has a concept called Autonomous System (AS) which is a hierarchical system that separates routers into an individual area. These areas can either be intra-area (within one area), inter-area (between separate areas) or external (from another AS).
Area 0:
It is considered the “Backbone” area. It is mandatory that all other areas must have a connection into Area 0, Area 0 is also called as the transit area to connect all other areas.
ABR:
OSPF routers can belong to multiple areas, and can have separate Topology based on their areas. But for every Area there should be router which is required to connect that Area with Area-0. So the router which is placed at the Border between Area and Area-0 are known as Area Border Routers (ABRs).
ASBR:
ASBR Stand for Autonomous System Border Router. A router can become an ASBR in one of two ways:
- By connecting to a separate Autonomous System, such as the Internet
- By redistributing another routing protocol into the process.
ASBRs provide access to external networks. It defines two “types” of external routes:
- Type 2 (E2) – Includes only the external cost to the destination network. External cost is the metric being advertised from outside the OSPF domain. This is the default type assigned to external routes.
- Type 1 (E1) – Includes both the external cost, and the internal cost to reach the ASBR, to determine the total metric to reach the destination network. Type 1 routes are always preferred over Type 2 routes to the same destination.
Thus, the four separate router types are as follows:
- Internal Routers – all router interfaces belong to only one Area.
- Area Border Routers (ABRs) – Mediator between communication two different Area routers
- Backbone Routers – contain at least one interface in Area 0
- Autonomous System Border Routers (ASBRs) – Connected with separate or External Autonomous System
In Above example we have:
- Internal Area: Router R1, R2 and R3 are part of Area-1 which is called Internal area.
- Backbone Area: Router R3, R4 and R5 are part of Area-0 which is called Backbone area.
How DR/BDR works
As we already discussed about DR/BDR (Designated and Back-up Designated Router).in above example lets check how we can check about DR/BDR.
DR/BDR can only be configured in same LAN network or NBMA network. Here Area-1 and Area-2 routers are part of same LAN.
Let verify the config for R1, R2 and R3.
R1#
router ospf 10
log-adjacency-changes
network 10.1.1.0 0.0.0.255 area 1
network 10.1.2.0 0.0.0.255 area 1
R2#
router ospf 10
log-adjacency-changes
network 10.1.1.0 0.0.0.255 area 1
network 10.1.3.0 0.0.0.255 area 1
network 10.10.20.2 0.0.0.0 area 1
R3#
router ospf 10
log-adjacency-changes
network 10.1.2.0 0.0.0.255 area 1
network 20.1.1.0 0.0.0.255 area 0
Below is “sh ip OSPF neighbor Details” command Output, which provide the detailed information about neighbor IP Address, Interface, DR IP and BDR IP address and state. State should be Full.
R1#sh ip ospf neighbour detail
Neighbour 20.1.1.1, interface address 10.1.2.1
In the area 1 via interface GigabitEthernet0/0
Neighbour priority is 1, State is FULL, 6 state changes
DR is 10.1.2.1 BDR is 10.1.2.2
Options is 0x00
Dead timer due in 00:00:33
Neighbour is up for 00:06:36
Index 1/1, retransmission queue length 0, number of retransmissions 0
First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0)
Last retransmission scan length is 0, maximum is 0
Last retransmission scan time is 0 msec, maximum is 0 msec
How to configure and identify ABR
There is no such specific configuration to make a router as ABR. A router which is part of Internal and Backbone Area (Area-0) is called ABR (Area Boarder Router).
In Above example R3 and R5 is ABR. Let check these router config.
R3#
router ospf 10
log-adjacency-changes
network 10.1.2.0 0.0.0.255 area 1
network 20.1.1.0 0.0.0.255 area 0
How to configure and identify ASBR
There is no such specific configuration to make a router as ASBR. A router which is part of External Network and Backbone Area (Area-0) is called ASBR (Autonomous System Border).
In Above example R4 is ASBR because EIGRP is running on Lo interface of the router and redistributed. Let check this router config.
R4#
router eigrp 100
redistribute ospf 10
network 192.168.1.0
network 192.168.0.0 0.0.255.255
!
router ospf 10
log-adjacency-changes
redistribute eigrp 100
redistribute connected subnets
network 20.1.1.0 0.0.0.255 area 0
network 20.1.2.0 0.0.0.255 area 0
Also, you can check ABR and ASBR or Boarder Routes with “Sh Ip OSPF Boarder-Routers” command output.
R3#sh ip ospf border-routers
OSPF Process 10 internal Routing Table
Codes: i – Intra-area route, I – Inter-area route
i 192.168.1.1 [1] via 20.1.1.2, GigabitEthernet0/2, ASBR, Area 0, SPF 1
i 30.1.3.1 [2] via 20.1.1.2, GigabitEthernet0/2, ABR, Area 0, SPF 2
R3#
R4#sh ip ospf border-routers
OSPF Process 10 internal Routing Table
Codes: i – Intra-area route, I – Inter-area route
i 20.1.1.1 [1] via 20.1.1.1, GigabitEthernet0/0, ABR/ASBR, Area0, SPF 1
i 30.1.3.1 [1] via 20.1.2.2, GigabitEthernet0/1, ABR/ASBR, Area0, SPF 1
R4#
R4 is acting as ABR and ASBR both.
OSPF Area Identification Code in Routing Table
R3#sh ip route
Codes: L – local, C – connected, S – static, R – RIP, M – mobile, B – BGP
D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
E1 – OSPF external type 1, E2 – OSPF external type 2, E – EGP
i – IS-IS, L1 – IS-IS level-1, L2 – IS-IS level-2, ia – IS-IS inter area
* – candidate default, U – per-user static route, o – ODR
P – periodic downloaded static route
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
O 10.1.1.0/24 [110/2] via 10.1.2.2, 00:21:07, GigabitEthernet0/1
C 10.1.2.0/24 is directly connected, GigabitEthernet0/1
L 10.1.2.1/32 is directly connected, GigabitEthernet0/1
C 10.1.3.0/24 is directly connected, GigabitEthernet0/0
L 10.1.3.2/32 is directly connected, GigabitEthernet0/0
O 10.10.20.2/32 [110/3] via 10.1.2.2, 00:21:07, GigabitEthernet0/1
20.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
C 20.1.1.0/24 is directly connected, GigabitEthernet0/2
L 20.1.1.1/32 is directly connected, GigabitEthernet0/2
O 20.1.2.0/24 [110/2] via 20.1.1.2, 00:04:51, GigabitEthernet0/2
30.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
O IA 30.1.1.0/24 [110/4] via 20.1.1.2, 00:04:51, GigabitEthernet0/2
O IA 30.1.2.0/24 [110/3] via 20.1.1.2, 00:04:51, GigabitEthernet0/2
O IA 30.1.3.0/24 [110/3] via 20.1.1.2, 00:04:51, GigabitEthernet0/2
O IA 30.30.10.2/32 [110/4] via 20.1.1.2, 00:04:51, GigabitEthernet0/2
O IA 30.30.30.1/32 [110/4] via 20.1.1.2, 00:04:51, GigabitEthernet0/2
O E2 192.168.0.0/16 [110/20] via 20.1.1.2, 00:45:38, GigabitEthernet0/2
Virtual Link
Aa we know,all areas should be connected to the backbone area or Area-0. But some time or in some design requirement, there may be instances when an area connected with an Internal Area and that Area does not have a direct link to Area 0.
That area needs to cross another interna area to reach the backbone area and then we need to create a virtual link.
Below is requirement to configure Virtual Link.
- Both routers must share a common area.
- The transit area cannot be a stub area.
- One of the routers must be connected to Area 0.
- A virtual link would essentially create a tunnel from Area 2 to Area 0, using Area 1 a transit area. One end of the Virtual Link must be connected to Area 0.
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