Networking

Why does OSPF require all traffic between non-backbone areas to pass through a backbone area (area 0)?   Because inter-area OSPF is distance vector, it is vulnerable to routing loops. It avoids loops by mandating a loop-free inter-area topology, in which traffic from one area can only reach another area through area 0. Ref : Jeff Doyle article at Network world http://www.networkworld.com/article/2348778/cisco-subnet/my-favorite-interview-question.html

Three types of traffic may be defined in relation to areas: Intra-area traffic consists of packets that are passed between routers within a single area. Inter-area traffic consists of packets that are passed between routers in different areas. External traffic consists of packets that are passed between a router within the OSPF domain and a router within another routing domain. Area ID 0 (or 0.0.0.0) is reserved for the backbone. The backbone is responsible for summarizing the topologies of each area to every other area. For this reason, all inter-area traffic must pass through the backbone; non-backbone areas cannot exchange packets directly. Ref: Routing TCP/IP volume 1

We also know about GRE tunnel at VPN topics. That can connect private LAN over public LAN. We can use GRE tunnel another way.In this scenario we can't use virtual link because area 2 is NSSA area so we try with GRE tunnel.        Figure 1.1 Just create tunnel at ABR Routers. Put the tunnel network to area 0. Figure 1.2 We imagine like figure 1.2. R2 int tun 0  ip add 172.16.1.1 255.255.255.252 tunnel source 10.1.23.2 tunnel destination 10.1.34.4 ip ospf 1 area 0 R4 int tunnel 0 ip add 172.16.1.2 255.255.255.252 tunnel source 10.1.34.4  tunnel destination 10.1.23.2  ip ospf 1 area 0 One of the point to notice tunnel source and destination are must ABR's network and same area. When you configure difference area network tunnel will flapping.This scenario I use the physical interfaces. After Tunnel is up we will see the Router 4 neighbor like that. Neighbor ID     Pri   State     ...

We know the virtual link for connect to OSPF backbone area 0. How about two that network have 2 backbone area?  Figure 1.1 R1 and R6 can't connect. R1 R1(config-router)#do sh ip route ospf      3.0.0.0/24 is subnetted, 1 subnets O IA    3.3.3.0 [110/3] via 10.1.12.2, 00:22:38, FastEthernet0/0      4.0.0.0/24 is subnetted, 1 subnets O IA    4.4.4.0 [110/4] via 10.1.12.2, 00:21:52, FastEthernet0/0      10.0.0.0/24 is subnetted, 3 subnets O IA    10.1.23.0 [110/2] via 10.1.12.2, 00:23:28, FastEthernet0/0 O IA    10.1.34.0 [110/3] via 10.1.12.2, 00:22:38, FastEthernet0/0 R6 R6(config-if)#do sh ip route ospf      3.0.0.0/24 is subnetted, 1 subnets O IA    3.3.3.0 [110/4] via 10.1.56.5, 00:16:06, FastEthernet0/0      4.0.0.0/24 is subnetted, 1 subnets O IA    4.4.4.0 [...

The following restrictions apply to stub areas:     You cannot create a virtual link through a stub area.   A stub area cannot contain an AS boundary router.   You cannot configure the backbone as a stub area.   You cannot configure an area as both a stub area and an not-so-stubby area (NSSA). Ref : Juniper.net

OSPF Virtual Link OSPF virtual link is use for that area far from backbone area 0. At this figure 1.1 area 2 is far from area 0 so area 2 can't reach other area. Figure 1.1 After configure OSPF, neighbor is up but router 2 don't see 3.3.3.0/24 network. R3#sh ip ospf nei Neighbor ID     Pri   State           Dead Time   Address         Interface 2.2.2.2           1   FULL/DR         00:00:39    10.1.2.1        FastEthernet0/0 R2#sh ip route      10.0.0.0/30 is subnetted, 2 subnets C       10.1.2.0 is directly connected, FastEthernet1/0 C       10.1.1.0 is directly connected, FastEthernet0/0 One of OSPF rule all area must connect to area 0.In this cause we can use virtual link at area 1. R2 router ospf 1  router-id 2.2.2.2  log-adjace...

OSPF Area 0 is a backbone area. Other Area must connect to Area 0. Virtual link use for connect to Area 0 pass from other area. When middle area is stub area we can use GRE tunnel.