OSPF Operation in a Multiple-Area NBMA Environment

OSPF activities across multiple areas include operations such as creating link information, flooding techniques, and routing table construction. If a packet is heading for an intra-area network, it will be forwarded from an internal router to the destination router within the single area. Refer to the Type 1 and Type 2 LSAs for Area 1, as shown in Figure 5.3. However, as a packet needs to exit the area to another area, it follows these three steps:

1. The packet is routed from the originating network to an area border router.

2. The area border router transmits the packet through the transit area (backbone) to the ABR servicing the target network.

3. The target area border router forwards the packet on through its area until it arrives at the destination network.

It is the responsibility of the ABR to produce routing information concerning each area to which it is connected. The ABR then assumes the task of flooding the data into the backbone area and onward to other connected areas.

The entire area must be converged and synchronized before an area border router can commence the process of flooding summary link-state advertisements.

An ABR sends out summary LSAs for each network that it knows about, by default. As mentioned, you can reduce the volume of LSAs by implementing solid route summarization techniques on the ABRs. Summary link-state updates are Type 3 and Type 4. These summary updates are put into an LSU packet and sent through all interfaces of the ABR. There are a few exceptions, however. For example, if an interface on the ABR is connected to a totally stubby area, or if the summary includes Type 5 external route information and is linked to a stub or totally stubby area, the packet will not be sent. After the destination ABR or ASBR obtains the LSU, it inserts it into the link-state database (topology table) and subsequently floods it into the local area. The internal area routers also add the entry to their local databases, so if you want to prevent this operation, make the area a stub area.

After all the various router types get their updates, they will inject the data into their topology tables and recalculate their routing tables. According to RFC 2328, there is an order of preference. Paths to intra-area routes (flagged as O in the route table) get added first. Again, these are Type 1 and Type 2 LSAs. Next, all the routers will then compute the optimal paths to the other areas inside the internetwork and generate the resulting Type 3 and Type 4 entries ( O IA in the route table), as shown in Listing 5.2. An intra-area route to a destination will take precedence over an inter-area route to that same destination and is consequently injected into the routing table. Finally, all routers, except for stub area routers, compute the autonomous system external Type 5 destinations, if applicable , and distribute them. These external routes are marked as O E1 and O E2 in the route table. Listing 5.2 shows all the OSPF codes highlighted in the top section. After these operations are complete, the routers are prepared to reach any network inside or outside the AS.

Listing 5.2 Routers Compute Paths to Other Areas and Generate Type 3 and Type 4 Entries

 ABR1#show ip route Codes:  C_- connected, S  static, I  IGRP, 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  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, * - candidate default         U  per-user static route, o  ODR         T  traffic engineered route Gateway of last resort is 10.14.0.1 to network 0.0.0.0         10.0.0.0/8 is variably subnetted,  9 subnets,  2 masks  O  IA     10.20.0.0/16 [110/128] via 10.14.0.1,  00:09:13,  Ethernet0  C         10.10.3.0/24 is directly connected, Serial0  O  IA     10.30.0.0/16 [110/148] via 10.14.0.1,  00:07:47,  Ethernet0  C         10.10.2.0/24 is directly connected, Serial1 <Output Omitted>