OSPF Topology Transparent Zones

Anyone who has worked with OSPF for any length of time has at least heard of areas—but perhaps before diving into Topology Transparent Zones (TTZs), a short review is in order.

In this diagram, routers A and B are in area 0, routers C and D are Area Border Routers (ABRs), and routers E, F, G, H, and K are all in area 1. The ABRs, C and D, do not advertise the existence of E, F, G, H, or K to the routers in area 0, nor the links to or between any of those routers. Any reachable destinations in area 1 are advertised using a em>summary LSA, or a type 3 LSA, towards A and B. From the perspective of A and B, 100::/64 and 101::/64 would be advertised by C and D as directly connected destinations, using the cost from C and D to each of these two destinations, based on a summary LSA.

What if you wanted to place H and K in their own area, with G as an ABR, behind the existing area 1? You cannot do this in OSPF using any form of a standard flooding domain, or area. There is no way to take information about a destination out of a summary LSA and place it in another summary LSA without risking the formation of a routing loop. Once the topology information is lost, which OSPF relies on to prevent the formation of routing loops, there is no way to gain it back.

Why might you want to do this? The primary reasons for stacking multiple areas in this way relate to mobile ad-hoc networks; you cannot control the way routers might be connected together, nor where flooding domain boundaries might be drawn. Assuming you have a requirement, you would be out of luck if the only tool you have is a standard OSPF area.

However, you can stack areas using TTZs. What is a TTZ? It is a flooding domain that does not appear to exist to the routers around it—or rather, connected to either side of the TTZ. In the illustration below, routers C, D, E, F, and G have been configured so they form a TTZ.

Note that when the network is configured this way, routers A, B, H, and K all believe they are in a single flooding domain. From the perspective of these four routers, C, D, and G appear to be directly connected by a set of point-to-point links. Traffic sent from B to 101::/64, for instance, would pass into the TTZ at D, through the TTZ, and back out the other side at G. This traffic would be forwarded normally through the TTZ, as the routers within the TTZ have a full view of the network topology, as well as reachable destinations.

The destination within the TTZ, 100::/64, would be advertised by each TTZ edge router as if it were a directly connected destination. Hence G, C, and D would each advertise 100::/64 as a connected interface in their router LSA, or their type 1 advertisement. This allows routers outside the TTZ to reach destinations within the TTZ, without revealing the internal topology of the TTZ.

Information about the topology within the TTZ is carried in a set of opaque LSAs which are not forwarded outside the TTZ. This allows the TTZ to maintain consistent internal state, without burdening the rest of the network with internal topology information, or topology changes.

TTZ’s are a rather narrowly focused solution; it is not likely you will see these used in an OSPF network near you any time soon. On the other hand, they are an interesting, experimental, addition to OSPF that can be used to solve a set of edge cases.

You can read about TTZs more in RFC8099.

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