Areas and Hierarchy in OSPF and IS-IS ..


This is required primarily for scalability issues wherein instabilities inside one small section of the network are hidden from the rest of the network. This also helps in reducing the size of the routing tables, etc. Both the protocols establish a two level hierarchy among the areas.

IS-IS

– Divides the whole routing domain into small areas and uses logical hierarchy based on routing levels called Level 1 and Level 2.

– Level 1 routing is within the area and L2 is between the areas.

– Original spec called for Level 1 routers to know only the topology inside their area and they were unaware of routers/destinations outside of their area. They simply forward all their traffic for outside their area to the nearest Level 2 router.

– Level 2 routers knew only the Level 2 topology and didn’t know any topology inside the area. This forced strict hierarchal routing between the areas where all inter-area data traffic originating from one area followed a default route to the Level 2 sub-domain, where it was forwarded by L2 routing to the destination area.

– This has now changed and RFC 2966 allows leaking L2 information inside L1 for more optimal routing.

– There was some work done in IS-IS for multi-level hierarchies but it wasn’t all that useful and was dropped in between. The idea was that if the networks use IDRP as well along with IS-IS then the 2 levels may not be enough.

– IS-IS routers are associated with a single area and the whole router then belongs to that particular area.

– Area boundaries intersect on links .

– can be extended to support higher levels of hierarchy based on the way routes are leaked in between the levels by setting the up/down bit, when routes are propagated down the hierarchy.

OSPF

– Divides the routing domain into regular areas and a backbone area that is designated as area 0.0.0.0 and all packets going from one area to the other must traverse through this backbone.

– The spec calls for the backbone to be contiguous and to be connected to all the areas through an ABR. There is however a provision to work with disconnected physically disparate backbone areas using virtual links

– Can be attached to multiple areas as its designed around links and uses a links based addressing scheme. It’s the links which are assigned to the areas and not the routers themselves.

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About Manav Bhatia

Manav Bhatia is a SDN/NFV dataplane architect at Ionos Networks and has co-authored several IETF standards on routing protocols, BFD, IPv6, security, etc. He is also a member of IETF Routing Area Directorate where he helps the Area Directors review the IETF standards for their impact on the Routing Area. View all posts by Manav Bhatia

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