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DES-3810 Series Layer 3 Managed Ethernet Switch Web UI Reference Guide
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The format of these link-state updates is described in more detail below.
Router link-state updates are flooded to all routers in the current area. These updates describe the destinations
reachable through all of the router’s interfaces.
Summary link-state updates are generated by Border Routers to distribute routing information about other networks
within the AS. Normally, all Summary link-state updates are forwarded to the backbone (area 0) and are then
forwarded to all other areas in the network. Border Routers also have the responsibility of distributing routing
information from the Autonomous System Border Router in order for routers in the network to get and maintain routes
to other Autonomous Systems.
Network link-state updates are generated by a router elected as the Designated Router on a multi-access segment
(with more than one attached router). These updates describe all of the routers on the segment and their network
connections.
External link-state updates carry routing information to networks outside the Autonomous System. The Autonomous
System Border Router is responsible for generating and distributing these updates.
OSPF Authentication
OSPF packets can be authenticated as coming from trusted routers by the use of predefined passwords. The default
for routers is to use no authentication.
There are two other authentication methods: Simple Password Authentication (key) and Message Digest
authentication (MD-5).
Simple Password Authentication
A password (or key) can be configured on a per-area basis. Routers in the same area that participate in the routing
domain must be configured with the same key. This method is possibly vulnerable to passive attacks where a link
analyzer is used to obtain the password.
Message Digest Authentication (MD-5)
MD-5 authentication is a cryptographic method. A key and a key-ID are configured on each router. The router then
uses an algorithm to generate a mathematical “message digest” that is derived from the OSPF packet, the key and the
key-ID. This message digest (a number) is then appended to the packet. The key is not exchanged over the wire and
a non-decreasing sequence number is included to prevent replay attacks.
Backbone and Area 0
OSPF limits the number of link-state updates required between routers by defining areas within which a given router
operates. When more than one area is configured, one area is designated as area 0, also called the backbone.
The backbone is at the center of all other areas, all areas of the network have a physical (or virtual) connection to the
backbone through a router. OSPF allows routing information to be distributed by forwarding it into area 0, from which
the information can be forwarded to all other areas (and all other routers) on the network.
In situations where an area is required, but is not possible to provide a physical connection to the backbone, a virtual
link can be configured.
Virtual Links
Virtual links accomplish two purposes:
1. Linking an area that does not have a physical connection to the backbone.
2. Patching the backbone in case there is a discontinuity in area 0.
Areas Not Physically Connected to Area 0
All areas of an OSPF network should have a physical connection to the backbone, but in some cases it is not possible
to physically connect a remote area to the backbone. In these cases, a virtual link is configured to connect the remote
area to the backbone. A virtual path is a logical path between two border routers that have a common area, with one
border router connected to the backbone.
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