Connected: An Internet Encyclopedia
15. Virtual Links
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RFC 1583
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15. Virtual Links
15. Virtual Links
The single backbone area (Area ID = 0.0.0.0) cannot be disconnected,
or some areas of the Autonomous System will become unreachable. To
establish/maintain connectivity of the backbone, virtual links can
be configured through non-backbone areas. Virtual links serve to
connect physically separate components of the backbone. The two
endpoints of a virtual link are area border routers. The virtual
link must be configured in both routers. The configuration
information in each router consists of the other virtual endpoint
(the other area border router), and the non-backbone area the two
routers have in common (called the transit area). Virtual links
cannot be configured through stub areas (see Section 3.6).
The virtual link is treated as if it were an unnumbered point-to-
point network (belonging to the backbone) joining the two area
border routers. An attempt is made to establish an adjacency over
the virtual link. When this adjacency is established, the virtual
link will be included in backbone router links advertisements, and
OSPF packets pertaining to the backbone area will flow over the
adjacency. Such an adjacency has been referred to in this document
as a "virtual adjacency".
In each endpoint router, the cost and viability of the virtual link
is discovered by examining the routing table entry for the other
endpoint router. (The entry's associated area must be the
configured transit area). Actually, there may be a separate routing
table entry for each Type of Service. These are called the virtual
link's corresponding routing table entries. The InterfaceUp event
occurs for a virtual link when its corresponding TOS 0 routing table
entry becomes reachable. Conversely, the InterfaceDown event occurs
when its TOS 0 routing table entry becomes unreachable.[19] In other
words, the virtual link's viability is determined by the existence
of an intra-area path, through the transit area, between the two
endpoints. Note that a virtual link whose underlying path has cost
greater than hexadecimal 0xffff (the maximum size of an interface
cost in a router links advertisement) should be considered
inoperational (i.e., treated the same as if the path did not exist).
The other details concerning virtual links are as follows:
- AS external links are NEVER flooded over virtual adjacencies.
This would be duplication of effort, since the same AS external
links are already flooded throughout the virtual link's transit
area. For this same reason, AS external link advertisements are
not summarized over virtual adjacencies during the Database
Exchange process.
- The cost of a virtual link is NOT configured. It is defined to
be the cost of the intra-area path between the two defining area
border routers. This cost appears in the virtual link's
corresponding routing table entry. When the cost of a virtual
link changes, a new router links advertisement should be
originated for the backbone area.
- Just as the virtual link's cost and viability are determined by
the routing table build process (through construction of the
routing table entry for the other endpoint), so are the IP
interface address for the virtual interface and the virtual
neighbor's IP address. These are used when sending OSPF
protocol packets over the virtual link. Note that when one (or
both) of the virtual link endpoints connect to the transit area
via an unnumbered point-to-point link, it may be impossible to
calculate either the virtual interface's IP address and/or the
virtual neighbor's IP address, thereby causing the virtual link
to fail.
- In each endpoint's router links advertisement for the backbone,
the virtual link is represented as a Type 4 link whose Link ID
is set to the virtual neighbor's OSPF Router ID and whose Link
Data is set to the virtual interface's IP address. See Section
12.4.1 for more information. Note that it may be the case that
there is a TOS 0 path, but no non-zero TOS paths, between the
two endpoint routers. In this case, both routers must revert to
being non-TOS-capable, clearing the T-bit in the Options field
of their backbone router links advertisements.
- When virtual links are configured for the backbone, information
concerning backbone networks should not be condensed before
being summarized for the transit areas. In other words, each
backbone network should be advertised into the transit areas in
a separate summary link advertisement, regardless of the
backbone's configured area address ranges. See Section 12.4.3
for more information.
- The time between link state retransmissions, RxmtInterval, is
configured for a virtual link. This should be well over the
expected round-trip delay between the two routers. This may be
hard to estimate for a virtual link; it is better to err on the
side of making it too large.
Next: 16. Calculation Of The Routing Table
Connected: An Internet Encyclopedia
15. Virtual Links