Connected: An Internet Encyclopedia
E.1 A fix for a problem with OSPF Virtual links
Up:
Connected: An Internet Encyclopedia
Up:
Requests For Comments
Up:
RFC 1583
Up:
E. Differences from RFC 1247
Prev: E. Differences from RFC 1247
Next: E.2 Supporting supernetting and subnet 0
E.1 A fix for a problem with OSPF Virtual links
E.1 A fix for a problem with OSPF Virtual links
In RFC 1247, certain configurations of OSPF virtual links can
cause routing loops. The root of the problem is that while there
is an information mismatch at the boundary of any virtual link's
Transit area, a backbone path can still cross the boundary. RFC
1247 attempted to compensate for this information mismatch by
adjusting any backbone path as it enters the transit area (see
Section 16.3 in RFC 1247). However, this proved not to be
enough. This memo fixes the problem by having all area border
routers determine, by looking at summary links, whether better
backbone paths can be found through the transit areas.
This fix simplifies the OSPF virtual link logic, and consists of
the following components:
- A new bit has been defined in the router links
advertisement, called bit V. Bit V is set in a router's
router links advertisement for Area A if and only if the
router is an endpoint of an active virtual link that uses
Area A as its Transit area (see Sections 12.4.1 and A.4.2).
This enables the other routers attached to Area A to
discover whether the area supports any virtual links (i.e.,
is a transit area). This discovery is done during the
calculation of Area A's shortest-path tree (see Section
16.1).
- To aid in the description of the algorithm, a new parameter
has been added to the OSPF area structure:
TransitCapability. This parameter indicates whether the area
supports any active virtual links. Equivalently, it
indicates whether the area can carry traffic that neither
originates nor terminates in the area itself.
- The calculation in Section 16.3 of RFC 1247 has been
replaced. The new calculation, performed by area border
routers only, examines the summary links belonging to all
attached transit areas to see whether the transit areas can
provide better paths than those already found in Sections
16.1 and 16.2.
- The incremental calculations in Section 16.5 have been
updated as a result of the new calculations in Section 16.3.
Next: E.2 Supporting supernetting and subnet 0
Connected: An Internet Encyclopedia
E.1 A fix for a problem with OSPF Virtual links