Features
The following sections provide a brief description of key OSPF features supported in our implementation of OSPF.
Intra-area, Interarea, and External Routes
You can split up an OSPF AS into areas. Doing this reduces the size of the link state database (LSDB). Each OSPF area runs as a separate network and maintains its own LSDB. OSPF computes routes only to destinations within the area and does not flood routes beyond the area boundaries.
Routing Priority
OSPF areas receive routes based on priority. Table 8-2 lists the routing priority.
If you use the redistribute command to import routes from other protocols or sources, the routes default to external type 2. You can specify a route map with the redistribute command to modify the type. Alternatively, you can use the metric-type keyword with the redistribute command to specify the type.
Virtual Links
Each OSPF area must be directly connected to the backbone area. The backbone is responsible for distributing routing information between nonbackbone areas. All routers in the backbone must be contiguous, but they need not be physically adjacent. You can configure backbone routers to be logically adjacent by creating OSPF virtual links.
Authentication
OSPF supports three modes of authentication:
- Null authentication - implies no authentication is in use
- Simple password authentication - requires a 64-bit unencrypted password in each OSPF packet
- Cryptographic authentication - uses a shared secret key that is configured on each router on a network. RFC 2328 defines the use of OSPF cryptographic authentication with the MD5 algorithm.
Opaque LSAs
OSPF opaque LSAs provide a generalized way of extending OSPF. The system generates opaque LSAs to carry traffic engineering (TE) information, accepts them from other routers, and floods them accordingly. OSPF uses the TE information to build a database from which paths can be computed for MPLS label-switched paths.
Route Leakage
Routes can be leaked into OSPF or from OSPF as follows:
- Route leakage into OSPF - When another routing protocol adds a new route to the routing table, or when a static route is added to the routing table, OSPF can be informed through the redistribute commands. When OSPF learns the new route, it floods the information into the routing domain using external LSAs.
- Route leakage from OSPF - OSPF adds routing information to the routing table, which is used in forwarding IP packets.
Equal-Cost Multipath
OSPF inherently supports the notion of equal-cost multipath (ECMP). When building the shortest-path tree, OSPF calculates all paths of equal cost to a given destination. If equal-cost paths exist, OSPF inserts into the routing table the next hops for all equal-cost paths to a destination.
OSPF MIB
See the ERX System Software CD, shipped with your system, for complete information on the OSPF Management Information Base (MIB) supported by your system. In the MIBs folder you will find information on all supported standard and Juniper Networks ERX enterprise (proprietary) MIBs. OSPF does not act as a host within the system and therefore does not support the ospfIfMetric and ospfHost tables.
Interacting with Other Routing Protocols
OSPF interacts seamlessly with other routing protocols.
With IS-IS
OSPF was developed originally from an early version of the IS-IS intradomain routing protocol. OSPF can receive IS-IS routing information. See Chapter 9, Configuring IS-IS.
With RIP
ERX systems can simultaneously run OSPF and RIP. When doing so, OSPF routes are preferred over RIP. In general, use of the OSPF protocol is preferred because of its robustness, responsiveness, and decreased bandwidth requirements. See Chapter 7, Configuring RIP.
With BGP
The default expectation is that your routing environment is an AS running OSPF and exchanging BGP routes with other ASs. See ERX Routing Protocols Configuration Guide, Vol. 2, Chapter 1, Configuring BGP Routing.