Assume we have a simple triangular network:
Now imagine the A-to-C link fails. How will OSPF react to the link failure as compared to EIGRP? Which one will converge faster? Try to answer the questions before pressing the Read more link ;)
EIGRP: Feasible successors
EIGRP tries to use feasible successors to speed up the convergence process. Whenever B reports its distance to X to A, A compares B’s reported distance to its current feasibility distance. Lower reported distance means that B doesn’t use A to get to X (A is not B's successor). B is also not A's successor for X (that’s C as long as the A-to-C link is operational), but it’s a feasible successor. Using B to get traffic to X will not result in a routing loop.
A feasible successor is evaluated for individual destinations. From A’s perspective, B is a feasible successor for X and C is a feasible successor for Y.
Faced with A-to-C link loss, A can switch immediately to the feasible successor (B) for destination X. Convergence is immediate.
OSPF: Let’s make sure we’re in sync
OSPF is way more lackadaisical:
- It changes the LSAs affected by the updates and floods them ... unless there’s been a recent topology change, in which case it goes and sits quietly in a corner until the timers throttle lsa hold-interval expires.
- It waits a bit more, silently lamenting its misery, until the timers throttle spf spf-start timer expires.
- It runs SPF algorithm, computes the new OSPF shortest-path tree, and copies the results in the IP routing table.
Total convergence time: 5+ seconds (unless you’ve done some serious tweaking).
Loop-Free Alternate Fast Reroute: Feasible Successor for OSPF
There’s no reason OSPF couldn’t have reacted faster – every single router knows the whole topology of all attached areas and can thus easily calculate which of its neighbors could be feasible successors. That’s exactly what LFA is doing:
- OSPF (or IS-IS) routing process runs SPF, computes its own best paths, and installs them in the IP routing table (RIB).
- After the network has converged, OSPF runs SPF algorithm from the perspective of its neighbors. If a neighbor’s SPF tree doesn’t use current router as the next hop for a specific destination, it’s safe to use that neighbor as a feasible successor.
- The feasible successor information calculated by OSPF is downloaded in RIB and FIB, where it can be used immediately after the link failure.
Can I use it?
Loop Free Alternate Fast Reroute is available for OSPF and IS-IS in Cisco IOS 15.1(3)S and XE 3.4S. The IS-IS implementation is pretty rudimentary; the OSPF implementation allows you to specify numerous alternate path selection criteria. For example, you might prefer alternate paths that don’t go over a shared risk link group, through the same LAN interface, or through the next-hop router.
- Fast Failover section in How Networks Really Work webinar
- Basic Specification for IP Fast Reroute: Loop-Free Alternates (RFC 5286)
- Understanding and Deploying Loop-Free Alternate Feature (Junos Implementation Guide)
- Nice introductory article by Tony Brown
- IP Fast Reroute Applicability – EuroNOG presentation by Pierre Francois (with lots and lots of details and topology analysis).