Cisco CCNA (200-301)IP ConnectivityHard
R1 has two equal-cost OSPF paths to network 10.5.5.0/24, each with a total cost of 20, learned via GigabitEthernet0/0 and GigabitEthernet0/1. Both appear in the routing table with the same [110/20] metric. What is the default behavior of R1 regarding these two routes?
- AR1 compares router IDs and installs only the path from the lowest router ID
- BR1 installs both routes and load-balances traffic across them, up to a default maximum of 4 equal-cost paths
- CR1 requires the 'variance' command to be configured before it will install more than one equal-cost OSPF path
- DR1 installs only the route learned first and discards the second as a duplicate
Show answer & explanationAnswer & explanation
Correct answer: B. R1 installs both routes and load-balances traffic across them, up to a default maximum of 4 equal-cost paths
OSPF supports equal-cost multipath (ECMP) by default, installing up to four equal-cost paths (configurable with 'maximum-paths') to the same destination and load-balancing traffic across them. Unlike EIGRP, OSPF does not use 'variance' for unequal-cost balancing; only truly equal-cost paths are used.
Why the other options are wrong
- A. Router ID is used for OSPF Router ID or DR/BDR elections, not for choosing among equal-cost paths.
- C. 'variance' is an EIGRP-specific command used for unequal-cost load balancing and does not apply to OSPF.
- D. OSPF does not discard equal-cost duplicates; it keeps multiple paths for load balancing.
OSPF Equal-Cost Multipath (ECMP)
When OSPF calculates multiple paths to the same destination with identical total cost, it installs all of them in the routing table (default maximum of 4) and load-balances traffic across them.
- Default maximum equal-cost paths is 4 (varies by platform, can be changed with 'maximum-paths')
- Only routes with identical cost are eligible; OSPF has no unequal-cost load balancing like EIGRP variance
- Traffic is typically load-balanced per-destination by default using CEF
Memory trick: Same cost, both roads open, up to four lanes.