A routing protocol must be able to identify the best path when several routes to the same network exist. This parameter or measurement is called metric. Each dynamic routing protocol, including OSPF, comes with its own formula for calculating route metrics in order to make the best routing choices. In fact, the OSPF metric differs from the metrics of the RIP and EIGRP routing protocols.
A Route metric relies on several parameters such as hop count, bandwidth, load, interface cost, delay, reliability, or a mix of these factors. For instance, the OSPF metric is calculated based on the OSPF costs of interfaces.
Besides, EIGRP uses a sophisticated formula that relies on bandwidth, delay, load, and reliability for route metric calculation, while OSPF and RIP consider the cost (based on interface bandwidth) and hop count as metric parameters, respectively.
Before we continue, here are the network diagram and the router configurations I will use in the rest of this guide.
|Router R1||Router R2||Router R3|
The OSPF metric for a particular route on a router is the sum of the OSPF costs of all outgoing interfaces IP packets must pass through to go from the router to reach the route’s corresponding destination. For example, the metric of the 10.0.2.0/24 on router R3 is equal to the OSPF costs of R3’s F0/0, R1’s S1/0, and R2’s F0/0 interfaces.
Let’s determine router R2’s OSPF metric for its connected subnet 10.0.2.0/24.
Since IPv4 packets sent to subnet 10.0.2.0/24 must exit the F0/0 interface only, the OSPF metric of the route 10.0.2.0/24 is the cost of the F0/0 interface, which is equal to the OSPF reference bandwidth divided by the bandwidth of the interface.
Note that the faster the interface (greater bandwidth), the smaller the OSPF cost. It is possible to find the value of the OSPF reference bandwidth using the show ip ospf command. Example 1 clearly indicates that the reference bandwidth is 100 Mbps, the default value. You can adjust that value using the auto-cost reference-bandwidth command in router configuration mode.
R2# show ip ospf Routing Process "ospf 1" with ID 10.0.12.2 Start time: 00:04:25.804, Time elapsed: 00:03:50.188 Supports only single TOS(TOS0) routes Supports opaque LSA Supports Link-local Signaling (LLS) Supports area transit capability Event-log enabled, Maximum number of events: 1000, Mode: cyclic Router is not originating router-LSAs with maximum metric Initial SPF schedule delay 5000 msecs Minimum hold time between two consecutive SPFs 10000 msecs Maximum wait time between two consecutive SPFs 10000 msecs Incremental-SPF disabled Minimum LSA interval 5 secs Minimum LSA arrival 1000 msecs LSA group pacing timer 240 secs Interface flood pacing timer 33 msecs Retransmission pacing timer 66 msecs Number of external LSA 0. Checksum Sum 0x000000 Number of opaque AS LSA 0. Checksum Sum 0x000000 Number of DCbitless external and opaque AS LSA 0 Number of DoNotAge external and opaque AS LSA 0 Number of areas in this router is 1. 1 normal 0 stub 0 nssa Number of areas transit capable is 0 External flood list length 0 IETF NSF helper support enabled Cisco NSF helper support enabled Reference bandwidth unit is 100 mbps Area BACKBONE(0) Number of interfaces in this area is 2 Area has no authentication SPF algorithm last executed 00:02:59.016 ago SPF algorithm executed 4 times Area ranges are Number of LSA 4. Checksum Sum 0x02C6D4 Number of opaque link LSA 0. Checksum Sum 0x000000 Number of DCbitless LSA 0 Number of indication LSA 0 Number of DoNotAge LSA 0 Flood list length 0
Example 1 – Displaying the OSPF reference bandwidth
Besides, you can check the bandwidth of the F0/0 interface using the show interface F0/0 statement, as shown in Example 2.
R2# show interfaces fastEthernet 0/0 FastEthernet0/0 is up, line protocol is up Hardware is i82543 (Livengood), address is ca02.06a3.0008 (bia ca02.06a3.0008) Internet address is 10.0.2.2/24 MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec, omitted output
Example 2 – Displaying interface bandwidth
Since the bandwidth is 100000 Kbps or 100Mbps, the OSPF cost of the interface is 100Mbps/100Mbps = 1, as mentioned in the show ip ospf interface command (Example 3).
R2# show ip ospf interface fastEthernet 0/0 FastEthernet0/0 is up, line protocol is up Internet Address 10.0.2.2/24, Area 0 Process ID 1, Router ID 10.0.12.2, Network Type BROADCAST, Cost: 1 omitted output
Example 3 – OSPF configuration of router R2’s F0/0 interface
Now, let’s compute the OSPF metric of the path that starts on router R1, and ends on router R2 to reach subnet 10.0.2.0/24. IP packets forwarded by R1 and destined to subnet 10.0.2.0/24 exit router R1’s Serial 1/0 interface, then pass through router R2’s F0/0 interface. Therefore, the OSPF metric of the route in question is 65, which is the sum of the OSPF costs of those interfaces.
Note that the costs of R1’s Serial 1/0 and R2’s FastEthernet 0/0 interfaces are 64 and 1, respectively (Examples 3 and 4). As shown in Example 5, S1/0’s bandwidth is 1.544Mbps, and thus the cost would be 100Mbps/1.544Kbps = 64.766839378. However, the real OSPF cost of interface Serial 1/0 is the integer part of the resulting value, which is 64 (Example 4).
R1# show ip ospf interface serial 1/0 Serial1/0 is up, line protocol is up Internet Address 10.0.12.1/24, Area 0 Process ID 1, Router ID 10.0.13.1, Network Type POINT_TO_POINT, Cost: 64 omitted output
Example 4 – OSPF configuration of router R1’s S1/0 interface
R1# show interfaces serial 1/0 Serial1/0 is up, line protocol is up Hardware is M4T Internet address is 10.0.12.1/24 MTU 1500 bytes, BW 1544 Kbit/sec, DLY 20000 usec, omitted output
Example 5 – OSPF configuration of router R1’s S1/0 interface
Now, let’s verify our analysis using the show ip route command.
R1# show ip route Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route, + - replicated route Gateway of last resort is not set 10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks O 10.0.2.0/24 [110/65] via 10.0.12.2, 00:14:06, Serial1/0 C 10.0.12.0/24 is directly connected, Serial1/0 L 10.0.12.1/32 is directly connected, Serial1/0 C 10.0.13.0/24 is directly connected, FastEthernet0/0 L 10.0.13.1/32 is directly connected, FastEthernet0/0
Example 6 – Displaying R1’s routing table
In fact, Example 6 shows that the OSPF metric of R1’s route leading to subnet 10.0.2.0/24 is 65. Without a doubt, the routing table includes several elements, and the metric is one of them. It is listed right after the administrative distance, and those values are separated by a forward slash (/).
To sum up, the OSPF metric of a specific path is the sum of OSPF costs for all outgoing interfaces in that path from the beginning to the end. If OSPF has two routes to the same destination network, the route with the lowest metric wins. Otherwise, OSPF performs equal load balancing.
Finally, the total OSPF cost of the path from router R3 to subnet 10.0.2.0/24 is 66 because outgoing interfaces, in that path, are R3’s F0/0, R1’s S1/0, and R2’s F0/0 interfaces, and their costs are 1,64, and 1, respectively.
Tuning The OSPF Metric on Cisco IOS by Modifying The OSPF Cost
To adjust the OSPF metric of a route, you need to change the OSPF cost of one or more interfaces that influence the metric value. To change the OSPF cost of an interface, use the ip ospf cost, bandwidth, or auto-cost reference bandwidth commands. To learn more about how to adjust the OSPF cost of an interface, read the post: OSPF Cost.
Related Lessons to OSPF Metric
- OSPF Router ID
- OSPF Null Authentication
- OSPF Plain Text Authentication
- OSPF Default Route
- Basic OSPF Configuration Lab for CCNA
- OSPF Configuration
- OSPF Passive Interface
- OSPF Virtual Link
- OSPF Stub Area
- OSPF LSA Types
- OSPF Graceful Restart
- OSPF Totally Stubby Area
- OSPF Reference Bandwidth
- OSPF Cost
- OSPF DR/BDR Election
- OSPF Hello and Dead Interval
- OSPF Metric
- OSPF MD5 Authentication
- OSPF HMAC-SHA Cryptographic Authentication
- OSPF Multi-Area
- OSPF TTL Security Check
- OSPF Graceful Shutdown
- Route Redistribution between OSPF and RIP
- OSPF Network Types
- OSPF Totally NSSA Area
- OSPF NSSA Area
- OSPF Summarization
- OSPF Route Filtering
- OSPF Type 5 LSA Filtering
- OSPF ABR Type 3 LSA Filtering
- OSPF Prefix Suppression
- OSPF Path Selection
- OSPF LSA Throttling
- OSPF SPF Throttling
- OSPF Incremental SPF
- OSPF Non-Broadcast Network Type
- OSPF Point-to-Point Network Type
- OSPF Broadcast Network Type
- OSPF Point-to-Multipoint Network Type
- OSPF vs RIP
- OSPF LSA Group Pacing
- OSPF LSA Flood Pacing
- OSPF LSA Retransmission Pacing
- Troubleshooting OSPF Neighbor Adjacency
- Troubleshooting OSPF Route Installation
- Troubleshooting OSPF Route Advertisement
- OSPF Stub Router
I hope this blog post helps you learn something.
Now I’d like to turn it over to you:
What did you like about this tutorial?
Or maybe you have an excellent idea that you think I need to add.
Either way, let me know by leaving a comment below right now.