OSPF cost could be set explicitly or calculated automatically using the link’s bandwidth and OSPF reference bandwidth. In this tutorial, I explain the definition of OSPF reference bandwidth, how to adjust it, and more.
Upon the completion of this lesson, you will be able to answer the following questions:
- What is reference bandwidth in OSPF?
- What is the function of the auto-cost reference-bandwidth command?
- Why would you want to change the OSPF default reference bandwidth?
- How do I change the reference bandwidth in OSPF?
- How does OSPF calculate its metric or cost?
- How does OSPF check reference bandwidth?
- How does OSPF determine the cost of an interface?
- How does OSPF reference bandwidth get used?
- How to use the auto-cost reference-bandwidth command?
In the rest of this tutorial, we use the following network topology consisting of 4 routers. All interfaces belong to area 0, and the bandwidths of some interfaces have been changed to demonstrate facts related to OSPF reference bandwidth.
Here are the links to download the configurations applied to the routers.
|Router R1||Router R2||Router R3||Router R4|
What is OSPF Reference Bandwidth?
OSPF reference bandwidth is a value OSPF uses along with bandwidth to calculate the link’s cost if the cost was not set manually. By default, OSPF divides the reference bandwidth by the bandwidth and assigns the resulting value to the link’s cost.
OSPF Cost = (OSPF Reference Bandwidth) / (Link Bandwidth)
100 Mbps is the default value for OSPF reference bandwidth on Cisco IOS, IOS-XE, and IOS-XR.
Link Bandwidth depends on the interface type (Loopback, T1, Ethernet, Fast Ethernet, Gigabit Ethernet, 10-Gigabit Ethernet, Serial, OC48, etc) by default. The table below shows the default bandwidth for several interface types and the corresponding cost based on different OSPF reference bandwidth values.
|Interface Type||Default Bandwidth||OSPF Cost|
|Reference bandwidth of 100Mbps||Reference bandwidth of 1Gbps||Reference bandwidth of 10Gbps|
When the value of OSPF Reference Bandwidth is lower than the link’s bandwidth, OSPF sets the cost to 1. For example, with OSPF reference bandwidth equal to 100 Mbps, loopback, Gigabit Ethernet, OC48, and 10-Gigabit Ethernet interfaces get an OSPF of 1 (Examples 1, 2, 3, and 4 )
R1# show interfaces loopback 0 Loopback0 is up, line protocol is up Hardware is Loopback Internet address is 10.0.1.1/24 MTU 1514 bytes, BW 8000000 Kbit/sec, DLY 5000 usec, omitted output
R1# show ip ospf interface loopback 0 Loopback0 is up, line protocol is up Internet Address 10.0.1.1/24, Area 0 Process ID 1, Router ID 10.0.1.1, Network Type LOOPBACK, Cost: 1 Loopback interface is treated as a stub Host
Example 1 – Displaying the bandwidth and OSPF cost of R1’s loopback 0 interface
R2# show interfaces gigabitEthernet 0/0 GigabitEthernet0/0 is up, line protocol is up Hardware is i82543 (Livengood), address is ca02.0888.0008 (bia ca02.0888.0008) Internet address is 10.0.12.2/24 MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec, reliability 255/255, txload 1/255, rxload 1/255 omitted output
R2# show ip ospf interface gigabitEthernet 0/0 GigabitEthernet0/0 is up, line protocol is up Internet Address 10.0.12.2/24, Area 0 Process ID 1, Router ID 10.0.24.2, Network Type BROADCAST, Cost: 1 Topology-MTID Cost Disabled Shutdown Topology Name 0 1 no no Base omitted output
Example 2 – Displaying the bandwidth and OSPF cost of R2’s GigabitEthernet 0/0 interface
R3# show interfaces fastethernet 0/1 FastEthernet0/1 is up, line protocol is up Hardware is AmdFE, address is cc03.089c.0001 (bia cc03.089c.0001) Internet address is 10.0.34.3/24 MTU 1500 bytes, BW 2500000 Kbit/sec, DLY 100 usec, reliability 255/255, txload 1/255, rxload 1/255 omitted output
R3# show ip ospf interface fastethernet 0/1 FastEthernet0/1 is up, line protocol is up Internet Address 10.0.34.3/24, Area 0 Process ID 1, Router ID 10.0.34.3, Network Type BROADCAST, Cost: 1 Transmit Delay is 1 sec, State BDR, Priority 1 omitted output
Example 3 – Displaying the bandwidth and OSPF cost of R3’s FastEthernet 0/1 interface
R4# show interfaces gigabitEthernet 0/0 GigabitEthernet0/0 is up, line protocol is up Hardware is i82543 (Livengood), address is ca04.063f.0008 (bia ca04.063f.0008) Internet address is 10.0.34.4/24 MTU 1500 bytes, BW 10000000 Kbit/sec, DLY 10 usec, omitted output
R4# show ip ospf interface gigabitEthernet 0/0 GigabitEthernet0/0 is up, line protocol is up Internet Address 10.0.34.4/24, Area 0 Process ID 1, Router ID 10.0.34.4, Network Type BROADCAST, Cost: 1 omitted output
Example 4 – Displaying the bandwidth and OSPF cost of R4’s GigabitEthernet 0/0 interface
Default OSPF Reference Bandwidth
The default OSPF reference bandwidth on Cisco IOS is 100Mbps. The table below provides the default OSPF reference bandwidth on different router operating systems.
|Router OS||Default OSPF Reference Bandwidth|
|Cisco IOS XE||100Mbps|
|Cisco IOS XR||100Mbps|
Adjusting OSPF Reference Bandwidth on Cisco IOS
The goal of adjusting OSPF reference bandwidth is to best calculate the OSPF cost according to the interface’s bandwidth. On Cisco IOS, OSPF reference bandwidth can range from 1 to 4294967 Mbps. Using a small value will make all interfaces equal to OSPF while using a too-high value leads to very high OSPF costs that may exceed the maximum cost of 65535 that can be assigned to an interface. However, setting the OSPF reference bandwidth to the speed of the fastest router interface in the current OSPF domain would be enough.
The auto-cost reference-bandwidth bw allows changing the default value of OSPF reference bandwidth, where bw is a number between 1 and 4294967 in Mbps.
R2(config-router)# auto-cost reference-bandwidth 10000
To display the current OSPF reference bandwidth on a particular router, use the show ip ospf command (Example 6).
Finally, make sure that OSPF reference bandwidth is the same on all routers in the current OSPF autonomous system.
R2# show ip ospf Routing Process "ospf 1" with ID 10.0.24.2 Start time: 00:03:49.320, Time elapsed: 00:41:10.548 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 10000 mbps Area BACKBONE(0) Number of interfaces in this area is 2 Area has no authentication SPF algorithm last executed 00:01:59.948 ago SPF algorithm executed 7 times Area ranges are Number of LSA 7. Checksum Sum 0x02FB20 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 5 – Displaying general information about OSPF
OSPF Issue Caused by OSPF Reference Bandwidth
When you adjust OSPF reference bandwidth manually, Cisco IOS displays the following message.
% OSPF: Reference bandwidth is changed. Please ensure reference bandwidth is consistent across all routers.
Basically, configuring different OSPF reference bandwidth values on routers in the same OSPF routing domain would not cause routing loops. A routing loop occurs when router A forwards a packet, destined for a particular network, to a next-hop router B, which sends the packet back to A.
However, suboptimal routing may occur because links with lower bandwidth gets lower OSPF cost.
Related Lessons to OSPF Reference Bandwidth
- 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.