BFD在我之前準備SP CCIE LAB時就曾經提及過,目前sp ccie lAB中ios vERSION暫時尚未支援此功能,但是這個新功能在未來的r&s OR sp CCIE中應該會是一個很好的題材,因為它可以達到比縮減HELLO TIMER的方式更快速的收歛。



by Ivan Pepelnjak

A few years ago, we measured network convergence times in seconds, and aimed to establish an alternate path across the network following a link or node failure within a few seconds. Ten seconds was a sensible number that wouldn’t upset users (assuming that the failures weren’t too common) and definitely wouldn’t break TCP sessions. (The DLSw local termination would take care of the leftovers of the SNA networks.) The only environments aiming at sub-second convergence times were real-time mission-critical applications, from industrial control to battlefield communications.

The introduction of Voice over IP (VoIP) into the data networks changed the rules completely; the human ear tends to be upset by even a sub-second loss of signal. While the quality-of-service requirements of VoIP deployment are usually well understood (and often implemented quite successfully), the convergence aspects are often forgotten—that is, until the first major failure, when even the proverbial phone doesn’t ring any more.

Until recently, we’ve had to struggle with a patchwork of (frequently proprietary) kludges implemented on top of unreliable data-link layers (for example, frame relay end-to-end keepalives) and routing protocol “solutions” that offered only shortened hello timers. Fortunately, after struggling for more than 25 years with the deficiencies of Layer 2 signaling, major router vendors introduced an interoperable standard protocol aimed exclusively at detecting two-way communication with the next-hop router: Bidirectional Forwarding Detection(BFD).

Introduction to BFD

The Bidirectional Forwarding Detection protocol has a single, well-defined goal: “[P]rovide low-overhead, short-duration detection of failures in the path between adjacent forwarding engines, including the interfaces, data links and (to the extent possible) the forwarding engines themselves” (source: BFD draft specifications).


Forwarding engines are commonly known as routers. The distinction between a router and a forwarding engine is significant only in distributed architectures with physically separate control and forwarding plane; for example, the Gigabit Switch Router (GSR).

The BFD protocol is very similar to the hello mechanisms used in the majority of the routing protocols, with a few significant differences:

BFD tests bidirectional communication. (The EIGRP hello protocol can have problems with unidirectional links.)

BFD packets are small (24 bytes on top of the UDP+IP header) and focused exclusively on path-failure detection. (Routing protocolhello packets perform a number of additional duties.)

In routers with a distributed architecture, BFD packets can be processed on interface modules (see Figure 1), whereas routing protocolhello packets are always processed by the control plane (main CPU).


Distributed platforms run BFD on the line cards


Due to the low overhead, it’s common to use BFD timers in the milliseconds range. (The minimum value accepted by Cisco IOS is 50 milliseconds.)

BFD can rely on control packets (similar to the hello packets used by the routing protocols), or use echo packets (IP packets addressed to the router itself but sent to the Layer 2 address of the next-hop node, as shown in Figure 2) in parallel with control packets. The echo packets thoroughly test the complete bidirectional forwarding path between adjacent routers, as they have to be transmitted by the originating router, propagated to the adjacent router, received by its interface logic, switched by its forwarding engine and sent back to the originator (similar to the way in which GRE keepalives work).

For example, when R1 sends a BFD echo packet in Figure 2, it sets the destination IP address in the packet to its own interface IP address and the MAC address in the Layer 2 frame header to the neighbor’s (R2) MAC address. When R2 receives the packet, it performs a Layer 3 lookup and sends the packet toward its final destination (back to R1).


BFD echo mode

The BFD protocol has no inherent discovery mechanisms to detect adjacent BFD nodes; it’s designed solely as an agent for other applications requiring fast failure-detection. Whenever a routing protocol (or any other application) configured to use BFD detects a new neighbor, it requests availability tracking from the BFD module, as shown in Figure 3. After the BFD session is established with the new neighbor, BFD signals adjacent node failure to the routing protocol, significantly reducing failure-detection time.


OSPF registers its neighbors with BFD

Using BFD in Cisco IOS

As of October 2008, the implementation of BFD on Cisco routers is still somewhat sporadic and depends on the hardware platform you’re using and the software that the platform is running. All modern routing protocols (OSPF, IS-IS, EIGRP and BGP), as well as Hot Standby Router Protocol (HSRP), work with BFD. Some platforms can also use BFD to track the status of static routes.

The hardware support is sketchier: high-end routers (including ASR) running specialized IOS releases (for example, the 12.2 SRC release) support almost anything you might need in the core of your network. Integrated Service Routers (1800 through 3800) support BFD on Ethernet interfaces, but not on serial links. Older low-end routers don’t support BFD at all.


If you want to run BFD on ISRs, use the latest maintenance build of the 12.4(15)T release or a later IOS release.

Configuring BFD is a two-step process:

Step 1.

Configure BFD timers on the interfaces on which you want to use BFD.

Step 2.

Configure routing protocols to use BFD.

Configuring BFD

BFD timers are configured with the bfd interval send-timer mix_rx receive-timer multiplier number interface configuration command. The send-timer specifies the frequency of BFD packets originated by the router, the receive-timer the minimum interval between packets accepted from BFD peers. (BFD adjacency will not form if the send-timer on one peer is lower than the receive-timer on another peer.) The multiplier number is the number of BFD packets that can be lost before the BFD peer is declared “down.”

If you want to use the BFD echo mode, you should configure bfd slow-timers to specify the interval at which the control packets are sent and bfd echo on the interface to enable BFD echo mode.

Listing 1 shows a typical BFD configuration with BFD echo mode.


Typical BFD configuration

bfd slow-timers 3000


interface FastEthernet0/0

 bfd interval 100 min_rx 100 multiplier 3

 bfd echo

Configuring Interior Routing Protocols

After you’ve configured BFD on individual interfaces, you have to tell the routing protocols to use it. In most cases, there’s no good reason that the routing protocols would not use BFD whenever possible; the only command you have to use in these scenarios is the bfd all-interfaces router configuration command. For example, to tell OSPF to use BFD as configured in Listing 1, use the OSPF configuration in Listing 2.


OSPF uses BFD on all interfaces

router ospf 1

 network area 0

 bfd all-interfaces

If you want to be more specific, you could enable BFD on individual interfaces with the specific command for that particular routing protocol (for OSPF, use the ip ospf bfd interface configuration command), as shown in Listing 3.


OSPF uses BFD on Fast Ethernet interface only

interface FastEthernet0/0

 ip address

 ip ospf bfd

 bfd interval 100 min_rx 100 multiplier 3

You could also enable BFD on all interfaces with the bfd all-interfaces router configuration command, and disable it on specific interfaces. (OSPF uses the ip ospf bfd disable interface configuration command.)


You’d disable BFD on an interface only if the interface is flaky but you want to retain routing adjacency across short failures. This shouldn’t be necessary in most well-designed networks.

Using BFD with BGP

BGP can use BFD to detect failure of directly connected neighbors. eBGP or iBGP neighbor failure is detected as long as the neighbor resides on a directly connected subnet. The per-neighbor loss detection is configured with the neighbor fall-over bfd router configuration command. The configuration in Listing 4 uses BFD to detect failures of two BGP neighbors. One is directly connected (even though it uses an iBGP session); the other uses the more common loopback-to-loopback iBGP session.


Using BFD with BGP

interface FastEthernet0/0

 ip address

 bfd interval 100 min_rx 100 multiplier 3


router bgp 65000

 no synchronization

 bgp log-neighbor-changes

 neighbor remote-as 65000

 neighbor fall-over bfd

 neighbor remote-as 65000

 neighbor update-source Loopback0

 neighbor fall-over bfd

 no auto-summary

When the router succeeds in establishing a BGP session with a neighbor configured with BFD failure detection, it tries to register the neighbor’s IP address with BFD. Directly connected neighbors are accepted by BFD. Destinations of multihop BGP sessions are obviously rejected (BFD for multihop paths is not yet supported by Cisco IOS), resulting in a syslog message, as shown in Listing 5.


BFD does not work for multihop BGP neighbors

%BGP-4-BFD_NOT_ONEHOP: BFD is supported for single hop neighbors. is not single hop neighbor

%BGP-5-ADJCHANGE: neighbor Up

BFD in Action

In Figure 4, I’ve set up a very simple lab network to illustrate BFD failure-detection capabilities: two OSPF routers connected over a Fast Ethernet link. I’ve synchronized the times with Network Time Protocol to ensure that the syslog messages will indicate proper chronological order of events.


BFD test network

In this example, I’ve configured BFD on the Fast Ethernet interfaces (remember that BFD doesn’t work on serial interfaces yet on the 2800 series routers) and configured the OSPF routing protocol to use it with the configuration commands from Listing 3. After configuring BFD and OSPF, I’ve used the show bfd neighbor command to check the BFD neighbors (Listing 6).


BFD neighbors registered by OSPF

a1#show ip ospf neighbor

Neighbor ID  Pri  State      Dead Time Address    Interface    0  FULL/  -   00:00:37 Serial0/0/0.100    1  FULL/BDR   00:00:31   FastEthernet0/0

a1#show bfd neighbor

OurAddr    NeighAddr  LD/RD  RH/RS  Holddown(mult)  State  Int    1/2    Up        0    (3 )   Up     Fa0/0

If you want to see which protocols use BFD to detect failure of a particular BFD neighbor, use the show bfd neighbors detailscommand. The printout in Listing 7 indicates that both OSPF and HSRP use BFD to improve neighbor failure-detection times.


BFD is used by OSPF and HSRP

a1#show bfd neighbors details

OurAddr    NeighAddr  LD/RD  RH/RS  Holddown(mult)  State  Int    1/2    Up        0    (3 )   Up     Fa0/0

Session state is UP and using echo function with 100 ms interval.

Local Diag: 0, Demand mode: 0, Poll bit: 0

MinTxInt: 1000000, MinRxInt: 1000000, Multiplier: 3

Received MinRxInt: 1000000, Received Multiplier: 3

Holddown (hits): 0(0), Hello (hits): 1000(866)

Rx Count: 525, Rx Interval (ms) min/max/avg: 1/23596/923 last: 176 ms ago

Tx Count: 869, Tx Interval (ms) min/max/avg: 1/1000/869 last: 624 ms ago

Elapsed time watermarks: -1 0 (last: 0)

Registered protocols: OSPF HSRP

Uptime: 00:05:59

Last packet: Version: 1            - Diagnostic: 0

             State bit: Up         - Demand bit: 0

             Poll bit: 0           - Final bit: 0

             Multiplier: 3         - Length: 24

             My Discr.: 2          - Your Discr.: 1

             Min tx interval: 1000000    - Min rx interval: 1000000

             Min Echo interval: 100000

Failure Detection with BFD

Without BFD, neighbor failure-detection relies exclusively on OSPF hello timers. A1 thus realized that A2 was no longer available approximately half a minute after I shut down the Fast Ethernet interface on A2 (Listing 8).


Neighbor failure-detection with OSPF

a2(config)#int fa 0/0



17:31:27.347: %LINK-5-CHANGED: Interface FastEthernet0/0, changed state to administratively down


17:32:01.723: %OSPF-5-ADJCHG: Process 1, Nbr on FastEthernet0/0 from FULL to DOWN, Neighbor Down: Dead timer expired

When using BFD, A1 detected that A2 was no longer reachable long before A2 had time to generate the syslog message indicating that the interface had been disabled (Listing 9).


OSPF neighbor loss-detection with BFD



17:34:10.304: %OSPF-5-ADJCHG: Process 1, Nbr on FastEthernet0/0 from FULL to DOWN, Neighbor Down: Interface down or detached

17:34:12.300: %LINK-5-CHANGED: Interface FastEthernet0/0, changed state to administratively down


17:34:10.615: %OSPF-5-ADJCHG: Process 1, Nbr on FastEthernet0/0 from FULL to DOWN, Neighbor Down: BFD node down


In this article, you’ve seen how the introduction of Bidirectional Forwarding Detection (BFD) can significantly reduce network convergence time in environments that don’t provide fast link or node failure-detection. BFD enables you to detect hop-by-hop Layer 3 failures in milliseconds without having to fine-tune routing protocol hello timers. The only changes required in the router configuration are the configuration of per-interface BFD parameters with the bfd interval configuration command and the enabling of BFD functionality in the routing protocols with the bfd all-interfaces router configuration command (or a protocol-specific configuration command, if you want to configure BFD only on particular interfaces).

Post a Comment

Popular posts from this blog

Pairwise Master Key (PMK) vs Parewise Transient Key(PTK) vs PseudoRandom Function(PRF) vs GTK (Groupwise Transient Key)

DSSS(直接序列展頻技術) vs OFDM(正交頻率多重分割)