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The BFD—BGP Multihop Client Support feature enables Border Gateway Protocol (BGP) to use multihop Bidirectional Forwarding Detection (BFD) support, which improves BGP convergence as BFD detection and failure times are faster than the Interior Gateway Protocol (IGP) convergence times in most network topologies.
The BFD—BGP cBIT feature allows BGP to determine if BFD failure is dependent or independent of the Control Plane. This allows BGP greater flexibility in handling BFD down events.
Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Since multihop BFD operates in software, the minimum timer recommended is 200 ms x 3.
For BGP IPv4 and BGP IPv6 peering sessions only, multihop BFD support is available for BGP for address-family IPv4 and IPv6 unicast.
For multihop BGP sessions using IPv6 Link Local addresses, BFD multihop support is not available.
Currently BFD Hardware offload is not supported for multihop BFD sessions and so C-bit will not be set for multihop sessions.
Multihop BFD for IPv6 Virtual Routing and Forwarding (VRF) is not supported.
Information About BFD - BGP Multihop Client Support and cBit (IPv4 and IPv6)
BFD is a detection protocol that is designed to provide fast forwarding path failure detection times for all media types, encapsulations, topologies, and routing protocols. In addition to fast forwarding path failure detection, BFD provides a consistent failure detection method for network administrators. Because the network administrator can use BFD to detect forwarding path failures at a uniform rate, rather than the variable rates for different routing protocol hello mechanisms, network profiling and planning is easier, and reconvergence time is consistent and predictable. The main benefit of implementing BFD for BGP is a significantly faster reconvergence time. For internal BGP (iBGP) sessions and external BGP (eBGP) sessions that are either single hop or multihop, BGP can use of the multihop BFD support to help improve the BGP convergence because BFD detection and failure times are faster than the IGP convergence times in most of the network topologies. BGP needs the support of multihop BFD as described in RFC5882, Generic Application of Bidirectional Forwarding Detection (BFD).
BGP by default will purge the routes received from a specific peer when a BFD down event occurs and BFD informs BGP about it. The cBit in BFD determines whether BFD is dependent or independent of the Control Plane. Clients like BGP, whose peers are enabled with fast fall over feature with BFD support, can use this BFD cBit support to provide a more deterministic mechanism to do nonstop forwarding (NSF) when BGP graceful restart is enabled along with BFD fast-fallover support for BGP sessions.
When BGP is using BFD for the fast fallover feature for remote connectivity detection, BFD can detect some of those failures. If BFD is independent of the control plane, a BFD session failure means that data cannot be forwarded anymore (due to link control failures) and so the BGP graceful restart procedures should be aborted to avoid traffic black holes. On the other hand, when BFD is dependent on the control plane, a BFD failure cannot be separated out from the other events taking place in the control plane. When the control plane crashes, a switchover happens and BFD restarts. It is best for the clients (like BGP) to avoid any aborts due to the graceful restart taking place.
The table below describes the handling of BFD down events by BGP.
BFD Down Event |
Failure—Control Plane Independent? |
BGP Action for NSF (when GR and BFD are enabled) |
---|---|---|
BGP control plane detection failure enabled |
Yes |
Purge Routes |
BGP control plane detection failure enabled |
No |
Carry on NSF and keep stale routes in Routing Information Base (RIB) |
BGP control plane detection failure disabled (the default behavior) |
Yes |
Purge Routes |
BGP control plane detection failure disabled (the default behavior) |
No |
Purge Routes |
BGP session establishment works independently from BFD state change, except for fast fall-over detection, that is, inaccessible next-hop and cause best path re-calculation. This means that the BGP session could be established while BFD state is down or dampened, even with neighbor fail-over bfd configured.
From the XE 3.17S release the new optional keyword strict-mode is introduced, which does not allow BGP session to become established, if BFD is in down state. When BFD is dampened or down the routing protocol states or sessions cannot come up.
How to Configure BFD - BGP Multihop Client Support and cBit (IPv4 and IPv6)
The multihop
BFD minimum detection time should be higher than IGP convergence times in your
network to ensure that down events are not mistakenly identified during
reconvergences, causing multihop BGP sessions to flap.
For the BFD strict mode to work, configure BFD on both the
neighboring devices.
Note
Note
1.
enable
2.
configure
terminal
3.
router
bgp
autonomous-system-number
4.
neighbor
ip-address
remote-as
autonomous-system-number
5.
neighbor
ip-address
update-source
interface-type interface-number
6.
neighbor
ip-address
remote-as
autonomous-system-number
7.
neighbor
ip-address
ebgp-multihop
ttl
8.
neighbor
ip-address
fall-over
bfd
[multi-hop|single-hop] [check-control-plane-failure]
[ strict-mode]
9.
end
Configuration Examples for BFD - BGP Multihop Client Support and cBit (IPv4 and IPv6)
R1 e0/0 ---------------e0/0 R2 Router 1 configuration hostname R1 ! bfd map ipv4 2.2.2.2/32 1.1.1.1/32 mh1 ! bfd-template multi-hop mh1 interval min-tx 50 min-rx 50 multiplier 3 ! interface Loopback1 ip address 1.1.1.1 255.255.255.255 ip ospf 1 area 0 ! interface Ethernet0/0 ip address 10.0.0.1 255.255.255.0 ip ospf 1 area 0 ! router ospf 1 ! router bgp 1 neighbor 2.2.2.2 remote-as 1 neighbor 2.2.2.2 update-source Loopback1 neighbor 2.2.2.2 fall-over bfd multi-hop check-control-plane-failure strict-mode ! address-family ipv4 neighbor 2.2.2.2 activate exit-address-family ! Router 2 configuration: hostname R2 ! bfd map ipv4 1.1.1.1/32 2.2.2.2/32 mh1 bfd-template multi-hop mh1 interval min-tx 50 min-rx 50 multiplier 3 ! interface Loopback1 ip address 2.2.2.2 255.255.255.255 ip ospf 1 area 0 ! interface Ethernet0/0 ip address 10.0.0.2 255.255.255.0 ip ospf 1 area 0 ! router ospf 1 ! router bgp 1 neighbor 1.1.1.1 remote-as 1 neighbor 1.1.1.1 update-source Loopback1 neighbor 1.1.1.1 fall-over bfd multi-hop check-control-plane-failure strict-mode ! address-family ipv4 neighbor 1.1.1.1 activate exit-address-family !
Related Topic |
Document Title |
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Cisco IOS commands |
|
BGP commands |
Description |
Link |
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The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Feature Name |
Releases |
Feature Information |
---|---|---|
BFD—BGP Multihop Client Support and cBit (IPv4/IPv6) |
The BFD—BGP Multihop Client Support feature enables Border Gateway Protocol (BGP) to use multihop Bidirectional Forwarding Detection (BFD) support, which improves BGP convergence as BFD detection and failure times are faster than the Interior Gateway Protocol (IGP) convergence times in most of network topologies. The BFD—BGP cBIT feature allows BGP to determine if BFD failure is dependent or independent of the Control Plane. This allows BGP greater flexibility in handling BFD down events. In Cisco IOS XE Release 3.7S, support was added for the Cisco ASR 903 router. The following commands were modified: neighbor fall-over and show ip bgp neighbors. |