Prior to the
introduction of Border Gateway Protocol (BGP) Nonstop Routing (NSR) with
Stateful Switchover (SSO), BGP required that all neighboring devices
participating in BGP Nonstop Forwarding (NSF) be configured to be either
NSF-capable or NSF-aware (by configuring the devices to support the BGP
graceful restart mechanism). BGP NSF, thus, required that all neighboring
devices be upgraded to a version of Cisco IOS software that supports BGP
graceful restart. However, in many Multiprotocol Layer Switching (MPLS) Virtual
Private Networks (VPN) deployments, there are situations where provider edge
(PE) devices engage in exterior BGP (eBGP) peering sessions with customer edge
(CE) devices that do not support BGP graceful restart and cannot be upgraded to
a software version that supports BGP graceful restart in the same time frame as
the PE devices.
BGP NSR with SSO
provides a high availability (HA) solution to service providers whose PE
devices engage in eBGP peering relationships with CE devices that do not
support BGP graceful restart. BGP NSR works with SSO to synchronize BGP state
information between the active and standby route processor (RP). SSO minimizes
the amount of time a network is unavailable to its users following a
switchover. When the BGP Support for NSR with SSO feature is configured, in the
event of an RP switchover, the PE device uses BGP NSR with SSO to maintain BGP
state for eBGP peering sessions with CE devices that are not NSF-aware (see the
illustration below).
Additionally, the BGP
Support for NSR with SSO feature dynamically detects NSF-aware peers and runs
graceful restart with those CE devices. For eBGP peering sessions with
NSF-aware peers and for internal BGP (iBGP) sessions with BGP route reflectors
(RRs) in the service provider core, the PE device uses NSF to maintain BGP
state. BGP NSR with SSO, thus, enables service providers to provide the
benefits of NSF with the additional benefits of NSR without requiring CE
devices to be upgraded to support BGP graceful restart.
Figure 1. BGP NSR with SSO Operations
During an RP Switchover
BGP NSR with SSO is
supported in BGP peer, BGP peer group, and BGP session template configurations.
To configure support for BGP NSR with SSO in BGP peer and BGP peer group
configurations, use the
neighbor
ha-mode
sso command in address family configuration mode
for IPv4 VRF address family BGP peer sessions. To include support for Cisco BGP
NSR with SSO in a peer session template, use the
ha-mode
sso command in
session-template configuration mode.
When BGP NSR with
graceful restart is configured, graceful restart takes precedence over BGP NSR
for all BGP peers, if graceful restart capability is received from the BGP
peer.
When you enable
graceful restart globally, the graceful restart capability exchange takes
effect after all current BGP sessions are reset. You must do a session
renegotiation for all BGP peers manually. To reset all current BGP sessions,
use the
clear ip bgp *
command in privileged EXEC mode. After the session reset, graceful restart
takes precedence over BGP NSR for all peers.
To configure a
particular BGP peer to choose NSR over graceful restart, you must disable
graceful restart for that particular BGP peer on a per neighbor basis. To
disable graceful restart, use the
neighbor
ip-address
ha-mode
graceful-restart disable command.