BGP Route Refresh

The information and configuration in this chapter are based on SR OS Release 20.5.R2. The option to manually trigger BGP ROUTE_REFRESH messages to a BGP peer is supported in SR OS Release 19.7.R1, and later.

In SR OS releases earlier than 19.7.R1, only the automatic route refresh mechanism for VPN routes that carry Route Target extended communities, such as VPN-IPv4, VPN-IPv6, L2-VPN, MVPN-IPv4, or MVPN-IPv6 routes, is supported.

In SR OS releases earlier than 19.7.R1, soft reconfiguration inbound is supported for all non-VPN and VPN address families, using a clear command with soft-inbound option. With soft reconfiguration inbound, incoming routes are continuously retained in memory (RIB-IN), exactly as they were originally received from a BGP peer. Therefore, when an import policy change happens, the reevaluation of these routes can happen locally. There is no need to involve the peer node, because no route-refresh is involved. The disadvantage is the extra resource consumption to retain a copy of all original routes in memory, even if they are not needed at the current time.

RFC 2918, Route Refresh Capability for BGP-4, describes the BGP ROUTE_REFRESH message type and capability for BGP-4. When BGP router PE-1 sends a route refresh message for a specific address family to its BGP peer PE-2, PE-2 re-advertises all its RIB-OUT routes for PE-1 belonging to that address family. Manually-triggered BGP route refresh can be used for any BGP address family. However, if PE-2 did not advertise the route refresh capability in the BGP OPEN message to PE-1, then PE-2 ignores the incoming ROUTE_REFRESH message from PE-1.

In this chapter, the following use cases are shown:

• Automatic route refresh for VPN-IP and L2-VPN routes after an import policy is modified

• Block automatic route refresh for VPN-IP routes (mp-bgp-keep option)

• Manual route refresh for BGP routes for different address families (soft-route-refresh option in clear command)

Example topology shows the example topology with two nodes.

The initial configuration on the nodes includes:

• Cards, MDAs, ports

• Router interfaces

• SR-ISIS

The following route policies are configured on PE-1; the policies on PE-2 are similar.

# on PE-1:
configure {
    policy-options {
        community "target:64500:1" {
            member "target:64500:1" { }
        }
        community "target:64500:2" {
            member "target:64500:2" { }
        }
        prefix-list "192.0.1.0/24" {
            prefix 192.0.1.0/24 type range {
                start-length 32
                end-length 32
            }
        }
        policy-statement "export-VPLS2" {
            entry 10 {
                action {
                    action-type accept
                    community {
                        add ["target:64500:2"]
                    }
                }
            }
        }
        policy-statement "export-VPRN1" {
            entry 10 {
                action {
                    action-type accept
                    next-hop 192.0.1.11
                    community {
                        add ["target:64500:1"]
                    }
                }
            }
        }
        policy-statement "export-bgp" {
            entry 10 {
                from {
                    prefix-list ["192.0.1.0/24"]
                }
                action {
                    action-type accept
                }
            }
        }
        policy-statement "import-VPLS2" {
            entry 10 {
                from {
                    family [l2-vpn]
                    community {
                        name "target:64500:2"
                    }
                }
                action {
                    action-type accept
                }
            }
            default-action {
                action-type reject
            }
        }
        policy-statement "import-VPRN1" {
            entry 10 {
                from {
                    community {
                        name "target:64500:1"
                    }
                    protocol {
                        name [bgp-vpn]
                    }
                }
                action {
                    action-type accept
                }
            }
            default-action {
                action-type reject
            }
        }

Two BGP groups are configured: one for the VPN-IPv4 and Label-IPv4 address families and another for the L2-VPN address family. The BGP configuration for the base router on PE-1 is as follows:

# on PE-1:
configure {
    router "Base" {
        autonomous-system 64500
        bgp {
            split-horizon true
            next-hop-resolution {
                labeled-routes {
                    transport-tunnel {
                        family label-ipv4 {
                            resolution-filter {
                                ldp false
                                sr-isis true
                            }
                        }
                    }
                }
            }
            group "iBGP-L2" {
                type internal
                local-address 192.0.1.11
                family {
                    l2-vpn true
                }
            }
            group "iBGPv4" {
                peer-as 64500
                family {
                    vpn-ipv4 true
                    label-ipv4 true
                }
            }
            neighbor "192.0.1.21" {
                group "iBGP-L2"
            }
            neighbor "192.0.2.2" {
                group "iBGPv4"
                export {
                    policy ["export-bgp"]
                }
            }
        }

The service configuration on PE-1 is as follows:

# on PE-1:
configure {
    service {
        pw-template "PW1" {
            pw-template-id 1
        }
        vpls "BGP-AD VPLS 3" {
            admin-state enable
            service-id 3
            customer "1"
            bgp 1 {
                route-distinguisher "64500:3"
                route-target {
                    export "target:64500:3"
                    import "target:64500:3"
                }
                pw-template-binding "PW1" {
                }
            }
            bgp-ad {
                admin-state enable
                vpls-id "64500:3"
                vsi-id-prefix 192.0.1.11
            }
            sap 1/2/1:3 {
            }
        }
        vpls "BGP-VPLS 2" {
            admin-state enable
            service-id 2
            customer "1"
            bgp 1 {
                route-distinguisher "64500:2"
                vsi-import ["import-VPLS2"]
                vsi-export ["export-VPLS2"]
                route-target {
                    export "target:64500:2"
                    import "target:64500:2"
                }
                pw-template-binding "PW1" {
                    import-rt ["target:64500:2"]
                }
            }
            bgp-vpls {
                admin-state enable
                maximum-ve-id 100
                ve {
                    name "PE-1"
                    id 1
                }
            }
            sap 1/2/1:2 {
            }
        }
        vprn "VPRN 1" {
            admin-state enable
            service-id 1
            customer "1"
            route-distinguisher "64500:1"
            vrf-target {
                community "target:64500:1"
            }
            vrf-import {
                policy ["import-VPRN1"]
            }
            vrf-export {
                policy ["export-VPRN1"]
            }
            auto-bind-tunnel {
                resolution filter
            }
            bgp {
                next-hop-resolution {
                    use-bgp-routes true
                }
            }
            interface "lo1" {
                loopback true
                ipv4 {
                    primary {
                        address 172.31.1.1
                        prefix-length 32
                    }
                }
            }
        }

The following BGP OPEN message sent by PE-1 includes the route refresh capability for two BGP address families:

1 2020/06/23 09:03:58.168 UTC MINOR: DEBUG #2001 Base BGP
"BGP: OPEN
Peer 1: 192.0.2.2 - Send (Passive) BGP OPEN: Version 4
   AS Num 64500: Holdtime 90: BGP_ID 192.0.2.1: Opt Length 26 (ExtOpt F)
   Opt Para: Type CAPABILITY: Length = 24: Data:
     Cap_Code GRACEFUL-RESTART: Length 2
       Bytes: 0x0 0x78
     Cap_Code MP-BGP: Length 4
       Bytes: 0x0 0x1 0x0 0x80                 # AFI / SAFI ; 1 / 128 ; vpn-ipv4
     Cap_Code MP-BGP: Length 4
       Bytes: 0x0 0x1 0x0 0x4                  # AFI / SAFI ; 1 / 4 ; label-ipv4
     Cap_Code ROUTE-REFRESH: Length 0
     Cap_Code 4-OCTET-ASN: Length 4
       Bytes: 0x0 0x0 0xfb 0xf4
"

The BGP session between PE-1 and PE-2 includes the route refresh capability, as follows. No route refresh messages have been triggered manually yet.

[]
A:admin@PE-1# show router bgp neighbor 192.0.2.2 | match RtRefresh
Input RtRefresh      : 0                Output RtRefresh     : 0
Local Capability     : RtRefresh MPBGP 4byte ASN
Remote Capability    : RtRefresh MPBGP 4byte ASN

PE-1 receives the following BGP Labeled Unicast (BGP-LU) route:

[]
A:admin@PE-1# show router bgp routes label-ipv4
===============================================================================
 BGP Router ID:192.0.2.1        AS:64500       Local AS:64500
===============================================================================
 Legend -
 Status codes  : u - used, s - suppressed, h - history, d - decayed, * - valid
                 l - leaked, x - stale, > - best, b - backup, p - purge
 Origin codes  : i - IGP, e - EGP, ? - incomplete

===============================================================================
BGP Routes
===============================================================================
Flag  Network                                            LocalPref   MED
      Nexthop (Router)                                   Path-Id     IGP Cost
      As-Path                                                        Label
-------------------------------------------------------------------------------
u*>i  192.0.1.21/32                                      100         None
      192.0.2.2                                          None        10
      No As-Path                                                     524274
-------------------------------------------------------------------------------
Routes : 1
===============================================================================

PE-1 receives the following VPN-IPv4 route for VPRN 1:

[]
A:admin@PE-1# show router bgp routes vpn-ipv4
===============================================================================
 BGP Router ID:192.0.2.1        AS:64500       Local AS:64500
===============================================================================
 Legend -
 Status codes  : u - used, s - suppressed, h - history, d - decayed, * - valid
                 l - leaked, x - stale, > - best, b - backup, p - purge
 Origin codes  : i - IGP, e - EGP, ? - incomplete

===============================================================================
BGP VPN-IPv4 Routes
===============================================================================
Flag  Network                                            LocalPref   MED
      Nexthop (Router)                                   Path-Id     IGP Cost
      As-Path                                                        Label
-------------------------------------------------------------------------------
u*>i  64500:1:172.31.1.2/32                              100         None
      192.0.1.21                                         None        0
      No As-Path                                                     524286
-------------------------------------------------------------------------------
Routes : 1
===============================================================================

PE-1 receives one L2-VPN route for BGP-VPLS 2 and one L2-VPN route for BGP-AD VPLS 3:

[]
A:admin@PE-1# show router bgp routes l2-vpn 
===============================================================================
 BGP Router ID:192.0.2.1        AS:64500       Local AS:64500      
===============================================================================
 Legend -
 Status codes  : u - used, s - suppressed, h - history, d - decayed, * - valid
                 l - leaked, x - stale, > - best, b - backup, p - purge
 Origin codes  : i - IGP, e - EGP, ? - incomplete

===============================================================================
BGP L2VPN Routes
===============================================================================
Flag  RouteType                   Prefix                             MED
      RD                          SiteId                             Label
      Nexthop                     VeId                   BlockSize   LocalPref
      As-Path                     BaseOffset             vplsLabelBa 
                                                         se          
-------------------------------------------------------------------------------
u*>i  VPLS                        -                      -           0
      64500:2                     -                                  -
      192.0.1.21                  2                      8           100
      No As-Path                  1                      524278       
u*>i  AutoDiscovery               192.0.1.21             -           0
      64500:3                     -                                  -
      192.0.1.21                  -                      -           100
      No As-Path                  -                      -            
-------------------------------------------------------------------------------
Routes : 2
===============================================================================

The soft-route-refresh option in the clear router bgp neighbor command keeps a BGP session up and sends one or more ROUTE_REFRESH messages to the peer, each requesting the peer to resend all RIB-OUT routes for a specific address family (or for all established address families for a BGP neighbor). This option can be used to debug and troubleshoot route advertisement issues.