Weighted ECMP for 6PE over RSVP-TE LSPs

This chapter provides information about Weighted Equal Cost Multipath (ECMP) for IPv6 Provider Edge (6PE) routers over Resource Reservation Protocol with Traffic Engineering (RSVP-TE) Label Switched Paths (LSPs).

Topics in this chapter include:

Applicability

The information and configuration in this chapter are based on SR OS Release 23.3.R2. Weighted ECMP for 6PE routers over RSVP-TE LSPs is supported in SR OS Release 15.0.R6, and later.

Chapter Weighted ECMP for VPRN over RSVP-TE and SR-TE LSPs in the 7450 ESS, 7750 SR, and 7950 XRS Layer 3 Services Advanced Configuration Guide for MD CLI is recommended reading.

Overview

Equal Load Balancing

In this chapter, ECMP refers to spraying traffic flows over multiple RSVP-TE LSPs within an ECMP set. ECMP spraying consists of hashing the relevant fields in the packet header and selecting the tunnel next-hop based on the modulo operation of the output of the hash and the number of RSVP-TE LSPs present in the ECMP set. The maximum number of RSVP-TE LSPs in the ECMP set is defined by the ecmp command.

Only RSVP-TE LSPs with the same lowest LSP metric can be part of the ECMP set. If the number of such RSVP-TE LSPs exceeds the maximum number of RSVP-TE LSPs allowed in the ECMP set as defined by the ecmp command, the RSVP-TE LSPs with the lowest tunnel IDs are selected first. By default, all RSVP-TE LSPs in the ECMP set have the same weight, and traffic flows are spread evenly over all RSVP-TE LSPs in the ECMP set, regardless of the bandwidth of the active path in the RSVP-TE LSPs. By default, ECMP is enabled and set to 1.

Unequal Load Balancing

Weighted ECMP sprays traffic flows over RSVP-TE LSPs proportionally to the load-balancing-weight <weight> value configured on each RSVP-TE LSP in the ECMP set. Weighted ECMP in AS 64496 shows that PE-1 forwards two thirds of the traffic flows on LSP-PE-1-PE-2-PE-3_strict with weight 2 and one third on LSP-PE-1-PE-4-PE-3_strict with weight 1.

Figure 1. Weighted ECMP in AS 64496

The LSP load balancing weight can be configured in an LSP template or on an RSVP-TE LSP. By default, the load balancing weight equals zero, in which case regular ECMP applies.

Weighted load balancing can be performed only when all the next-hops are associated with the same neighbor and all the RSVP-TE LSPs are configured with a non-zero load balancing weight. If one or more RSVP-TE LSPs in the ECMP set toward a specific next-hop do not have a load balancing weight configured, regular ECMP spraying is used.

The following command is used to configure the weight in an LSP template:

configure {
    router "Base" {
        mpls {
            lsp-template "LSPtemplate1" {
                load-balancing-weight ?

 load-balancing-weight <number>
 <number> - <1..4294967295>

    Load balancing weight for an MPLS LSP template

    Warning: Modifying this element toggles
    'configure router "Base" mpls lsp-template "LSPtemplate1" admin-state' automatically
    for the new value to take effect.

The following command is used to configure the weight on an LSP (for example on "LSP-PE-1-PE-2-PE-3_strict"):

configure {
    router "Base" {
        mpls {
            lsp "LSP-PE-1-PE-2-PE-3_strict" {
                load-balancing-weight ?

 load-balancing-weight <number>
 <number> - <1..4294967295>

    Load balancing weight for an MPLS LSP

The LSP load balancing weight on LSP-PE-1-PE-2-PE-3_strict is configured with a value of 2, as follows:

configure {
    router "Base" {
        mpls {
            path "path-PE-1-PE-2-PE-3_strict" {
                admin-state enable
                hop 10 {
                    ip-address 192.168.12.2
                    type strict
                }
                hop 20 {
                    ip-address 192.168.23.2
                    type strict
                }
            }
            lsp "LSP-PE-1-PE-2-PE-3_strict" {
                admin-state enable
                type p2p-rsvp
                to 192.0.2.3
                path-computation-method local-cspf
                metric 100
                load-balancing-weight 2
                primary "path-PE-1-PE-2-PE-3_strict" {
                }
            }

Weighted ECMP for 6PE over RSVP-TE LSPs is enabled in the bgp next-hop-resolution context as follows:

configure {
    router "Base" {
        bgp {
            next-hop-resolution {
                weighted-ecmp true

The weighted-ecmp option controls load balancing to the same next-hop only.

Configuration

Example Topology for 6PE over RSVP-TE LSPs shows the example topology with four PEs. IES 1 is configured on PE-1 and PE-3. A traffic generator is connected to IES 1 SAP 2/1/c36/2 on PE-1 and IES 1 SAP 2/1/c36/2 on PE-3. The traffic generator generates multiple IPv6 traffic flows with random IP addresses and TCP/UDP port numbers. As a result, these flows are sprayed over different MPLS LSPs between PE-1 and PE-3.

Figure 2. Example Topology for 6PE over RSVP-TE LSPs

Initial Configuration

The initial configuration on the PEs includes the following:

  • Cards, MDAs, ports

  • Router interfaces

  • IS-IS as IGP (alternatively, OSPF can be used) with traffic engineering enabled

  • MPLS and RSVP enabled on all router interfaces

  • MPLS paths with strict hops from PE-1 to PE-3 and the other way around: one via PE-2 and the other via PE-4. The LSP via PE-2 gets a load balancing weight of 2, whereas the LSP via PE-4 gets a load balancing weight of 1. Both LSPs have the same metric.

The initial configuration on PE-1 is as follows.

configure {
    router "Base" {
        interface "int-PE-1-PE-2" {
            port 1/1/c1/1
            ipv4 {
                primary {
                    address 192.168.12.1
                    prefix-length 30
                }
            }
        }
        interface "int-PE-1-PE-4" {
            port 1/1/c2/1
            ipv4 {
                primary {
                    address 192.168.14.1
                    prefix-length 30
                }
            }
        }
        interface "system" {
            ipv4 {
                primary {
                    address 192.0.2.1
                    prefix-length 32
                }
            }
        }
        isis 0 {
            admin-state enable
            area-address [49.0001]
            traffic-engineering true
            interface "system" {
            }
            interface "int-PE-1-PE-2" {
                interface-type point-to-point
            }
            interface "int-PE-1-PE-4" {
                interface-type point-to-point
            }
        }
        mpls {
            admin-state enable
            interface "int-PE-1-PE-2" {
            }
            interface "int-PE-1-PE-4" {
            }
            path "path-PE-1-PE-2-PE-3_strict" {
                admin-state enable
                hop 10 {
                    ip-address 192.168.12.2
                    type strict
                }
                hop 20 {
                    ip-address 192.168.23.2
                    type strict
                }
            }
            path "path-PE-1-PE-4-PE-3_strict" {
                admin-state enable
                hop 10 {
                    ip-address 192.168.14.2
                    type strict
                }
                hop 20 {
                    ip-address 192.168.34.1
                    type strict
                }
            }
            lsp "LSP-PE-1-PE-2-PE-3_strict" {
                admin-state enable
                type p2p-rsvp
                to 192.0.2.3
                path-computation-method local-cspf
                metric 100
                load-balancing-weight 2
                primary "path-PE-1-PE-2-PE-3_strict" {
                }
            }
            lsp "LSP-PE-1-PE-4-PE-3_strict" {
                admin-state enable
                type p2p-rsvp
                to 192.0.2.3
                path-computation-method local-cspf
                metric 100
                load-balancing-weight 1
                primary "path-PE-1-PE-4-PE-3_strict" {
                }
            }
        }
        rsvp {
            admin-state enable
            interface "int-PE-1-PE-2" {
            }
            interface "int-PE-1-PE-4" {
            }
        }

The configuration on PE-3 is similar.

With the preceding configuration, MPLS and RSVP are enabled on all interfaces, including the system interface, which is added automatically.

Weighted ECMP for 6PE over RSVP-TE LSPs

BGP is configured for the label-IPv6 address family and the next-hop resolution is set to RSVP; see the 6PE Next-Hop Resolution chapter.

In this example, the traffic generator sends IPv6 traffic to the SAP in IES 1. The IPv6 packets are tunneled through the IPv4 network between PE-1 and PE-3. The service configuration on PE-1 is as follows:

configure {
    service {
        ies "IES-1" {
            admin-state enable
            service-id 1
            customer "1"
            description "6PE-1"
            interface "int-PE-1-STC" {
                sap 2/1/c36/2 {
                }
                ipv6 {
                    address 2001:db8::11:1 {
                        prefix-length 120
                    }

The configuration on PE-3 is similar.

On PE-1, the following BGP configuration defines next-hop resolution with weighted ECMP and the resolution filter only allows RSVP-TE LSPs. BGP is configured for the label-IPv6 address family and BGP multipath is configured in the bgp context.

configure {
    router "Base" {
        autonomous-system 64496
        bgp {
            ibgp-multipath true
            split-horizon true
            next-hop-resolution {
                weighted-ecmp true
                labeled-routes {
                    transport-tunnel {
                        family label-ipv6 {
                            resolution-filter {
                                ldp false
                                rsvp true
                            }
                        }
                    }
                }
            }
            group "iBGP" {
                path-mtu-discovery true
                peer-as 64496
                export {
                    policy ["export-6PE-1"]
                }
            }
            neighbor 192.0.2.3 {
                group "iBGP"
                family {
                    label-ipv6 true
                }
            }
        }

The configuration on PE-3 is similar.

On PE-1 and PE-3, the following export policy is configured:

configure {
    policy-options {
        policy-statement "export-6PE-1" {
            entry 10 {
                from {
                    protocol {
                        name [direct]
                    }
                }
                action {
                    action-type accept
                }
            }
            default-action {
                action-type reject
            }
        }

The following command enables ECMP in the base router.

configure {
    router "Base" {
        ecmp 2

On PE-1, the route table in the base router shows that the remote prefix 2001:db8::33:0/120 has flag [2], meaning that the next-hop 192.0.2.3 occurs twice for this prefix, as follows:

[/]
A:admin@PE-1# show router route-table 2001:db8::33:0/120

===============================================================================
IPv6 Route Table (Router: Base)
===============================================================================
Dest Prefix[Flags]                            Type    Proto     Age        Pref
      Next Hop[Interface Name]                                    Metric   
-------------------------------------------------------------------------------
2001:db8::33:0/120 [2]                        Remote  BGP_LABEL 00h01m10s  170
       192.0.2.3 (tunneled:RSVP:2)                                  100
2001:db8::33:0/120 [2]                        Remote  BGP_LABEL 00h01m10s  170
       192.0.2.3 (tunneled:RSVP:4)                                  100
-------------------------------------------------------------------------------
No. of Routes: 2
Flags: n = Number of times nexthop is repeated
       B = BGP backup route available
       L = LFA nexthop available
       S = Sticky ECMP requested
===============================================================================

The route table on PE-3 shows a similar route with flag [2] for prefix 2001:db8::11:0/120.

On PE-1, the following detailed route table info (using keyword extensive) for prefix 2001:db8::33:0/120 shows that RSVP-TE tunnel 2 and RSVP-TE tunnel 4 are used to reach the next-hop 192.0.2.3. Both RSVP-TE tunnels have metric 100, but the weight of RSVP-TE tunnel 2 is twice as much as the weight of RSVP tunnel 4, so the load on RSVP-TE LSP 2 is twice as high as the load on RSVP LSP 4.

[/]
A:admin@PE-1# show router route-table 2001:db8::33:0/120 extensive

===============================================================================
Route Table (Router: Base)
===============================================================================
Dest Prefix             : 2001:db8::33:0/120
  Protocol              : BGP_LABEL
  Age                   : 00h01m10s
  Preference            : 170
  Indirect Next-Hop     : 192.0.2.3
    Label               : 2
    QoS                 : Priority=n/c, FC=n/c
    Source-Class        : 0
    Dest-Class          : 0
    ECMP-Weight         : N/A
    Resolving Next-Hop  : 192.0.2.3 (RSVP tunnel:2)
      Metric            : 100
      ECMP-Weight       : 2
    Resolving Next-Hop  : 192.0.2.3 (RSVP tunnel:4)
      Metric            : 100
      ECMP-Weight       : 1
-------------------------------------------------------------------------------
No. of Destinations: 1
===============================================================================

The following tunnel table on PE-1 shows that RSVP-TE tunnel 2 has PE-2 as next-hop (192.168.12.2) and RSVP-TE tunnel 4 has next-hop PE-4 (192.168.14.2):

[/]
A:admin@PE-1# show router tunnel-table

===============================================================================
IPv4 Tunnel Table (Router: Base)
===============================================================================
Destination           Owner     Encap TunnelId  Pref   Nexthop        Metric
   Color                                                              
-------------------------------------------------------------------------------
192.0.2.3/32          rsvp      MPLS  2         7      192.168.12.2   100
192.0.2.3/32          rsvp      MPLS  4         7      192.168.14.2   100
---snip---
-------------------------------------------------------------------------------
Flags: B = BGP or MPLS backup hop available
       L = Loop-Free Alternate (LFA) hop available
       E = Inactive best-external BGP route
       k = RIB-API or Forwarding Policy backup hop
===============================================================================

Traffic Verification

The traffic generator sends IPv6 traffic flows to SAP 2/1/c36/2 of IES 1 on PE-1. The packets are tunneled over the available RSVP-TE LSPs present in the ECMP set. The traffic is load balanced unevenly: two thirds of the traffic flows is tunneled via PE-2 (port 1/1/c1/1) while one third of the traffic flows is tunneled via PE-4 (port 1/1/c2/1). The load on the ports is as follows:

[/]
A:admin@PE-1# monitor port 1/1/c1/1 rate interval 3 repeat 3

===============================================================================
Monitor statistics for Port 1/1/c1/1
===============================================================================
                                                   Input                 Output
-------------------------------------------------------------------------------
---snip---
-------------------------------------------------------------------------------
At time t = 6 sec (Mode: Rate)
-------------------------------------------------------------------------------
Octets                                               101                 444150
Packets                                                1                    431
Errors                                                 0                      0
Bits                                                 808                3553200
Utilization (% of port capacity)                   ~0.00                   0.03
---snip---
===============================================================================

[/]
A:admin@PE-1# monitor port 1/1/c2/1 rate interval 3 repeat 3

===============================================================================
Monitor statistics for Port 1/1/c2/1
===============================================================================
                                                   Input                 Output
-------------------------------------------------------------------------------
---snip---
-------------------------------------------------------------------------------
At time t = 6 sec (Mode: Rate)
-------------------------------------------------------------------------------
Octets                                               226                 186190
Packets                                                2                    182
Errors                                                 0                      0
Bits                                                1808                1489520
Utilization (% of port capacity)                   ~0.00                   0.01
---snip---
===============================================================================

[/]
A:admin@PE-1# monitor port 2/1/c36/2 rate interval 3 repeat 3

===============================================================================
Monitor statistics for Port 2/1/c36/2
===============================================================================
                                                   Input                 Output
-------------------------------------------------------------------------------
---snip---
-------------------------------------------------------------------------------
At time t = 6 sec (Mode: Rate)
-------------------------------------------------------------------------------
Octets                                            602112                      0
Packets                                              588                      0
Errors                                                 0                      0
Bits                                             4816896                      0
Utilization (% of port capacity)                    0.04                   0.00
---snip---
===============================================================================

This can also be verified as follows:

[/]
A:admin@PE-1# show port 1/1/c1/1 statistics

===============================================================================
Port Statistics on Slot 1
===============================================================================
Port                        Ingress Packets                  Ingress Octets
Id                           Egress Packets                   Egress Octets
-------------------------------------------------------------------------------
1/1/c1/1                                 47                            4863    
                                      14730                        15157578    
===============================================================================

[/]
A:admin@PE-1# show port 1/1/c2/1 statistics

===============================================================================
Port Statistics on Slot 1
===============================================================================
Port                        Ingress Packets                  Ingress Octets
Id                           Egress Packets                   Egress Octets
-------------------------------------------------------------------------------
1/1/c2/1                                 44                            4423    
                                       6266                         6424681    
===============================================================================

[/]
A:admin@PE-1# show port 2/1/c36/2 statistics

===============================================================================
Port Statistics on Slot 2
===============================================================================
Port                        Ingress Packets                  Ingress Octets
Id                           Egress Packets                   Egress Octets
-------------------------------------------------------------------------------
2/1/c36/2                             20904                        21405696    
                                          0                               0    
===============================================================================

Conclusion

Operators can control how 6PE traffic is load balanced unequally over multiple RSVP-TE LSPs by defining a load balancing weight value on each LSP.