Equal Cost Multipath Load Sharing

Cumulus Linux enables ECMP by default. Load sharing occurs automatically for IPv4 and IPv6 routes with multiple installed next hops. The hardware or the routing protocol configuration determines the maximum number of routes for which load sharing occurs.

ECMP operates only on equal cost routes in the RIB.

For Cumulus Linux to consider routes equal, the routes must:

• Originate from the same routing protocol. Routes from different sources are not considered equal. For example, a static route and an OSPF route are not considered for ECMP load sharing.
• Have equal cost. If two routes from the same protocol are unequal, only the best route installs in the routing table.

When multiple routes are in the routing table, a hash determines through which path a packet follows. To prevent out of order packets, ECMP hashes on a per-flow basis; all packets with the same source and destination IP addresses and the same source and destination ports always hash to the same next hop. ECMP hashing does not keep a record of packets that hash to each next hop and does not guarantee that traffic to each next hop is equal.

Next Hop Groups

ECMP routes resolve to next hop groups, which identify one or more next hops. To view next hop information, run the NVUE nv show router nexthop rib or nv show router nexthop rib <id> commands, or the ip nexthop show or ip nexthop show <id> kernel commands.

cumulus@leaf01:mgmt:~$ nv show router nexthop rib
Installed - Install state 
ID   Installed  Uptime                Vrf      Valid  Via                        ViaIntf        ViaVrf   Depends
---  ---------  --------------------  -------  -----  -------------------------  -------------  -------  -------
12              2024-10-22T18:35:53Z  default  on     swp53                                     default         
13              2024-10-22T18:35:53Z  default  on     swp51                                     default         
14   on         2024-10-22T18:35:53Z  default  on     swp54                                     default         
15   on         2024-10-22T18:36:00Z  default  on     lo                                        default         
16   on         2024-10-22T18:35:53Z  default  on     eth0                                      mgmt            
17   on         2024-10-22T18:35:53Z  default  on     eth0                                      mgmt            
18              2024-10-22T18:36:00Z  default  on                                                               
19   on         2024-10-22T18:36:00Z  default  on     192.168.200.1              eth0           mgmt            
20   on         2024-10-22T18:36:00Z  default  on                                                               
21   on         2024-10-22T18:36:00Z  default  on                                                               
22   on         2024-10-22T18:36:00Z  default  on                                                               
24              2024-10-22T18:35:53Z  default  on     swp52                                     default         
52              2024-10-22T18:35:55Z  default  on     peerlink.4094                             default         
62   on         2024-10-22T18:36:02Z  default  on     fe80::4ab0:2dff:feb5:3daa  peerlink.4094  default         
74              2024-10-22T18:35:59Z  default  on     br_default                                default         
75              2024-10-22T18:35:59Z  default  on     vlan10v0                                  RED             
76   on         2024-10-22T18:35:59Z  default  on     vlan10                                    RED             
77              2024-10-22T18:35:59Z  default  on     vlan10v0                                  RED             
78              2024-10-22T18:35:59Z  default  on     vlan4063_l3                               RED             
79              2024-10-22T18:35:59Z  default  on     vlan20                                    RED             
80   on         2024-10-22T18:35:59Z  default  on     vlan10                                    RED             
81   on         2024-10-22T18:35:59Z  default  on     vlan20                                    RED             
82   on         2024-10-22T18:35:59Z  default  on     vlan30                                    BLUE            
83              2024-10-22T18:35:59Z  default  on     vlan4006_l3                               BLUE            
84   on         2024-10-22T18:35:59Z  default  on     vlan30                                    BLUE            
91              2024-10-22T18:35:59Z  default  on     vlan20v0                                  RED             
92              2024-10-22T18:35:59Z  default  on     vlan4063_l3v0                             RED             
93              2024-10-22T18:35:59Z  default  on     vlan20v0                                  RED             
94              2024-10-22T18:35:59Z  default  on     vlan30v0                                  BLUE            
95              2024-10-22T18:35:59Z  default  on     vlan4006_l3v0                             BLUE            
96              2024-10-22T18:35:59Z  default  on     vlan30v0                                  BLUE            
100             2024-10-22T18:36:01Z  default  on     vxlan48                                   default         
107  on         2024-10-22T18:36:04Z  default  on     fe80::4ab0:2dff:fe32:2a3f  swp52          default         
110  on         2024-10-22T18:36:04Z  default  on     10.10.10.63                vlan4063_l3    RED             
111  on         2024-10-22T18:36:04Z  default  on     10.10.10.63                vlan4006_l3    BLUE            
115  on         2024-10-22T18:36:04Z  default  on     fe80::4ab0:2dff:fe41:6b79  swp51          default         
125  on         2024-10-22T18:42:21Z  default  on                                                        107    
                                                                                                         115    
126  on         2024-10-22T18:36:04Z  default  on                                                        111    
                                                                                                         127    
127  on         2024-10-22T18:36:04Z  default  on     10.10.10.64                vlan4006_l3    BLUE            
128  on         2024-10-22T18:36:04Z  default  on                                                        110    
                                                                                                         129    
129  on         2024-10-22T18:36:04Z  default  on     10.10.10.64                vlan4063_l3    RED             
140  on         2024-10-22T18:42:34Z  default  on     10.0.1.34                  vlan4006_l3    BLUE            
142  on         2024-10-22T18:42:36Z  default  on     10.0.1.34                  vlan4063_l3    RED
...

The following example shows information for next hop group 108:

cumulus@leaf01:mgmt:~$ nv show router nexthop rib 129
                 operational         
---------------  --------------------
type             zebra               
ref-count        2                   
vrf              default             
valid            on                  
installed        on                  
interface-index  74                  
uptime           2024-10-22T18:36:04Z

Via
======
                                                                          
    Flags - u - unreachable, r - recursive, o - onlink, i - installed, d -          
    duplicate, c - connected, A - active, Type - Type of nexthop, Weight - Weight to
    be used by the nexthop for purposes of ECMP, VRF - VRF to use for egress.       
                                                                                
    Nexthop      Flags  Type        Weight  VRF  Interface  
    -----------  -----  ----------  ------  ---  -----------
    10.10.10.64  oA     ip-address  1       RED  vlan4063_l3

Via BackupNexthops
=====================
No Data

Depends
==========
No Data

Dependents
=============
    Nexthop-group
    -------------
    128

ECMP Hashing

You can configure custom hashing to specify what to include in the hash calculation during load balancing between:

• Multiple next hops of a layer 3 route (ECMP hashing).
• Multiple interfaces that are members of the same bond (bond or LAG hashing). For bond hashing, see Bonding - Link Aggregation.

For ECMP load balancing between multiple next hops of a layer 3 route, you can hash on these fields:

The following example commands omit the source port and destination port from the hash calculation:

cumulus@switch:~$ nv set system forwarding ecmp-hash source-port off
cumulus@switch:~$ nv set system forwarding ecmp-hash destination-port off
cumulus@switch:~$ nv config apply

cumulus@switch:~$ sudo nano /etc/cumulus/datapath/traffic.conf
...
# HASH config for  ECMP to enable custom fields
# Fields will be applicable for ECMP hash
# calculation
#Note : Currently supported only for MLX platform
# Uncomment to enable custom fields configured below
hash_config.enable = true

#hash Fields available ( assign true to enable)
#ip protocol
hash_config.ip_prot = true
#source ip
hash_config.sip = true
#destination ip
hash_config.dip = true
#source port
hash_config.sport = false
#destination port
hash_config.dport = false
...

GTP Hashing

GTP carries mobile data within the core of the mobile operator’s network. Traffic in the 5G Mobility core cluster, from cell sites to compute nodes, have the same source and destination IP address. The only way to identify individual flows is with the GTP TEID. Enabling GTP hashing adds the TEID as a hash parameter and helps the Cumulus Linux switches in the network to distribute mobile data traffic evenly across ECMP routes.

Cumulus Linux supports TEID-based ECMP hashing for:

• GTP-U packets ingressing physical ports.
• VXLAN encapsulated GTP-U packets terminating on egress VTEPs.

For TEID-based load balancing for traffic on a bond, see Bonding - Link Aggregation.

GTP TEID-based ECMP hashing is only applicable if the outer header egressing the port is GTP encapsulated and if the ingress packet is either a GTP-U packet or a VXLAN encapsulated GTP-U packet.

To enable TEID-based ECMP hashing:

cumulus@switch:~$ nv set system forwarding ecmp-hash gtp-teid on
cumulus@switch:~$ nv config apply

To show that TEID-based ECMP hashing is on, run the command:

cumulus@switch:~$ nv show system forwarding ecmp-hash
                   applied  description
-----------------  -------  -----------------------------------
destination-ip     on       Destination IPv4/IPv6 Address
destination-port   on       TCP/UDP destination port
gtp-teid           on       GTP-U TEID
...

ECMP Hash Buckets

When there are multiple routes in the routing table, Cumulus Linux assigns each route to an ECMP bucket. When the ECMP hash executes, the result of the hash determines which bucket to use.

In the following example, four next hops exist. Three different flows hash to different hash buckets. Each next hop goes to a unique hash bucket.

The addition of a next hop creates a new hash bucket. The assignment of next hops to hash buckets, as well as the hash result, sometimes changes with the addition of next hops.

With the addition of a new next hop, there is a new hash bucket. As a result, the hash and hash bucket assignment changes, so the existing flows go to different next hops.

When you remove a next hop, the remaining hash bucket assignments can change, which can also change the next hop selected for an existing flow.

A next hop fails, which removes the next hop and hash bucket. It is possible that Cumulus Linux reassigns the remaining next hops.

In most cases, modifying hash buckets has no impact on traffic flows as the switch forwards traffic to a single end host. In deployments where multiple end hosts use the same IP address (anycast), you must use resilient hashing.

Unique Hash Seed

You can configure a unique hash seed for each switch to prevent hash polarization, a type of network congestion that occurs when multiple data flows try to reach a switch using the same switch ports.

You can set a hash seed value between 0 and 4294967295. If you do not specify a value, switchd creates a randomly generated seed.

To configure the hash seed:

cumulus@switch:~$ nv set system forwarding hash-seed 50
cumulus@switch:~$ nv config apply

cumulus@switch:~$ sudo nano /etc/cumulus/datapath/traffic.conf
...
#Specify the hash seed for Equal cost multipath entries
# and for custom ecmp and lag hash
# Default value: random
# Value Range: {0..4294967295}
ecmp_hash_seed = 50
...

cumulus@switch:~$ sudo systemctl restart switchd.service

cl-ecmpcalc

Run the cl-ecmpcalc command to determine a hardware hash result. For example, you can see which path a flow takes through a network. You must provide all fields in the hash, including the ingress interface, layer 3 source IP, layer 3 destination IP, layer 4 source port, and layer 4 destination port.

cumulus@switch:~$ sudo cl-ecmpcalc -i swp1 -s 10.0.0.1 -d 10.0.0.1 -p tcp --sport 20000 --dport 80
ecmpcalc: will query hardware
swp3

If you omit a field, cl-ecmpcalc fails.

cumulus@switch:~$ sudo cl-ecmpcalc -i swp1 -s 10.0.0.1 -d 10.0.0.1 -p tcp
ecmpcalc: will query hardware
usage: cl-ecmpcalc [-h] [-v] [-p PROTOCOL] [-s SRC] [--sport SPORT] [-d DST]
                   [--dport DPORT] [--vid VID] [-i IN_INTERFACE]
                   [--sportid SPORTID] [--smodid SMODID] [-o OUT_INTERFACE]
                   [--dportid DPORTID] [--dmodid DMODID] [--hardware]
                   [--nohardware] [-hs HASHSEED]
                   [-hf HASHFIELDS [HASHFIELDS ...]]
                   [--hashfunction {crc16-ccitt,crc16-bisync}] [-e EGRESS]
                   [-c MCOUNT]
cl-ecmpcalc: error: --sport and --dport required for TCP and UDP frames

Resilient Hashing

In Cumulus Linux, when a next hop fails or you remove the next hop from an ECMP pool, the hashing or hash bucket assignment can change. Resilient hashing is an alternate way to manage ECMP groups. Cumulus Linux assigns next hops to buckets using their hashing header fields and uses the resulting hash to index into the table of 2^n hash buckets. Because all packets in a given flow have the same header hash value, they all use the same flow bucket.

The NVIDIA Spectrum ASIC assigns packets to hash buckets and assigns hash buckets to next hops. The ASIC also runs a background thread that monitors buckets and can migrate buckets between next hops to rebalance the load.

• When you remove a next hop, Cumulus Linux distributes the assigned buckets to the remaining next hops.
• When you add a next hop, Cumulus Linux assigns no buckets to the new next hop until the background thread rebalances the load.
• The load rebalances when the active flow timer expires only if there are inactive hash buckets available; the new next hop can remain unpopulated until the period set in active flow timer expires.
• When the unbalanced timer expires and the load is not balanced, the thread migrates buckets to different next hops to rebalance the load.

Any flow can migrate to any next hop, depending on flow activity and load balance conditions. Over time, the flow can get pinned, which is the default setting and behavior.

When you enable resilient hashing, Cumulus Linux assigns next hops in round robin fashion to a fixed number of buckets. In this example, there are 12 buckets and four next hops.

Unlike default ECMP hashing, when you need to remove a next hop, the number of hash buckets does not change.

With 12 buckets and four next hops, instead of reducing the number of buckets, which impacts flows to known good hosts, the remaining next hops replace the failed next hop.

After you remove the failed next hop, the remaining next hops replace it. This prevents impact to any flows that hash to working next hops.

Resilient hashing does not prevent possible impact to existing flows when you add new next hops. Because there are a fixed number of buckets, a new next hop requires reassigning next hops to buckets.

As a result, some flows hash to new next hops, which can impact anycast deployments.

Cumulus Linux does not enable resilient hashing by default. When you enable resilient hashing, all ECMP groups share 65,536 buckets. An ECMP group is a list of unique next hops that multiple ECMP routes reference.

All ECMP routes must use the same number of buckets (you cannot configure the number of buckets per ECMP route).

To enable resilient hashing:

cumulus@switch:~$ sudo systemctl restart switchd.service

cumulus@switch:~$ sudo vtysh
switch# configure terminal
switch(config)# zebra nexthop proto only
switch(config)# exit
switch# write memory
switch# exit
cumulus@switch:~$

Considerations

When the router adds or removes ECMP paths, or when the next hop IP address, interface, or tunnel changes, the next hop information for an IPv6 prefix can change. FRR deletes the existing route to that prefix from the kernel, then adds a new route with all the relevant new information. In certain situations, Cumulus Linux does not maintain resilient hashing for IPv6 flows.

To work around this issue, you can enable IPv6 route replacement.

To enable the IPv6 route replacement option:

1. In the /etc/frr/daemons file, add the configuration option --v6-rr-semantics to the zebra daemon definition. For example:

   cumulus@switch:~$ sudo nano /etc/frr/daemons
   ...
   vtysh_enable=yes
   zebra_options=" -M cumulus_mlag -M snmp -A 127.0.0.1 --v6-rr-semantics -s 90000000"
   bgpd_options=" -M snmp  -A 127.0.0.1"
   ospfd_options=" -M snmp -A 127.0.0.1"
   ...
   

2. Restart FRR with this command:

   cumulus@switch:~$ sudo systemctl restart frr.service

   Restarting FRR restarts all the routing protocol daemons that are enabled and running.

To verify that IPv6 route replacement, run the systemctl status frr command:

cumulus@switch:~$ systemctl status frr

    ● frr.service - FRRouting
      Loaded: loaded (/lib/systemd/system/frr.service; enabled; vendor preset: enabled)
      Active: active (running) since Mon 2020-02-03 20:02:33 UTC; 3min 8s ago
        Docs: https://frrouting.readthedocs.io/en/latest/setup.html
      Process: 4675 ExecStart=/usr/lib/frr/frrinit.sh start (code=exited, status=0/SUCCESS)
      Memory: 14.4M
      CGroup: /system.slice/frr.service
            ├─4685 /usr/lib/frr/watchfrr -d zebra bgpd staticd
            ├─4701 /usr/lib/frr/zebra -d -M snmp -A 127.0.0.1 --v6-rr-semantics -s 90000000
            ├─4705 /usr/lib/frr/bgpd -d -M snmp -A 127.0.0.1
            └─4711 /usr/lib/frr/staticd -d -A 127.0.0.1

Adaptive Routing

Adaptive routing is a load balancing feature that improves network utilization for eligible IP packets by selecting forwarding paths dynamically based on the state of the switch, such as queue occupancy and port utilization.

The benefits of using adaptive routing include:

• The switch can forward adaptive routing eligible IP packets over all the available ECMP member ports to maximize the total traffic throughput, while removing potential ECMP flow collisions.
• The switch distributes incoming traffic equally (or according to their weights) between the available IP next hops, which helps to minimize latency and network congestion.
• If the cumulative rate of one or more flows exceeds the link bandwidth of the individual uplink port, adaptive routing can distribute the traffic dynamically between multiple uplink ports; the available bandwidth for these flows is not limited to the link bandwidth of an individual uplink port.

With adaptive routing, the switch forwards packets to the less loaded path on a per packet basis to best utilize the fabric resources and avoid congestion. The change decision for port selection is set to one microsecond; you cannot change it.

Cumulus Linux supports ECMP resource optimization for adaptive routing, which addresses the requirement of large numbers of ECMP groups during routing protocol convergence in transient scenarios.

Cumulus Linux supports adaptive routing with:

• Switches with the Spectrum-4 ASIC at 400G and 200G speeds.
• RoCE2 unicast traffic.
• VXLAN-encapsulated RoCE traffic.
• Layer 3 interfaces.
• Next hop router interfaces in the default VRF.

Cumulus Linux also supports BGP W-ECMP with adaptive routing; see BGP Weighted Equal Cost Multipath.

Enable Adaptive Routing

To enable adaptive routing:

cumulus@switch:~$ nv set interface swp51 router adaptive-routing enable on
cumulus@switch:~$ nv set interface swp52 router adaptive-routing enable on
cumulus@switch:~$ nv config apply

cumulus@switch:~$ sudo nano /etc/cumulus/switchd.d/adaptive_routing.conf
## Global adaptive-routing enable/disable setting
adaptive_routing.enable = TRUE
...
## Per-port configuration
interface.swp51.adaptive_routing.enable = TRUE
interface.swp51.adaptive_routing.link_util_thresh = 70
interface.swp52.adaptive_routing.enable = TRUE
interface.swp52.adaptive_routing.link_util_thresh = 70
...

When you enable adaptive routing, Cumulus Linux uses the default profile settings for your switch ASIC type. You cannot change the default profile settings. If you need to make adjustments to the settings, contact NVIDIA Customer Support.

Link Utilization

Link utilization, when crossing a threshold, is one of the parameters in the adaptive routing decision. The default link utilization threshold percentage on an interface is 70. You can change the percentage to a value between 1 and 100.

Link utilization is off by default; you must enable the global link utilization setting to use the link utilization thresholds set on adaptive routing interfaces. You cannot enable or disable link utilization per interface.

cumulus@switch:~$ nv set router adaptive-routing link-utilization-threshold on
cumulus@switch:~$ nv config apply

cumulus@switch:~$ nv set interface swp51 router adaptive-routing link-utilization-threshold 100
cumulus@switch:~$ nv set router adaptive-routing link-utilization-threshold on
cumulus@switch:~$ nv config apply

cumulus@switch:~$ sudo nano /etc/cumulus/switchd.d/adaptive_routing.conf
## Global adaptive-routing enable/disable setting
adaptive_routing.enable = TRUE

## Global Link-utilization-threshold on/off
adaptive_routing.link_utilization_threshold_disabled = FALSE

## Per-port configuration
interface.swp51.adaptive_routing.enable = TRUE
interface.swp51.adaptive_routing.link_util_thresh = 100

Example Configuration

cumulus@switch:~$ nv set interface swp51 router adaptive-routing enable on
cumulus@switch:~$ nv set interface swp52 router adaptive-routing enable on
cumulus@switch:~$ nv config apply

cumulus@switch:~$ nv set interface swp51 router adaptive-routing enable on
cumulus@switch:~$ nv set interface swp52 router adaptive-routing enable on
cumulus@switch:~$ nv set interface swp51 router adaptive-routing link-utilization-threshold 100
cumulus@switch:~$ nv set interface swp52 router adaptive-routing link-utilization-threshold 100
cumulus@switch:~$ nv set router adaptive-routing link-utilization-threshold on
cumulus@switch:~$ nv config apply 

cumulus@switch:~$ sudo nano /etc/cumulus/switchd.d/ad.aptive_routing.conf
## Global adaptive-routing enable/disable setting
adaptive_routing.enable = TRUE

## Global Link-utilization-threshold on/off
adaptive_routing.link_utilization_threshold_disabled = TRUE

## Per-port configuration
interface.swp51.adaptive_routing.enable = TRUE
interface.swp51.adaptive_routing.link_util_thresh = 0
interface.swp52.adaptive_routing.enable = TRUE
interface.swp52.adaptive_routing.link_util_thresh = 0
...

cumulus@switch:~$ sudo nano /etc/cumulus/switchd.d/adaptive_routing.conf
## Global adaptive-routing enable/disable setting
adaptive_routing.enable = TRUE

## Global Link-utilization-threshold on/off
adaptive_routing.link_utilization_threshold_disabled = FALSE

## Per-port configuration
interface.swp51.adaptive_routing.enable = TRUE
interface.swp51.adaptive_routing.link_util_thresh = 100
interface.swp52.adaptive_routing.enable = TRUE
interface.swp52.adaptive_routing.link_util_thresh = 100

Show Adaptive Routing Settings

To show adaptive routing settings, run the nv show router adaptive-routing command:

cumulus@leaf01:mgmt:~$ nv show router adaptive-routing
                            operational   applied
--------------------------  ------------  -------
enable                      on            off

To show adaptive routing configuration for an interface, run the nv show interface <interface> router adaptive-routing.

Considerations

IPv6 Next Hop Preference

Cumulus Linux uses IPv6 link-local addresses as BGP next hops when receiving a route with both link-local and global next hops. To configure a BGP peering to prefer global next hop addresses, configure the ipv6-nexthop-prefer-global option in an inbound route map applied to the peer. Use this configuration when there are multiple BGP peerings to the same router with adaptive routing enabled, or multiple peerings to the same router on interfaces that share the same MAC address or physical interface. Refer to Set IPv6 Prefer Global.