Segment Routing
Cumulus Linux supports source based routing with SRv6.
The NICs connected to the switch fabric perform SRv6 origination and termination, and the switches act as SRv6-aware nodes. SRv6 allows NICs to directly control the path that traffic takes throughout the fabric by encoding an ordered list of SRv6 segment identifiers (uSIDs) in the packet header.
Cumulus Linux supports uN (End with NEXT-CSID) and uA (End.X with NEXT-CSID) endpoint behaviors, defined in RFC9800.
Cumulus Linux supports segment routing:
- On the Spectrum-4 switch only.
- In the default VRF only.
Configure Segment Routing
To configure segment routing:
- Enable segment routing.
- Configure the SRv6 locator settings and the static IDs. You can configure a maximum of 256 locators.
- Configure the SRv6 locator prefix. The prefix length must match the sum of block length and the node length.
- Configure the SRv6 locator block length. You can specify a value between 16 and 64. The default value is 16.
- Configure the SRv6 locator function length. You can specify a value between 0 and 64. The default value is 0.
- Configure the SRv6 locator node length. You can specify a value between 0 and 64. The default value is 16.
- Configure the static segment identifier locator name. The static segment identifier must be part of the locator prefix.
- Configure the static segment identifier endpoint behavior. You can specify uA or uN. For uA segment identifiers, next hop (peer link-local) learning occurs with router advertisements. Spectrum switches enable router advertisements on the interface automatically when you configure a uA segment identifier; however, if the adjacent device is a non-Spectrum switch, you need to enable router advertisements on the adjacent device on the connected interface to ensure proper next hop discovery.
The following table provides the supported formats for block, node, and function length.
| Format | Block Length | Node Length | Function Length |
|---|---|---|---|
| uN | 32 | 16 | 0 |
| uA + uN | 16 | 16 | 16 |
| uN only | 16 | 16 | 0 |
| uA only | 16 | 0 | 16 |
Avoid reusing IPv6 prefixes for both static routes and static segment identifiers. Use distinct prefixes to prevent routing table conflicts and ensure correct segment routing behavior. If you configure a static route that overrides a segment identifier or a segment identifier that overrides a static route, unset, then reset the static segment identifier.
The following example enables segment routing, configures the SRv6 locator called LEAF with the prefix fcbb::/16, and sets the static segment identifier fcbb:fe8::/32:
cumulus@switch:~$ nv set router segment-routing srv6 state enabled
cumulus@switch:~$ nv set router segment-routing srv6 locator LEAF prefix fcbb::/16
cumulus@switch:~$ nv set router segment-routing srv6 locator LEAF block-length 16
cumulus@switch:~$ nv set router segment-routing srv6 locator LEAF node-length 0
cumulus@switch:~$ nv set router segment-routing srv6 locator LEAF func-length 16
cumulus@switch:~$ nv set router segment-routing static srv6-sid fcbb:fe8::/32 behavior uA
cumulus@switch:~$ nv set router segment-routing static srv6-sid fcbb:fe8::/32 interface swp1
cumulus@switch:~$ nv set router segment-routing static srv6-sid fcbb:fe8::/32 locator-name LEAF
cumulus@switch:~$ nv config apply
- To disable segment routing, run the
nv set router segment-routing srv6 state disabledcommand. - To unset all SRv6 locators, run the
nv unset router segment-routing srv6 locatorcommand. - To unset all static segment identifiers, run the
nv unset router segment-routing static srv6-sidcommand. - To unset a static segment identifier, run the
nv unset router segment-routing static srv6-sid <prefix>command.
cumulus@switch:~$ sudo vtysh
...
leaf01# configure t
leaf01(config)# segment-routing
leaf01(config-sr)# srv6
leaf01(config-srv6)# static-sids
leaf01(config-srv6-sids)# sid fcbb:fe8::/32 locator LEAF behavior uA interface swp1
leaf01(config-srv6-sids)# exit
leaf01(config-srv6)# locators
leaf01(config-srv6-locators)# locator LEAF
leaf01(config-srv6-locator)# prefix fcbb::/16 block-len 16 node-len 0 func-bits 16
leaf01(config-srv6-locator)# end
leaf01# exit
Show Segment Routing Configuration
To show if segment routing is enabled, and to show the configured locators and SRv6 segment identifiers, run the nv show router segment-routing command:
cumulus@switch:~$ nv show router segment-routing
applied
------------ --------------------
srv6
state enabled
[locator] LEAF å
static
[srv6-sid] fcbb:fe8::/32
To show the configuration for all SRv6 locators, run the nv show router segment-routing srv6 locator command or the vtysh show segment-routing srv6 locator command:
cumulus@switch:~$ nv show router segment-routing srv6 locator
SRv6 locator name prefix block length node length function length status
----------------- ---------------- ------------ ----------- --------------- ------
LEAF fcbb::/16 16 0 16 up
To show the configuration for a specific SRv6 locator, run the NVUE nv show router segment-routing srv6 locator <locator-id> command or the vtysh show segment-routing srv6 locator <locator> detail command:
cumulus@switch:~$ nv show router segment-routing srv6 locator LEAF
operational applied
------------ --------------- -----------------
prefix fcbb::/16. fcbb::/16
block-length 16 16
node-length 0 0
func-length 16 16
status up
To show the configuration for a specific SRv6 static segment identifier, run the NVUE nv show router segment-routing static srv6 sid <sid> command or the vtysh show segment-routing srv6 sid <sid> command:
cumulus@switch:~$ nv show router segment-routing srv6 sid fcbb:fe8::/32
operational applied
------------ ------------------- -------
locator-name LEAF
behavior End.X
interface swp1
nexthop-v6 fe80::202:ff:fe00:2
protocol static
Show Segment Routing Endpoints
Segment routing endpoints are installed as IPv6 routes into the RIB and FIB. To show segment routing endpoints, view the
IPv6 RIB with the nv show vrf <vrf-id> router rib ipv6 route command. You can view a specific route with the nv show vrf <vrf-id> router rib ipv6 route <route-id> command.
Show Segment Routing Statistics
To show SRv6 statistics, run the nv show router segment-routing srv6 stats command
cumulus@switch:~$ nv show router segment-routing srv6 stats
Hit Counters
------------------------------
SID Packets
--------------------------------------- ----------
fcbb:fe8::/32 0
Drop Counters
------------------------------
Total no-sid-dropped packets
To show information about a specific SRv6 segment identifier, run the NVUE nv show router segment-routing srv6 stats sid <sid> command or the vtysh show segment-routing srv6 sid command:
cumulus@switch:~$ nv show router segment-routing srv6 stats sid fcbb:fe8::/32
To show information about non segment identifier dropped packets, run the nv show router segment-routing srv6 stats no-sid-drop command:
cumulus@switch:~$ nv show router segment-routing srv6 stats no-sid-drops
operational
---------------------- -----------
no-sid-dropped-packets 0
When you enable packet trimming with segment routing, Cumulus Linux counts the trimmed packet twice in the SRv6 statistics.
Clear SRv6 Statistics
To clear all segment routing statistics, run the nv action clear router segment-routing srv6 stats command:
cumulus@switch:~$ nv action clear router segment-routing srv6 stats
To clear SRv6 statistics for a specific segment identifier, run the nv action clear router segment-routing srv6 stats sid <sid> command:
cumulus@switch:~$ nv action clear router segment-routing srv6 stats sid fcbb:fe8::/32
To clear SRv6 statistics for non segment identifier dropped packets, run the nv action clear router segment-routing srv6 stats no-sid-drops command:
cumulus@switch:~$ nv action clear router segment-routing srv6 stats no-sid-drops