Routing

This chapter discusses routing on switches running Cumulus Linux.

Manage Static Routes

Static routes are added to the FRRouting routing table and then the kernel routing table.

To add static routes:

cumulus@switch:~$ net add routing route 203.0.113.0/24 198.51.100.2
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

cumulus@switch:~$ sudo vtysh

switch# configure terminal
switch(config)# ip route 203.0.113.0/24 198.51.100.2
switch(config)# exit
switch# write memory
switch# exit
cumulus@switch:~$

The NCLU and vtysh commands save the configuration in the /etc/frr/frr.conf file. For example:

...
!
ip route 203.0.113.0/24 198.51.100.2
!
...

To delete a static route:

cumulus@switch:~$ net del routing route 203.0.113.0/24 198.51.100.2
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

cumulus@switch:~$ sudo vtysh

switch# configure terminal
switch(config)# no ip route 203.0.113.0/24 198.51.100.2
switch(config)# exit
switch# write memory
switch# exit
cumulus@switch:~$

To view static routes, run the NCLU net show route static command or the vtysh show ip route command. For example:

cumulus@switch:~$ net show route static
RIB entry for static
====================
Codes: K - kernel route, C - connected, S - static, R - RIP,
        O - OSPF, I - IS-IS, B - BGP, P - PIM, T - Table,
        > - selected route, * - FIB route
 S>* 203.0.113.0/24 [1/0] via 198.51.100.2, swp3

Static Multicast Routes

To add a static multicast route (mroute):

cumulus@switch:~$ net add routing mroute 230.0.0.0/24
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

cumulus@switch:~$ sudo vtysh

switch# configure terminal
switch(config)# ip mroute 203.0.0.0/24
switch(config)# exit
switch# write memory
switch# exit
cumulus@switch:~$

The NCLU and vtysh commands save the configuration in the /etc/frr/frr.conf file. For example:

...
!
ip mroute 230.0.0.0/24
!
...

To delete an mroute:

cumulus@switch:~$ net del routing mroute 230.0.0.0/24
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

cumulus@switch:~$ sudo vtysh

switch# configure terminal
switch(config)# no ip mroute 203.0.0.0/24
switch(config)# exit
switch# write memory
switch# exit
cumulus@switch:~$

To view mroutes, run the following command from the vtysh shell:

cumulus@switch:~$ sudo vtysh
switch# show ip rpf 230.0.0.0
Routing entry for 230.0.0.0/24 using Multicast RIB
  Known via "static", distance 1, metric 0, best
  * directly connected, swp31s0

Static Routing via ip route

You can also create a static route by adding the route to a switch port configuration. For example:

cumulus@switch:~$ net add interface swp3 ip address 198.51.100.1/24
cumulus@switch:~$ net add interface swp3 post-up routing route add 203.0.113.0/24 via 198.51.100.2
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

cumulus@switch:~$ sudo vtysh

switch# configure terminal
switch(config)# interface swp3
switch(config-if)# post-up ip route 203.0.113.0/24 198.51.100.2
switch(config-if)# exit
switch(config)# exit
switch# write memory
switch# exit
cumulus@switch:~$

The NCLU and vtysh commands save the configuration in the /etc/network/interfaces file. For example:

...
auto swp3
iface swp3
    address 198.51.100.1/24
    post-up ip route add 203.0.113.0/24 via 198.51.100.2
...

The ip route command allows you to manipulate the kernel routing table directly from the Linux shell. See man ip(8) for details. FRRouting monitors the kernel routing table changes and updates its own routing table accordingly.

To display the routing table:

cumulus@switch:~$ ip route show
default via 10.0.1.2 dev eth0
10.0.1.0/24 dev eth0  proto kernel  scope link  src 10.0.1.52
192.0.2.0/24 dev swp1  proto kernel  scope link  src 192.0.2.12
192.0.2.10/24 via 192.0.2.1 dev swp1  proto zebra  metric 20
192.0.2.20/24  proto zebra  metric 20
     nexthop via 192.0.2.1  dev swp1 weight 1
     nexthop via 192.0.2.2  dev swp2 weight 1
192.0.2.30/24 via 192.0.2.1 dev swp1  proto zebra  metric 20
192.0.2.40/24 dev swp2  proto kernel  scope link  src 192.0.2.42
192.0.2.50/24 via 192.0.2.2 dev swp2  proto zebra  metric 20
192.0.2.60/24 via 192.0.2.2 dev swp2  proto zebra  metric 20
192.0.2.70/24  proto zebra  metric 30
     nexthop via 192.0.2.1  dev swp1 weight 1
     nexthop via 192.0.2.2  dev swp2 weight 1
198.51.100.0/24 dev swp3  proto kernel  scope link  src 198.51.100.1
198.51.100.10/24 dev swp4  proto kernel  scope link  src 198.51.100.11
198.51.100.20/24 dev br0  proto kernel  scope link  src 198.51.100.21

Apply a Route Map for Route Updates

To apply a route map to filter route updates from Zebra into the Linux kernel:

cumulus@switch:~$ net add routing protocol static route-map myroutemap

cumulus@switch:~$ sudo vtysh

switch# configure terminal
switch(config)# ip protocol static route-map myroutemap
switch(config)# exit
switch# write memory
switch# exit
cumulus@switch:~$

The NCLU and vtysh commands save the configuration in the /etc/frr/frr.conf file. For example:

...
!
ip protocol static route-map myroutemap
!
...

Configure a Gateway or Default Route

Consider creating a gateway or default route on each switch for traffic destined outside the switch’s subnet or local network. All such traffic passes through the gateway, which is a host on the same network that routes packets to their destination beyond the local network.

In the following example, you create a default route in the routing table 0.0.0.0/0, which indicates any IP address can be sent to the gateway, which is another switch with the IP address 10.1.0.1.

cumulus@switch:~$ net add routing route 0.0.0.0/0 10.1.0.1
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

cumulus@switch:~$ sudo vtysh

switch# configure terminal
switch(config)# ip route 0.0.0.0/0 10.1.0.1
switch(config)# exit
switch# write memory
switch# exit
cumulus@switch:~$

The NCLU and vtysh commands save the configuration in the /etc/frr/frr.conf file. For example:

...
!
ip route 0.0.0.0/0 10.1.0.1
!
...

Control Link-local Multicast

Cumulus Linux provides a configuration option on Broadcom switches to disable forwarding of link-local multicast packets to the CPU so that such packets only flood the ASIC, which reduces CPU usage.

To disable forwarding of link local multicast packets to the CPU on a Broadcom switch, run the following command:

cumulus@switch:~$ echo TRUE > /cumulus/switchd/config/hal/bcm/ll_mcast_punt_disable

The configuration above takes effect immediately, but does not persist if you reboot the switch.

To apply the configuration so that it is persistent, edit the /etc/cumulus/switchd.conf file and uncomment the hal.bcm.ll_mcast_punt_disable = TRUE option. For example:

cumulus@switch:~$ sudo nano /etc/cumulus/switchd.conf
#
# /etc/cumulus/switchd.conf - switchd configuration file
#
...
# Bridge L2MC Link-Local multicast punt disable
hal.bcm.ll_mcast_punt_disable = TRUE
...

A switchd restart is not required.

Supported Route Table Entries

Cumulus Linux (via switchd)advertises the maximum number of route table entries that are supported on a given switch architecture, including:

• Layer 3 IPv4 LPM (longest prefix match) entries that have a mask less than /32
• Layer 3 IPv6 LPM entries that have a mask of /64 or less
• Layer 3 IPv6 LPM entries that have a mask greater than /64
• Layer 3 IPv4 neighbor (or host) entries that are the next hops seen in ip neighbor
• Layer 3 IPv6 neighbor entries that are the next hops seen in ip -6 neighbor
• ECMP next hops, which are IP address entries in a router’s routing table that specify the next closest/most optimal router in its routing path
• MAC addresses

In addition, switches on the Tomahawk, Trident II, Trident II+, and Trident3 platforms are configured to manage route table entries using Algorithm Longest Prefix Match (ALPM). In ALPM mode, the hardware can store significantly more route entries.

You can use either the NCLU net show system asic command or the cl-resource-query to determine the current table sizes on a given switch.

Forwarding Table Profiles

On Mellanox Spectrum and some Broadcom ASICs, you can configure the allocation of forwarding table resources and mechanisms. Cumulus Linux provides a number of generalized profiles for the platforms described below. These profiles work only with layer 2 and layer 3 unicast forwarding.

Cumulus Linux defines these profiles as default, l 2-heavy, v4-lpm-heavy and v6-lpm-heavy. Choose the profile that best suits your network architecture and specify the profile name for the forwarding_table.profile variable in the /etc/cumulus/datapath/traffic.conf file.

cumulus@switch:~$ cat /etc/cumulus/datapath/traffic.conf | grep forwarding_table -B 4
# Manage shared forwarding table allocations
# Valid profiles -
# default, l2-heavy, v4-lpm-heavy, v6-lpm-heavy
#
forwarding_table.profile = default

After you specify a different profile, restart switchd for the change to take effect. You can see the forwarding table profile when you run cl-resource-query.

Number of Supported Route Entries By Platform

The following tables list the number of MAC addresses, layer 3 neighbors, and LPM routes validated for each forwarding table profile for the various supported platforms. If you do not specify any profiles as described above, the default values are the ones that the switch will use.

Mellanox Spectrum Switches

Broadcom Tomahawk/Tomahawk+ Switches

Broadcom Trident II/Trident II+/Trident3 Switches

Broadcom Helix4 Switches

Helix4 switches do not have profiles.

TCAM Resource Profiles for Spectrum Switches

On the Mellanox Spectrum ASIC, you can configure TCAM resource allocation, which is shared between IP multicast forwarding entries and ACL tables. Cumulus Linux provides a number of general profiles for this platform: default, ipmc-heavy and acl-heavy. Choose the profile that best suits your network architecture and specify that profile name in the tcam_resource.profile variable in the /usr/lib/python2.7/dist-packages/cumulus/__chip_config/mlx/datapath.conf file.

cumulus@switch:~$ cat /usr/lib/python2.7/dist-packages/cumulus/__chip_config/mlx/datapath.conf | grep -B3 "tcam_resource"
#TCAM resource forwarding profile

    1. Valid profiles -
    2. default, ipmc-heavy, acl-heavy, ipmc-max
       tcam_resource.profile = default

After you specify a different profile, restart switchd for the change to take effect.

When nonatomic updates are enabled (acl.non_atomic_update_mode is set to TRUE in the /etc/cumulus/switchd.conf file), the maximum number of mroute and ACL entries for each profile are:

When nonatomic updates are disabled (acl.non_atomic_update_mode is set to FALSE in the /etc/cumulus/switchd.conf file), the maximum number of mroute and ACL entries for each profile are:

Route Entry Takes Precedence Over Neighbor Entry

On Broadcom switches with Cumulus Linux 4.0 and later, when there is a /32 IPv4 or /128 IPv6 route and the same prefix is also a neighbor entry in the linux kernel, the route entry takes precedence over the neighbor entry in the forwarding lookup. To change this behavior, update the route_preferred_over_neigh variable to FALSE in the /etc/cumulus/switchd.conf file.

Caveats and Errata

Do Not Delete Routes through Linux Shell

Do not use the Linux shell to delete static routes added via FRRouting (with vtysh commands). Delete the routes with the vtysh commands; otherwise FRRouting might not be able to clean up its internal state completely, which can result in incorrect routing.

Using NCLU Commands to Delete Routing Configuration

When you use NCLU commands to delete routing (FRR) configuration, such as static routes or route map rules (multiples of which can exist in a configuration), commit ten or fewer delete commands at a time to avoid commit failures.

Add IPv6 Default Route with src Address on eth0 Fails without Adding Delay

Attempting to install an IPv6 default route on eth0 with a source address fails at reboot or when running ifup on eth0.

The first execution of ifup -dv returns this warning and does not install the route:

cumulus@switch:~$ sudo ifup -dv eth0
warning: eth0: post-up cmd '/sbin/ip route add default via 2001:620:5ca1:160::1 /
src 2001:620:5ca1:160::45 dev eth0' failed (RTNETLINK answers: Invalid argument)<<<<<<<<<<

Running ifup a second time on eth0 successfully installs the route.

To work around this issue, either add a two second delay or exclude the src parameter to the ip route add that causes the need for the delay:

• Add a delay to the eth0 interface:

cumulus@switch:~$ net add interface eth0 ipv6 address 2001:620:5ca1:160::45/64 post-up /bin/sleep 2s
cumulus@switch:~$ net add interface eth0 post-up /sbin/ip route add default via 2001:620:5ca1:160::1 src 2001:620:5ca11:160::45 dev eth0

• Exclude the src parameter to the ip route add that causes the need for the delay. If the src parameter is removed, the route is added correctly.

cumulus@switch:~$ net add interface eth0 post-up /sbin/ip route add default via 2001:620:5ca1:160::1 dev eth0

cumulus@switch:~$ ifdown eth0
Stopping NTP server: ntpd.
Starting NTP server: ntpd.
cumulus@switch:~$ ip -6 r s
cumulus@switch:~$ ifup eth0
Stopping NTP server: ntpd.
Starting NTP server: ntpd.
cumulus@switch:~$ ip -6 r s
2001:620:5ca1:160::/64 dev eth0  proto kernel  metric 256
fe80::/64 dev eth0  proto kernel  metric 256
default via 2001:620:5ca1:160::1 dev eth0  metric 1024

Use the Same Neighbor Cache Aging Timer for IPv4 and IPv6

Cumulus Linux does not support different neighbor cache aging timer settings for IPv4 and IPv6.

For example, see the two settings for neigh.default.base_reachable_time_ms in /etc/sysctl.d/neigh.conf:

cumulus@switch:~$ sudo cat /etc/sysctl.d/neigh.conf

...

net.ipv4.neigh.default.base_reachable_time_ms=1080000
net.ipv6.neigh.default.base_reachable_time_ms=1080000

...

Related Information

• Linux IP - ip route command
• FRRouting docs - static route commands