Lab 8-1: Interface Metrics, Floating Static Routes, and Multiple Equal-Cost Routes (ECMP)

Example 8-8 demonstrates the use of different metrics (the least preferable is referred to as a floating static route) for the same prefix as well as equal metrics for load balancing. The BSD world only supplies metrics in context with interfaces (see Example 8-9). This is not a deficiency, but is instead a design choice to leave these issues to dynamic routing protocols.

If you use two routes with an equal metric value, load balancing is done on a per-connection basis; if you specify the Linux equalize keyword on two routes, load balancing is done on a per-packet basis. In this case, the route is just recomputed for every packet. Without it, the route stays cached and bound to a specific next hop as long as it is up and alive.

Example 8-8 starts with adding two unequal-cost routes to the same destination prefix via the route and different prefixes via the ip route command sequence to demonstrate a floating static route setup. This is different on a Cisco router: The floating static route is added only to the routing table "on demand," when the preferable prefix route fails, hence the name floating. The second command sequence of Example 8-8 establishes a mix of per-destination and per-packet (equalize) load-balanced ECMP. As you can see from the following output in Example 8-9, both unequal metric routes are placed in the routing table. The ECMP routes have the same weight, 1, and the two per-packet load-balanced ECMP routes are labeled with the equalize flag (shaded text).

Example 8-8. Linux ECMP Setup Example

[root@callisto:~#] route add ?net 11.1.1.0/24 metric 2 gw 192.168.1.254

[root@callisto:~#] route add ?net 11.1.1.0/24 metric 1 gw 192.168.14.254

[root@callisto:~#] ip route add 11.1.2.0/24 via 192.168.1.254 metric 2

[root@callisto:~#] ip route add 11.1.2.0/24 via 192.168.14.254 metric 1

[root@callisto:~#] ip route add 10.1.1.0/24 equalize nexthop via 192.168.1.254 dev eth1

                   nexthop via 192.168.14.254 dev eth0

[root@callisto:~#] ip route add 10.1.5.0/24 nexthop via 192.168.1.254 nexthop via

                    192.168.14.254

Example 8-9. Linux ECMP Setup Result

[root@callisto:~#] ip route help

Usage: ip route { list | flush } SELECTOR

       ip route get ADDRESS [ from ADDRESS iif STRING ]

                            [ oif STRING ]  [ tos TOS ]

       ip route { add | del | change | append | replace | monitor } ROUTE

SELECTOR := [ root PREFIX ] [ match PREFIX ] [ exact PREFIX ]

            [ table TABLE_ID ] [ proto RTPROTO ]

            [ type TYPE ] [ scope SCOPE ]

ROUTE := NODE_SPEC [ INFO_SPEC ]

NODE_SPEC := [ TYPE ] PREFIX [ tos TOS ]

             [ table TABLE_ID ] [ proto RTPROTO ]

             [ scope SCOPE ] [ metric METRIC ]

INFO_SPEC := NH OPTIONS FLAGS [ nexthop NH ]...

NH := [ via ADDRESS ] [ dev STRING ] [ weight NUMBER ] NHFLAGS

OPTIONS := FLAGS [ mtu NUMBER ] [ advmss NUMBER ]

           [ rtt NUMBER ] [ rttvar NUMBER ]

           [ window NUMBER] [ cwnd NUMBER ] [ ssthresh REALM ]

           [ realms REALM ]

TYPE := [ unicast | local | broadcast | multicast | throw |

          unreachable | prohibit | blackhole | nat ]

TABLE_ID := [ local | main | default | all | NUMBER ]

SCOPE := [ host | link | global | NUMBER ]

FLAGS := [ equalize ]

NHFLAGS := [ onlink | pervasive ]

RTPROTO := [ kernel | boot | static | NUMBER ]





[root@callisto:~#] ip route show

192.168.1.0/24 dev eth1  scope link

192.168.1.0/24 dev ipsec0  proto kernel  scope link  src 192.168.1.1

10.1.5.0/24

        nexthop via 192.168.1.254  dev eth1 weight 1

        nexthop via 192.168.14.254  dev eth0 weight 1

192.168.14.0/24 dev eth0  scope link

11.1.2.0/24 via 192.168.14.254 dev eth0  metric 1

11.1.2.0/24 via 192.168.1.254 dev eth1  metric 2

10.1.1.0/24 equalize

        nexthop via 192.168.1.254  dev eth1 weight 1

        nexthop via 192.168.14.254  dev eth0 weight 1

11.1.1.0/24 via 192.168.14.254 dev eth0  metric 1

11.1.1.0/24 via 192.168.1.254 dev eth1  metric 2

127.0.0.0/8 dev lo  scope link

default via 192.168.1.254 dev eth1



[root@callisto:~#] route -n

Kernel IP routing table

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface

192.168.1.0     0.0.0.0         255.255.255.0   U     0      0        0 eth1

192.168.1.0     0.0.0.0         255.255.255.0   U     0      0        0 ipsec0

10.1.5.0        192.168.1.254   255.255.255.0   UG    0      0        0 eth1

192.168.14.0    0.0.0.0         255.255.255.0   U     0      0        0 eth0

11.1.2.0        192.168.14.254  255.255.255.0   UG    1      0        0 eth0

11.1.2.0        192.168.1.254   255.255.255.0   UG    2      0        0 eth1

10.1.1.0        192.168.1.254   255.255.255.0   UG    0      0        0 eth1

11.1.1.0        192.168.14.254  255.255.255.0   UG    1      0        0 eth0

11.1.1.0        192.168.1.254   255.255.255.0   UG    2      0        0 eth1

127.0.0.0       0.0.0.0         255.0.0.0       U     0      0        0 lo

0.0.0.0         192.168.1.254   0.0.0.0         UG    0      0        0 eth1

As previously mentioned, BSD Unices do not provide metrics in context with the route command. However, you can assign metrics to interfaces, as demonstrated in Example 8-10 (shaded text).

Example 8-10. Example for OpenBSD Interface Metrics

[root@ganymed:~#] ifconfig ne4 metric 5

[root@ganymed:~#] ifconfig -A

...

ne4: flags=8863<UP,BROADCAST,NOTRAILERS,RUNNING,SIMPLEX,MULTICAST> metric 5 mtu 1500

        media: Ethernet 10baseT full-duplex

        inet 192.168.2.254 netmask 0xffffff00 broadcast 192.168.2.255

        inet6 fe80::5054:5ff:fee3:e42f%ne4 prefixlen 64 scopeid 0x2

...