Link State Propagation is a feature on which an interface in an in-path interface pair will follow the state of the Ethernet link of the opposite interface.
Without Steelhead appliances, the WAN and LAN routers are directly connected. When the interface on the LAN router goes down (cable disconnected, router rebooting) then the interface on the WAN router will lose the Ethernet link. The routing process will be immediately informed about this and inform the rest of the routing cloud that the networks learned from the LAN router are unreachable.
With a Steelhead appliance between the two routers, when the interface on the LAN router goes down, it is only the LAN interface on the in-path interface which notices it. Since the WAN router will be unaware of this, the routing process on the WAN router will not get alerted and has to wait for a timeout on protocol level. With Link State Propagation enabled, when the LAN interface loses its Ethernet link, the in-path interface will also bring down the WAN interface and thus the interface on the WAN router will lose its Ethernet link too. Now the routing process on the WAN router will immediately be informed that path to the LAN router is down instead of having to wait for routing protocol timeouts.
In this example the output of the
show interface lan0_0 brief
and
show interface wan0_0 brief
commands show that the lan0_0 went down
(Up: yes)
and that wan0_0 is brought down
(Up: no).
Figure 4.21. Compare the "Up" status to see which interface was brought down.
SH # show in-path lsp Link State Propagation Enabled: yes SH # show interface lan0_0 brief Interface lan0_0 state Up: yes Interface type: ethernet Speed: UNKNOWN Duplex: UNKNOWN MTU: 1500 HW address: 00:0E:B6:8C:7C:42 Link: no SH # show interface wan0_0 brief Interface wan0_0 state Up: no Interface type: ethernet Speed: UNKNOWN Duplex: UNKNOWN MTU: 1500 HW address: 00:0E:B6:8C:7C:41 Link: no
The LSP feature still works when you have multiple Steelhead appliances in a serial cluster.
During installation a lot of swapping of cables might happen until
everything is working properly. It could be easier to disable LSP at
this time with the command
no in-path lsp enable
so that the persons on-site have direct visual feedback about link
states when cables are plugged in. When all the cables are plugged
in and all the Ethernet links are established, then LSP should be
enabled again with the command
in-path lsp enable
.