4.10. Link State Propagation (LSP)

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.

4.10.1. How to determine which interface went down first

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.

4.10.2. LSP during installation

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.