Autoroute Announce Metric (XE)

Load mpls.te.autoroute.announce.metric.init.cfg

#IOS-XE
config replace flash:mpls.te.autoroute.announce.metric.init.cfg

#IOS-XR
configure
load bootflash:mpls.te.base.config.with.isis.cfg
commit replace
y

The link between CSR9 and XRv12 is shut down.

A TE tunnel is setup from CSR8 to CSR10 via the path CSR8-CSR9-CSR10. Use autoroute relative metric so the tunnel is not used to get to 11.11.11.11

Answer

When you use autoroute announce, the tunnel becomes an available link to the local IGP (which in this case is ISIS). The metric of the tunnel takes the IGP best cost to the tunnel destination, even if the tunnel goes over what would be a higher IGP cost path.

The tunnel is available for use with ECMP if the the IP-forwarding path does not contain the destination itself.

For example, let’s use autoroute annouce on the existing tunnel and examine the RIB on CSR8.

#CSR8
int tun0
 mpls traffic-eng tunnel autoroute announce

We see that tunnel0 is used in three routes. It is used as ECMP to 5.5.5.5, ECMP to 11.11.11.11, and the only path to CSR10.

i L2     5.5.5.5 [115/30] via 132.8.9.9, 00:05:43, GigabitEthernet2.589
                 [115/30] via 10.10.10.10, 00:05:43, Tunnel0

i L2     10.10.10.10 [115/20] via 10.10.10.10, 00:05:43, Tunnel0

i L2     11.11.11.11 [115/30] via 132.6.8.6, 00:00:18, GigabitEthernet2.568
                     [115/30] via 10.10.10.10, 00:00:18, Tunnel0

No other ECMP IP path can be used to CSR10 because CSR10 is always in the IP forwarding path. Likewise, the IP path to CSR11 via CSR9 would have been another ECMP candidate, but CSR10 is in that forwarding path, so only the IP forwarding path CSR6-XRv12-XRv11 is valid for ECMP. The ECMP path for 5.5.5.5/32 via CSR9-CSR2-CSR5 is also valid.

We are asked to use relative metric to prevent the tunnel from being used on the path to 11.11.11.11/32. Right now the IGP IP path is 30 via CSR6, XRv12, XRv11. The tunnel to CSR10 has a cost of 20, plus 10 from CSR10 to XRv11. If we increase the cost of the tunnel by 1, it should no longer be available for ECMP.

#CSR8
int tunnel0
 tunnel mpls traffic-eng autoroute metric relative +1

Now the tunnel is no longer used to get to 11.11.11.11. In fact, the IGP path via CSR10 is now valid because it is shorter (20) than the tunnel (21):

i L2     11.11.11.11 [115/30] via 132.8.9.9, 00:00:02, GigabitEthernet2.589
                     [115/30] via 132.6.8.6, 00:00:02, GigabitEthernet2.568

Interestingly, the path to CSR10 itself is no longer using the tunnel, as it has a higher metric than the IGP cost to CSR10. Therefore we would probably want to use autoroute destination instead. This will create a /32 route to 10.10.10.10 via tunnel0 instead of announcing the tunnel to the local IGP process.

#CSR8
int tunnel0
 no tunnel mpls traffic-eng autoroute announce
 tunnel mpls traffic-eng autoroute destination

The use of a positive relative metric doesn’t quite make sense. If you use this, you will simply never use the tunnel in the RIB, so I don’t really see any difference compared to just not annoucing it to the IGP in the first place.