Backdoor Link
Last updated
Last updated
Load isis.inter.area.cfg
Configure an additional link between R1 and XR2, using vlan 120 with subnet 20.1.20.0/24. Configure this link so that area 49.1234 and 49.1920 can reach each other directly without traversing the L2 backbone. Do not let traffic destined for any other areas run over this link.
The backdoor feature configures a L1/L2 router to do two things:
Not set the ATT bit in L1 LSPs
Continue installing a default route via other L1 routers that have the ATT bit set
On IOS-XE this is configured with is-type level-1-2 backdoor. On IOS-XR this is configured using attached-bit send never-set.
It seems that this feature leaked L2 routes into L1 in past software releases, but this does not seem to happen by default on current software. Nevertheless we can manually do this with L2 into L1 redistribution on the two routers. We don’t need to worry about leaking any other L2 routes, because this is a completely separate, disjointed L2 adjacency, only between these two routers.
On R1, notice that it now has an L2 adjacency but it does not set the ATT bit in its L1 LSP. It is not truly attached to the L2, it is just running a standalone backdoor link with another area.
On R2, notice that it now learns L1 inter-area routes to area 49.1920 prefixes:
However, R3 and R4 do not learn these prefixes. This is because they are truly attached to the L2, so they reject L1 routes that have the Up/Down bit set.
Likewise, XR1 rejects the area 49.1234 inter-area L1 prefixes.
However, L1-only routers in each area can use the backdoor link to directly reach each other.
