- Task
Set the overload bit on routers XE7, XE5, and XR1:
- On XE7, set overload bit on startup, wait for BGP, and supress all external routes
- On XE5, set overload bit right now, and advertise all prefixes
- On XR1, set overload bit on startup, wait for BGP, and suppress both inter-area and external routes
- Answer
XE7
router isis
set-overload-bit on-startup wait-for-bgp suppress external
XE5
router isis
set-overload-bit
XR1
router isis 1
set-overload-bit on-startup wait-for-bgp
- Explanation
The overload bit in ISIS is set in a router’s LSP. It can be displayed using show isis database. For example, verify that XE5 is currently setting the OL bit in its LSP:
ATT/P/OL means Attached bit/Partition repair/Overload bit. Parition repair is not implemented in any modern ISIS implementation. The attached bit is used in a L1 LSP to signal that the router is attached to the L2 backbone.
The original purpose of the OL bit was for an ISIS router to indicate that it has run out of memory, and that its LSDB might be out of sync. Therefore, other routers should not use this router as transit in their paths. Note that when the OL bit is set, the router is never used for transit, even if no other paths exist. The node can still be used to reach leaf prefixes advertised directly from the node itself, such as its own loopback.
- Verification
XE7 should no longer have ECMP routes to XR1. It should only be using XE6 for transit. It will only use XE5 for traffic XE5 itself is advertising - its loopback and connected routes.
Note that the OL bit does not set XE5’s metrics to max, as it works in OSPF. Instead, the OL means “don’t use me for transit.” The OL bit does not impact the actual link metrics. Also note that even if no other paths exist, a node with the OL bit set will never be used for transit.
Neither XE7 or XR1 have set the OL bit yet. They will on startup, and they will clear the bit once BGP sessions come up.
I tried to verify that XRv1 will wait for BGP, but it doesn’t seem to work. Instead, simply doing on-startup seconds does work. The XRv will set OL=1 upon bootup, and will wait the specified amount of time before clearing the OL bit.
Nenhum comentário:
Postar um comentário