Information Technology Reference
In-Depth Information
Figure 3-4
IGP/BGP Convergence Scenario
R4
R5
AS 200
AS 65000
R3
R1
AS 100
R2
If R2 fails, the traffic reroutes, taking the longer path through R4, R5, and R3. The issue
arises when R2 returns to service. The IGP reconverges, and the next hop on R1 for prefixes
in AS 200 is R2. However, BGP has not reconverged on R2, which means that R2 does not
know how to reach any destinations in AS 200. Traffic destined for AS 200 from AS 65000
is sent to R2. However, without the necessary routing information, the traffic is discarded
until R2 learns the prefix information for AS 200.
The solution is to have a method for a router that has just rebooted to advertise in the IGP
that it should not be used as a transit router. This means that the newly rebooted router must
be a leaf on the Shortest Path Tree. The only traffic that should be sent to this router is traffic
destined for a directly connected prefix. The prefixes on the router itself must be reachable,
or the BGP sessions can't form.
The IS-IS and OSPF routing protocols both provide a transient black hole avoidance
mechanism.
IS-IS Overload Bit
The IS-IS protocol provides a feature called the
Overload bit (OL-bit)
. The OL-bit is a
value in the Link State PDU (LSP) that was originally intended to signal that the router was
having a problem, such as resource starvation, and to specify that the router should not be
included in the topology as a transit-capable router. However, the directly connected prefix
information still can be reached through a router with the OL-bit set, much like a router
acting as a multihomed host. This allows for remote router management.
The issue of IGP convergence black-holing traffic presents another use of the OL-bit. The
OL-bit can be set for the newly rebooted router, which allows the BGP sessions to form and
BGP to reconverge. The router, however, is not be used for transit. After the BGP sessions
converge, the router issues a new copy of its LSP with the OL-bit removed. The fully con-
verged router then becomes part of the transit topology.
