Juniper IGP - CONFIGURATION GUIDE V11.1.X, Configuration Manual

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bit in the LSP header, and transmits the LSP with the current neighbor information.
By sending the updated LSP with the overload bit set immediately after forming the
first adjacency, IS-IS reduces the convergence time across the network.
If IS-IS waits for all adjacencies to be up before it sends the updated LSP with the
overload bit set, the other routers in the domain still have the transit router's old LSP
and continue to forward transit traffic to the transit router until all adjacencies are
formed. That traffic is lost.
Waiting for BGP Convergence
When BGP converges, the transit router again has the reachability information it
needs to forward traffic to destinations that are not directly connected. Typically,
you then want the transit router to clear the overload bit in its LSP and retransmit
the LSP to inform the other routers in the domain that they can use it as a transit
router.
BGP is assumed to have converged when all of the following conditions have been
met:
■
90 percent of BGP peers have reached an established state,
■
The transit router has received an end-of-rib marker from all IBGP peers that
advertise the graceful-restart capability.
■
The average rate of learning new routes has dropped to a low level.
Example Topology
Figure 21 on page 370 shows a sample topology where source end system A is
communicating with destination end system B through routers 1, 2, 3, and 4.
Figure 21: Transit Router Topology
The transit routers, 2 and 3, learn the route to B from BGP. In a steady state
environment, the BGP routing tables are synchronized on all the transit routers.
Suppose the traffic forwarding path is currently A –> 1 –> 2 –> 4 –> B. If transit
router 2 goes down, the network converges to the alternative path, A –> 1 –> 3
370 ■
Configuring Global IS-IS Parameters
JUNOSe 11.0.x IP, IPv6, and IGP Configuration Guide