Fault-Tolerant IP and MPLS Networks

Review of Chapter 6 Concepts 167

Figure 7-2 Detrimental Effects of BGP with MPLS Control-Plane Restart

For BGP with MPLS restart, to avoid negative impacts on the MPLS forwarding plane you need

to extend the BGP graceful restart mechanism to account for label information. The BGP restart

procedure that considers the MPLS forwarding state in the graceful restart capability exchange

is referred to as the BGP with an MPLS graceful restart mechanism.

Review of Chapter 6 Concepts

Because this chapter extends the BGP restart concepts of Chapter 6 to MPLS, this section

contains a summary of the relevant concepts from that chapter. Chapter 6 described the harmful

effects of the BGP control-plane restart on the IP control and forwarding planes. You learned

that when BGP in a router restarts, the restarting router loses its BGP sessions. On discovering

a BGP session failure with the restarting router, neighbors withdraw routes derived from the

failed BGP session. If the BGP restarts quickly, after restarting the router attempts to reestablish

BGP sessions with peers and exchange routing information. The peers relearn routes from the

restarting router and as a result re-advertise routes that were withdrawn a short time before.

A route withdrawal that is abruptly followed by re-advertisement is termed a route flap. As

discussed previously, BGP route flaps are undesirable for a number of reasons, including the

initiation of forwarding that leads to packet loss and the injection of extra control traffic that

consumes link bandwidth and control processor resources.

Chapter 6 also discussed the main reason that BGP neighbors withdraw routes from the

restarting peer—to avoid forwarding data traffic into the black hole created by the restarting

router. The assumption is that the restarting router is incapable of preserving the forwarding

state across the restart and hence cannot forward across the restart. Under this assumption, the

correct course of action is to withdraw failed routes as soon as possible to inform other BGP

Forwarding state lost.

Data traffic disrupted.

LSR1 detects the failure.

Removes the forwarding state.

Withdraws routes/label from peers.

Ripple effect continues

(not shown here).

LSR1

LSR2

BGP Session

BGP BGP

FIB LFIB

LSP

BGP in LSR2 restarts

and loses BGP sessions.

FIB LFIB

Forwarding state removed.

Data traffic disrupted.

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