Bgp; Is-Is; Ospf And Ospfv3 - Juniper EX9200 Features Manual

Chapter 10: Understanding How Graceful Restart Enables Uninterrupted Packet Forwarding When a Router Is Restarted

BGP

IS-IS

OSPF and OSPFv3

Copyright © 2017, Juniper Networks, Inc.
PIM Sparse Mode on page 68
RIP and RIPng on page 69
When a router enabled for BGP graceful restart restarts, it retains BGP peer routes in its
forwarding table and marks them as stale. However, it continues to forward traffic to
other peers (or receiving peers) during the restart. To reestablish sessions, the restarting
router sets the "restart state" bit in the BGP OPEN message and sends it to all participating
peers. The receiving peers reply to the restarting router with messages containing
end-of-routing-table markers. When the restarting router or switch receives all replies
from the receiving peers, the restarting router performs route selection, the forwarding
table is updated, and the routes previously marked as stale are discarded. At this point,
all BGP sessions are reestablished and the restarting peer can receive and process BGP
messages as usual.
While the restarting router does its processing, the receiving peers also temporarily retain
routing information. When a receiving peer detects a TCP transport reset, it retains the
routes received and marks the routes as stale. After the session is reestablished with the
restarting router or switch , the stale routes are replaced with updated route information.
Normally, IS-IS routers move neighbor adjacencies to the down state when changes
occur. However, a router enabled for IS-IS graceful restart sends out Hello messages
with the Restart Request (RR) bit set in a restart type length value (TLV) message. This
indicates to neighboring routers that a graceful restart is in progress and to leave the
IS-IS adjacency intact. The neighboring routers must interpret and implement restart
signaling themselves. Besides maintaining the adjacency, the neighbors send complete
sequence number PDUs (CSNPs) to the restarting router and flood their entire database.
The restarting router never floods any of its own link-state PDUs (LSPs), including
pseudonode LSPs, to IS-IS neighbors while undergoing graceful restart. This enables
neighbors to reestablish their adjacencies without transitioning to the down state and
enables the restarting router to reinitiate a smooth database synchronization.
When a router enabled for OSPF graceful restart restarts, it retains routes learned before
the restart in its forwarding table. The router does not allow new OSPF link-state
advertisements (LSAs) to update the routing table. This router continues to forward
traffic to other OSPF neighbors (or helper routers), and sends only a limited number of
LSAs during the restart period. To reestablish OSPF adjacencies with neighbors, the
restarting router must send a grace LSA to all neighbors. In response, the helper routers
enter helper mode and send an acknowledgement back to the restarting router. If there
are no topology changes, the helper routers continue to advertise LSAs as if the restarting
router had remained in continuous OSPF operation.
When the restarting router receives replies from all the helper routers, the restarting
router selects routes, updates the forwarding table, and discards the old routes. At this
point, full OSPF adjacencies are reestablished and the restarting router receives and
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