Users Guide

Graceful Restart
Graceful restart is a protocol-based mechanism that preserves the forwarding table of the restarting router and its neighbors for a
specified period to minimize the loss of packets. A graceful-restart router does not immediately assume that a neighbor is permanently
down and so does not trigger a topology change.
Normally, when an IS-IS router is restarted, temporary disruption of routing occurs due to events in both the restarting router and the
neighbors of the restarting router. When a router goes down without a graceful restart, there is a potential to lose access to parts of the
network due to the necessity of network topology changes.
IS-IS graceful restart recognizes that in a modern router, the control plane and data plane are functionally separate. Restarting the control
plane functionality (such as the failover of the active route processor module (RPM) to the backup in a redundant configuration) should
not necessarily interrupt data packet forwarding. This behavior is supported because the forwarding tables previously computed by an
active RPM have been downloaded into the forwarding information base (FIB) on the line cards (the data plane). For packets that have
existing FIB/content addressable memory (CAM) entries, forwarding between ingress and egress ports can continue uninterrupted while
the control plane IS-IS process comes back to full functionality and rebuilds its routing tables.
A new TLV (the Restart TLV) is introduced in the IIH PDUs, indicating that the router supports graceful restart.
Timers
Three timers are used to support IS-IS graceful restart functionality. After you enable graceful restart, these timers manage the graceful
restart process.
There are three times, T1, T2, and T3.
• The T1 timer specifies the wait time before unacknowledged restart requests are generated. This is the interval before the system
sends a Restart Request (an IIH with the RR bit set in Restart TLV) until the complete sequence number PDU (CSNP) is received
from the helping router. You can set the duration to a specific amount of time (seconds) or a number of attempts.
• The T2 timer is the maximum time that the system waits for LSP database synchronization. This timer applies to the database type
(level-1, level-2, or both).
• The T3 timer sets the overall wait time after which the router determines that it has failed to achieve database synchronization (by
setting the overload bit in its own LSP). You can base this timer on adjacency settings with the value derived from adjacent routers
that are engaged in graceful restart recovery (the minimum of all the Remaining Time values advertised by the neighbors) or by setting
a specific amount of time manually.
Implementation Information
IS-IS implementation supports one instance of IS-IS and six areas.
You can configure the system as a Level 1 router, a Level 2 router, or a Level 1-2 router. For IPv6, the IPv4 implementation has been
expanded to include two new type, length, values (TLVs) in the PDU that carry information required for IPv6 routing. The new TLVs are
IPv6 Reachability and IPv6 Interface Address. Also, a new IPv6 protocol identifier has also been included in the supported TLVs. The new
TLVs use the extended metrics and up/down bit semantics.
Multi-topology IS-IS adds TLVs:
• MT TLV — contains one or more Multi-Topology IDs in which the router participates. This TLV is included in IIH and the first fragment
of an LSP.
• MT Intermediate Systems TLV — appears for every topology a node supports. An MT ID is added to the extended IS reachability
TLV type 22.
• MT Reachable IPv4 Prefixes TLV — appears for each IPv4 an IS announces for a given MT ID. Its structure is aligned with the
extended IS Reachability TLV Type 236 and it adds an MT ID.
• MT Reachable IPv6 Prefixes TLV — appears for each IPv6 an IS announces for a given MT ID. Its structure is aligned with the
extended IS Reachability TLV Type 236 and add an MT ID.
By default, Dell Networking OS supports dynamic host name exchange to assist with troubleshooting and configuration. By assigning a
name to an IS-IS NET address, you can track IS-IS information on that address easier. Dell Networking OS does not support ISO CLNS
routing; however, the ISO NET format is supported for addressing.
To support IPv6, the Dell Networking implementation of IS-IS performs the following tasks:
• Advertises IPv6 information in the PDUs.
• Processes IPv6 information received in the PDUs.
• Computes routes to IPv6 destinations.
• Downloads IPv6 routes to the RTM for installing in the FIB.
Intermediate System to Intermediate System
401