As per Relevance of the word destination, we have this rfc below:
Network Working Group D.
Request for Comments: 3277
Category: Informational April 2002
Intermediate System to Intermediate System (IS-IS
Transient Blackhole
Status of this
This memo provides information for the Internet community. It
not specify an Internet standard of any kind. Distribution of
memo is unlimited
Copyright
Copyright (C) The Internet Society (2002). All Rights Reserved
This document describes a simple, interoperable mechanism that can
employed in Intermediate System to Intermediate System (IS-IS
networks in order to decrease the data loss associated
deterministic blackholing of packets during transient
conditions. The mechanism proposed here requires no IS-IS
changes and is completely interoperable with the existing IS-
specification
1.
When an IS-IS router that was previously a transit router
unavailable as a result of some transient condition such as a reboot
other routers within the routing domain must select an
path to reach destinations which have previously transited the
router. Presumably, the newly selected router(s) comprising the
have been available for some time and, as a result, have
forwarding information bases (FIBs) which contain a full set
reachability information for both internal and external (e.g., BGP
destination networks
When the previously failed router becomes available again, it is
seconds before the paths that had previously transited the router
again selected as the optimal path by the IGP. As a result
forwarding tables are updated and packets are once again
along the path. Unfortunately, external destination
information (e.g., learned via BGP) is not yet available to
router, and as a result, packets bound for destinations not
via the IGP are unnecessarily discarded
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RFC 3277 IS-IS Transient Blackhole Avoidance April 2002
A simple interoperable mechanism to alleviate the offshoot
with this deterministic behavior is discussed below
2.
This document describes a simple, interoperable mechanism that can
employed in IS-IS [1, 2] networks in order to avoid transition to
newly available path until other associated routing protocols such
BGP have had sufficient time to converge
The benefits of such a mechanism can be realized when considering
following scenario depicted in Figure 1.
D.1
|
+-------+
| RtrD |
+-------+
/ \
/ \
+-------+ +-------+
| RtrB | | RtrC |
+-------+ +-------+
\ /
\ /
+-------+
| RtrA |
+-------+
|
S.1
Figure 1: Example Network
Host S.1 is transmitting data to destination D.1 via a primary
of RtrA->RtrB->RtrD. Routers A, B and C learn of reachability
destination D.1 via BGP from RtrD. RtrA's primary path to D.1
selected because when calculating the path to BGP NEXT_HOP of RtrD
the sum of the IS-IS link metrics on the RtrA-RtrB-RtrD path is
than the sum of the metrics of the RtrA-RtrC-RtrD path
Assume RtrB becomes unavailable and as a result the RtrC path to
is used. Once RtrA's FIB is updated and it begins forwarding
to RtrC, everything should behave properly as RtrC has
forwarding information regarding destination D.1's availability
BGP NEXT_HOP RtrD
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RFC 3277 IS-IS Transient Blackhole Avoidance April 2002
Assume now that RtrB comes back online. In only a few seconds, IS-
neighbor state has been established with RtrA and RtrD and
synchronization has occurred. RtrA now realizes that the best
to destination D.1 is via RtrB, and therefore updates it
appropriately. RtrA begins to forward packets destined to D.1
RtrB. Though, because RtrB has yet to establish and synchronize
BGP neighbor relationship and routing information with RtrD, RtrB
no knowledge regarding reachability of destination D.1, and
discards the packets received from RtrA destined to D.1.
If RtrB were to temporarily set its LSP Overload bit
synchronizing BGP tables with its neighbors, RtrA would continue
use the working RtrA->RtrC->RtrD path, and the LSP should only
used to obtain reachability to locally connected networks (
than for calculating transit paths through the router, as defined
[1]).
However, it should be noted that when RtrB goes away, its LSP
still present in the IS-IS databases of all other routers in
routing domain. When RtrB comes back it establishes adjacencies.
soon as its neighbors have an adjacency with RtrB, they
advertise their new adjacency in their new LSP. The result is
all the other routers will receive new LSPs from RtrA and
containing the RtrB adjacency, even though RtrB is still
its synchronization and therefore has not yet sent its new LSP
At this time SPF is computed and everyone will include RtrB in
tree since they will use the old version of RtrB LSP (the new one
not yet arrived). Once RtrB has finished establishing all
adjacencies, it will then regenerate its LSP and flood it. Then
other routers within the domain will finally compute SPF with
correct information. Only at that time will the Overload bit
taken into account
As such, it is recommended that each time a router establishes
adjacency, it will update its LSP and flood it immediately,
before beginning database synchronization. This will allow for
Overload bit setting to propagate immediately, and remove
potential for an older version of the reloaded routers LSP to
used
After synchronization of BGP tables with neighboring routers (
expiry of some other timer or trigger), RtrB would generate a
LSP, clearing the Overload bit, and RtrA could again begin using
optimal path via RtrB
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RFC 3277 IS-IS Transient Blackhole Avoidance April 2002
Typically, in service provider networks IBGP connections are done
peerings with 'loopback' addresses. As such, the newly
router must advertise its own loopback (or similar) IP address,
well as associated adjacencies, in order to make the
accessible to other routers within the routing domain. It is
of this that simply flooding an empty LSP is not sufficient
3. Deployment
Such a mechanism increases overall network availability and
network operators to alleviate the deterministic blackholing
introduced in this scenario. Similar mechanisms [3] have
defined for OSPF, though only after realizing the usefulness
from that of the IS-IS Overload bit technique
This mechanism has been deployed in several large IS-IS networks
a number of years
Triggers for setting the Overload bit as described are left to
implementer. Some potential triggers could perhaps include "
seconds after booting", or "N number of BGP prefixes in the BGP Loc
RIB".
Unlike similar mechanisms employed in [3], if the Overload bit is
in a router's LSP, NO transit paths are calculated through
router. As such, if no alternative paths are available to
destination network, employing such a mechanism may actually have
negative impact on convergence (i.e., the router maintains the
available path to reach downstream routers, but the Overload
disallows other nodes in the network from calculating paths via
router, and as such, no feasible path exists to the routers).
Finally, if all systems within an IS-IS routing domain haven'
implemented the Overload bit correctly, forwarding loops may occur
4. Potential
Alternatively, it may be considered more appealing to
something more akin to [3] for this purpose. With this model,
transient conditions a node advertises excessively high link
to serve as an indication, to other nodes in the network that
transiting the router are "less desirable" than existing paths
The advantage of a metric-based mechanism over the Overload
mechanism model proposed here is that transit paths may still
calculated through the router. Another advantage is that a metric
based mechanism does not require that all nodes in the IS-IS
correctly implement the Overload bit
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RFC 3277 IS-IS Transient Blackhole Avoidance April 2002
However, as currently deployed, IS-IS provides for only 6 bits
space for link metric allocation, and 10 bits aggregate path metric
Though extensions proposed in [4] remove this limitation, they
not yet been widely deployed. As such, there's currently
flexibility when using link metrics for this purpose. Of course
both methods proposed in this document are backwards-compatible
5. Security
The mechanisms specified in this memo introduces no new
issues to IS-IS
6.
The author of this document makes no claim to the originality of
idea. Thanks to Stefano Previdi for valuable feedback on
mechanism discussed in this document
7.
[1] ISO, "Intermediate system to Intermediate system
information exchange protocol for use in conjunction with
Protocol for providing the Connectionless-mode Network
(ISO 8473)," ISO/IEC 10589:1992.
[2] Callon, R., "OSI IS-IS for IP and Dual Environment," RFC 1195,
December 1990.
[3] Retana, A., Nguyen, L., White, R., Zinin, A. and D. McPherson
"OSPF Stub Router Advertisement", RFC 3137, June 2001.
[4] Li, T. and H. Smit, "IS-IS extensions for Traffic Engineering",
Work in Progress
8. Author's
Danny
Phone: 303.470.9257
EMail: danny@tcb.
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RFC 3277 IS-IS Transient Blackhole Avoidance April 2002
9. Full Copyright
Copyright (C) The Internet Society (2002). All Rights Reserved
This document and translations of it may be copied and furnished
others, and derivative works that comment on or otherwise explain
or assist in its implementation may be prepared, copied,
and distributed, in whole or in part, without restriction of
kind, provided that the above copyright notice and this paragraph
included on all such copies and derivative works. However,
document itself may not be modified in any way, such as by
the copyright notice or references to the Internet Society or
Internet organizations, except as needed for the purpose
developing Internet standards in which case the procedures
copyrights defined in the Internet Standards process must
followed, or as required to translate it into languages other
English
The limited permissions granted above are perpetual and will not
revoked by the Internet Society or its successors or assigns
This document and the information contained herein is provided on
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED,
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE
Funding for the RFC Editor function is currently provided by
Internet Society
McPherson Informational [Page 6]
if you see any problems within the linking, don't worry be happy,
this is version 0.1 of the Relevance System and you gotta expect some crappy subroutines sometimes,
just be content we did not write this in Java, which would have made this "bigger and better" HAHAHHA.
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