Transcript Document

• OSPF
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IS-IS - Comparison with OSPF
Both IS-IS and OSPF are link state
protocols, and both use the same
Dijkstra algorithm for computing the
best path through the network. As a
result, they are conceptually similar.
Both support variable length subnet
masks, can use multicast to discover
neighboring routers using hello
packets, and can support
authentication of routing updates.
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IS-IS - Comparison with OSPF
1
To operate with IPv6 networks, the
OSPF protocol was rewritten in OSPF
v3 (as specificed in RFC 2740).
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IS-IS - Comparison with OSPF
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IS-IS differs from OSPF in the way that "areas" are
defined and routed between
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IS-IS - Comparison with OSPF
In OSPF, areas are delineated on the
interface such that an area border
router (ABR) is actually in two or more
areas at once, effectively creating the
borders between areas inside the ABR,
whereas in IS-IS area borders are in
between routers, designated as Level 2
or Level 1-2. The result is that an IS-IS
router is only ever a part of a single
area.
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IS-IS - Comparison with OSPF
IS-IS also does not require Area 0 (Area
Zero) to be the backbone area through which
all inter-area traffic must pass. The logical
view is that OSPF creates something of a
spider web or star topology of many areas all
attached directly to Area Zero and IS-IS by
contrast creates a logical topology of a
backbone of Level 2 routers with branches of
Level 1-2 and Level 1 routers forming the
individual areas.
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IS-IS - Comparison with OSPF
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IS-IS also differs from OSPF in the
methods by which it reliably floods
topology and topology change
information through the network.
However, the basic concepts are
similar.
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IS-IS - Comparison with OSPF
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Given the same set of resources, IS-IS can
support more routers in an area than OSPF
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Open Shortest Path First - OSPF router types
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The router type is an attribute of an OSPF
process. A given physical router may have
one or more OSPF processes. For
example, a router that is connected to
more than one area, and which receives
routes from a BGP process connected to
another AS, is both an area border router
and an autonomous system boundary
router.
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Open Shortest Path First - OSPF router types
Each router has an identifier, customarily
written in the dotted decimal format (e.g.,
1.2.3.4) of an IP address. This identifier must
be established in every OSPF instance. If not
explicitly configured, the highest logical IP
address will be duplicated as the router
identifier. However, since the router identifier
is not an IP address, it does not have to be a
part of any routable subnet in the network,
and often isn't to avoid confusion.
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Open Shortest Path First - OSPF v3 Packet Formats
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The "Main OSPF Packet Header" is
the same for all 5 types of packets
(with exception of the Type field)
whereas the following sub-headers
will vary from type to type and are
shown below the Main OSPF Packet
Header.
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Open Shortest Path First - OSPF v3 Packet Formats
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Type 3: The OSPF Link State
Request Packet
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Open Shortest Path First - OSPF v3 Packet Formats
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Type 4: The OSPF Link State
Update Packet
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Open Shortest Path First - OSPF v3 Packet Formats
1
This "Options Field" is used in OSPF
Hello packets, Database Description
packets, and certain LSAs (routerLSAs, network-LSAs, inter-area-routerLSAs, and link-LSAs).
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Open Shortest Path First - OSPF v3 Packet Formats
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(Note: Previous OSPF versions {v1 & v2} DO
NOT support all of the options/fields listed
here.)
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Open Shortest Path First - OSPF v3 Packet Formats
1
x-bit: This is currently
deprecated. It was
previously used by
MOSPF.
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Open Shortest Path First - OSPF in broadcast and non-broadcast networks
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For non-broadcast multiple-access
networks (NBMA), RFC 2328 defined
the following two official modes for
OSPF:
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OSPF
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'Open Shortest Path First' ('OSPF') is a linkstate routing protocol for Internet Protocol
(IP) networks. It uses a link-state routing
protocol|link state routing algorithm and falls
into the group of interior routing protocols,
operating within a single autonomous system
(Internet)|autonomous system (AS). It is
defined as OSPF Version 2 in RFC 2328
(1998) for IPv4. The updates for IPv6 are
specified as OSPF Version 3 in RFC 5340
(2008).
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OSPF
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OSPF is perhaps the most widely used
interior gateway protocol (IGP) in large
enterprise networks. IS-IS, another linkstate dynamic routing protocol, is more
common in large service provider
networks. The most widely used
exterior gateway protocol is the Border
Gateway Protocol (BGP), the principal
routing protocol between autonomous
systems on the Internet.
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OSPF - Overview
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OSPF was designed to support variablelength subnet masking (VLSM) or
Classless Inter-Domain Routing (CIDR)
addressing models.
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OSPF - Overview
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OSPF detects changes in the topology,
such as link failures, and
Convergence (routing)|converges on a
new loop-free routing structure within
seconds. It computes the shortest path
tree for each route using a method
based on Dijkstra's algorithm, a
shortest path first algorithm.
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OSPF - Overview
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The OSPF routing policies to construct a
route table are governed by link cost
factors (external metrics) associated with
each routing interface. Cost factors may
be the distance of a router (round-trip
time), network throughput of a link, or link
availability and reliability, expressed as
simple unitless numbers. This provides a
dynamic process of traffic load balancing
between routes of equal cost.
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OSPF - Overview
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An OSPF network may be structured,
or subdivided, into routing areas to
simplify administration and optimize
traffic and resource utilization. Areas
are identified by 32-bit numbers,
expressed either simply in decimal,
or often in octet-based dot-decimal
notation, familiar from IPv4 address
notation.
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OSPF - Overview
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Each additional area must have a direct or
virtual connection to the backbone OSPF area
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OSPF - Overview
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OSPF does not use a TCP/IP transport
protocol (UDP, TCP), but is encapsulated
directly in IP datagrams with protocol
number 89. This is in contrast to other
routing protocols, such as the Routing
Information Protocol (RIP), or the Border
Gateway Protocol (BGP). OSPF handles
its own error detection and correction
functions.
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OSPF - Overview
OSPF reserves the multicast
addresses 224.0.0.5 for IPv4 or FF02::5
for IPv6 (all SPF/link state routers,
also known as AllSPFRouters) and
224.0.0.6 for IPv4 or FF02::6 for IPv6
(all Designated Routers, AllDRouters),
as specified in RFC 2328 and RFC
5340.
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OSPF - Overview
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For routing multicast IP traffic, OSPF
supports the Multicast Open Shortest
Path First protocol (MOSPF) as defined
in RFC 1584.RFC 1584, Multicast
Extensions to OSPF, J. Moy, The Internet
Society (March 1994) Cisco does not
include MOSPF in their OSPF
implementations. PIM (Protocol
Independent Multicast) in conjunction
with OSPF or other IGPs, (Interior
Gateway Protocol), is widely deployed.
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OSPF - Overview
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The OSPF protocol, when running on
IPv4, can operate securely between
routers, optionally using a variety of
authentication methods to allow only
trusted routers to participate in
routing. OSPFv3, running on IPv6, no
longer supports protocol-internal
authentication. Instead, it relies on
IPv6 protocol security (IPsec).
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OSPF - Overview
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All IP prefix information has been
removed from the link-state
advertisements and from the Hello
discovery packet making OSPFv3
essentially protocol-independent
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OSPF - Neighbor relationships
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OSPF uses both unicast and multicast to send
hello packets and link state updates.
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OSPF - Neighbor relationships
Provided that OSPF is configured
correctly, OSPF forms neighbor
relationships only with the routers directly
connected to it
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OSPF - Area types
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While most OSPF implementations will
right-justify an area number written in a
format other than dotted decimal format
(e.g., area 1), it is wise to always use
dotted-decimal formats
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OSPF - Backbone area
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Note that in OSPF the term Autonomous
System Boundary Router (ASBR) is
historic, in the sense that many OSPF
domains can coexist in the same Internetvisible autonomous system, RFC1996 .
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OSPF - Backbone area
All OSPF areas must
connect to the backbone
area
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OSPF - Proprietary extensions
Several vendors (Cisco, Allied Telesis,
Juniper, Alcatel-Lucent, Huawei, Quagga),
now implement the two extensions below
to stub and NSSA area. Although not
covered by RFC, they are considered by
many to be standard features in OSPF
implementations.
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OSPF - Proprietary extensions
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:Redistribution into an NSSA area creates
a special type of LSA known as type 7,
which can exist only in an NSSA area. An
NSSA ASBR generates this LSA, and an
NSSA ABR router translates it into type 5
LSA which gets propagated into the OSPF
domain.
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OSPF - Proprietary extensions
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In such a case, the ASBR does send
externals into the totally stubby area,
and they are available to OSPF
speakers within that area
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OSPF - Transit area
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A transit area is an area with two or more
OSPF border routers and is used to pass
network traffic from one adjacent area to
another. The transit area does not
originate this traffic and is not the
destination of such traffic.
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OSPF - Path preference
OSPF uses path cost as its basic
routing metric, which was defined by
the standard not to equate to any
standard value such as speed, so the
network designer could pick a metric
important to the design
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OSPF - Traffic engineering
OSPF-TE is an extension to OSPF
extending the expressivity to allow for
traffic engineering and use on non-IP
networks (RFC 3630). More information
about the topology can be exchanged
using opaque link-state
advertisement|LSA carrying type-lengthvalue elements. These extensions allow
OSPF-TE to run completely out of band of
the data plane network. This means that it
can also be used on non-IP networks,
such as optical networks.
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OSPF - Traffic engineering
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OSPF-TE is used in GMPLS networks as
a means to describe the topology over
which GMPLS paths can be established.
GMPLS uses its own path setup and
forwarding protocols, once it has the full
network map.
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OSPF - Traffic engineering
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In the Resource Reservation Protocol
(RSVP), OSPF-TE is used for recording
and flooding RSVP signaled bandwidth
reservations for label switched paths
within the link-state database.
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OSPF - Other extensions
RFC 3717 documents work in optical
routing for IP, based on constraint-based
extensions to OSPF and IS-IS.
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OSPF - Internal router
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An internal router is a router that has
OSPF neighbor relationships with
interfaces in the same area.
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OSPF - Designated router
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Do not confuse the
DR with an OSPF
router type
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OSPF - Designated router
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* If the priority setting on an OSPF router
is set to 0, that means it can NEVER
become a DR or BDR (Backup Designated
Router).
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OSPF - Designated router
* The priority values range between 0 255,
http://www.cisco.com/en/US/docs/ios/iprou
te_ospf/command/reference/iro_cr_book.p
df with a higher value increasing its
chances of becoming DR or BDR.
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OSPF - Designated router
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* IF a HIGHER priority OSPF router
comes online AFTER the election has
taken place, it will not become DR or
BDR until (at least) the DR and BDR
fail.
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OSPF - Designated router
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DRs and BDRs are
always setup/elected
on OSPF broadcast
networks
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OSPF - Backup designated router
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A backup designated router (BDR) is a
router that becomes the designated
router if the current designated router
has a problem or fails. The BDR is the
OSPF router with second highest
priority at the time of the last election.
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OSPF - OSPF v3 Packet Formats
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The Main OSPF Packet Header is the
same for all 5 types of packets (with
exception of the Type field) whereas
the following sub-headers will vary
from type to type and are shown below
the Main OSPF Packet Header.
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OSPF - OSPF v3 Packet Formats
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::This Options Field is used in OSPF Hello
packets, Database Description packets,
and certain LSAs (router-LSAs, networkLSAs, inter-area-router-LSAs, and linkLSAs).
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OSPF - OSPF v3 Packet Formats
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::(Note: Previous OSPF versions DO NOT
support all of the options/fields listed here.)
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OSPF - OSPF v3 Packet Formats
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:::x-bit: This is currently
deprecated. It was
previously used by
MOSPF.
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OSPF - Implementations
* Allied Telesis
implements OSPFv2
OSPFv3 in Allied Ware
Plus (AW+)
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OSPF - Implementations
* Bird Internet routing
daemon|BIRD implements
both OSPFv2 and OSPFv3
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OSPF - Implementations
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* GNU Zebra, a GNU General Public License|GPL
routing suite for Unix-like systems supporting OSPF
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OSPF - Implementations
* NetWare
implements OSPF in
its Multi Protocol
Routing module.
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OSPF - Implementations
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* OpenBSD includes an
OpenOSPFD
implementation within the
OpenBGPD protocol.
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OSPF - Implementations
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* XORP, a routing suite implementing RFC2328
(OSPFv2) and RFC2740 (OSPFv3) for both IPv4
and IPv6
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OSPF - Implementations
* Windows NT 4.0 Server, Windows
2000 Server and Windows Server 2003
implement OSPFv2 in the Routing and
Remote Access Service, although the
functionality was removed in Windows
Server 2008.
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OSPF - Applications
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There are some historical accidents
that made IS-IS the preferred IGP for
ISPs, but ISP's today may well choose
to use the features of the now-efficient
implementations of OSPF, after first
considering the pros and cons of IS-IS
in service provider environments.
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OSPF - Applications
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As mentioned, OSPF can provide better
load-sharing on external links than other
IGPs. When the default route to an ISP is
injected into OSPF from multiple ASBRs
as a Type I external route and the same
external cost specified, other routers will
go to the ASBR with the least path cost
from its location. This can be tuned further
by adjusting the external cost.
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OSPF - Applications
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The only real limiting factor that may
compel major ISPs to select IS-IS over
OSPF is if they have a network with
more than 850 routers. There is
mention of an OSPF network with over
1000 routers, but that is quite
uncommon and the network must be
specifically designed to minimize
overhead to achieve stable operation.
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Link-state advertisement - OSPF v3 LSA Packet Header Formats
As per [
http://tools.ietf.org/html/rfc5340#appen
dix-A.4 Appendix A.4] of RFC 5340
(OSPFv3 for IPv6) depending upon the
LS Type, there are nine major LSA
Packet formats as follows (actually
eight as one has been deprecated):
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