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Which Routing Protocol: Comparison between OSPF & ISIS
February 24, 2011 | By Cisco
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Transcript
Comparison between OSPF & ISIS Faraz Shamim
Technical Leader
Cisco Systems Inc
Issues and Comparison
. OSPF top down view . ISIS top down view
.OSPF is.for the most part more .IS­IS was not designed from the start “optimized” (and therefore as an IP routing protocolsignificantly more complex)
.Adjacency is reported once two­way.Uses.complex, multistate connectivity has been ensured process.to synchronize
.IS­IS essentially uses its regular
databases.between neighbors
flooding techniques to synchronize
.Intended to minimize neighbors
transient routing problems.by
. Coarse database granularity
Coarse database granularity
ensuring that a newborn
ensuring that a newborn
router has.nearly complete makes this easy (just a few routing information before it CSNPs) begins.carrying traffic .Transient routing issues can be
.Accounts.for a significant reduced (albeit nonportion of OSPF’s deterministically) by judiciousimplementation complexity use of the “overload” bit
.Partially a side effect of
granular database (requires.
many DBD packets)
Issues and Comparison
. Encapsulation . Encapsulation
OSPF runs on top of IP IS­IS runs directly over L2 (next to IP)
.Traditional IP routing
protocol approach .Sort of makes sense (ISIS was originally
..AAllows llows virtual links nks (if
virtual li(if designed for CLNS)
designed for CLNS)you like them) .Partition repair requires .Relies on IP tunneling (rarelyfragmentation for large implemented) LSAs .More difficult to spoof or .Subject to spoofingattack
and DoS attacks (use
of authentication is
strongly advised)
Terminology
OSPF:
. Host

Router Link Packet
Packet
.
Designated router (DR)
.
Backup DR (BDR)

Link­stateadvertisement(LSA)

Hello packet
.
DatabaseDescription (DBD)
ISIS:
.
End System (ES)
.
IntermediateSystem(IS)

Circuit
.
Data Un(PDU)

ProtocoProtocol l Data Unit it (PDU)
.
Designated IS (DIS)

N/A(noBDISisused)
.
Link­statePDU(LSP)

IIH PDU
.
Complete Sequence NumberPDU (CSNP)
Terminology (cont.)
OSPF:

LS update
.
LS acknowledgement Area
.
Non­backbonearea

Backbonearea
.
Area Border Router (ABR)
.
Virtual link . V

AS Boundary Router (ASBR)
.
Router ID
.
Link­state ID . N

Advertising router ID . N
ISIS:

LSP(ISISrunsover layer­2)
.
Partial Sequence Number PDU (PSNP)
.
Subdomain (area)
.
Level­1 area
.
Level­2 area

L1L2 router irtual link (not used though)
.
any IS
.
System ID
/A
/A
Packets
OSPF basic header is Commonheader is onlyfixed20bytes 8bytes
Version Type Packet Length
Router ID
Area ID
Checksum Autype
Authentication
Authentication
1 1
1 1 1 R
PDU Type
1
Which.Routing.Protocol? ⓒ2011.Cisco.Systems,.Inc..All.rights.reserved.
Packets
Packet Encoding
OSPF is “efficiently” encoded
.
Positional fields

Holy 32­bit alignment
provides tidy packet
provides tidy packet pictures,but not much else
.
Only LSAs areextensible (not Hellos, etc.)

UnrecognizedLSA types not flooded(thoughopaque LSAscan suffice, ifimplemented universally)
Which.Routing.Protocol? ⓒ2011.Cisco.Systems,.Inc..All.rights.reserved.
Packet Encoding
IS­IS is mostly Type­
Length­Value encoded
.
No particular alignment
.
Extensible from the start (unknowntypes ignored but still flooded)

All packet types areextensible
.
Nested TLVs provide structure for more granularextension(thoughbase spec doesnot use them; OSPF is starting to do so)
Packets
OSPF
5typeof basic packets
1
Hello
2
DBD
2
DBD
3
LS Request
4
LS Update
5
Link State.Ack
ISIS
. 3.types of basic packetsgranularity within
1
Hello.(3.types L1.LAN,
L2.LAN Point­to­point)
L2.LAN, Point­to­point)
2
Link statepacket (L1,L2)
3
Sequencenumberpacket (CSNP, PSNP)
Hello
OSPF:

Fixed format
.
Sent every.10 sec.by.default
default.
.
Intelligent sending on NBMA
.
Suppressed for demand circuits
ISIS:
.
TLVs.(extendable)
.
Sent every10 secsbydefault
default
.
DIS sends3 timesfaster
OSPF LSAs ISIS LSPs
.
Up to 256 LSPs per IS
.
EachLSP isconstructedwithTLVs:
Type LSA
1 Router
2 Network
3 Summary Network
4 Summary ASBR
4 Summary ASBR
5 External
6 Group Membership
7 NSSA
8 External Attributes
9.11 Opaque
TLV Purpose
2 Neighbor announcement
10 Authentication
22 Extended neighbor info(TE)
22 Extended neighbor info(TE)
128 Internal IP Routing info
129 NLPID announcement (IP)
130 External IP Routing info
132 IP Interface addresses
135 Wide scale metrics
Adjacency Establishment
OSPF:
.
LSDB synchronisation is performedbefore a neighbor isreported in the router­LSA
.
Onpop int­to­pop int links adjacenciesare established between every pair of neighbors that can see each other

On LAN segmentsadjacencies are established with the DR and BDR
.
MTU mismatch is detected
ISIS:
.
Adjacency is reported once two­way connectivity has been ensured

Point­to­point linksaretreated
the same way as in OSPF
the same way as in OSPF
.
OnLANsegments,adjacencies are established with the DIS (no BDIS is elected)

MTU mismatch is detected
Database Granularity
OSPF database node is an LSAdvertisement

LSAs are mostly numerous
and small (one external per
LSA, one summary per LSA)

Networkand Router LSAscan become large
.
LSAs grouped into
.
LSAs grouped intoLSUpdates during flooding

LSUpdates are built
individually at each hop

Small changes can yield smallpackets(but Router, NetworkLSAs can be large)
.IS­IS database node is an LSPacket

LSPs are clumps of topology information organized by the originating router
.
Always flooded intact,unchangedacrossall flooding hops (so hops (so LSP LSP MTU
flooding MTU is an architecturalconstant­­it must fit across all links)

Small topology changes always yield entire LSPs(thoughpacket size turns out to be much less of an issue than packet count)

Implementationscan
attempt clever packing
Designated Routers
Both protocols elect a designated router on multiaccess networks toremove O(N^2) link problem (by creating a pseudonode)and to reduce flooding traffic (DR ensures flooding
(DR ensures flooding
reliability)
OSPF elects both a DR and aBackup DR, each of which becomes adjacent with allother routers
.
BDR takes over if DR fails

DRship is sticky, not deterministic
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