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LSA/TLV TLV shares common Headers Different LSA for different routes.
PPP Different control protocol negotiate IPCP negotiate between the peers.
ICPC being negotiate for the IP traffic and OSICP being negotiate for the ISIS traffic.
Interface Type P2P, Broadcast, Unnumbered Broadcast P2P, Broadcast, P2MP, NBMA
Datastructure Data structure of ISIS is TLV ( Time Length Value ). OSPF Datastructure called as LSA. And we have different types of LSA.
Generally ISIS designed in such a way that if it receive any TLV it does not understand, then it will flood to the neighboring router. Where as if a LSA which is received by a router, If doesn’t understand then it will drop the packet.
Security can never be routed beyond the immediate next hop and hence  shielded from IP spoofing and similar Denial of Service attacks It is possible to attack router running OSPF remotely.
Scalability So ISIS is more scalable as it inhenritently  separate the topology and IP addressing seperately. So any change in the IP addressing will be invisible to the topology calculation. OSPF mix the topology and the addressing in the same LSA and every time its change IP addresses it calculate SPF calculation in it.
ISIS do PRC OSPF do full SPF calculation
If a link flap then ISIS will do only PRC If a link flap OSPF will do a complete SPF when there is a change in the network.
ISIS has decoupled addressing OSPFv3 have decoupled addressing
SPF Calculation SPF for a given level is computed in a single phase by taking all ISIS LSP TLV togther. Performed in three phases.  First the intra area routes by building the shortest path trees. The second phase calculates the inter area routes by examine the summary LSA and last to examine the external LSA.
In ISIS a single can have both Level 1 and Level 2 In OSPF a single link can be in area 0 or in non backbone area
Design Difference In ISIS the router is the member of a area In OSPF the links are the member of a area not the router.
Route Propagation Level 1 to Level 2 route will propagate, but vice versa by default it will not In OSPF all the route will propagate in each and every area.
Default route In ISIS we need to configure in all the level 1 router to generate default route. Only one command to generate a default route into the non backbone area.
IS-IS runs directly over layer 2 and hence   – cannot support virtual links unless some explicit tunneling is implemented Supports virtual link
Packets are intentionally kept small so that they don’t required hop-by-hop fragmentation Support IP fragmentation services
Fragmentation The concern regarding scalability of the link state routing protocols is the flooding and it is belived that preventing fragmentation during flooding is the reason why ISIS fragments only at the originating router. OSPF does not provide any explicit fragmentation is necessary, IP fragmentation/ reassembly is used. OSPF packets has been designed so that large protocol packets can be generally be split into several smaller protocol packets.
Designated router concept DR election is  deterministic DR election is non deterministic
Explanation : Deterministic DIS election makes the possibiity of predecting the router that will be elected as DIS from the same set of router. ISIS DIS can be preempted at any time by a router with higher priroity coming alive. OSPF DR : After a router elected as a DR no other router can take over the position unless the original router goes down. When ever the router comes up it accept the DR if it has the highest priroity.
Back up DR/DIS A backup DIS is redundant in ISIS beacue all the routers are sync with each other and also because of shorter hello interval used by the DIS allow for faster detection of failures and subsequent replacement of DIS. The presence of BDR make the replacement of of the DR transperent in case of DR goes down.
LSA / LSP Flushing When LSP purged the header alone is flooded with the remaining lifetime set to 0 and  the value of the checksum set to 0. When a LSA is flushed the age is set to maxAge and the LSA flooded.
ISIS spec allows in certain cases for non self  originated the LSP  to be purged OSPF only allows self originated LSA to be flushed.


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