OSPF vs ISIS - The Basics

OSPF operates directly over IP, with a protocol number of 89. The source address of all OSPF messages is always the local end of an adjacency, and all messages are either multicast to one of two reserved multicast addresses,224.0.0.5 and 224.0.0.6, or unicast to the distant end of an adjacency. At no time does OSPF broadcast its

messages.

IS-IS operates not over the network layer like OSPF, but over the data link layer. But like OSPF, IS-IS messages are either unicast or multicast never broadcast. The source address of IS-IS messages is always the data link layer address (the MAC address, for example) of the local end of the adjacency, and the destination address is either the data link layer address of the distant end of the adjacency or, on broadcast media such as Ethernet, one of two reserved multicast MAC addresses: 0180:c200:0014 or 0180:c200:0015.

Because it runs over IP, OSPF can be and has been the target of spoofing and denial-of-service (DoS) attacks. Attacking IS-IS requires direct access to a network link or router.

Because IS-IS is not an IP protocol, prioritizing its messages is more problematic. Some router manufacturers, such as Juniper Networks and Cisco Systems, use proprietary internal mechanisms to tag IS-IS messages in such a way that they can be added to the same network control queue as OSPF

OSPF uses five message types:

1. Hello
2. Database Description (DD)
3. Link State Request
4. Link State Acknowledgement
5. Link State Update

IS-IS uses four basic message types:

1. IS-IS Hello (IIH)
2. Complete Sequence Number PDU (CSNP)
3. Partial Sequence Number PDU (PSNP)
4. Link State PDU (LSP)

Unlike OSPF, IS-IS messages have subtypes. There are LAN and Point-to-Point Hellos, used as the names imply on either broadcast or point-to-point media. The LAN Hellos are also subdivided into level 1 and level 2 types and are sent over level 1 and level 2 adjacencies. Likewise, Sequence Number PDUs (CSNPs and PSNPs) and LSPs are also subdivided into level 1 and level 2 types. So although there are only four basic types of IS-IS messages, when divided by function, there are nine actual types:

1. Level 1 LAN IIH
2. Level 2 LAN IIH
3. Point-to-Point IIH
4. Level 1 CSNP
5. Level 2 CSNP
6. Level 1 PSNP
7. Level 2 PSNP
8. Level 1 LSP
9. Level 2 LSP

Each router must be able to uniquely identify itself within the routing domain. This is the purpose of the OSPF router ID
(RID) and the IS-IS system ID (SysID). In addition, the router must be able to identify its general position within the routing domain. This is the purpose of the area ID (AID).


A loopback interface is a logical interface it exists only in software and has no physical presence on the router it is not susceptible to physical failures. So, there is no risk that an interface failure or shutdown on a router could force


OSPF to find a new RID and re-advertise its LSAs using the new RID, which in turn causes SPF runs on routers throughout the area and contributes to network instability.


There are two approaches by which a particular OSPF implementation could handle the loss of a RID. One approach is that the failure of an interface will have no effect on the RID. After all, the OSPF process just needs to know some 32-bit value, with some confidence that the value is unique within the OSPF routing domain, to use as its


RID at start-up. Once the value is known, it can be remembered, and the subsequent failure of the interface from which the RID was derived is irrelevant. The problem with this approach is that the loss of an IP address on an interface might not have been accidental. What if the IP address is intentionally removed from an interface and is reused on another router, and that router selects that IP address as its own RID? If the first router retains the same address as its RID, you now have duplicate RIDs in your network.
The second approach avoids the problem just described and is therefore the lesser of two evils. This approach to the loss of an IP address from which the RID is derived is to force the router to acquire a new RID from its remaining IP addresses.


When you have duplicate RID’s in the network the ospf database would show different next hops for the same subnet intermittently thus it may look like a routing loop


Some implementations (Juniper)will fill in the leading 0s in
configuration of area ID’s output displays, whereas others display the configuration exactly as typed (Cisco)


If the decimal number entered for the AID is greater than 255, JUNOS expresses the binary equivalent of the number in dotted-decimal format


In contrast to the OSPF AID and RID, which are expressed separately, the IS-IS AID and SysID are specified together in the Network Entity Title(NET). The NET is a special version of an ISO network service access point (NSAP) address,