Tag Archives: CSNP

Database Granularity in OSPF vs IS-IS ..

This post compares how the two link state protocols hold their routing information in their databases as this affects their behavior in how they flood/distribute the change of routing information and the internal implementation complexity.


o Organization of Routing Information

OSPF encodes the routing information into small chunks, which it calls Link State Advertisement (LSA). Each LSA has its own 20-byte header in order to be identified uniquely. This header is called the LSA Header. There is no limitation on the size of a LSA, though the actual LSA size is limited by IP packet size limitation: 65,535 bytes minus the LSA Header size and IP packet header size. The database access in OSPF is per LSA basis.

In OSPF routing, the information within an area is described by type 1 and type 2 LSAs (known as Router-LSA and Network-LSA respectively). These LSAs can become big depending upon the number of adjacencies to be advertised and prefixes to be carried inside an area. In other words, the routing information with respect to a single node (either router or network node) is encoded inside a single LSA. On the other hand, each inter-area or external prefix is advertised in a separate LSA (AS-External LSA).

An OSPFv2 router may originate only one Router-LSA for itself, while in OSPFv3, a router is allowed to originate multiple Router-LSAs. A router may originate a Network-LSA for each IP subnet on which the router acts as a designated router (DR). A router may originate one LSA for each inter-area and external prefix, with no limitations on the number of LSAs that it may originate.


Originating a new and a unique LSA for each inter-area route and an external prefix implies that there is a LSA Header overhead involved while the information is kept in the database or is flooded to the neighbors. There is thus some extra memory and bandwidth consumed in total.

o Carrying Routing Information

LSAs are carried in Link State Update packets (called LS Updates or LSUs). Each LS Update packet has its own header, consists of a 24 byte OSPF protocol header, and a 4-bytes field indicating the number of LSAs contained in the packet. Thus multiple LSAs can be packed into a single LS Update packet. Some implementations may not do this as its considered difficult achieving this during flooding.


In the face of network changes, OSPF floods only the updated LSAs. Therefore, even if an implementation does not pack multiple LSAs into a single LS Update packet (and so bandwidth is consumed by LS Update header for each update of a single LSA), the bandwidth consumption for each network change can be considered adequately small.


o Organization of the Routing Information

In IS-IS, protocol packets are called Protocol Data Units or PDUs. IS-IS encodes the link state information into the set of TLVs and packs these TLVs into one or more Link State PDUs (LSPs). The size limit of a LSP is configurable. The Routing database consists of these PDUs and the access to the database is per PDU basis. The original IS-IS specification places an upper bound on the number of LSPs a router can originate to 255. There are however techniques which enable a router to originate more than 255 LSPs, by using multiple system-id’s for itself.


Since routing information in IS-IS for each router is packed in fewer LSPs, the memory consumed for bookkeeping of the routing data within the database is less and is more efficient.

o Carrying Routing Information

Each LSP is flooded independently, without being modified all the way from the originator through the routers till the very end. This results in all the routers having the same LSPs as that originated by the first router.


Since LSPs are not modified in any way and are not allowed to be fragmented, in order to be flooded successfully over all links existing in the IS-IS network, great care must be ensured when configuring the size limit of LSP that routers can originate and receive.

If the size limit of the LSP is set without taking into account the minimum value of the MTUs throughout the network, or if the size limit of LSPs conflict among some the routers in the network, the database synchronization may not be achieved, and this can result in routing loops and/or blackholes.

When a change occurs to a LSP, the whole LSP needs to be flooded, and therefore the bandwidth usage can be non-optimal. There is however a solution which exists in theory. If an implementation finds some of the entities to be flapping, then they may be packed into smaller LSPs or may be isolated from the other stable entities. This way one needs to only advertise the unstable LSP/LSPs. I have not btw come across any implementation that does that. Leave a comment if you know one that does this!

Database granularity also affects when two routers need to synchronize their databases. In OSPF, because of its high database granularity there are a lot of items which it needs to synchronize and that process is somewhat complicated with a lot of DBD packets being exchanged back and forth. This gets worse if the router trying to sync is being inundated with a lot of other data traffic also. This is not much of an issue these days as any router worth its salt would prioritize the OSPF control packets.

This is however much simpler in case of IS-IS and there isn’t any finite state machine that the neighbors need to go through to synchronize their databases. It just uses it regular flooding mechanism (a couple of CSNPs describe their entire topology information) to exchange its entire database. You plug in the new IS-IS router and before you realize the router is already sync’ed up with all the other IS-IS routers in the network!

Checks on HELLOs for OSPF and IS-IS during Adjacency Formation ..

The HELLOs (or IIHs in IS-IS parlance)  are responsible bringing up the adjacencies between the two (or multiple) routers. Forming adjacencies is an integral part of all link state routing protocols as all protocol packets other than HELLOs are flooded only over the adjacencies. The rules for formation of such adjacencies however differ between IS-IS, OSPF v2 and OSPF v3.


Besides the basic checks to verify the integrity of the packet, IS-IS does a few checks before forming any adjacency upon receiving the IIHs.

o It allows multiple area addresses to be configured on a router. During the IIH exchange the adjacency is formed only if at least one of the area address matches. The advantage of having multiple areas is explained in the further posts. NOTE that Level 2 only adjacencies would be formed even if the area addresses are not matching.

o To prevent the LSPs and CSNPs from being dropped due to different values for originatingLSPBufferSize and ReceiveLSPBufferSize, all IIHs are padded till the maximum MTU when the adjacency comes up. This check verifies consistent settings between the adjacent routers. This is however not a sufficient check.

o Adjacencies are formed without regard to interface addressing or asymmetric HOLD timer values. Values of IIH interval are not sent in IIH packets. While the IS-IS protocol provides sufficient routing information for relaying packets between adjacent routers, many implementations nonetheless require ARP support to do this. These implementations typically refuse to form an adjacency unless the neighbor interface IP address is on the local interface’s IP subnet.

o IS-IS can carry addressing information of different protocols in its TLV’s. However, the protocol supported field must be sent in Dual and IP-Only routers. RFC1195 specifies no checks for the protocol supported field for adjacency formation. It places topology restrictions on multi-protocol networks. In networks that conform to these restrictions, neighboring routers will always have a protocol in common. Therefore, it does not state whether adjacency formation should take protocols supported into account. However, many implementations, do not form an adjacency with a neighbor unless they share at least one protocol in common.

o Not matching Hold Timer values has advantages wherein the administrator can set different Hold times for different routers. This helps in cases where the going down of a DIS or some router needs to be detected faster. For such routers the hold timer can be set to a lower value.


The checks for formation of adjacencies are stricter in OSPFv2 as compare to that of IS-IS.

o The area-id of the received packet should always match the incoming interface (with the exception of virtual links). Area type is strictly checked by checking the E-bit (not set for non-default areas) and the N- bit (not-set for non-NSSA areas).

o The values of the Hello interval, the Router Dead Interval and network mask received in Hellos are matched with those on the configured interface. Any mismatch in the values causes the Hello packet to be dropped and hence prevents formation of adjacencies. The disadvantage of this approach is that Hello Interval and Router Dead Interval changes need to be done within the Router Dead Interval, to prevent breaking adjacencies. The advantage is we would not form adjacency in case there is a router that has been mis-configured with a large value and which could cause problems later. The network mask check however does not apply to point to point links. That allows the two ends of a Point-to-Point link to have different addresses.

o MTU check is not done in the hellos. It is done in the during the Database (DB) Exchange process.


Most of the checks for OSPFv3 are similar to that of OSPFv2.

o OSPFv3 runs on a per link basis instead of a per subnet basis. The check for a network mask is thus not done.

o Instance ID field (non-existent in OSPFv2) on the link is matched with the incoming ID in Hellos. The adjacency is formed only if the Instance ID matches. This allows multiple instances of OSPFv3 to run on a single link.