ISIS Continued…

Some more details about ISIS:

Packet types in ISIS :

Hello Packets : Just like other IGPs ISIS also uses hello packets to form the adjacency and as a keepalive mechanism. ISIS has many hello packets intermediate system to intermediate system hello(IIHs), End system to intermediate system hellos(EIHs- sent to mac 09-00-2B-00-00-05), Intermediate system to end system hello (IEHs- sent to mac 09-00-2B-00-00-04). IIHs are of two types L1 hellos(sent to mac 01-80-C2-00-00-14) and Level 2 hellos (sent to mac 01-80-C2-00-00-15).

LSPs(Link state packets) : Unlike OSPF in ISIS there is no different LSA types …here just one packet consists complete information, this packet called as LSP packet and sent at the beginning when adjacency formed, then also sent when any triggered update occur and also sent when the lifetime for any LSP expires.

CSNP (Complete Sequence number PDU): This packet is sent periodically on multi-access network. This PDU is similar to the DBD packet in OSPF and informs all the neighbors about the LSDB information present at the DIS(Designated intermediate system like DR in OSPF). This is sent every 10 sec on multi-access network. There is no periodic CSNP on p2p interfaces.

PSNP(Partial sequence network PDU): On receiving CSNP if the router finds any prefix information which is not present in its LSDB table then it sends the PSNP packet to request this information. PSNP is also used to acknowledgement.

How LSDB sync happens in P2P and Multi-access network.

In multi-access network the DIS keeps on sending the CSNP packet(multicast) to all the routers in network. If any router finds any information missing in its LSDP comparing with the CSNP received , it makes a request by PSNP and the DIS would sent the LSP. The same would be acked by PSNP.

On the other hand over the P2P network there is no CSNP after the adjacency forms. Any triggered update would be sent by CSNP and would be acked by PSNP.

ISIS Metric :

By default the metric value in ISIS is 10 and it adds 10 for each link it crosses.   Earlier ISIS metirc was just 6 bit long and could not accommodate the metric for big ISPs where there could be 1000s of routers…Later the metric field was enhanced with 24 bits which has enough bits to accommodate large metric. 6 bit metric is called Narrow metric and 24 bit is called Wide metric.

Few facts :

ISIS is very scalable protocol …somewhere I read that in ISIS there could be 1000 routers in a single area unlike ospf where we can just have 50 routers in one area.

The default hello interval for the DIS is three times faster (that is, three times smaller) than the interval for the other routers so that DIS failures can be quickly detected.

Unlike OSPF, the DIS process is preemptive, that means if new device comes into the network with better priority then it would be elected as DIS and unlike ospf it doesn’t require clearing of isis process etc ..

My next post would cover the packet captures and some more detail …then we would complete the tasks I have mentioned in my previous post….happy learning!

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