Isis vs ospf


i am studying the ISIS routing protocol. and i have read that ISIS is more suitable to use in ISP environment rather than OSPF. Please Explain why ISIS is more preferable than OSPF.


Hi Faizan,

There are a lot of discussions why IS-IS is better than OSPF or vice versa.

IS-IS used to have “partial route calculation” which made it more efficient than OSPF. However, OSPF, now also has “incremental SPF”.

IS-IS uses TLVs which makes it easy to route any protocol…IPv4, IPv6, or anything else. That’s one advantage over OSPF where you had to run OSPFv2 for IPv4 and OSPFv3 for IPv6 (nowadays OSPFv3 can do both IPv4 and IPv6).

The main reason that ISPs use IS-IS is probably a historic reason. Back in the 90s, IP wasn’t the dominant protocol it is today. IS-IS was a good choice since it uses its own encapsulation and thanks to its TLVs, it can route anything.

When IPv6 came around, nothing changed to IS-IS except that a new TLV was added. IS-IS works and most ISPs have been using it for a long time now…


ISP’s would also like to use IS-IS inside an MPLS enviroment network which utilize Metro Ethernet devices.

that way, the metro ethernet makes switching decisions on the packets for a vlan per customer and that makes the configuration easier.
that way you don’t have to configure ip addresses in the l3 switches interfaces, only one ip for loopback which could also be a private address.

that way you have a routing protocols for MPLS environment and you could create L3 VPNS services while also create L2 VPN services and also create much better redundancy using VSS / LACP and have much higher capacity for the network for less money.

for me, the main difference between IS-IS and OSPF is that you can configure IS-IS on L3 switches w/o assining those ip addreses for the actual interfaces.

I had a realy hard tieme understanding IS-IS because it is realy simple compared to OSPF:

  1. OSPF area 0 - backbone
    this area has the whole information of its own and connects any other areas

  2. OSPF area 1+ - just another area
    not a backbone and also a concept which doesn’t exist in IS-IS

  3. OSPF stub area
    an area which knows only routes from inside of it and a default route

  4. OSPF ASBR / ABR - some router which connects to more than 1 ospf area

in IS-IS the equivalent values are:

  1. IS-IS Level-2 Router - backbone
    those routers configure as level-2 are functioning the exact same as would area 0 routers in OSPF - everything need to connect into the level-2 backbone and you can’t have the same IS-IS network being seperated by having some level-2 router which don’t connect to the backbone directly.

  2. IS-IS level-1 router - a stub area
    this type of router is the exact same concept of stub area in OSPF.
    the routers only knows their own network and must exist in the same area number (as you know, IS-IS also utilize area numbers but they are used differently from OSPF) and they would have a default route toward an level-1-2 router.

  3. IS-IS level 1-2 router - a connection between level-1 and level-2 sides of the IS-IS network
    you will have to make an level-1 adjacency between the level-1-2 router with the level-1 roiter, but also you will have to make an level-2 adjacency between the leve-1-2 router and the level-2 router so the connectiuon looks like that:

in IS-IS you can redistribute level-2 databse into the level-1 database by redistributing the L2 into the L1.

in OSPF totaly stuby area the only way to make such a leak is by using a leak-map (works for eigrp, not quite sure if it also true about ospf).