ISIS and BGP Design

Hello, and Welcome! To IS-IS and BGP design.

When I first saw that Cisco put IS-IS and BGP on the CCDA exam, I thought, there’s no way. IS-IS is a little known protocol not even designed to use the TCP/IP stack, and BGP. I mean BGP is the routing protocol of the internet! Service provider networks, transit autonomous systems, millions of routes! It’ll make your head explode. There’s no way we can cover either in depth in a general design series, especially not at the CCDA level!

And well I found that Cisco is not looking for any kind of depth here. We’ll probably be going into both of these protocols deeper than is needed but they’re both very cool and exciting to work with. In this video we’ll be covering an overview of how IS-IS works from a design perspective and what a scaled network looks like. We’ll also be going over some key ideas about BGP and how it might look to use that as an interior gateway protocol. So let’s get started.

ISIS qualities

  • Link state protocol
  • Designed for the OSI protocol suite
  • Uses areas in a similar manner as OSPF
  • neighborship forms at layer 2

Most people when they earn their CCNA and are learning about routing protocols, you learn there’s 2 types. Distance vector and link state protocols. So in distance vector you have RIP, EIGRP, and BGP, and in the link state side you have OSPF, and usually really small they’ll include IS-IS because nobody wants to try and unpack the bag of holding that is IS-IS. Well, just like you may have learned then, IS-IS is a link state routing protocol and it was designed for the OSI protocol suite. Now you may say , ‘wait, don’t you mean OSI model?’ no, so back when TCP/IP was in its infancy it had a competitor, the OSI protocol stack. Just like if anyone remembers Betamax or laserdiscs? Back when VHS was first coming out, those were really the better medium, but for whatever reason VHS won out. Well, this is very similar honestly, OSI feels like the better protocol stack in my opinion although it’s a bit more complicated.

So IS-IS was the routing protocol at the time that was developed for the OSI protocol suite, but it was designed in such a way that it was highly extendable. It had it’s basic functionality but also allowed for TLV fields, that’s ‘type length value’ where you can define arbitrary information in those fields. So because of this, IS-IS was easily converted, or integrated for use with IP routing and the resultant protocol was called ‘integrated IS-IS’. Now before I get too far, let be explain briefly. In OSI, routers are called intermediate systems, and hosts like workstations or servers are called end systems, hence IS and ES. Now IS-IS uses the concept of areas just like OSPF, and actually it uses the same algorithm to calculate the best path around the network, Dijkstra’s ‘shortest path first’ algorithm. However there’s no such thing as an ABR because all routers are wholly in a single area and are not split, the boundary is on a link rather than a router, but we’ll get into hat a bit more in a moment.

 The OSI equivalent to an IP address is an NSAP address, that’s network service access point. Each router has a single NSAP address, and neighborships for IS-IS form at layer 2, so only for routers on the same segment.

Now, like I mentioned a moment ago, the routers in IS-IS are always wholly within a single area, there are not any area border routers like with OSPF. However, there are 2 different databases or types of routers and that’s level 1 routers and level 2 routers. In this way IS-IS is a hierarchical routing protocol. Level 1 routers only contain the link state database for the area they are within. There’s a L1/L2 router, which is a lot like or ABR in OSPF, which contains both he link state database for level 2 and of the area for which it is a level 1 router. The level 1 routers in the area have no knowledge of the L1 database, they just know how to get to the nearest L1/L2 router in the area. The L1/L2 routers have the ability to summarize their area’s network into the L2 database. The L2 routers are intended to be the network backbone. That rather than having an area 0, you have L2 routers. However it’s a lot more extendible. You can have multiple backbones if your design ends up that way.

One of the major issues with using IS-IS as an interior gateway protocol is that it requires a fair bit of manual tuning. As far as IS-IS is concerned, all links have a cost of 10. Just 10-. Basically just hop count, we’re getting like into RIP world here. So you as the network admin will need to go into each and define a cost for each link, you’ll likely have your spreadsheet with link speeds to cost and apply that to each link in IS-IS. It’s more manual, but because of the TLV fields, it’s no extendible that it keeps on coming in handy and really works well and is overall much more flexible than OSPF.

Another note is that currently IS-IS doesn’t support anything but cleartext authentication. There’s a draft for an RFC to modify IS-IS to use only MD5 authentication, but at this time that hasn’t bee published yet to everything’s still cleartext.

So like I mentioned, the backbone of IS-IS is the L2 routers. There’s not a backbone area like OSPF’s area 0. The ‘backbone’’ of IS-IS is a continuous path of adjacent L2 routers. These are intended to be the spine to your sine and leaf topology and truly act like a backbone. The L1/L2 routers which are like the ABRs of IS-IS don’t flood any link updates into their area they are part of, that includes L2 and any other L1 areas. Here we have an example I got from the CCDA cisco press book showing a sample IS-IS design. You can see it’s set up very much like OSPF with your areas, an L1 area in the bottom left and top right with a backbone L2 area in the middle. If your network is large enough, you may have a line of only L2 areas, however often people opt for just connecting a bunch of L1/L2 routers together because your network has to be rather large to benefit from having only L2 routers.

Now here we are to the big wooly mammoth and all powerful he-man that is BGP. BGP is the routing protocol of the internet, if you didn’t know. It’s what ISPs use to exchange routes and generally what is used to keep track of the millions of routes that make up the internet as we know it. This is really for a few reasons. First, it’s rather slow. You can imagine what might happen if the world used something like OSPF, the routers of the world would explode! With an insane number of LSAs being flooded into the worlds’ routing areas every second it would be impossible to handle the amount of information that’s being transferred. So BGP, when it sees a network go down, it sits and waits a moment and sees if it comes back up.

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