[c-nsp] IS-IS question

Mon Nov 27 04:44:18 EST 2006

All . 

We are actually thinking of migrating from Eigrp to IS-IS in our Mpls Core 
.. 
I already did a documentation on why we choose ISIS compare to Opsf etc... 

If anyone has already done such a migration i would appreciate if he can 
share the knowledge or give some tips and tricks ........
Of course any feedbacks on the following are more than welcome . 

Basically why we choose IsIs compare to Ospf : 

- Simple protocol: IS-IS has less packet types (four) than OSPF which 
makes it a simpler protocol. In fact packet type depends on router type 
and not on the prefix nature. 
There are four different LSP types in IS-IS: 

1.      Level-1 non-pseudonode LSP:  generated by an intra-area router.
2.      Level-1 pseudonode LSP:  generated by an intra-area DIS on 
broadcast media.
3.      Level-2 non-pseudonode LSP:  generated by an area router.
4.      Level-2 pseudonode LSP:  generated by an area router DIS on 
broadcast media.

- Different lifetime per database element:  IS-IS allows the association 
of a specific refresh-timer per LSP (different from OSPF LSA).
- Flooding more optimal:  all IP prefixes are, with IS-IS, advertised 
within one packet instead of generating one packet per database element 
for OSPF.
- Partial Route Calculation (PRC):  SPF (Dijkstra) is run when topology 
has to be calculated (SPF tree) but when only IP routing information has 
to be calculated IS-IS performs a Partial Route Calculation (PRC) which 
consumes less CPU.
- Summarization rules not limited:  it is possible to summarize external 
routes in any point of the network, and not only on the originating device 
like in OSPF

The scalability of IS-IS might drives our choice of this IGP for the MPLS 
backbone versus OSPF.

Internet
sthaug at nethelp.no

Sent by: cisco-nsp-bounces at puck.nether.net
26/11/2006 12:45

To
robhass
cc
cisco-nsp
Subject
Re: [c-nsp] IS-IS question

> All routers (CRs and PE1) are IS-IS Level-2-only routers. All links
> are OC48 POS.
> If link between CR1 and CR2 fail then traffic between CR2 and CR1 will
> be transited over PE1.
> 
> My solution to bypass this is set much higher metrics at links between
> CRs and PEs than metrics inside core. Is my method is good ? Do you
> have any other recommendations regarding migration from EIGRP to IS-IS
> ? (routers are mostly GSR and 6500/7600 + 3750 L3 for Datacenter + new
> Junipers will be few M320).

The simplest solutions are often the best, and adjusting the IGP metrics
should work just fine for this case. Since you mention PE routers, you
possibly have an MPLS network - in which case there are several other
tools available (LSPs with explicit paths, link coloring, etc).

Steinar Haug, Nethelp consulting, sthaug at nethelp.no
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