[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Section 16.2 of RFC2328 and ABR's behaviour

Srinivas Goli wrote:





-----Original Message-----
From: Mailing List [mailto:OSPF at PEACH.EASE.LSOFT.COM] On Behalf Of Acee Lindem
Sent: Tuesday, February 14, 2006 5:18 PM
To: OSPF at PEACH.EASE.LSOFT.COM
Subject: Re: Section 16.2 of RFC2328 and ABR's behaviour

Srinivas Goli wrote:



Folks,

Section 16.2 states the following

  16.2.  Calculating the inter-area routes

The inter-area routes are calculated by examining

summary-LSAs.


If the router has active attachments to multiple areas, only
backbone summary-LSAs are examined. Routers attached to a
single area examine that area's summary-LSAs. In

either case,


the summary-LSAs examined below are all part of a

single area's


      link state database (call it Area A).

From this it is clear that the an ABR will only examine the summary


LSAs


from the backbone area. This is done to avoid the counting
to infinity

problem since exchanging routes between areas is done using Distance Vector mechanisms.

Now some of vendors seems to skip this check i.e examining

summary LSAs
from all attached areas. Can any one explain why this would be 


considered useful, assuming this is done intentionally.




Hi Srinivas,
See RFC 3509.
Thanks,
Acee



Hi Acee,

I had looked at RFC3509 as possible explanation of what I saw but the
particular case I am looking at is a router with two actively attached
areas both of which are non backbone areas. The router would identify
itself as ABR by setting B bit in the router-lsa. The router also
re-advertises the summary LSAs in one area into the other areas. 

Hi Srinivas,

Note that RFC 2328 considers any router "attached" to multiple areas an ABR. While I
can't find a precise definition in the RFC, "attached" is generally considered to mean
one or more UP interfaces.

Area border routers
           A router that attaches to multiple areas.  Area border
           routers run multiple copies of the basic algorithm, one copy
           for each attached area. Area border routers condense the
           topological information of their attached areas for
           distribution to the backbone.  The backbone in turn
           distributes the information to the other areas.


So, the router is not misbehaving. What is wrong is that the backbone is not contiguous (if the router doesn't have any
backbone interfaces).

This ABR definition has been debated over the years and was the
primary motivation for the RFC 3509 functionality. 


Thanks,
Acee

Thanks, 




Regards,