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aaabbb16@hotmail.com 03-24-2008 04:31 AM

route aggregation and summarization
 
Two more questions here:
1. Anyone can explain route aggregation and summarization. my question
they are same.
Also how they are related to CIDR and VLSM.

2. Right now does CIDR for route summarization
only ? Anyone use it to assisgn a classless ip addr. such as
192.168.2.1 255.0.0.0?

st


Trendkill 03-24-2008 11:18 AM

Re: route aggregation and summarization
 
On Mar 24, 12:31 am, aaabb...@hotmail.com wrote:
> Two more questions here:
> 1. Anyone can explain route aggregation and summarization. my question
> they are same.
> Also how they are related to CIDR and VLSM.
>
> 2. Right now does CIDR for route summarization
> only ? Anyone use it to assisgn a classless ip addr. such as
> 192.168.2.1 255.0.0.0?
>
> st


Route aggregation is the grouping of smaller routes together into
advertisements that are smaller in number (count) then all of the
individual routes. Generally speaking, you would use 10.0.0.0/16 as
an aggregate to 10.0.1.0, 10.0.2.0, 10.0.3.0, etc, which you may have
configured as vlans in a datacenter. Summarization can refer to many
things, but can be configured on specific routing protocols to
automatically summarize by the Class of the network (based on first
octet). Summarization can also be configured manually, either by
commands like 'ip summary-route eigrp', or in BGP by creating a route
to null0 for the summary and putting in the network statement for the
aggregate network or supernet.

To answer your second question, 192.168.2.1 with a 255.0.0.0 subnet
mask would mean that router is advertising 192.0.0.0 - 192.255.255.255
via its routing protocol,. Presuming you are asking about a situation
where 192.168.2.1 255.255.255.0 is a interface address, and you want
to summarize a larger range than this, you would put summaries towards
the WAN or Internet (usually summaries configured in the core and then
passed into the WAN routers, although some folks prefer to put the
summaries on the WAN routers themselves). Using a mask of 255.0.0.0
would most likely be a very bad idea, unless of course you really have
a datacenter with 16,000,000 hosts.

At their core, summaries are used to keep the routing table manageable
and clean. If you have 10 datacenters, and you were being clean, you
could have about 10 or so routes in your table, before WAN links or
default routes. If you did not summarize, you would have 10 x each
vlan or subnetwork in each datacenter. Particularly when you are
dealing with retail companies with hundreds or thousands of stores
connected via point to point links, it was easy in the old days to
degrade performance on a router or link with all of the updates from
the various routes. This is why it is best to use stub,
summarization, and default information originate to keep your tables
short and succinct.

Tilman Schmidt 03-24-2008 01:32 PM

Re: route aggregation and summarization
 
aaabbb16@hotmail.com schrieb:
> 2. Right now does CIDR for route summarization
> only ? Anyone use it to assisgn a classless ip addr. such as
> 192.168.2.1 255.0.0.0?


To add to Trendkill's very valid answer:

Classes are dead. It's only out of habit that people still frequently
use a /24 mask with addresses in the 192.168 range. All networks are
classless nowadays in the sense that you cannot reliably deduce the
netmask from the first few bits of the address - you need to be told
what it is. In particular, networks using addresses from the 10.* and
172.16 to 172.31 private ranges rarely use the /8 or /16 mask
respectively that would have been associated with them in the old
classful world.

HTH
T.

News Reader 03-24-2008 04:35 PM

Re: route aggregation and summarization
 
Summarization also has benefits in terms of convergence.

Using Trendkill's example, if you had a summary route of 10.0.0.0/16 in
your table, you would not be aware of the status of individual
"summarized" routes such as 10.0.1.0, 10.0.2.0, 10.0.3.0, which means
you would not have to reconverge every time there was a change in status
of one of these routes.

A flapping interface on a "summarized" network would not result in you
having to recalculate an entire OSPF topology (for example).

It provides stability, and security.

Best regards,
News Reader


aaabbb16@hotmail.com wrote:
> Two more questions here:
> 1. Anyone can explain route aggregation and summarization. my question
> they are same.
> Also how they are related to CIDR and VLSM.
>
> 2. Right now does CIDR for route summarization
> only ? Anyone use it to assisgn a classless ip addr. such as
> 192.168.2.1 255.0.0.0?
>
> st
>



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