Hello Vinod,
Why are you using /30s that take up address space and increase the size of
your IGP? Use IP Unnumbered. IP Unnumbered is the BCP of running an Internet
operations.
As to your question below, use the following as references:
ISP Essentials Power Session
http://www.cisco.com/public/cons/isp/documents/IOSEssentials_Seminar.zip
BGP Routing Workshop
http://www.cisco.com/public/cons/workshops/bgp/
In the BGP Routing Workshop, look specifically for the BCP (Best Common
Practice) session.
In you example below, given that you have only allocated two /30s out of the
/18 .... the BCP would be to advertise only the /18. Nothing more specific
than the /18. So you would have (assuming the /18 is 202.9.0/18):
router bgp 1076
no auto-summary
no synchronization
neighbor a.b.c.d remote-as x
neighbor a.b.c.d prefix-list out-filter out
network 202.9.0.0 mask 255.255.192.0
!
ip route 202.9.0.0 255.255.192.0 Null 0 250
!
ip prefix-list out-filter permit 202.9.0.0/18
ip prefix-list out-filter deny 0.0.0.0/0 le 32
!
There are three major techniques for advertising you aggregate to your
upstream. (see the first BGP session in the BGP Workshop materials). We
recommend this one in the workshop. We've found - through experience - to be
the easiest for young ISPs to implement.
On the NAT question, is the public addresses inside the /18 allocation? If
not, the easiest thing to do is to to renumber the public NAT address pool
to the /18 block.
Now for the interesting recommendation. If you check out IP Unnumbered and
still decided to use the /30 technique, we recommend that these /30s NOT go
into your IGP. The would be advertised and generated into your iBGP. Why?
Consider these routes as "external" to your network. "External" routes are
the job of the EGP (BGP in this case). By keeping these external routes out
of the IGP, you help keep the IGP lean - increasing the convergence time
(recovery speed) of you network.
This may not seem like a big deal when you have two /30 links. But, it is a
big deal when you have 10,000 /30 links in your IGP.
Now some would say that you can use the synergistic relationship of your
routing protocol, addressing plan, and network plan to insure that you do
IGP summarization at the area border router. This is the goal in an ideal
network. It is what we teach. But .... I have yet to see any network that
keep the discipline to insure summarization would happen. Scaling, growth,
and customer pressures eventually forces the wholes to be punched into the
summarizations - allow /30s all over the place.
This is the key reason we recommend IP Unnumbered. With IP Unnumbered, you
can have 10,000 lease line customer - with no IGP entries on the circuits
between you and your customer.
Check out the workshop materials. We have this covered in the materials.
Barry
-----Original Message-----
From: Vinod Anthony Joseph Cherunni [mailto:vac@antarix.net]
Sent: Friday, February 02, 2001 11:13 PM
To: cisco-nsp@puck.nether.net
Subject: [nsp] BGP Advertisements
Dear All,
Once again some queries in my mind. Out of a /18 address block that has
been allocated to me, which I am further subnetting to achieve multiple /30
prefixes to allocate addresses for all my links. The following queries arise
in my mind.
(a) How would I announce those /30 prefixes to an upstream provider. I
mean that If I am using the following /24 block "202.9.4.0/24" to break it
into multiple /30 prefixes which would effectively give me 64 of such
prefixes, & assuming only two /30 prefixes are put into use.. This means
that only two /30 prefixes will be in my IGP routing table. Now how will I
aggregate these multiple /30 prefixes into a single probably /24 block &
announce it
If I have currently only allocated 202.9.4.4/30 & 202.9.4.8/30 for two
links & the remaining /30 prefixes are unused. Can I summarize them as
follows in BGP
router bgp 1076
no auto-summary
no synchronization
neighbor a.b.c.d remote-as x
aggregate-address 202.9.4.0 255.255.255.0 summary-only (Is this okay or
else will I have to list the two /30's using "network" statements. Feel it
won't scale since the number will grow large.)
(b) Also I am starting off with using private addresses at small portions
of my internal network due to some reasons, & use NAT for translating an
internal private IP Prefix to a valid IP /24 prefix. How could I announce it
to an external peer. Because I would'nt have any entry in the internal
routing table for the NAT public prefix.
(c) Lastly is it advicable to announce the entire /18 allocated to me
using an ip route to null for the entire /18 even though its not being fully
utilized, if I would need to do that.
Kindly enlighten me.
With warm regards,
Vinod.
This archive was generated by hypermail 2b29 : Sun Aug 04 2002 - 04:12:27 EDT