Does this help:
Credits for this is at the bottom..
Basics of BGP v.02
1: What is BGP, and when should I use it?
2: How does BGP work?
3: What are the basic steps to running BGP?
4: What type of router do I need to run BGP?
5: This sounds complicated, do I need another person on staff just to
manage this?
6: About the author.
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1: What is BGP, and when should I use it?
BGP, short for "Border Gateway Protocol", is a protocol used by Internet
routers to communicate which networks they can access. You should use BGP
if you have more than one Internet connection, and are trying to achieve
load balancing and/or redundancy.
2: How does BGP work?
Before you run BGP, you must be assigned an ASN ("Autonomous System
Number"). This number identifies your network and routers to the Internet.
When you announce (also known as advertising) that you have a block of IPs
in your ASN, you are sending information about that IP block to your
"Neighbor", usually your ISP's BGP router. Your neighbor receives the
advertisement and adds it to their BGP tables. It will then add its' ASN to
your ASN in the AS Path and copy the advertisement to it's neighbors. This
process continues until your route has been copied (propagated) to every
router on the Internet. Since many Internet routers have several
connections, and thus several different paths to your network, they have to
decide on the best path to your network. This is where it uses the ASN
information. The Internet routers select the path to your network that has
the shortest number of ASNs to cross. This is called the "ASN hop Count",
and is different from the number of hops you see in a traceroute.
Hypothetical Example:
Bob (ASN 12345) has a connection to UUNet (ASN 701) and Genuity (ASN 1).
UUNet has a direct connection to Bill (ASN 23456). Genuity doesn't have a
direct connection to Bill, but instead has to cross Buddy (ASN 7654).
Diagram:
Bob(12345) - UUNet(701) - Bill(23456)
+- Genuity(1) - Buddy(7654) -+
In this scenario, Bill is going to receive two paths to Bob.
The first path Via UUNet looks like this: 701 12345
The second path Via Genuity/Buddy looks like this: 7654 1 12345
Bill would select the UUNet path because of the shorter number of ASNs in
the AS PATH.
If Bob's UUNet connection went down, then Bill would still have the longer
Genuity path in his BGP tables and would send data to Bob via that path.
This is why BGP is used for redundancy.
3: What are the basic steps to running BGP?
1. Talk to your ISP's and see if they will let you peer with them. This
isn't a question that the average helpdesk person is going to be able to
answer, so you may have to work your way up the support chain. If the
answer is no, then you need to find a different ISP. Most large ISP's have
no problem with this. I won't suggest a specific ISP in the FAQ, but I have
personally worked with UUNet, Sprint, PsiNet, Savvis, SBC, Broadwing, and
Cable & Wireless. I was pleased with the service from all of these.
2. Apply for an Autonomous System Number from ARIN. (www.arin.net)This
Requires ARIN membership ($750 one-time, $35 annual). If you are not within
the continental US you will have to apply for an ASN from a different
agency, probably RIPE or APNIC.
3. Acquire a minimum of a /24 (256 IPs) from at least one of your ISP's.
4. Acquire the following information from each of your ISP's.
4a. IP Address of the Peer
4b. AS# of the peer
4c. Authentication (if any)
5. Provide each of your ISP's with the following info:
5a. Your Peer IP address
5b. Your AS#
5c. Authentication (if any, not recommended)
5d. The Subnets you will be advertising (Good ISP's filter incoming
advertisements to prevent accidents from impacting the entire Internet
routing community. Why? Lookup "Blackholing" The process where you
accidentally or intentionally cause another network to become unreachable.)
6. Configure BGP on your router. (See the hardware section of the FAQ for
recommendations)
6a. Most people want to maximize the usage they get out of both internet
connections by load-balancing or selecting the shortest AS Path to traffic's
destination. The number of AS Paths your router can hold in memory limits
load balancing and intelligent path selection.
- If you have less than 32 MB of Ram, then accept only each provider's
default route.
- If you have less than 64 MB of Ram, then accept a default route, and
"customer routes" from each provider. This gives you most of the advantages
of Path selection and load balances the other traffic.
- If you have 128mb or more of Ram, you can accept full routes from each
provider and let the router select the best AS-path. In this scenario, you
need to monitor your memory usage because the BGP tables grow and shrink as
networks are added and removed. Additionally, you may have to "tweak" the
routing information you are receiving to make efficient use of you links.
6b. If you have a primary and a secondary link, and want to prefer one over
the other, or want more control over traffic entering and exiting, then use
one of the following strategies.
6b1. As-Prepend to cause the Internet to prefer one link or the other
6b2. Set the Local Preference (universal) or weight (Cisco specific) on
the preferred link
6b3. Filter to accept summary routes only from one provider
6b3. Use BGP's metrics to recommend preferences to your ISP's
* These are slightly more complex and will take up too much room in the FAQ.
If you have questions, you can email the author [EMAIL PROTECTED] or
4: What type of router do I need to run BGP?
Most people running BGP use Cisco routers. Juniper, Lucent, Foundry, and
many other companies also manufacture BGP capable routers, but Cisco has the
largest market share.
Here are the things to look for in a BGP router:
1. Processing Power - It should be able to easily handle the amount of
traffic you want to push through it.
2. Memory - Running BGP with Full tables is memory intensive. Currently, it
takes approximately 90mb of Ram to accept full tables from two providers.
This number is growing, so it is best to keep an eye towards future
expandability.
3. Interfaces - It seems obvious, but if you are going to have two t-1's and
a DS-3, your router needs to be able to accept two t-1 and a Ds-3 interface.
4. Cost - This is obvious. High-end routers are not cheap. If your company
structure allows it, it is often possible to pick up 2 or 3 used routers for
the cost of a single new one. Ask around to find a good used equipment
vendor, or try ebay.
In August of 2002, I see most BGP deployments on Cisco 3640's with the
Maximum amount of memory. In the near future, I see most people staying
with this platform and accepting less than full routing tables to overcome
the memory limitation. The next step up from the 3640 is the 3660, the 7200
series, and the 7500 series.
5: This sounds complicated, do I need another person on staff just to
manage this?
Usually, No. Once BGP is setup, it doesn't require much in the way of
regular maintenance. The only thing I usually recommend is having the
network administrator check occasionally (once a day) to verify that all of
your links are still up. This can be made easier using a graphical tool
like MRTG to display the status of the router in a web page. If you are
interested in speaking to the author for information about BGP consulting,
or general questions, see my contact information below.
6: About The Author.
Ejay Hire has been working in IT since 1996. He provides BGP, Network
Management, Network Design, and Disaster recovery consulting services.
Please email with any questions, I'd be glad to help.
-----Original Message-----
From: Luke Levis [mailto:[EMAIL PROTECTED]
Sent: Friday, June 13, 2003 7:38 AM
To: NT 2000 Discussions
Subject: RE: off topic - routing
what if I am using 2 different ISP's?
-----Original Message-----
From: Robert Gonzaga (306) [mailto:[EMAIL PROTECTED]
Sent: Friday, June 13, 2003 10:32 AM
To: NT 2000 Discussions
Subject: RE: off topic - routing
I believe the ISP will give you the AN for BGP.
-----Original Message-----
From: Luke Levis [mailto:[EMAIL PROTECTED]
Sent: Friday, June 13, 2003 7:05 AM
To: NT 2000 Discussions
Subject: RE: off topic - routing
well I stared at the config until hyperterminal actually started talking to
me.. so I sucked it up and called Sprint and they had forgot to add routing
for the class C they gave me through that s0 interface, and within seconds
it was up and running.... guess I should have called them first.... on
another note I'm looking into getting a BGP number is that something I get
from an ISP or do I have to go through someone like network solutions?
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]
Sent: Wednesday, June 11, 2003 9:39 PM
To: NT 2000 Discussions
Subject: Re: off topic - routing
Luke,
You've seen the ISP default route issue, so I won't cover that here. You
got the subnet mask thing too I see. Do a 'show ip route' and email it to
me.
2nd. Do an extended ping. just type 'ping'
You want extended commands 'y' source will be the IP address of e0
See if that goes, email the results.
Is this a new install or was it working for the longest time...
For safety, send a 'show interface' .
The ISP gave you the usable range of 208.4.66.0-255 or 144.232.255.26
Let's go from there.
-James
At 12:59 6/11/2003, Luke Levis wrote:
>been racking my brain here and can't seem to figure out what I have
>missed...
>
>I have a cisco 2524 with 1 serial interface and one ethernet interface, and
>like I have always done I plugged the t-1 into the serial interface and
>plug the ethernet interface into a hub and hung a laptop off the hub.. I
>gave the serial interface the assigned IP that my ISP gave me and gave the
>ethernet interface one of the adresses in my class C that my ISP gave me
and
>gave the laptop an address in that class C range and set it's gateway to
the
>ethernet interface.... now from the router I can traceroute, ping, etc. to
>my heart's content and get anywhere I want, and from the laptop I can get
to
>ethernet interface on the router but no further.. I put in my default
route
>as
>ip route 0.0.0.0 0.0.0.0 s0 and even put in a specific route for the class
c
>space, but the laptop cannot get past the E0 interface here is the
>interface and route config:
>
>interface Ethernet0
> ip address 208.4.66.1 255.255.254.0
>!
>interface Serial0
> ip address 144.232.255.26 255.255.255.128
>!
>ip classless
>ip route 0.0.0.0 0.0.0.0 Serial0
>ip route 208.4.66.0 255.255.254.0 Serial0
>
>the laptops address is 208.4.66.10 subnet 255.255.254.0 gateway 208.4.66.1
>and I can ping the laptop from the router
>
>any help is appreciated, sorry for the off-topic
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