Internet is a set of Autonomous Systems (AS) that interconnect each other.  Autonomous System is a network under the same administration.

As using route reflector is a way simplifying and scaling iBGP configuration, it is recommended to use if you have many routers in your AS.

IBGP require all BGP systems within the AS are a fully meshed, so that any external routing information is redistributed among all routers within the AS. 

Look here for complete TelisSonera BGP Community: 

http://www.db.ripe.net/whois?searchtext=-a+-r+-T+aut-num+as1299

Communities of Tiscali International Network (TINet) - AS3257

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This is the list of BGP communities that can be set by customers

(after engineering@ip.tiscali.net approval):

• 3257:1030 do not announce to upstream.
• 3257:1031 prepend 1x to upstream.
• 3257:1032 prepend 2x to upstream.
• 3257:1033 prepend 3x to upstream.

Customer Setup Information

Multi-homing with BGP (Border Gateway Protocol) is the practice of connecting to multiple service providers and having simultaneous external BGP peering sessions with each provider. A Multi-homed customer typically owns an Autonomous System Number and exchanges routing table information with two or more upstream Internet
Service Providers (ISPs).

In iBGP, there is a rule to avoid routing loop by not advertise any route receive from other iBGP neighbor.  The rule have consequence that every router in the AS needs peer with all router so every router have routing from other router in the AS.  This is called full mesh configuration. 

This post will shows applications of BGP Community.  It doesn't show deep description about BGP Community.  BGP communities are tags, or attributes that can be attached to BGP prefixes announcement.  Based on that community, policy can be define to do something to that routes.  BGP Communities are descibed in RFC 1997.  According to RFC 1997, BGP Communities describe as a group of destinations which share some common property.  Each autonomus system administrator may define which communities a destination belongs to.  By default, all destinations belong to the general Internet community.  BGP Community have 32 bit value.

Peer group in a set of BGP neighbor that share some policy.  Policy that can be the same, for example, route-map, filter-list, prefix-list, update-source, route-reflector client.  Peer group can reduce cpu process consumption, also configuration effort.

1. Overview/Introduction
This document is meant to concisely describe the signalling options that Deutsche Telekom's AS3320 makes available to customer networks supported by BGP routing (transit service products). The intended audience is a customer's network administrator who is familiar with BGP and general public Internet routing techniques as well as how to configure and control their routers. This version of the document describes the options as defined for the initial roll out of the advanced routing policy definitions (implemented using the TROPOS configuration generator) identified as TSD3320 v1.0 targetted for January 17th 2005.

Verizon Business supports a user configurable routing policy by allowing customers to fine tune their routing announcements via the BGP community attribute. The following information outlines the policy and demonstrates how customers can implement the routing policy of their choice.

Cogent will send the customer a complete BGP table without aggregating the internal Cogent routes. This is useful for customers who are multi-homed within the Cogent Network.

1. SAVVIS Received BGP Community Attribute Values (BGP community String)

SAVVIS Prepend/Suppression BGP Community Attribute Values (3561:30PPN)

SAVVIS allows customers to control certain traffic flows off-net with the implementation of BGP community attribute values that prepend route/prefix paths with additional AS hops. These BGP community attribute values, which the customer applies, affects the way SAVVIS peers choose the degree of preference of a given prefix/route, because the number of AS hops within the route-path has been lengthened.

Level 3 BGP Communities 

Level 3 (AS3356) does not allow any part of 4.0.0.0/8 to be multihomed. If you are a customer who is currently using address space from 4.0.0.0/8 and if you need to multihome your network to another provider, please contact Level 3 for new address space which can be multihomed.

Global Crossing (GBLX) allows customer to control their routes announced within GBLX/AS3549 network using bgp community string. These community strings can be used various attributes of their prefixes announced within GBLX network, and can be useful if you are multihomed with another IP transit.

Global Crossing has standard policy applied for its customers and peers.

Customers:

Local preference : 300

Metric policy : Accept customer's metric

BGP community attribute is such a way for tagging routes.  It is widely used for route manipulating.  Administrator can tag a community to a route then set a policy when the community is match.  BGP community can be used to influence routes at the upstream, so it affect the way routes back to our network or inbound traffic.

Sample BGP regular expressions that can be used on Cisco router.
_100_ Going through AS 100
^100$ Directly connected to AS 100
_100$ Originated in AS 100
^100. Networks behind AS 100
^[0-9]+$ AS paths one AS long
^([0-9]+)(_\1)*$ Networks originating in neighboring AS
^$ Networks originated in local AS
.* Matches everything

BGP route selection criteria (on Cisco router) :

1. Exclude route with inaccessible next-hop
2. Prefer highest weight (local to router)
3. Prefer highest local preference (globa within AS)
4. Prefer routes that router originated
5. Prefer shortest AS paths (only length is compared)
6. Prefer lowest origin code (IGP
7. Prefer lowest MED
8. Prefer external (EBGP) path over internal (IBGP)
9. For IBGP paths, prefer path through closest IGP neighbor
10. For EBGP paths, prefer oldest (most stable) path
11. Prefer paths from router with the lowest BGP router-id

Before configuring BGP, you need gather information about your network and peer. Basically, configuring BGP on Juniper router is the same with Cisco or another router. Basic configuration is just configure BGP session the advertise our network.

Configuring BGP on Cisco router is simple enough. Before start configuring BGP, you need two consideration :
1. BGP only advertise routing entry that exist in IGP routing table
2. BGP is classfull routing protocol, you must include netmask in network statement BGP configuration if the network is not classfull.