Global Crossing BGP Community String

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

Peers:

Local preference : 200

Metric policy : do not accept peers metric

BGP community string that can be used for customers to control routes:

BGP Community String	Action
3549:100	set local preference 100
3549:200	set local preference 200
3549:275	set local preference 275
3549:300	set local preference 300
3549:350	set local preference 350

Control Route Propagation

GBLX provides the customer limited control over how their prefixes are propagated to various network peers. This is accomplished using as-path prepending at the GBLX-Peer border. The following BGP communities may be sent to prepend customer announced prefixes:

BGP Community String	Action
3549:600	Deny inter-continental export of tagged prefix [iBGP].
3549:666	Deny inter-as export of tagged prefix (deny to peers, send to customers) [eBGP].

For a limited subset of GBLX peering connections, more granular control of announcements is provided. If GBLX sees a BGP community matching 3549:8..., routing announcements sent to the following listed ASNs will be modified according to these rules :

ASN	Peer	No Export	Prepend +1	Prepend +2	Prepend +3
174	Cogent	8280	8281	8282	8283
209	Qwest	8010	8011	8012	8013
577	Bellnexxia	8090	8091	8092	8093
701	MCI	8030	8031	8032	8033
1239	Sprint	8060	8061	8062	8063
1257	Tele2	8110	8111	8112	8113
1299	TeliaSonera	8250	8251	8252	8253
1668	AOL	8070	8071	8072	8073
2497	JPNIC	8080	8081	8082	8083
2516	KDDI	8100	8101	8102	8103
2828	XO	8260	8261	8262	8263
2914	NTT Verio	8120	8121	8122	8123
3257	Tiscali	8240	8241	8242	8243
3300	InfoNet Europe	8130	8131	8132	8133
3303	Swisscom	8140	8141	8142	8143
3320	DTAG	8150	8151	8152	8153
3356	Level 3	8160	8161	8162	8163
3561	Savvis	8170	8171	8172	8173
4134	ChinaNet	8230	8231	8232	8233
5511	OpenTransit	8190	8191	8192	8193
6453	Teleglobe	8210	8211	8212	8213
6461	AboveNet	8200	8201	8202	8203
6762	Seabone (TI)	8050	8051	8052	8053
6830	UPC/Chello	8180	8181	8182	8183
7018	AT&T (US)	8220	8221	8222	8223
7473	Singtel	8040	8041	8042	8043
7911	Wilcom	8020	8021	8022	8023
12956	Telefonica	8270	8271	8272	8273

Use of GBLX BGP community string will be shown below.

You are multihomed with GBLX and Verizon. You want some prefixes is using your link to Verizon for incoming traffic. This also means that you want GBLX prefer routes from Verizon that from you. As shown above, GBLX give routes form its customer with local preference 300, it will be choosen that form its peers. To have the routes from peers more prefer, you must tagged your routes sends to GBLX with community string that have value lower than default value. In the list above, community string 3549:100 will set the routes with local preference 100, this value is lower than default community for GBLX peers.

Example to set this in Cisco router:

Your prefix that want verizon as incoming is 200.200.200.0/24 and your AS is 100.

You will set prefix-list, route-map, and apply to BGP configuration.

Create prefix-list

router#config t

router(config)#ip prefix-list TO-VERIZON permit 200.200.200.0/24

Create route-map to set community

router#config t

router(config)#route-map SET_COMM permit 10

router(config-route-map)#match ip address prefix-list TO-VERIZON

router(config-route-map)#set community 3549:100

router(config-route-map)#route-map SET_COMM permit 15

Apply route-map at peer to GBLX

router#config t

router(config)#router bgp 100

router(config-router)#neigh 201.201.201.2 remote-as 3549

router(config-router)#neigh 201.201.201.2 send-community both

router(config-router)#neigh 201.201.201.2 route-map SET_COMM out

Then you can check effect of your community in GBLX route server. If it's correct, you will see your prefix have local preference 100.

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