Level 3 BGP Communites

Level 3 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.
If you are a provider who has been asked to route a more-specific network block that is part of 4.0.0.0/8, please ask your customer to contact Level 3 to obtain a new network block. Level 3 will not accept advertisements for any networks that are part of 4.0.0.0/8 from any non-customer peer.

Note that the import/export designations above are a simplification of our actual policies which cannot be properly described given the limitations of RPSL
They would also be rather tedious to read if we denoted all of the common policy bits applied to every peer, so we have only noted the peer-specific bits.
We have also only documented our customer peers in this object at this time.

The following import actions are common to every Level 3 customer peering session:

- RFC1918 and other reserved networks and subnets are not permitted.
- Advertisements with reserved ASes in the path (ie 64512 - 65535) are not permitted.
- Prefixes shorter than /3 or longer than /28 are not permitted.
- Advertisements containing the AS of a network with which we have a non-customer peering relationship are not permitted (ie customers are not allowed to advertise transit for UUnet, CW, etc).
- Advertisements tagged with our own "internal use only" BGP communities (ie the city/country/region/etc BGP communities outlined below) will have all of their BGP communities stripped from them at ingress, and any BGP communities meant to affect localpref, prepending, etc are thus ignored.
- Prefixes not matching the per-peer import policy as documented above are not permitted.
- Localpref will be set to 100 by default, subject to modification based on received BGP communities as outlined below.
- A hard limit is placed on the number of routes a peer is allowed to announce to us. This number is based on their registered routes plus a bit of extra overhead.

The following import actions are common to every Level 3 non-customer peering session:

- RFC1918 and other reserved networks and subnets are not permitted.
- Advertisements with reserved ASes in the path (ie 64512 - 65535) are not permitted.
- Prefixes shorter than /3 or longer than /24 are not permitted.
- Any subnet of any Level 3 CIDR block (as documented in rs-Level3) is not accepted unless routing for the subnet in question has been explicitly requested by the multihomed customer in question.
- Advertisements tagged with our own "internal use" or "customer use" BGP communities (as outlined below) will have all of their BGP communities stripped from them at ingress.
- MEDs are overwritten unless special arrangements have been made.
- Peers who register their routes with meaningful policies may have a prefix filter applied based on this policy.
- Localpref is set to 86 or lower (depending on the nature of the facility over which the peering is taking place and several other factors).
- A hard limit is placed on the number of routes a peer is allowed to announce to us. This number is based on their registered routes or on a historical view of the number of routes they have been advertising plus a bit of extra overhead.

The following export actions are common to every Level 3 peering session:

- RFC1918 and other reserved networks and subnets are not announced.
- Advertisements with reserved ASes in the path (ie 64512 - 65535) are not announced.
- Prefixes shorter than /3 or longer than /24 are not announced.
- Any subnet of any Level 3 CIDR block (as documented in rs-Level3) is not announced unless routing for the subnet in question has been explicitly requested by the multihomed customer in question.
- Suppression and prepend actions as outlined in the BGP community list below are taken for announcements to non-customer peers.

 <font size="2">
</font>

BGP Communities applied at ingress

--------------------------------------------------------------------------------
prefix type BGP communities
--------------------------------------------------------------------------------

--------------------------------------------------------------------------------
error type BGP communities
--------------------------------------------------------------------------------

--------------------------------------------------------------------------------
city BGP communities
--------------------------------------------------------------------------------

--------------------------------------------------------------------------------
country BGP communities
--------------------------------------------------------------------------------

--------------------------------------------------------------------------------
regional BGP communities
--------------------------------------------------------------------------------

BGP Communities accepted from customers

--------------------------------------------------------------------------------
Customer traffic engineering (TE) notes
--------------------------------------------------------------------------------

--------------------------------------------------------------------------------
Customer traffic engineering BGP communities - Suppression
--------------------------------------------------------------------------------

--------------------------------------------------------------------------------
Customer traffic engineering BGP communities - Prepending
--------------------------------------------------------------------------------

--------------------------------------------------------------------------------
Customer traffic engineering BGP communities - Regional
--------------------------------------------------------------------------------

--------------------------------------------------------------------------------
Customer traffic engineering BGP communities - LocalPref
--------------------------------------------------------------------------------

--------------------------------------------------------------------------------
Customer traffic engineering BGP communities - Blackhole
--------------------------------------------------------------------------------

--------------------------------------------------------------------------------
trouble: 24 Hour Call +44-08000-927-729
trouble: Abuse reports to This e-mail address is being protected from spam bots, you need JavaScript enabled to view it
Peering issues to This e-mail address is being protected from spam bots, you need JavaScript enabled to view it
http://www.level3.com/

Applying BGP Community string with sample configuration

2. Pick the best BGP community string for your traffic shaping plan (mainly incoming traffic). Most of ISPs are providing BGP community string with local preference and AS prepending option. Cannot tell which one is better than the other. It will depend on your global traffic shaping plan.

3. Follow the below commands ( Cisco only )

The below Sample configuration will tag the 10.0.0.0/24 route with [ISP AS]:120 or [ISP AS]:3 and will not tag any other routes.

router#config t
router(config)#ip bgp-community new-format
router(config)#access-list 10 permit 10.0.0.0 0.0.0.255
router(config)#access-list 10 deny any

router(config)#route-map [to-ISP] permit 10
router(config-route-map)#match ip address 10
router(config-route-map)#set community [ISP AS]:120 <---- using Local Preference

or

router(config-route-map)#set community [ISP AS]:3 <------- using AS prepending
router(config-route-map)#route-map [to-ISP] permit 20
router(config-route-map)#exit

router(config)#router bgp [xxxx] <------------------------------- xxxx = customer's ASN
router(config-router)#neighbor x.x.x.x send-community
router(config-router)#neighbor x.x.x.x route-map [to-ISP] out
router(config-router)#exit
router(config)#exit
router#copy running-config startup-config

4. And then, go to www.CiscoNET.com and pick one of route server on the map to see your announcement. If you are using AS prepending option, you will see your AS prepends on route servers. Sometime you might not see your route with particular ISP path.
In most of case it might not be any routing problem, just the route path was dropped at somewhere by BGP best path selection scheme. Try Oregon route server, if you can see your route. The Oregon route server is providing many possible and available paths between BGP speakers and neighbors.
If you don't see your route on there? check other route servers and also check your
BGP configuration. You might need to contact your upstream provider to check what they are learning BGP route from you.

Add as favourites (20) | Views: 286 | E-mail

Be first to comment this article

Write Comment
Please keep the topic of messages relevant to the subject of the article. Personal verbal attacks will be deleted. Please don't use comments to plug your web site. Such material will be removed. Just ensure to *Refresh* your browser for a new security code to be displayed prior to clicking on the 'Send' button. Keep in mind that the above process only applies if you simply entered the wrong security code.
Name:
E-mail
Title:
BBCode:
Comment:
Code:*

Powered by AkoComment Tweaked Special Edition v.1.4.3

---

Please share this article with ;