| Network Working Group | X. Xu |
| Internet-Draft | K. Bi |
| Intended status: Standards Track | Huawei |
| Expires: June 27, 2017 | December 24, 2016 |
BGP Neighbor Autodiscovery
draft-xu-idr-neighbor-autodiscovery-00
BGP has been used as an underlay routing protocol in many hyper-scale data centers. This document proposes a BGP neighbor autodiscovery mechanism which can be used to simplify the BGP deployment greatly. This mechanism is very useful for those hyper-scale data centers where BGP is used as an underlay routing protocol.
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BGP has been used as an underlay routing protocol instead of IGP in many hyper-scale data centers [RFC7938]. Furthermore, there is an attempt to leverages BGP Link-State distribution and the Shortest Path First algorithm similar to Internal Gateway Protocols (IGPs) such as OSPF [I-D.keyupate-idr-bgp-spf]. In a word, there is a strong motivation to replace IGP by BGP in hyper-scale data centers.
However, BGP is not good as IGP from the perspective of deployment automation and simplicity. For instance, the IP address and Autonomous System Number (ASN) of each BGP neighbor have to be manually configured on BGP routers although these BGP peers are directly connected. In addition, for those directly connected BGP routers, it's usually not ideal to establish BGP sessions over their directly connected interface addresses due to the following reasons: 1) it's not convient to do trouble-shooting; 2) the BGP update volume is unnecessarily increased when there are multiple physical links between them and those links couldn't be configured as a Link Aggregtion Group (LAG) due to whatever reason (e.g., diffferent link type or speed). As a result, it's more common that loopback interface addresses of those directly connected BGP peers are used for BGP session establishment. To make those loopback addresses of directly connected BGP peers reachable from one another, either static routes have to be configured or some kind of IGP has to be enabled. The former is not good from the automation perspective while the latter is in conflict with the original intention of using BGP as IGP.
This draft specifies a BGP neighbor autodiscovery mechanism by borrowing some ideas from the Label Distribution Protocol (LDP) [RFC5036] . More specifically, directly connected BGP routers could automatically discovery the loopback address and the ASN of one other through the exchange of the to-be-defined BGP HELLO messages. The BGP session establishment process as defined in [RFC4271] is triggered once directly connected BGP neighbors are discovered from one another. Note that the BGP session should be established over the discovered loopback address of the BGP neighbor. In addition, to elimnate the need of configing static routes or enabling IGP for the loopback addresses, a certain type of routes towards the BGP neighbor's loopback addresses are dymatically created once the BGP neighbor has been discovered. The administritive distance of such type of routes MUST be smaller than their equivalents which are learnt via the normal BGP update messages . Otherwise, circular dependency problem would occur once these loopback addresses are advertised via the normal BGP update messages as well.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
This memo makes use of the terms defined in [RFC4271].
To automatically discover directly connected BGP neighbors, a BGP router periodically sends BGP HELLO messages out those interfaces on which BGP neighbor autodiscovery are enabled. The BGP HELLO message is a new BGP message which has the same fixed-size BGP header as the exiting BGP messages. However, the HELLO message MUST sent as UDP packets addressed to the to-be-assigned BGP discovery port (179 is the suggested port value) for the "all routers on this subnet" group multicast address (i.e., 224.0.0.2 in the IPv4 case and FF02::2 in the IPv6 case. The IP source address is set to the address of the interface over which the message is sent out.
In addition to the fixed-size BGP header, the HELLO message contains the following fields:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version | Hold Time | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLVs |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: BGP Hello Message
The ASN TLV format is show as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=TBD2 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AS Number (2-octet or 4-octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: ASN TLV
The Connection Address TLV format is shown as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=TBD3 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Connection Address (4-octet or 16-octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Connection Address TLV
A BGP peer receiving Hellos from another peer maintains a Hello adjacency corresponding to the Hellos. The peer maintains a hold timer with the Hello adjacency, which it restarts whenever it receives a Hello that matches the Hello adjacency. If the hold timer for a Hello adjacency expires the peer discards the Hello adjacency.
We recommend that the interval between Hello transmissions be at most one third of the Hello hold time.
A BGP session with a peer has one or more Hello adjacencies.
A BGP session has multiple Hello adjacencies when a pair of BGP peers is connected by multiple links that have the same connection address; for example, multiple PPP links between a pair of routers. In this situation, the Hellos a BGP peer sends on each such link carry the same Connection Address. In addition, to elimnate the need of configing static routes or enabling IGP for the loopback addresses, a certain type of routes towards the BGP neighbor's loopback addresses (e.g., carried in the Connection Address TLV) are dymatically created once the BGP neighbor has been discovered. The administritive distance of such type of routes MUST be smaller than their equivalents which are learnt via the normal BGP update messages. Otherwise, circular dependency problem would occur once these loopback addresses are advertised via the normal BGP update messages as well.
BGP uses the regular receipt of BGP Discovery Hellos to indicate a peer's intent to keep BGP session identified by the Hello. A BGP peer maintains a hold timer with each Hello adjacency that it restarts when it receives a Hello that matches the adjacency. If the timer expires without receipt of a matching Hello from the peer, BGP concludes that the peer no longer wishes to keep BGP session for that link or that the peer has failed. The BGP peer then deletes the Hello adjacency. When the last Hello adjacency for an BGP session is deleted, the BGP peer terminates the BGP session by sending a Notification message and closing the transport connection.
TBD
The authors would like to thank
TBD.
TBD
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC4271] | Rekhter, Y., Li, T. and S. Hares, "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, DOI 10.17487/RFC4271, January 2006. |
| [I-D.keyupate-idr-bgp-spf] | Patel, K., Lindem, A., Zandi, S. and G. Velde, "Shortest Path Routing Extensions for BGP Protocol", Internet-Draft draft-keyupate-idr-bgp-spf-02, December 2016. |
| [RFC5036] | Andersson, L., Minei, I. and B. Thomas, "LDP Specification", RFC 5036, DOI 10.17487/RFC5036, October 2007. |
| [RFC7938] | Lapukhov, P., Premji, A. and J. Mitchell, "Use of BGP for Routing in Large-Scale Data Centers", RFC 7938, DOI 10.17487/RFC7938, August 2016. |