BGP Neighbor Autodiscovery

BGP has been used as the underlay routing protocol instead of IGP in many hyper-scale data centers . Furthermore, there is an ongoing effort to leverage BGP link-state distribution mechanism to achieve BGP-SPF . However, BGP is not good as an IGP from the perspective of deployment automation and simplicity. For instance, the IP address and the Autonomous System Number (ASN) of each and every BGP neighbor have to be manually configured on BGP routers although these BGP peers are directly connected. Furthermore, for those BGP routers with multiple physical links being connected, it's usually not ideal to establish BGP sessions over their directly connected interface addresses because the BGP update volume would be unnecessarily increased, meanwhile, it may not be suitable to configure those links as a Link Aggregation Group (LAG) due to some reasons. As a result, it's more common that loopback interface addresses of those directly connected BGP peers are used for BGP session establishment purpose. 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 network automation perspective while the latter is not good from the network simplification perspective (i.e., running less routing protocols). This draft specifies a BGP neighbor autodiscovery mechanism by borrowing some ideas from the Label Distribution Protocol (LDP) . More specifically, directly connected BGP routers could automatically discovery each other through the exchange of the to-be-defined BGP Hello messages. The BGP session establishment process as defined in could be triggered once directly connected BGP neighbors are discovered from one another. Note that the BGP session should be established over the discovered the peering address of the BGP neighbor and in most cases the peering address is a loopback address. In addition, to eliminate the need of configuring static routes or enabling IGP for the loopback addresses, a certain type of routes towards the BGP neighbor's loopback addresses as advertised as peering addresses are dynamically instantiated once the BGP neighbor has been discovered. The administrative distance of such type of routes MUST be smaller than their equivalents that are learnt by the regular BGP update messages . Otherwise, circular dependency would occur once these loopback addresses are advertised via the regular BGP updates.

This memo makes use of the terms defined in .

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 MUST sent as a UDP packet with a destination port of TBD (179 is the preferred port number value) addressed 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. The HELLO message contains the following fields:

Version: This 1-octet unsigned integer indicates the protocol version number of the message. The current BGP version number is 4. Type: The type of BGP message (Hello - TBD value from BGP Message Types Registry) Message Length: This 2-octet unsigned integer specifies the length in octets of the TLVs field. AS number: AS Number of the Hello message sender. BGP Identifier: BGP Identifier of the Hello message sender. Hold Time: Hello hold timer in seconds. Hello Hold Time specifies the time the receiving BGP peer will maintain its record of Hellos from the sending BGP peer without receipt of another Hello. The RECOMMENDED default value is 15 seconds. A value of 0 means that the receiving BGP peer should maintain its record until the link is UP. Reserved: SHOULD be set to 0 by sender and MUST be ignored by receiver. TLVs: This field contains one or more TLVs as described below. The Accepted ASN List TLV is an optional TLV that is used to signal the AS numbers from which the router would accept BGP sessions. When not signaled, it indicates that the router will accept BGP peering from any ASN from its neighbors. Only a single instance of this TLV is included and its format is shown below.

Type: TBD1 Length:Specifies the length of the Value field in octets. Accepted ASN-List: This variable-length field contains one or more accepted 4-octet ASNs. The Peering Address TLV is used to indicate to the neighbor the address to which they should establish BGP session. For each peering address, the router can specify its supported AFI/SAFI(s). When the AFI/SAFI values are specified as 0/0, then it indicates that the neighbor can attempt for negotiation of any AFI/SAFIs. The indication of AFI/SAFI(s) in the Peering Address TLV is not intended as an alternative for the MP capabilities negotiation mechanism. The Peering Address TLV format is shown below and at least one instance of this TLV MUST be present.

Type: TBD2 Length:Specifies the length of the Value field in octets. Flags : Current defined bits are as follows. All other bits SHOULD be cleared by sender and MUST be ignored by receiver. Bit 0x1 - address is IPv6 when set and IPv4 when clear Number of AFI/SAFI: indicates the number of AFI/SAFI pairs that the router supports on the given peering address. Reserved: sender SHOULD set to 0 and receiver MUST ignore. Address: This 4 or 16 octect field indicates the IPv4 or IPv6 address which is used for establishing BGP sessions. AFI/SAFI : one or more pairs of these values that indicate the supported capabilities on the peering address. Sub-TLVs : currently none defined When the Peering Address used is not the directly connected interface address (e.g. when it is a loopback address) then local prefix(es) that cover the peering address(es) MUST be signaled by the router. This allows the neighbor to learn these local prefix(es) and to program routes for them over the directly connected interfaces over which they are being signalled. The Local Prefixes TLV is used to only signal prefixes that are locally configured on the router and its format is as shown below.

Type: TBD3 Length:Specifies the length of the Value field in octets No. of IPv4 Prefixes : specifies the number of IPv4 prefixes. When value is 0, then it indicates no IPv4 Prefixes are present. No. of IPv6 Prefixes : specifies the number of IPv6 prefixes. When value is 0, then it indicates no IPv6 Prefixes are present. IPv4 Prefix Address & Prefix Mask: Zero or more pairs of IPv4 prefix address and their mask. IPv6 Prefix Address & Prefix Mask: Zero or more pairs of IPv6 prefix address and their mask. Sub-TLVs : currently none defined The Link Attributes TLV is a mandatory TLV that signals to the neighbor the link attributes of the interface on the local router. A single instance of this TLV MUST be present in the message. The Link Attributes TLV is as shown below.

Type: TBD4 Length:Specifies the length of the Value field in octets Local Interface ID : the local interface ID of the interface (e.g. the MIB-2 ifIndex) Flags : Currently defined bits are as follows. Other bits SHOULD be cleared by sender and MUST be ignored by receiver. Bit 0x1 - indicates link is enabled for IPv4 Bit 0x2 - indicates link is enabled for IPv6 Reserved: SHOULD be set to 0 by sender and MUST be ignored by receiver. No. of IPv4 Addresses : specifies the number of IPv4 local addresses on the interface. When value is 0, then it indicates no IPv4 Prefixes are present or the interface is IP unnumbered. No. of IPv6 Addresses : specifies the number of IPv6 Global addresses on the interface. When value is 0, then it indicates no IPv6 Global Prefixes are present or the interface is only configured with IPv6 link-local addresses IPv4 Address & Mask: Zero or more pairs of IPv4 address and their mask. IPv6 Address & Mask: Zero or more pairs of IPv6 address and their mask. Sub-TLVs : currently none defined The Neighbor TLV is used by a BGP router to indicate the peering address and information about the neighbors that have been discovered by the router on the specific link and their status. The BGP session establishment process begins when both the neighbors accept each other over at least one underlying inter-connecting link between them. The Neighbor TLV format is as shown below.

Type: TBD5 Length:Specifies the length of the Value field in octets Flags : Currently defined 0x1 bit is clear when Peering Address is IPv4 and set when IPv6. Other bits SHOULD be clear by sender and MUST be ignored by receiver. Status : Indicates the status code of the peering for the particular session over this link. The following codes are currently defined 0 - Indicates 1-way detection of the peer 1 - Indicates rejection of the peer due to local policy reasons (i.e. local router would not be initiating or accepting session to this neighbor) 2 - Indicates 2-way detection of the peering by both neighbors 3 - Indicates that the BGP peering session has been established between the neighbors and that this link would be utilized for forwarding to the peer BGP nexthop Reserved: SHOULD be set to 0 by sender and MUST be ignored by receiver. Neighbor Peering Address: This 4 or 16 octect field indicates the IPv4 or IPv6 peering address of the neighbor for which peering status is being reported. Sub-TLVs : currently none defined

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 (e.g., multiple point-to-point links between a pair of routers). In this situation, the Hellos a BGP peer sends on each such link carry the same Peering Address. In addition, to eliminate the need of configuring static routes or enabling IGP for advertising the loopback addresses, a certain type of routes towards the BGP neighbor's loopback addresses (i.e. carried in the Local Prefixes TLV) could be dynamically created once the BGP neighbor has been discovered. The administrative 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 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. The route towards the BGP neighbor's loopback address that had been dynamically created due to that BGP Hello adjacency SHOULD be deleted accordingly. When the last Hello adjacency for an BGP session is deleted, the BGP peer terminates the BGP session and closing the transport connection.

The authors would like to thank Enke Chen for his valuable comments and suggestions on this document.

This document requests IANA to allocate a new UDP port (179 is the preferred number ) and a BGP message type code for BGP Hello message.

Contact: IETF Chair . Description: BGP Hello Message. Reference: This document -- draft-xu-idr-neighbor-autodiscovery. Port Number: TBD1 (179 is the preferred value) -- To be assigned by IANA.]]>

This document requests IANA to create a new registry "TLVs of BGP Hello Message" with the following registration procedure:

For security purposes, BGP speakers usually only accept TCP connection attempts to port 179 from the specified BGP peers or those within the configured address range. With the BGP neighbor auto-discovery mechanism, it's configurable to enable or disable sending/receiving BGP hello messages on the per-interface basis and BGP hello messages are only exchanged between physically connected peers that are trustworthy. Therefore, the BGP neighbor auto-discovery mechanism doesn't introduce additional security risks associated with BGP. In addition, for the BGP sessions with the automatically discovered peers via the BGP hello messages, the TTL of the TCP/BGP messages (dest port=179) MUST be set to 255. Any received TCP/BGP message with TTL being less than 254 MUST be dropped according to .