Advertisement of Algorithm in BGP

proposes a solution that allows IGPs themselves to compute constraint-based paths over the SR network. allows flex-algo to be deployed in any IP network, even in the absence of SR-MPLS and SRv6. However, the algorithm-based path can only be used in the IGP domain. In the BGP-based inter-domain scenario, end-to-end path based on algorithms cannot be supported. This document proposes extensions to BGP to support algorithm-based end-to-end path establishment.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

This document defines a new transitive BGP Extended Communities Attribute. This new Extended Community has the following encoding, where:

Type: 1 octet. The value is 0x03. Sub-Type: 1 octet. TBA1 Flags: 1 octet. Unused, MUST be set to 0 and ingored on receipt. Algorithm: 1 octet specifying IGP Algorithm Types. Value from 0 to 255. Reserved: 4 Octets. MUST be set to 0, ignored at reception

| |<-----IGP Domain2---->| ]]> As shown in , node 8 is configured with two loopback addresses, loopback-1 and loopback-2, they belong to the flex-algo 128 plane and the flex-algo 129 plane respectively. In IGP domain 2, the routes to loopback-1 will be generated on the nodes (e.g, node 5,6,8) in the Flex-algo 128 plane and the routes to loopback-2 will be generated on the nodes (e.g, node 5,7,8) in the Flex-algo 129 plane. Node 5 can advertise prefix loopback-1 and prefix loopback-2 to node 4 through BGP. Node 4 can import the BGP routes into IGP and continue to advertise the routes to its neighbors in IGP domain1. Or, node 4 directly advertises the routes to node 1 through BGP. In both cases, the corresponding algorithm information from IGP domain2 is lost during the advertisement. As a result, node 4 does not know which Flex-algo plane to import loopback-1 or loopback-2 into IGP domain 1. With the Algorithm Extended Community, the algorithm information can be carried in the BGP route of loopbacks advertised from node 5 to node 4. The administrator can configure algorithms in each IGP domain in the network. A simple configuration method is that algorithms in each IGP domain are consistent. If the algorithm configurations in each IGP domain are inconsistent, the ASBR needs to know the mapping relationship of the algorithms and carry the converted algorithm information in Algorithm Extended Community when advertising the BGP route. A BGP speaker can advertise multiple paths for the same address prefix, each path is identified by a Path Identifier in addition to the address prefix . By leveraging add-path, multiple loopbacks on the egress node can be avoided. Same approach is applicable for SRv6 locator which is also advertised via , SR-MPLS BGP Prefix SID advertisement and BGP Labeled Unicast(BGP-LU). If any Router Reflector existed in the network, it SHOULD support this new Extended Community.

TBD

IANA is requested to allocate the sub-type TBA1 for "Algorithm Extended Community" under the "BGP Transitive Opaque Extended Community"