BGP for SR-MPLS

BGP EPE

Figure 2-41 BGP EPE networking

On the network shown in Figure 2-41, ASBR1 and ASBR3 are directly connected through two physical links. An EBGP peer relationship is established between ASBR1 and ASBR3 through loopback interfaces. ASBR1 runs BGP EPE to assign the Peer-Node SID 28001 to its peer (ASBR3) and to assign the Peer-Adj SIDs 18001 and 18002 to the physical links. For an EBGP peer relationship established between directly connected physical interfaces, BGP EPE allocates a Peer-Node SID rather than a Peer-Adj SID. For example, on the network shown in Figure 2-41, BGP EPE allocates only Peer-Node SIDs 28002, 28003, and 28004 to the ASBR1-ASBR5, ARBR2-ASBR4, and ASBR2-ASBR5 peer relationships, respectively.

Peer-Node SIDs and Peer-Adj SIDs are effective only on local devices. Different devices can have the same Peer-Node SID or Peer-Adj SID. Currently, BGP EPE supports only EBGP peers. For multi-hop EBGP peers, they must be directly connected through physical links. This is because transit nodes do not have BGP peer SID information, resulting in forwarding failures.

Fault Association

On the network shown in Figure 2-42, intra-AS SR-MPLS TE tunnels are deployed for AS 65001, and SBFD for SR-MPLS TE tunnel is configured. BGP EPE is configured among ASBR1, ASBR2, and ASRB3.

Two inter-AS SR-MPLS TE tunnels are deployed between CSG1 and ASBR3. The tunnels are orchestrated using intra-AS SIDs and inter-AS BGP SIDs. If a BGP EPE link between ASBRs fails, CSG1, the ingress of the SR-MPLS TE tunnel, cannot detect the failure, and therefore a traffic black hole may occur.

Currently, two methods are available for you to solve the preceding problem:

1. If a link between ASBR1 and ASBR3 fails, BGP EPE triggers SBFD for SR-MPLS TE tunnel in the local AS to go down. This enables the ingress of the SR-MPLS TE tunnel to rapidly detect the failure and switch traffic to another normal tunnel, such as tunnel 2.
2. If a link between ASBR1 and ASBR3 fails, ASBR1 pops the BGP EPE label from the received SR packet and searches the IP routing table based on the destination address of the packet. In this way, the packet may be forwarded to ASBR3 through the IP link ARBR1-ASBR2-ASBR3. This method applies when no backup inter-AS SR-MPLS TE tunnels exist on the network.

1. You can select either method as needed. Selecting both of the methods is not allowed.
2. The preceding methods are also supported if all the links corresponding to a BGP Peer-Set SID fail.

Figure 2-42 BGP EPE fault association

BGP-LS

Inter-AS E2E SR-MPLS TE tunnels can be established by manually specifying explicit paths, or they can be orchestrated by a controller. In a controller-based orchestration scenario, intra- and inter-AS SIDs are reported to the controller using BGP-LS. Inter-AS links must support TE link attribute configuration and reporting to the controller. This enables the controller to compute primary and backup paths based on link attributes. BGP EPE discovers network topology information and allocates SID information. BGP-LS then packages this information into the Link network layer reachability information (NLRI) field and reports it to the controller. Figure 2-43 and Figure 2-44 show Link NLRI formats.

Figure 2-43 Link NLRI format for Peer-Node and Peer-Adj SIDs advertised by BGP-LS

Figure 2-44 Link NLRI format for Peer-Set SIDs advertised by BGP-LS

Table 2-21 Link NLRI fields

Field	Description
NLRI	Network layer reachability information. It consists of the following parts: LocalDescriptor: local descriptor, which contains a local router ID, a local AS number, and a BGP-LS ID RemoteDescriptor: remote descriptor, which contains a peer router ID and a peer AS number LinkDescriptor: link descriptor, which contains addresses used by a BGP session
LinkAttribute	Link information. It is a part of the Link NLRI. Peer-Node SID: Peer-Node SID TLV Peer-Adj SID: Peer-Adj SID TLV Peer-Set SID: Peer-Set SID TLV Administrative Group: link management group attribute Max Link BW: maximum link bandwidth Max Reservable Link BW: maximum reservable link bandwidth Unreserved BW: remaining link bandwidth Shared Risk Link Group: SRLG

A Peer-Node SID TLV and a Peer-Adj SID TLV have the same format, as shown in Figure 2-45.

Figure 2-45 Peer SID TLV format

Table 2-22 Fields in a peer SID TLV

Field	Length	Description
Type	16 bits	TLV type.
Length	16 bits	Packet length.
Flags	8 bits	Flags field used in a Peer-Adj SID TLV. Figure 2-46 shows the format of this field. Figure 2-46 Flags field In this field: V: value flag. If set, the Adj-SID carries a value. By default, the flag is set. L: local flag. If set, the value or index carried by the Adj-SID has local significance. By default, the flag is set.
Weight	8 bits	Weight of the Adj-SID, used for load balancing purposes.
SID/Index/Label (variable)	Variable length	This field contains either of the following information based on the V and L flags: A 3-octet local label where the 20 rightmost bits are used for encoding the label value. In this case, the V and L flags must be set. A 4-octet index defining the offset in the Segment Routing global block (SRGB).