BGP for SR-MPLS

IGP for SR-MPLS allocates SIDs only within an autonomous system (AS). Proper SID orchestration in an AS facilitates the planning of an optimal path in the AS. However, IGP for SR-MPLS does not work if paths cross multiple ASs on a large-scale network. BGP for SR-MPLS is an extension of BGP for Segment Routing. This function enables a device to allocate BGP peer SIDs based on BGP peer information and report the SID information to a controller. SR-MPLS TE uses BGP peer SIDs for path orchestration, thereby obtaining the optimal inter-AS E2E SR-MPLS TE path. BGP for SR-MPLS involves the BGP egress peer engineering (EPE) extension and BGP-LS extension.

BGP EPE

BGP EPE allocates BGP peer SIDs to inter-AS paths. BGP-LS advertises the BGP peer SIDs to the controller. A forwarder that does not establish a BGP-LS peer relationship with the controller can run BGP-LS to advertise a peer SID to a BGP peer that has established a BGP-LS peer relationship with the controller. The BGP peer then runs BGP-LS to advertise the peer SID to the network controller. As shown in Figure 1, BGP EPE can allocate peer node segments (Peer-Node SIDs), peer adjacency segments (Peer-Adj SIDs), and Peer-Set SIDs to peers.

A Peer-Node SID identifies a peer node. The peers at both ends of each BGP session are assigned Peer-Node SIDs. An EBGP peer relationship established based on loopback interfaces may traverse multiple physical links. In this case, the Peer-Node SID of a peer is mapped to multiple outbound interfaces.
A Peer-Adj SID identifies an adjacency to a peer. An EBGP peer relationship established based on loopback interfaces may traverse multiple physical links. In this case, each adjacency is assigned a Peer-Adj SID. Only a specified link (mapped to a specified outbound interface) can be used for forwarding.
A Peer-Set SID identifies a group of peers that are planned as a set. BGP allocates a Peer-Set SID to the set. Each Peer-Set SID can be mapped to multiple outbound interfaces during forwarding. Because one peer set consists of multiple peer nodes and peer adjacencies, the SID allocated to a peer set is mapped to multiple Peer-Node SIDs and Peer-Adj SIDs.

Figure 1 BGP EPE networking
Click to enlarge

On the network shown in Figure 1, 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 1, 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 IPv4 EBGP peers. Multi-hop EBGP peers must be directly connected through physical links.

BGP EPE allocates SIDs to BGP peers and links, but cannot be used to construct a forwarding tunnel. BGP peer SIDs must be used with IGP SIDs to establish an E2E tunnel. SR-MPLS mainly involves SR LSPs (SR-MPLS BE) and SR-MPLS TE.

SR LSPs are dynamically computed by forwarders based on intra-AS IGP SIDs. Because peer SIDs allocated by BGP EPE cannot be used by an IGP, inter-AS SR LSPs are not supported.
E2E SR-MPLS TE tunnels can be established by manually specifying an explicit path. They can also be orchestrated by a network controller provided that the specified path contains inter-AS link information.

Fault Association

On the network shown in Figure 2, 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:

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.
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.

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

Figure 2 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 3 and Figure 4 show Link NLRI formats.

Figure 3 Link NLRI format for Peer-Node and Peer-Adj SIDs advertised by BGP-LS

Figure 4 Link NLRI format for Peer-Set SIDs advertised by BGP-LS

**Table 1** 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. Figure 5 shows the format of the Peer-Node SID TLV and Peer-Adj SID TLV.

Figure 5 Peer-Node SID TLV and Peer-Adj SID TLV format

**Table 2** Peer-Node SID TLV and Peer-Adj SID TLV fields
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 6 shows the format of this field. Figure 6 Flags field In this field: V: value flag. If this flag is set, an Adj-SID carries a label value. By default, the flag is set. L: local flag. If this flag is 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 Peer-Adj SID, used for load balancing purposes.
Reserved	16 bits	Reserved field.
SID/Label/Index (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).