BGP Link-State¶
Overview¶
BGP Link-State, or BGP-LS, is an extension to BGP designed to carry Link-State information from an IGP. BGP-LS is primarily used in scenarios like TE, where a centralized controller (like a SDN controller) needs visibility into the network topology to make more intelligent routing decisions. Importantly, BGP-LS is solely an information carrier; it does not replace Link-State routing protocols like OSPF or IS-IS. Its purpose is not to contribute to the RIB or to influence routing decisions directly.
BGP-LS provides network controllers with an efficient means of gathering information from an IGP database, which was commonly achieved by connecting the controller to the IGP as a standard router. BGP-LS sidesteps the requirement for IGP Layer 2 adjacencies, which traditionally necessitated the use of Layer-2 tunnels like GRE to link IGP routers with centralized controllers. Unlike IGPs, BGP-LS is not involved in providing candidate paths to the RIB, serving purely as an information conduit. Moreover, BGP's inherent ability to relay information over multiple hops and across various domains offers flexibility, allowing network administrators to easily integrate BGP-LS into existing BGP setups.
Data Encoding¶
BGP-LS employs AFI 16388 and SAFI 71 to transport IGP information like OSPF or IS-IS to a centralized controller. The data conveyed includes key elements of the network topology, which are called descriptors:
Nodes: Representing routers in the network.
Link: Detailing the connections between these nodes.
IPv4 Prefix: IPv4 network addresses associated with these elements.
IPv6 Prefix: IPv6 network addresses associated with these elements.
These elements are encapsulated within the NLRI field in BGP-LS updates. To complement these descriptors, BGP-LS introduces a new set of BGP attributes, known as “BGP-LS attributes”, to carry additional context associated with the descriptors.
Both the NLRI descriptors and the BGP-LS attributes use the TLV encoding scheme which is already used by IS-IS in its LSPs and OSPF in its opaque LSAs. Nevertheless, BGP-LS TLV code-points and value formats are different from their equivalents in IS-IS and OSPF.
See also
The descriptors format is specified in in RFC 7752. For the attribute formats, the list of TLV , managed by the IANA provides the RFCs associated with the code points.
BGP-LS in Virtual Service Router¶
Virtual Service Router allows for participation in BGP-LS sessions using the designated AFI and SAFI to communicate with third-party routers. However, Virtual Service Router does not have the ability to build BGP-LS data based on OSPF or IS-IS information. In other words, it can act as a relay for BGP-LS information but not as a speaker. This makes it a suitable solution in designs where the objective is to relay BGP-LS information originating from equipment provided by third-party vendors. The Virtual Service Router BGP-LS can forward data to other BGP-LS-compatible peers or to the SDN controller directly.
Virtual Service Router forwards BGP-LS information encapsulated in NLRI and additional attributes without any modification. This is significant as the BGP-LS TLV codes are frequently updated through new RFCs. The Virtual Service Router's approach of leaving data untouched ensures its adaptability to any future changes or additions to the BGP-LS standard.
Configuration¶
The following example configures a BGP-LS peering with the 192.0.2.1
neighbor within AS 65002:
router running config# / vrf main routing bgp
router running bgp# as 65002
router running bgp# address-family link-state-non-vpn enabled true
router running bgp# neighbor 192.0.2.1
router running neighbor 192.0.2.1#! remote-as 65002
router running neighbor 192.0.2.1# address-family link-state-non-vpn
router running link-state-non-vpn# enabled true
router running link-state-non-vpn# soft-reconfiguration-inbound true
router running link-state-non-vpn# commit
Configuration committed.
The received BGP-LS prefixes are displayed as follows:
router> show bgp link-state link-state all
BGP table version is 8, local router ID is 192.0.2.2, vrf id 0
Default local pref 100, local AS 65002
Network Next Hop Metric LocPrf Weight Path
*> Link OSPFv3 ID:0xff {Local {AS:65002 ID:0 Area:1 Rtr:10.10.10.11:2.2.2.2} Remote {AS:65002 ID:0 Area:1 Rtr:10.10.10.10:1.1.1.1} IPv4:10.1.0.1 Neigh-IPv4:10.1.0.2 IPv6:2001::1 Neigh-IPv6:2001::2 MT:0,2}/166
0 65002 i
*> IPv6-Prefix OSPFv3 ID:0x20 {Local {AS:65002 ID:0 Area:0 Rtr:10.10.10.10} MT:2 OSPF-Route-Type:1 IPv6:12:12::12:12/128}/77
0 65002 i
*> IPv6-Prefix ISIS-L2 ID:0x20 {Local {AS:65002 ID:0 Rtr:0000.0000.1003.00} MT:2 IPv6:12:12::12:12/128}/67
0 65002 i
*> IPv4-Prefix OSPFv2 ID:0x20 {Local {AS:65002 ID:0 Area:0 Rtr:10.10.10.10:1.1.1.1} IPv4:89.10.11.0/24}/57
0 65002 i
*> Node OSPFv2 ID:0x20 {Local {AS:65002 ID:0 Area:0 Rtr:10.10.10.10:1.1.1.1}}/49
0 65002 i
*> Node OSPFv2 ID:0x20 {Local {AS:65002 ID:0 Area:0 Rtr:10.10.10.10}}/45
0 65002 i
*> Node ISIS-L1 ID:0x20 {Local {AS:65002 ID:0 Rtr:0000.0000.1003.00}}/40
0 65002 i
*> Link ISIS-L1 ID:0x20 {Local {AS:65002 ID:0 Rtr:0000.0000.1001} Remote {AS:65002 ID:0 Rtr:0000.0000.1000} IPv4:10.1.0.1 Neigh-IPv4:10.1.0.2 IPv6:2001::1 Neigh-IPv6:2001::2 MT:0,2}/133
0 65002 i
Displayed 8 routes and 8 total paths
With the details:
router> show bgp link-state link-state all detail-routes
BGP table version is 8, local router ID is 192.0.2.2, vrf id 0
Default local pref 100, local AS 65002
[..]
BGP routing table entry for Node OSPFv2 ID:0x20 {Local {AS:65002 ID:0 Area:0 Rtr:10.10.10.10}}/45, version 3
Paths: (1 available, best #1)
Advertised to non peer-group peers:
192.0.2.1 192.0.2.3
65002
:: from 192.0.2.1 (192.0.2.1)
Origin IGP, valid, external, best (First path received)
Last update: Wed Sep 6 15:33:15 2023
BGP-LS attributes:
IGP Flags: 0b10000000
Opaque Link Attribute:
TLV type 11: 0x0101
TLV type 5: 0x0101
RTM Capability: Flags: 0b110 Values: 0x1123ed
L2 Bundle Member Attributes: Descriptor: 0xdc12fceb
Administrative group: 0x000f0001
Unidirectional Link Loss: 50.3316%
Flexible Algorithm Definition:
Flex-Algo: 130
Metric-Type: 0
Calc-Type: 0
Priority: 128
Flexible Algorithm Include Any Affinity: 0x00000001
Flexible Algorithm Exclude SRLG: 10
Flexible Algorithm Definition Flags: 0b10000000 00000000 00000000 00000000
Flexible Algorithm Prefix Metric:
Flex-Algo: 130
Flags: 0b10000000
Metric: 1000
Graceful-Link-Shutdown TLV: Enabled
Source Router Identifier: 12:12::12:12
TLV type 1871: 0x0012001200000000 0000000000
BGP routing table entry for Node ISIS-L1 ID:0x20 {Local {AS:65002 ID:0 Rtr:0000.0000.1003.00}}/40, version 2
Paths: (1 available, best #1)
Advertised to non peer-group peers:
192.0.2.1 192.0.2.3
65002
:: from 192.0.2.1 (192.0.2.1)
Origin IGP, valid, external, best (First path received)
Last update: Wed Sep 6 15:33:15 2023
BGP-LS attributes:
Multi-Topology ID: 0, 2
Node MSD:
Type: 8 Value: 1
Node Name: r3
IS-IS Area Identifier: 49.1000
IPv4 Router-ID of Local Node: 3.3.3.3
SR Capabilities:
Flags: 0b11000000
Range: 4000
SID/Label: From Label: 20000
SR Algorithm: 0, 128, 130
SR Local Block:
Flags: 0b00000000
Range: 1000
SID/Label: From Label: 15000
BGP routing table entry for Link ISIS-L1 ID:0x20 {Local {AS:65002 ID:0 Rtr:0000.0000.1001} Remote {AS:65002 ID:0 Rtr:0000.0000.1000} IPv4:10.1.0.1 Neigh-IPv4:10.1.0.2 IPv6:2001::1 Neigh-IPv6:2001::2 MT:0,2}/133, version 1
Paths: (1 available, best #1)
Advertised to non peer-group peers:
192.0.2.1 192.0.2.3
65002
:: from 192.0.2.1 (192.0.2.1)
Origin IGP, valid, external, best (First path received)
Last update: Wed Sep 6 15:33:15 2023
BGP-LS attributes:
IPv4 Router-ID of Local Node: 1.1.1.1
IPv4 Router-ID of Remote Node: 10.10.10.10
Maximum link bandwidth: 1410.07 Mbps
Max. reservable link bandwidth: 1410.07 Mbps
Unreserved bandwidth:
[0]: 1410.07 Mbps [1]: 1410.07 Mbps
[2]: 1410.07 Mbps [3]: 1410.07 Mbps
[4]: 1410.07 Mbps [5]: 1410.07 Mbps
[6]: 1410.07 Mbps [7]: 1410.07 Mbps
TE Default Metric: 100
IGP Metric: 10
Adjacency SID:
Flags: 0b00110000
Weight: 0
SID: 15000
Unidirectional Link Delay: 8500 microseconds
Min/Max Unidirectional Link Delay: 8000/9000 microseconds
Application-Specific Link Attributes:
SABM Flags : 0b00010000 00000000 00000000 00000000
UDABM Flags: 0b00000000 00000000 00000000 00000000
Administrative group: 0x00000001
TE Default Metric: 100
Min/Max Unidirectional Link Delay: 8000/9000 microseconds
Extended Administrative Group: 0x00000001
Displayed 8 routes and 8 total paths
Note
Virtual Service Router supports the display of the TLVs from the following RFCs:
RFC 5130 : A Policy Control Mechanism in IS-IS Using Administrative Tags
RFC 5305 : IS-IS Extensions for Traffic Engineering
RFC 5305 : IS-IS Extensions for Traffic Engineering
RFC 5307 : IS-IS Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS)
RFC 6119 : IPv6 Traffic Engineering in IS-IS
RFC 7752 : North-Bound Distribution of Link-State and Traffic Engineering (TE) Information Using BGP
RFC 8169 : Residence Time Measurement in MPLS Networks
RFC 8379 : OSPF Graceful Link Shutdown
RFC 8571 : BGP - Link State (BGP-LS) Advertisement of IGP Traffic Engineering Performance Metric Extensions
RFC 8814 : Signaling Maximum SID Depth (MSD) Using the Border Gateway Protocol - Link State
RFC 9085 : Border Gateway Protocol - Link State (BGP-LS) Extensions for Segment Routing
RFC 9086 : Border Gateway Protocol - Link State (BGP-LS) Extensions for Segment Routing BGP Egress Peer Engineering
RFC 9104 : Distribution of Traffic Engineering Extended Administrative Groups Using the Border Gateway Protocol - Link State (BGP-LS)
RFC 9247 : BGP - Link State (BGP-LS) Extensions for Seamless Bidirectional Forwarding Detection (S-BFD)
RFC 9294 : Application-Specific Link Attributes Advertisement Using the Border Gateway Protocol - Link State (BGP‑LS)
RFC 9351 : Border Gateway Protocol - Link State (BGP-LS) Extensions for Flexible Algorithm Advertisement
Unknown TLVs are displayed in the format: “TLV type <code-point>: <hexadecimal data>”.