Segment routing (SR) is describe in RFC 8402 and offer flexible way to provide traffic engineering based on the source routing. With SR there is no longer needs to maintain a per-application and per-flow state, and traffic can obey the forwarding instruction provided in the packet.
SR relies on some extension to the routing protocols ISIS and OSPF. The SR can support any type of control plane such as distributed, centralized or hybrid.
Each control plane model provide different set of capabilities and how instructions are allocated and signaled. In a distributed scenario, the segments are allocated and signaled by IS-IS or OSPF or BGP and each node individually decides how to proceed with a packet.
In a centralized deployment, the segments are allocated by a SR controller and the SR controller is responsible for computes the source-route policies.
In the hybrid model two model mentioned above can be use to compute the source-routing policies.
How it works
Each segment has an identifier (Segment Identifier) that is distributed throughout the network using new IGP extensions. The extensions are equally applicable to IPv4 and IPv6 control planes. Unlike the case for traditional MPLS networks, routers with SR do not require (LDP) and (RSVP-TE) to provide traffic traffic engineering.
SR can support following forwarding planes:
- MPLS where an ordered list of segments is represented as a stack of
- IPv6 where an ordered list of segments is encoded as an IPv6 header
Each router and each link has an associated SID (Segment Identifier) where SID for each node in the routing domain participating with SR must be globally unique and is assigned by a network administrator and represent the shortest path to the router. Unlike node SID an adjacency SID is locally significant and represents a specific adjacency. Early implementation of SR did not allow to change or allocate adjacency-SID manually. It has been change recently and adjacency-SID can be define manually from the range 15000-15999. Manual assignment has several benefits over dynamic allocation such as:
- Support manually provisioned adj-SID that is persistent over reload and restart
- Support same adj-SID to be provisioned for multiple adjacencies to the same neighbor
- Support same adj-SID to be provisioned for multiple adjacencies going to different neighbors
- Multiple manual Adj-SIDs can be configured for a single adjacency.
NOTE: As I am ware only IS-IS supports manual Adj-SID allocation unless something has change recently and OSPF does it as well so please refer to the latest documentation for more information.
With regard to the IGP control plane there are two basic blocks called IGP-Adjacency segment and IGP-Prefix segment .
In the context of the BGP these segments are called BGP-Peering segment and BGP-Prefix segment.
Segment Routing has several benefits such as:
- SDN ready
- Simplify network deployment as there is no requirement additional protocols apart IGP
- Scalability as avoid TE tunnels configuration, avoid large amount of LSP for MPLS-TE, avoid large amount of LDP labels.
This is a simple network setup I will use to show how to configure SR and what command to use to verify it. In addition, I will show some packet capture outputs to verify how control-plane and date plane looks like. That setup is based on Cisco IOS-XE.
This is basic segment-routing configuration for the diagram above. The only difference on each router is the IP address with the index number under segment-routing section, which refers to the Loopback interface.
interface Loopback0 ip address 18.104.22.168 255.255.255.255 segment-routing mpls connected-prefix-sid-map address-family ipv4 22.214.171.124/32 index 201 range 1 exit-address-family router ospf 1 segment-routing area 0 mpls segment-routing prefix-sid-map advertise-local
The index number must be unique on each devices as described early otherwise a router generates similar error message.
%OSPF-4-SR_DUP_SID: OSPF SR Duplicate Node SID index detected, SID 201, prefix 126.96.36.199/32 (R)
As soon as SR is enabled on the router, MPLS application must registered as a client with LSD (Label Switching Database) to allocate labels. SRGB label allocation range by default is between 16000-23999. The first command below shows how to verify the status of the segment routing and the second one how to verify label range allocation on the box.
CSR1#sh segment-routing mpls state Segment Routing MPLS State : ENABLED CSR1#sh segment-routing mpls gb LABEL-MIN LABEL_MAX STATE DEFAULT 16000 23999 ENABLED Yes
In this example OSPF is used for segment routing and if you want to verify OSPF segment routing status, you can run following command ip ospf segment-routing . This command gives you some basic information about relationship between OSPF and SR. Also it shows you what is the forwarding data plane for SR, what is the OSPF area that SR is enable for, what label allocation method is preferred, label range etc.
CSR1#sh ip ospf segment-routing OSPF Router with ID (188.8.131.52) (Process ID 1) Global segment-routing state: Enabled Segment Routing enabled: Area Topology name Forwarding 0 Base MPLS SR Attributes Prefer non-SR (LDP) Labels Do not advertise Explicit Null Local MPLS label block (SRGB): Range: 16000 - 23999 State: Created Registered with SR App, client handle: 2 Connected map notifications active (handle 0x0), bitmask 0x1 Local export map notifications active (handle 0x1), bitmask 0x2 Active policy map notifications active (handle 0x2), bitmask 0x6 Registered with MPLS, client-id: 100 Bind Retry timer not running Adj Label Bind Retry timer not running
With regard to the index number it must be unique as mentioned above due to that prefix-SID is advertised as domain-wide unique index. The prefix-SID points to specific label within the SRGB. The label equals prefix-SID index + SRGB base. For example prefix-SID for the CSR3 router is 203 so based on the label allocation formula, the label for 184.108.40.206 should be 16203. Let’s verify this on the CSR1.
CSR1#sh mpls forwarding-table Local Outgoing Prefix Bytes Label Outgoing Next Hop Label Label or Tunnel Id Switched interface 16 Pop Label 220.127.116.11-Gi1-10.1.12.2 \ 0 Gi1 10.1.12.2 17 Pop Label 18.104.22.168-Gi2-10.1.13.3 \ 0 Gi2 10.1.13.3 16203 Pop Label 22.214.171.124/32 0 Gi2 10.1.13.3 16204 16204 126.96.36.199/32 0 Gi1 10.1.12.2 16205 16205 188.8.131.52/32 0 Gi2 10.1.13.3
As shown above the prefix 184.108.40.206/32 got assigned the label 16203. All other Loopback IP’s got label allocation following the same logic. At this moment I am not going to discuss meaning of other two remaining labels (16 and 17), these two labels are associated with adjacency-SID which will be explain later on.
Now if you know some details about segment-routing let’s look at the packet capture to see how control-plane works for SR with OSPF and how OSPF advertises N-SID and adjacency-SID. In terms of N-SID all necessary information are encoded into LSA-type 10 and as you can see below the prefix 220.127.116.11/32 got allocated SID 201 which is the index number configured for CSR1 router as shown on the diagram.In addition to prefix-SID allocation you can see that Node flag was set to 1 as well, which indicates that this is N-SID. Another information encoded into LSA-type 10 is SRGB SID label, which in this case is equal 16000 as highlighted below.
In terms of adjacency-SID these information are also encoded into LSA-type 10 where you can find information about the link ID and label allocation. Some interesting information are highlighted below for reference.
The above packet capture output shows that label allocation for adjacency-SID advertised by CSR1 to CSR2 is 16. The flag indicates that that label is local significant only. The range for adjacency-SID is different then SRGB and in this example is assigned automatically by the system. That label is used for static MPLS labels allocation. It can be verify by checking mpls forwarding-table on CSR2 router as shown below. Even the CSR1 router advertised adjacency-SID encoded into LSA as the label 16, the CSR2 router shows Pop Label for CSR1 due to that it was received as implicit null label.
CSR2#sh mpls forwarding-table Local Outgoing Prefix Bytes Label Outgoing Next Hop Label Label or Tunnel Id Switched interface 16 Pop Label 18.104.22.168-Gi2-10.1.24.4 \ 0 Gi2 10.1.24.4 17 Pop Label 22.214.171.124-Gi1-10.1.12.1 \ 0 Gi1 10.1.12.1 16201 Pop Label 126.96.36.199/32 0 Gi1 10.1.12.1 16203 16203 188.8.131.52/32 0 Gi1 10.1.12.1 16204 Pop Label 184.108.40.206/32 0 Gi2 10.1.24.4 16205 16205 220.127.116.11/32 0 Gi2 10.1.24.4
Also, you can verify OSPF database to confirm what labels are sent from neighbor as shown below. On the CSR2 router following command was run ip ospf database opaque-area adv-router 18.104.22.168 to display all LSA-type 10 received from neighbor 22.214.171.124. The first LSA contains information about the SRGB range. The second LSA contains information about SID index number assign to the prefix 126.96.36.199/32, Flag N which indicates N-SID and also prefix-SID is included. The last LSA shows what is the adjacency-SID and who is the neighbor on that P2P link. Please note that this output is just a snipped of OSPF database and show some information related to SR advertised by CSR1 to CSR2.
CSR2#sh ip ospf database opaque-area adv-router 188.8.131.52 OSPF Router with ID (184.108.40.206) (Process ID 1) Type-10 Opaque Area Link States (Area 0) LS age: 1574 Options: (No TOS-capability, DC) LS Type: Opaque Area Link Link State ID: 220.127.116.11 Opaque Type: 4 (Router Information) Opaque ID: 0 Advertising Router: 18.104.22.168 LS Seq Number: 80000007 Checksum: 0xBF17 Length: 52 TLV Type: Router Information Length: 4 Capabilities: Graceful Restart Helper Stub Router Support TLV Type: Segment Routing Algorithm Length: 1 Algorithm: IGP metric based SPT TLV Type: Segment Routing Range Length: 12 Range Size: 8000 Sub-TLV Type: SID/Label Length: 3 Label: 16000 LS age: 1575 Options: (No TOS-capability, DC) LS Type: Opaque Area Link Link State ID: 22.214.171.124 Opaque Type: 7 (Extended Prefix) Opaque ID: 0 Advertising Router: 126.96.36.199 LS Seq Number: 80000007 Checksum: 0x732A Length: 44 TLV Type: Extended Prefix Length: 20 Prefix : 188.8.131.52/32 AF : 0 Route-type: Intra Flags : N-bit Sub-TLV Type: Prefix SID Length: 8 Flags : None MTID : 0 Algo : IGP metric based SPT SID : 201 LS age: 1576 Options: (No TOS-capability, DC) LS Type: Opaque Area Link Link State ID: 184.108.40.206 Opaque Type: 8 (Extended Link) Opaque ID: 7 Advertising Router: 220.127.116.11 LS Seq Number: 80000006 Checksum: 0x60CF Length: 48 TLV Type: Extended Link Length: 24 Link connected to : another Router (point-to-point) (Link ID) Neighboring Router ID: 18.104.22.168 Sub-TLV Type: Adj SID Length : 7 Flags : L-Bit, V-bit MTID : 0 Weight : 0 Label : 16
To verify Node-SID you can run following command sh ip ospf segment-routing sid-database which gives you information about index number used by specific prefix and which prefix is localy originated.
CSR2#sh ip ospf segment-routing sid-database OSPF Router with ID (22.214.171.124) (Process ID 1) OSPF Segment Routing SIDs Flags: L - local, N - label not programmed, M - mapping-server SID Prefix/Mask -------- ------------------ 201 126.96.36.199/32 202 (L) 188.8.131.52/32 203 184.108.40.206/32 204 220.127.116.11/32 205 18.104.22.168/32
When segment routing is enable for an OSPF, it enables MPLS for any interface on which the OSPF topology is active and then MPLS becomes responsible for data-plane forwarding.
Lets look at CSR1 mpls forwarding-table to see how it looks especially focus on the prefix 22.214.171.124. To reach this prefix the label 16205 is used and that label number is consistent through the network. Each node has exactly the same label allocation number for that prefix 126.96.36.199, because that number is allocated based on the prefix-SID + SRGB as describe above. Each node received exactly the same information encoded into LSA-type 10 and based on those information label is generated. Because every router has exactly the same OSPF database topology view that’s why each router allocated the same label 16205 for the prefix 188.8.131.52/32. Below is output from two routers where prefix 184.108.40.206/32 has the same label.
CSR1#sh mpls forwarding-table Local Outgoing Prefix Bytes Label Outgoing Next Hop Label Label or Tunnel Id Switched interface 16 Pop Label 220.127.116.11-Gi1-10.1.12.2 \ 0 Gi1 10.1.12.2 16202 Pop Label 18.104.22.168/32 0 Gi1 10.1.12.2 16203 16203 22.214.171.124/32 0 Gi1 10.1.12.2 16204 16204 126.96.36.199/32 0 Gi1 10.1.12.2 16205 16205 188.8.131.52/32 0 Gi1 10.1.12.2 CSR2#sh mpls forwarding-table 184.108.40.206 Local Outgoing Prefix Bytes Label Outgoing Next Hop Label Label or Tunnel Id Switched interface 16205 16205 220.127.116.11/32 1670 Gi2 10.1.24.4 CSR2#
To confirm that, let’s check again OSPF database running following command sh ip ospf database opaque-area adv-router 18.104.22.168 on the CSR1 router. As you can see there are two very important information here SRGB label which is set to 16000 and SID number setup to 205. Based on these information label 16205 is generated and programmed into data-plane for MPLS forwarding towards 22.214.171.124 prefix.
CSR1#sh ip ospf database opaque-area adv-router 126.96.36.199 OSPF Router with ID (188.8.131.52) (Process ID 1) Type-10 Opaque Area Link States (Area 0) LS age: 1344 Options: (No TOS-capability, DC) LS Type: Opaque Area Link Link State ID: 184.108.40.206 Opaque Type: 4 (Router Information) Opaque ID: 0 Advertising Router: 220.127.116.11 LS Seq Number: 80000008 Checksum: 0xA52C Length: 52 TLV Type: Router Information Length: 4 Capabilities: Graceful Restart Helper Stub Router Support TLV Type: Segment Routing Algorithm Length: 1 Algorithm: IGP metric based SPT TLV Type: Segment Routing Range Length: 12 Range Size: 8000 Sub-TLV Type: SID/Label Length: 3 Label: 16000 LS age: 1344 Options: (No TOS-capability, DC) LS Type: Opaque Area Link Link State ID: 18.104.22.168 Opaque Type: 7 (Extended Prefix) Opaque ID: 0 Advertising Router: 22.214.171.124 LS Seq Number: 80000008 Checksum: 0xF996 Length: 44 TLV Type: Extended Prefix Length: 20 Prefix : 126.96.36.199/32 AF : 0 Route-type: Intra Flags : N-bit Sub-TLV Type: Prefix SID Length: 8 Flags : None MTID : 0 Algo : IGP metric based SPT SID : 205
This is packet capture output taken for interface Gig1 at CSR4 router and you can see that incoming ICMP packet from CSR2 towards prefix 188.8.131.52 was label as 16205. Every router on the network has exactly the same label for the destination prefix.
For IPv4 with SR there is still requirement to rely on MPLS data-plane forwarding however there is no more prerequisite to run LDP, as all data-plane capabilities and label ranges are advertised using the SR-capabilities sub-TLV inserted into the OSPF LSA-type 10. With SR we can reduce number of protocols needed to reach the same goal as everything is carried by means of single IGP protocol. SR allows us to reduce network complexity by reducing control-plane protocols and simplify troubleshooting.
The next blog series will be L3VPN and SR.