BGP 3107 Architecture for Large ISP-Telecoms

BGP Label Update: Carrier Support Carrier - RFC 3107 post was describing how BGP RFC 3107 help service provider to share the VPN routes across the boundaries without sharing the PE routes. BGP Label Update feature is mainly used during inter-AS communication so that the changes occurring in one network couldn't impact the other.

Now consider in case of LTE network, there are thousand of thousand of eNODEB with all IP connectivity. Telecom provider is using IGP+LDP within the single domain. In this case, all the routes of eNODEB will flood across the domain however which is not required. Even we can't perform the summarization on loopbacks to reduce the number of routes within single area. How the scalability of PE will work in this kind of network?

Probably, BGP-3107 based architecture can be utilized within single Service Provider domain also to carry PE reachability. BGP IPv4-label address-family sessions between PE and P routers. IGP+LDP still runs within areas but does not carry PE reachability across areas. Remote PE loopback is a BGP ipv4 labeled route in RIB and next-hop for BGP service prefix is a BGP 3107 route.

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BGP Label Update: Carrier Support Carrier - RFC 3107

BGP Label Update allows you to set up a Virtual Private Network (VPN) network so that the autonomous system boundary routers (ASBRs) exchange IPv4 routes with Multiprotocol Label Switching (MPLS) labels of the provider edge (PE) routers. In this scenario, Route reflectors (RRs) exchange VPNv4 routes and ASBR get relaxed to store those routes.

This results in improved scalability and simplifies the configuration. By using this feature, you can use non MPLS network as transit network, this helps you to transport all the IPv4 routes with labels over non MPLS network.

When you issue the neighbor send-label command under BGP configuration, the routers advertise to each other that they can then send MPLS labels with the routes. If the routers successfully negotiate their ability to send MPLS labels, the routers add MPLS labels to all outgoing BGP updates. This eliminates the need for using any label distribution protocol between the LSRs.(IS LDP Required For VPNv4 Labels)

In the above scenario, route reflector can reflect the IPv4 routes and MPLS labels learned from the ASBR to the PE routers in the VPN. This is accomplished by enabling the ASBR to exchange IPv4 routes and MPLS labels with the route reflector. The route reflector also reflects the VPNv4 routes to the PE routers in the VPN. ASBRs exchange IPv4 routes and MPLS labels for the PE routers by using EBGP.

MPLS labels are included in the update messages. Routers exchange the following types of BGP messages:
1. Open Messages
2. Update Messages
3. Keepalive Messages
4. Notification Messages

Out of four, Update Messages contains the Network Layer Reachability Information (NLRI), which has IP addresses of the usable routes. The update message also includes path attributes and the lengths of both the usable and unusable paths. Labels for VPNv4 routes are encoded in the update message as specified in RFC 2858. The labels for the IPv4 routes are encoded in the update message as specified in RFC 3107.
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Loop Free Alternate: IP Fast Reroute (FRR)

We can use the IPv4 Loop-Free Alternate Fast Reroute feature to reduce the Outages duration when the primary uplinks fail on a router.

When a link or a router fails, routing algorithms like ISIS or OSPF compute new routes that take into account the failure. The time taken for computation is called routing transition. Until the transition is complete and all routers are converged on a common view of the network, the connectivity between the source and destination pairs is interrupted. Routing transition depends on the numbers of routers present in that area. The bigger the area; more the time is. This may impact on revenue of the customer also. We can use the IPv4 Loop-Free Alternate Fast Reroute feature to reduce the routing transition time to less than 50 milliseconds using a pre-calculated alternate next hop. When a router is notified of a link failure, the router immediately switches over to the repair path to reduce traffic loss.

The repair path computation is done by the Intermediate System-to-Intermediate System (IS-IS) or OSPF and the resulting repair paths are sent to the Routing Information Base (RIB). The repair path installation is Forwarding Information Base(FIB).

Eigrp has it's own Loop Free Alternate.
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Segment Routing Traffic Engineering

In my previous post of Segment Routing: Alternate of LDP and RSVP, already discussed about the advantages of using SR w.r.t LDP. The utmost advantage of using Segment Routing is to steer the traffic which is normally called “Traffic Engineering”.

In case of MPLS we called Traffic Engineering as MPLS-TE but in case of Segment Routing, it could be SR-TE (Segment Routing Traffic Engineering Tunnel. RSVP is required to build the MPLS-TE tunnels along with LDP but in Segment Routing, all you need is the Segment IDs only. It is free from heavy RSVP signaling.

There are different ways to create Segment Routing Traffic Engineering Tunnels:-
1. Manually Create SR-TE tunnel and explicitly define the path. This is equivalent to MPLS-TE tunnel and define explicitly path by giving the next hop addresses.

2. Manually Create SR-TE tunnel but for path computation use PCE (Path Control Element). In this case headend router will become the PCC (Path Computation Client) and some software which sits outside of the network but has full view of topology will become a PCC. PCEP (Path Computation Element Protocol) is used between PCC and PCE. For reference http://tools.ietf.org/html/draft-sivabalan-pce-segment-routing-00

3. Dynamically Create SR-TE tunnel and compute path by using PCE. PCE is having API where user can push the headend and tailend routers information with some kind of SLAs. PCE will dynamically create the tunnel push it to router (PCC). Once this is done, PCC will ask for dynamic path also. For reference http://www.enog.org/presentations/enog-6/201-SR_ENOG.pdf

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