In BGP dynamic SR-TE,
the label Switched Path (LSP) is enabled on demand when defined criteria and
policies are met and that is the key difference between manually enabled SR-TE
and BGP dynamic SR-TE. Policies, for example, low latency path, minimum cost
path, and so on are carried via BGP and matches on a given customer prefix.
SR-TE tunnel used for L3VPN or Virtual Private LAN Services (VPLS) using BGP
for auto-discovery and signaling is referred to as BGP-TE Dynamic.
BGP SR-TE dynamic
assumes the on-demand auto-tunnel resides in single IGP domain. In this case
path computation is done via IGP. SR-TE auto-tunnel created based on the
request from BGP is a dynamic SR-TE tunnel. In other words, tunnel path
information, or label stack, is computed based on the BGP next-hop and TE
attribute configuration. BGP dynamic SR-TE functions to trigger an On-demand
LSP (auto-tunnel). The functions include:
SR-TE profile is
locally configured in attribute-set to define certain SR-TE parameters, for
example, latency, disjoint path and so on. Once the BGP customer prefixes are
mapped to an SR-TE-profile, a tunnel is dynamically created (auto-tunnel or On
demand Label Switched Path (LSP)) using the parameters defined in the
attribute-set, for each specified BGP next-hop and attribute-set pair
associated with the prefixes. A binding SID is associated with each SR-TE
auto-tunnel and passed to BGP. The binding SID or binding label is installed
into Routing Information Base (RIB) and Forwarding Information Base (FIB). FIB
resolves BGP path via the binding SID or binding label, which forwards over the
On demand SR-TE auto-tunnel. The binding-SID is also used to steer the customer
traffic over the SR-TE LSP.
It must be noted that
BGP only carries the SR-TE policy in this case, while path computation is done
via IGP in a single IGP domain. In a single IGP domain the headend node has
full visibility of the end to end path and the topology engineering database
(Traffic Engineering Database or TED). Also it is assumed with BGP Dynamic
SR-TE that all the nodes reside within single AS and single IGP domain.
Figure 1. BGP-TE Dynamic
The above figure
depicts the workflow for BGP-TE dynamic using multiple routing domains use
equipment 2 (CPE) sends BGP update for Prefix-X and adds LL community, for
AC1 announces a
VPN route for prefix X with LL community.
BGP update of the VPN route matching community LL, ToR1 sends a request to PCE
controller for LSP path towards AC1 with low latency TE policy.
element (PCE) controller replies with a label stack, for example, 17003, 1600.
SR-TE auto-tunnel and installs the route for Prefix-X in VRF of this VPN.