PIC Edge in the Global Table
Last updated
Last updated
Topology: ine-spv4
Load bgp.pic.global.init.cfg
IPv4 BGP routing is currently setup in the network. R2 is a dual-homed CE that announces 1.1.1.1/32 and 2.2.2.2/32 to R3 and R4. XR1 is single-homed to R5, announcing 19.19.19.19/32 and 20.20.20.20/32. R6 is the RR.
Configure PIC so that R5 pre-installs a backup route in the FIB for 1.1.1.1/32 and 2.2.2.2/32.
By default, a RR hides multiple paths for the same prefix. It only reflects the best path for any given prefix. In a VPNv4/v6 environment, the RD can be used to “trick” the RR into reflecting multiple paths for the same prefix. However, this is the global table, so we cannot use this technique.
Instead, we must use add-path. Add-path is a capability that is exchanged in the OPEN message. So when you first configure the RR to send additional paths using bgp additional-paths send, it will tear down all existing sessions. On the PEs, you use bgp additional-paths receive, which will again tear down the session. Once the capability is configured on both sides, you can verify it in the show neighbor output:
Next, we need to configure which paths to select on the RR, and which paths to advertise to neighbors. The command bgp additional-paths select best 2 selects the best 2 paths as candidate to advertise to neighbors. The command advertise additional-paths best 2 advertises these to the neighbors. These must match. If you use something like select all and then advertise additional-paths best 2 you get this error:
This is because there are unique flags used when selecting all/best 2/etc. So if you select all, you cannot advertise best-2 because the best 2 routes are not marked.
Finally, the bgp additional-paths install command instructs R5 to install backup routes as repair routes in the FIB.
Before we make any changes, we can see that R5 only has one route to prefixes advertised by R2.
Once we configure add paths, we now see that R6 has selected the 2nd best route as an additional-path.
On R5, we see the additional path and that the backup path has been selected and installed in the FIB.
