Maximum path
In the RIB + FIB ECMP (multipath)
Is PIC supported by default ?
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/xe-3s/irg-xe-3s-book/irg-bgp-mp-pic.html
=> With BGP Multipath, the BGP prefix-independant convergence (PIC) feature is supported
Attribut that should be identical
- Weight
- LP
- AS Path (AS Number unless relax us used, AS length)
- Origin Code
- MED
- IGP Metric Next hop should be different
Add Path
If the path are equal, allow to advertise more than one bes oath (need to test in eBGP).
Mostly used with BGP without MPLS. If MPLS is used it’s better to have an RD different for each VRF of each router (easier to troubleshoot).
https://orhanergun.net/wp-content/uploads/2019/11/BGP-Add-path-vs-Shadow-RR-vs-Shadow-Session-vs-Unique-RD.pdf
PIC
https://www.ciscolive.com/c/dam/r/ciscolive/us/docs/2016/pdf/BRKRST-3321.pdf I don’t neded it at the moment because we don’t need this level if convergence (couple of minutes vs ms if you have hundres of thousand of prefixes)
https://blog.ipspace.net/2012/01/prefix-independent-convergence-pic.html
The generic optimization of the RIB-to-FIB update process is known as Prefix-Independent Convergence (PIC) - if the routing protocols can pre-compute alternate paths, suitably designed FIB can use that information to cache alternate next hops. Updating such a FIB no longer involves numerous updates to individual prefixes; you have to change only the next hop reachability information.
Best External
Allow a router to advertise it’s best external path even if in it’s BGP table it does have a beter route from inside
LABEL ALLOCATION
Per VRF
Cons
- IP Lookup needed after label lookup (can be a benefit, cf route sum issue)
- No granular load balancing because the bottom label is the same for all prefixes, if platform load balances on bottom label
- Potential forwarding loop during local traffic diversion to support PIC (Transient loop)
- No support for EIBGP multipath
Pros
- 1 label per vrf (less label used)
Per CE
Cons
- No granular load balancing because the bottom label is the same for all prefixes from one CE, if platform load balances on bottom label
- eBGPload balancing & BGP PIC is not supported (it makes usage of label diversity), unless resilient per-ce label
- Only single hop eBGPsupported, no multihop
Pros
- No IP lookup needed after label lookup
- Per-CE : one MPLS label per next-hop (so per connected CE router)(Number of MPLS labels used is very low)