BGP - Internet's
Secret Language.

1. RIB Display Differences: Cisco IOS XR displays both received and locally originated routes in its BGP RIB. In contrast, Nokia SR OS typically only shows received routes in the Adj-RIB-In; local routes must be verified via the Adj-RIB-Out or advertised-routes commands for specific neighbors.

2. Cisco Weight Attribute: Weight is a Cisco-proprietary path attribute local to the router that is not advertised to peers. It is the highest priority factor in the Cisco path selection process, where the route with the highest numerical weight is preferred.

3. The Attached Bit: When an L1L2 router has neighbors in other areas (the backbone), it sets the "A-bit" in its Level-1 LSP. When a Level-1-only router sees this bit, it automatically installs a default route pointing toward that L1L2 router to reach inter-area destinations.

4. SPRING Label Calculation: The egress label is calculated as the sum of the destination node's index (SID) and the first label in the label block of the next-hop router. This allows routers to calculate labels independently without the need for traditional label signaling protocols like LDP.

5. IPv6 BGP Options: One option is to add the IPv6 unicast address family to an existing IPv4 peering session, which requires manual next-hop manipulation. The second option is to establish a completely independent BGP session between IPv6 addresses, which increases management overhead but keeps the families separated.

6. Policy Application: Nokia SR OS uses a single export or import policy per attach point (neighbor, group, or global) that covers all address families simultaneously. Cisco IOS XR decouples these, allowing specific route policies to be applied per neighbor, per group, and per specific address family.

7. DIS and Pseudonodes: In broadcast segments, the DIS is elected to generate a separate LSP representing a "pseudonode." This simplifies the link-state database by making all routers on that segment appear as if they have point-to-point connections to the pseudonode rather than each other.

8. OSPF Distribution of SR: Segment Routing information is distributed in OSPF using Type 10 Opaque LSAs, which have an area-wide scope. These LSAs contain TLVs that describe the router's SR capabilities, label ranges, and Prefix SIDs.

9. IS-IS Route Leaking: Route leaking is the process of propagating Level-2 routes into a Level-1 area. This is used to provide Level-1 routers with specific reachability information for inter-area prefixes, which they would otherwise only reach via a default route.

10. Summary-Only Impact: Using the "summary-only" keyword during BGP aggregation filters out the more specific component routes from being advertised, sending only the aggregate prefix. While this reduces the size of the routing table, it can cause traffic drops if internal routers require the more specific routes for proper forwarding.