Expose policy backup address in fboss2

Summary:
# Context
The BGP FRR stack associates protected routes with a locally configured backup address, but operators need to verify both the BGP association and the backup nexthop received by FBOSS Agent. Design: https://docs.google.com/document/d/1wRTKEsbN4NAxEmF4PRrj-6UabvE4X_hTnrgdChgWh34/edit

# This Diff
Add optional `backup_addr` fields to the legacy and canonical BGP route API models, populate them from `BgpPath`, and preserve the field in both Python canonical-RIB decoders when they reconstruct `TBgpPath`. `fboss2 show bgp` renders the configured address as `Backup`; `fboss2 show route` marks backup nexthops as `(BACKUP)` while leaving ordinary primary output unchanged. This is observability only and does not affect path selection, FIB programming, or switchover enablement.

# This Stack
1. **D114950612** applies and caches the ingress policy action.
2. **D114950610** carries the backup address from Adj-RIB-In into Loc-RIB.
3. **D114950611** programs the backup address to FBOSS.
4. **This diff** exposes the resulting state in BGP and FBOSS CLI output.

Reviewed By: jaiharil

Differential Revision: D115002980

fbshipit-source-id: abe1ab4bb526cdda9ead8224223b73a00e7c5f61

Author: Yikai Lin

fboss/cli/fboss2/commands/show/bgp/CanonicalRibResolver.cpp

@@ -121,6 +121,7 @@ std::optional<TBgpPath> resolveCanonicalPath(
   assignIfSet(path.aggregator(), dedupPath.aggregator());
   assignIfSet(path.topologyInfo(), dedupPath.topology_info());
   assignIfSet(path.weight(), dedupPath.weight());
+  assignIfSet(path.backup_addr(), dedupPath.backup_addr());
 
   // Peer attribution resolves through the shared peers pool by index.
   if (canonPath.peer_idx().has_value()) {

fboss/cli/fboss2/oss/CmdShowUtils.cpp

@@ -419,13 +419,25 @@ void printRIBEntries(
           extCommunitiesStr = printExtCommunities(*extCommunities);
         }
 
+        std::string backupAddrStr;
+        if (const auto* backupAddr =
+                apache::thrift::get_pointer(path.backup_addr())) {
+          backupAddrStr = fmt::format(
+              " (backup: {})",
+              IPAddress::fromBinary(
+                  folly::ByteRange(
+                      folly::StringPiece(*backupAddr->prefix_bin())))
+                  .str());
+        }
+
         std::string pathToPrint = fmt::format(
-            "{} from {} ({}) via {} | LBW: {} | Origin: {} | "
+            "{} from {} ({}) via {}{} | LBW: {} | Origin: {} | "
             "LP: {} | ASP: {} | LM: {} | NH Weight: {} | MED: {} | ID: {} (rcvd) {} (sent) | Weight: {}{} | IgpCost: {}",
             marker,
             peerIdStr,
             peerDescriptionStr,
             nextHopStr,
+            backupAddrStr,
             lbwStr,
             (originStr != nettools::bgplib::kNullMessage)
                 ? originStr.substr(11) // Trim BGP_ORIGIN_ from origin string

fboss/cli/fboss2/test/CanonicalRibResolverTest.cpp

@@ -31,6 +31,7 @@ namespace {
 // Mirrors facebook::bgp::kBestPathGroup / kDefaultPathGroup -- the group keys
 // bgpd uses; the resolver must reconstruct best_group as kBestGroup.
 constexpr auto kBestGroup = "best";
+constexpr auto kBackupAddr = "2001:db8::1";
 
 // A canonical state with a single prefix and sparse path/peer IDs, optionally
 // marked as the selected best.
@@ -40,6 +41,7 @@ TCanonicalRibState makeSingleEntryState(int64_t med, bool markBest) {
   TBgpDedupedPath deduped;
   deduped.next_hop() = getPrefix(kNextHop);
   deduped.med() = med;
+  deduped.backup_addr() = getPrefix(kBackupAddr);
   state.deduped_paths()->emplace(7, deduped);
 
   TCanonicalPeer peer;
@@ -88,6 +90,7 @@ TEST(CanonicalRibResolverTest, ResolvesPathPeerAndBestPath) {
   EXPECT_EQ(kPeerDescription, path.peer_description().value());
   EXPECT_EQ(1234, path.router_id().value());
   EXPECT_EQ(100, path.med().value());
+  EXPECT_EQ(getPrefix(kBackupAddr), path.backup_addr().value());
   EXPECT_TRUE(path.is_best_path().value());
 }
 

fboss/cli/fboss2/test/CmdShowBgpTableDetailTest.cpp

@@ -42,6 +42,11 @@ class CmdShowBgpTableDetailTestFixture : public CmdHandlerTestBase {
     entriesIPv4_ = {buildEntry()};
     entriesIPv6_ = {
         buildEntry("2001::1/64", "2001::2", "2001::3", "two00one::three", 7)};
+    entriesIPv6_.front()
+        .paths()
+        ->at(entriesIPv6_.front().best_group().value())
+        .front()
+        .backup_addr() = getPrefix("2001:db8::1");
     combineEntries();
   }
 
@@ -114,7 +119,7 @@ TEST_F(CmdShowBgpTableDetailTestFixture, printOutput) {
       "    ExtCommunities: Type(64):SubType(2):AS(3):Value(4)\n"
       "    BestPath Rejection Reason: Router-Id, Filter Criterion: Choose Lowest Value\n"
       "\n> 2001::1/64, Selected 1/1 paths (1 active, 0 inactive)\n"
-      "*@  from 2001::3 (two00one::three) via 2001::2 | LBW: None | Origin: INCOMPLETE | LP: DEPRIO/25 | ASP: 65301 | LM: # | NH Weight: 7 | MED: 10 | ID: 5 (rcvd) 6 (sent) | Weight: 20 | IgpCost: 100"
+      "*@  from 2001::3 (two00one::three) via 2001::2 (backup: 2001:db8::1) | LBW: None | Origin: INCOMPLETE | LP: DEPRIO/25 | ASP: 65301 | LM: # | NH Weight: 7 | MED: 10 | ID: 5 (rcvd) 6 (sent) | Weight: 20 | IgpCost: 100"
       "\n    Router/Originator: 2.2.2.3 | ClusterList: [1.1.1.2]\n"
       "    Communities: FABRIC_POD_RSW_LOOP/65527:12705\n"
       "    ExtCommunities: Type(64):SubType(2):AS(3):Value(4)\n"

neteng/fboss/bgp/if/bgp_route_types.thrift

@@ -137,6 +137,8 @@ struct TBgpPath {
   // Set by the loc-RIB getters; left unset by shadow-RIB views, which only hold
   // selected paths.
   28: optional bool is_inactive;
+  // Locally configured backup address. This is not a BGP wire attribute.
+  29: optional bgp_attr.TIpPrefix backup_addr;
 }
 
 /**
@@ -386,6 +388,8 @@ struct TBgpDedupedPath {
   12: optional map<string, i64> topology_info;
   /** Cisco-style local BGP weight (path-selection attr). Mirrors TBgpPath.weight. */
   13: optional i32 weight;
+  /** Locally configured backup address. Mirrors TBgpPath.backup_addr. */
+  14: optional bgp_attr.TIpPrefix backup_addr;
 }
 
 /**