0. Introduction This is the design specification for next hop tracking feature in Frr. 1. Background Recursive routes are of the form: p/m --> n [Ex: 1.1.0.0/16 --> 2.2.2.2] where 'n' itself is resolved through another route as follows: p2/m --> h, interface [Ex: 2.2.2.0/24 --> 3.3.3.3, eth0] Usually, BGP routes are recursive in nature and BGP nexthops get resolved through an IGP route. IGP usually adds its routes pointing to an interface (these are called non-recursive routes). When BGP receives a recursive route from a peer, it needs to validate the nexthop. The path is marked valid or invalid based on the reachability status of the nexthop. Nexthop validation is also important for BGP decision process as the metric to reach the nexthop is a parameter to best path selection process. As it goes with routing, this is a dynamic process. Route to the nexthop can change. The nexthop can become unreachable or reachable. In the current BGP implementation, the nexthop validation is done periodically in the scanner run. The default scanner run interval is one minute. Every minute, the scanner task walks the entire BGP table. It checks the validity of each nexthop with Zebra (the routing table manager) through a request and response message exchange between BGP and Zebra process. BGP process is blocked for that duration. The mechanism has two major drawbacks: (1) The scanner task runs to completion. That can potentially starve the other tasks for long periods of time, based on the BGP table size and number of nexthops. (2) Convergence around routing changes that affect the nexthops can be long (around a minute with the default intervals). The interval can be shortened to achieve faster reaction time, but it makes the first problem worse, with the scanner task consuming most of the CPU resources. "Next hop tracking" feature makes this process event-driven. It eliminates periodic nexthop validation and introduces an asynchronous communication path between BGP and Zebra for route change notifications that can then be acted upon. 2. Goal Stating the obvious, the main goal is to remove the two limitations we discussed in the previous section. The goals, in a constructive tone, are the following: - fairness: the scanner run should not consume an unjustly high amount of CPU time. This should give an overall good performance and response time to other events (route changes, session events, IO/user interface). - convergence: BGP must react to nexthop changes instantly and provide sub-second convergence. This may involve diverting the routes from one nexthop to another. 3. Overview of the changes The changes are in both BGP and Zebra modules. The short summary is the following: - Zebra implements a registration mechanism by which clients can register for next hop notification. Consequently, it maintains a separate table, per (VRF, AF) pair, of next hops and interested client-list per next hop. - When the main routing table changes in Zebra, it evaluates the next hop table: for each next hop, it checks if the route table modifications have changed its state. If so, it notifies the interested clients. - BGP is one such client. It registers the next hops corresponding to all of its received routes/paths. It also threads the paths against each nexthop structure. - When BGP receives a next hop notification from Zebra, it walks the corresponding path list. It makes them valid or invalid depending on the next hop notification. It then re-computes best path for the corresponding destination. This may result in re-announcing those destinations to peers. 4. Design 4.1. Modules The core design introduces an "nht" (next hop tracking) module in BGP and "rnh" (recursive nexthop) module in Zebra. The "nht" module provides the following APIs: bgp_find_or_add_nexthop() : find or add a nexthop in BGP nexthop table bgp_find_nexthop() : find a nexthop in BGP nexthop table bgp_parse_nexthop_update() : parse a nexthop update message coming from zebra The "rnh" module provides the following APIs: zebra_add_rnh() : add a recursive nexthop zebra_delete_rnh() : delete a recursive nexthop zebra_lookup_rnh() : lookup a recursive nexthop zebra_add_rnh_client() : register a client for nexthop notifications against a recursive nexthop zebra_remove_rnh_client(): remove the client registration for a recursive nexthop zebra_evaluate_rnh_table(): (re)evaluate the recursive nexthop table (most probably because the main routing table has changed). zebra_cleanup_rnh_client(): Cleanup a client from the "rnh" module data structures (most probably because the client is going away). 4.2. Control flow The next hop registration control flow is the following: <==== BGP Process ====>|<==== Zebra Process ====> | receive module nht module | zserv module rnh module ---------------------------------------------------------------------- | | | bgp_update_ | | | main() | bgp_find_or_add_ | | | nexthop() | | | | | | | zserv_nexthop_ | | | register() | | | | zebra_add_rnh() | | | The next hop notification control flow is the following: <==== Zebra Process ====>|<==== BGP Process ====> | rib module rnh module | zebra module nht module ---------------------------------------------------------------------- | | | meta_queue_ | | | process() | zebra_evaluate_ | | | rnh_table() | | | | | | | bgp_read_nexthop_ | | | update() | | | | bgp_parse_ | | | nexthop_update() | | | 4.3. zclient message format ZEBRA_NEXTHOP_REGISTER and ZEBRA_NEXTHOP_UNREGISTER messages are encoded in the following way: /* * 0 1 2 3 * 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ * | AF | prefix len | * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ * . Nexthop prefix . * . . * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ * . . * . . * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ * | AF | prefix len | * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ * . Nexthop prefix . * . . * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ */ ZEBRA_NEXTHOP_UPDATE message is encoded as follows: /* * 0 1 2 3 * 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ * | AF | prefix len | * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ * . Nexthop prefix getting resolved . * . . * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ * | metric | * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ * | #nexthops | * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ * | nexthop type | * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ * . resolving Nexthop details . * . . * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ * . . * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ * | nexthop type | * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ * . resolving Nexthop details . * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ */ 4.4. BGP data structure Legend: /\ struct bgp_node: a BGP destination/route/prefix \/ [ ] struct bgp_info: a BGP path (e.g. route received from a peer) _ (_) struct bgp_nexthop_cache: a BGP nexthop /\ NULL \/--+ ^ | : +--[ ]--[ ]--[ ]--> NULL /\ : \/--+ : | : +--[ ]--[ ]--> NULL : _ : (_)............. 4.5. Zebra data structure rnh table: O / \ O O / \ O O struct rnh { u_char flags; struct route_entry *state; struct list *client_list; struct route_node *node; }; 5. User interface changes frr# show ip nht 3.3.3.3 resolved via kernel via 11.0.0.6, swp1 Client list: bgp(fd 12) 11.0.0.10 resolved via connected is directly connected, swp2 Client list: bgp(fd 12) 11.0.0.18 resolved via connected is directly connected, swp4 Client list: bgp(fd 12) 11.11.11.11 resolved via kernel via 10.0.1.2, eth0 Client list: bgp(fd 12) frr# show ip bgp nexthop Current BGP nexthop cache: 3.3.3.3 valid [IGP metric 0], #paths 3 Last update: Wed Oct 16 04:43:49 2013 11.0.0.10 valid [IGP metric 1], #paths 1 Last update: Wed Oct 16 04:43:51 2013 11.0.0.18 valid [IGP metric 1], #paths 2 Last update: Wed Oct 16 04:43:47 2013 11.11.11.11 valid [IGP metric 0], #paths 1 Last update: Wed Oct 16 04:43:47 2013 frr# show ipv6 nht frr# show ip bgp nexthop detail frr# debug bgp nht frr# debug zebra nht 6. Sample test cases r2----r3 / \ / r1----r4 - Verify that a change in IGP cost triggers NHT + shutdown the r1-r4 and r2-r4 links + no shut the r1-r4 and r2-r4 links and wait for OSPF to come back up + We should be back to the original nexthop via r4 now - Verify that a NH becoming unreachable triggers NHT + Shutdown all links to r4 - Verify that a NH becoming reachable triggers NHT + no shut all links to r4 7. Future work - route-policy for next hop validation (e.g. ignore default route) - damping for rapid next hop changes - prioritized handling of nexthop changes ((un)reachability vs. metric changes) - handling recursion loop, e.g. 11.11.11.11/32 -> 12.12.12.12 12.12.12.12/32 -> 11.11.11.11 11.0.0.0/8 -> - better statistics