Inadvertent change to unnumbered bgp next-hop with "set ip next-hop" route-map

[dslicenc]

• [X ] Did you check if this is a duplicate issue?
• [X ] Did you test it on the latest FRRouting/frr master branch?

Description

Found that if a route-map with "set ip address x.x.x.x" is applied outbound to an unnumbered peer, the update is sent with a single link-local ipv6 address (16 bytes) instead of the 32 byte link-local previously sent. This change was the result of a change made to allow setting ip next-hop for mpls-vpn routes (per the comment) but has the side-effect of modifying behavior not pertaining to that use-case.

Steps to Reproduce

1. Bring up bgp unnumbered peering between two routers
2. Define a route-map with "set ip next-hop x.x.x.x"
3. Apply the route-map to outbound updates to the unnumbered peer
4. Display the received bgp table entry for the change

Configuration on sender

router bgp 10
 bgp router-id 90.0.0.1
 no bgp default ipv4-unicast
 neighbor swp6 interface remote-as 80
 neighbor swp6 description r8
 neighbor swp6 advertisement-interval 0
address-family ipv4 unicast
  neighbor swp6 activate
  neighbor swp6 route-map FOO out
!
route-map FOO permit 10
 set ip next-hop 9.9.9.9

Expected behavior:
Before commit commit d54600f, the received table entry looked like this:

r8# sh ip bgp 10.0.0.4
BGP routing table entry for 10.0.0.4/32
Paths: (1 available, best #1, table default)
  Advertised to non peer-group peers:
  r1(swp1)
  10 40
    fe80::202:ff:fe00:6 from r1(swp1) (90.0.0.1) 
    (fe80::202:ff:fe00:6) (used)                         
      Origin IGP, valid, external, bestpath-from-AS 10, best
      Last update: Wed Mar  6 13:51:05 2019

Pcap file snippet of the above update:

Path attributes
    Path Attribute - MP_REACH_NLRI
        Flags: 0x90, Optional, Extended-Length, Non-transitive, Complete
        Type Code: MP_REACH_NLRI (14)
        Length: 49
        Address family identifier (AFI): IPv4 (1)
        Subsequent address family identifier (SAFI): Unicast (1)
        Next hop network address (32 bytes)
            Next Hop: Unknown address
        Number of Subnetwork points of attachment (SNPA): 0
        Network layer reachability information (12 bytes)
            10.0.2.0/24
                MP Reach NLRI prefix length: 24
                MP Reach NLRI IPv4 prefix: 10.0.2.0
            172.16.10.0/24
                MP Reach NLRI prefix length: 24
                MP Reach NLRI IPv4 prefix: 172.16.10.0
            172.16.1.0/24
                MP Reach NLRI prefix length: 24
                MP Reach NLRI IPv4 prefix: 172.16.1.0
    Path Attribute - ORIGIN: INCOMPLETE
        Flags: 0x40, Transitive, Well-known, Complete
        Type Code: ORIGIN (1)
        Length: 1
        Origin: INCOMPLETE (2)
    Path Attribute - AS_PATH: 65000 65002 
        Flags: 0x50, Transitive, Extended-Length, Well-known, Complete
        Type Code: AS_PATH (2)
        Length: 10
        AS Path segment: 65000 65002

Actual behavior:
After the above commit, the following bgp table entry on the receiver:

r8# sh ip bgp 10.0.0.4
BGP routing table entry for 10.0.0.4/32
Paths: (1 available, best #1, table default)
  Advertised to non peer-group peers:
  r1(swp1)
  10 40
    fe80::202:ff:fe00:6 from r1(swp1) (90.0.0.1)
      Origin IGP, valid, external, bestpath-from-AS 10, best
      Last update: Wed Mar  6 20:05:11 2019

And pcap file snippet

Border Gateway Protocol - UPDATE Message
    Marker: ffffffffffffffffffffffffffffffff
    Length: 71
    Type: UPDATE Message (2)
    Withdrawn Routes Length: 0
    Total Path Attribute Length: 48
    Path attributes
        Path Attribute - MP_REACH_NLRI
            Flags: 0x90, Optional, Extended-Length, Non-transitive, Complete
            Type Code: MP_REACH_NLRI (14)
            Length: 26
            Address family identifier (AFI): IPv4 (1)
            Subsequent address family identifier (SAFI): Unicast (1)
            Next hop network address (16 bytes)
                Next Hop: fe80::202:ff:fe00:6
            Number of Subnetwork points of attachment (SNPA): 0
            Network layer reachability information (5 bytes)
                10.0.0.4/32
                    MP Reach NLRI prefix length: 32
                    MP Reach NLRI IPv4 prefix: 10.0.0.4
        Path Attribute - ORIGIN: IGP
            Flags: 0x40, Transitive, Well-known, Complete
            Type Code: ORIGIN (1)
            Length: 1
            Origin: IGP (0)
        Path Attribute - AS_PATH: 10 40 
            Flags: 0x50, Transitive, Extended-Length, Well-known, Complete
            Type Code: AS_PATH (2)
            Length: 10
            AS Path segment: 10 40

Portion of commit causing the changed behavior

diff --git a/bgpd/bgp_routemap.c b/bgpd/bgp_routemap.c
index f7c79f873..f9f5142cd 100644
--- a/bgpd/bgp_routemap.c
+++ b/bgpd/bgp_routemap.c
@@ -1298,6 +1298,9 @@ static route_map_result_t route_set_ip_nexthop(void *rule,
                        bgp_info->attr->nexthop = *rins->address;
                        SET_FLAG(bgp_info->attr->rmap_change_flags,
                                 BATTR_RMAP_IPV4_NHOP_CHANGED);
+                       /* case for MP-BGP : MPLS VPN */
+                       bgp_info->attr->mp_nexthop_global_in = *rins->address;
+                       bgp_info->attr->mp_nexthop_len = sizeof(*rins->address);
                }
        }

Components

bgpd

Versions

• FRR: frr_7.1-dev_amd64.deb