Apply route-map for default-originate in FRRouting

blog | Jul 27, 2018

If you use Clos topology you should be aware that there are no direct links between leaves or spines. So, if you announce the default route unconditionally, you know what happens - traffic destined to 0.0.0.0/0 (or ::/0) is blackholed. And what happens if you want to announce the default route conditionally? Let’s say I need to announce 0.0.0.0/0 only if an arbitrary community 123:1 exists. For instance:

ip community-list standard upstream permit 123:1
!
route-map default-route permit 1
  match community upstream
  set as-path prepend 65031 65031

This looks legit and should work as expected, but unfortunately not. FRRouting just creates dummy attributes and uses them instead of applying attributes from the specified route-map. We can double check this using tcpdump:

12:05:46.149474 IP (tos 0xc0, ttl 1, id 52011, offset 0, flags [DF], proto TCP (6), length 97)
    10.0.0.1.53746 > 10.0.0.2.179: Flags [P.], cksum 0xc2af (correct), seq 108:153, ack 130, win 229, options [nop,nop,TS val 832288 ecr 732246], length 45: BGP
    Update Message (2), length: 45
      Origin (1), length: 1, Flags [T]: IGP
        0x0000:  00
      AS Path (2), length: 6, Flags [TE]: 65031
        0x0000:  0201 0000 fe07
      Next Hop (3), length: 4, Flags [T]: 10.0.0.1
        0x0000:  0a00 0001
      Updated routes:
        0.0.0.0/0

Notice AS Path attribute. It’s not 65031 65031, but just 65031.

As always I tried to debug and verify if it’s really hits the actual code in the source:

stap -e 'probe process("/usr/lib/frr/bgpd").statement("subgroup_default_originate@/root/frr/bgpd/bgp_updgrp_adv.c:720")
{ printf("route-map type for default-originate: %d", $bgp->peer_self->rmap_type); }'

route-map type for default-originate: 16

16 means PEER_RMAP_TYPE_DEFAULT. Looks good.

I wanted to achieve that set in route-map would work with default-originate.

Configuration is simple:

spine

router bgp 65031
 bgp router-id 10.0.0.1
 timers bgp 10 30
 neighbor 10.0.0.2 remote-as 65032
 !
 address-family ipv4 unicast
  redistribute kernel
  neighbor 10.0.0.2 default-originate route-map default
 exit-address-family
!
route-map default permit 10
 set as-path prepend 65031 65031
 set metric 200
!

leaf

router bgp 65032
 bgp router-id 10.0.0.2
 neighbor 10.0.0.1 remote-as 65031
!

I don’t usually have enough time to contribute to the community, but this project is my favorite one, thus why don’t invest my time after work and don’t try!

Conclusion

Contribution to community software is awesome. You learn new stuff, you get new ideas from other smart people. It’s absolutely worth spending your so expensive time on contributions. What I liked most was that FRRouting uses CI/CD quite heavy including clang static analysis (which is really cool, I never used it and now I understand how and when to use it).

I personally did tests on kitchen / vagrant.

So finally the feature request, landed on the master in FRRouting:

leaf1-debian-9# show ip bgp 0.0.0.0
BGP routing table entry for 0.0.0.0/0
Paths: (1 available, best #1, table Default-IP-Routing-Table)
  Advertised to non peer-group peers:
  10.0.0.1
  65031 65031 65031
    10.0.0.1 from 10.0.0.1 (10.0.0.1)
      Origin IGP, metric 200, localpref 100, valid, external, best
      AddPath ID: RX 0, TX 16
      Last update: Tue Jul 24 13:20:08 2018

Apply route-map for default-originate in FRRouting

spine

leaf

Conclusion

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