Arista EOS - BGP Selective Route Download

 In this post, I will share my experience with a relatively old (was released back in 2015) but very useful Arista EOS feature - BGP Selective Route Download (SRD). 

The use cases are quite straightforward:

• Program the necessary routes on the routers with small hardware resources. In the above TOI link, only 30K prefixes of 520K (back in 2015) cover 99% traffic. The left small traffic can be directed by the default route. 
• Another useful case (for me) is to control what routes be programmed, or even not installed at all. At meanwhile the BGP runs transparently, which processes, receives and advertises the BGP prefixes. A good example is the RR which is not in the data path.  Or hardness router in the lab, it just sends bgp updates. The traffic can be handled by a couple of static routes. 

In the below example, BGP only installs /24 IPv4 routes within 110.0.0/8 range and /64 IPv6 routes in 2000:110:1::/48. 

router bgp 65501

   bgp route install-map part-peer-v46

!

route-map part-peer-v46 permit 10

   match ip address prefix-list part-peer-route

!

route-map part-peer-v46 permit 20

   match ipv6 address prefix-list part-peer-route-v6

!

ip prefix-list part-peer-route seq 10 permit 110.0.0.0/8 eq 24

!

ipv6 prefix-list part-peer-route-v6

    seq 10 permit 2000:110:1::/48 eq 64

bn309#show ip route summary

...

VRF: default

   Route Source                                Number Of Routes

------------------------------------- -------------------------

...

   ospfv3                                                     0

   bgp                                                     1814

     External: 1814 Internal: 0

...

   Total Routes                                            1871

Number of routes per mask-length:

   /8: 2         /12: 2        /16: 1        /24: 1816     /25: 1

   /30: 7        /32: 42

bn309#show ip bgp installed | egrep '^ \* ' | wc -l

1816

At this time actually, this router receives/accepts over 1.4M prefixes. 

bn309#show ip bgp summary

BGP summary information for VRF default

Router identifier 192.168.230.2, local AS number 65501

Neighbor Status Codes: m - Under maintenance

  Description              Neighbor         V  AS           MsgRcvd   MsgSent  InQ OutQ  Up/Down State   PfxRcd PfxAcc

  IpTransit#1-7504         100.101.1.1      4  12083          44303    119708    0    0 01:08:12 Estab   690049 690049

  Local-Aris-Simu          192.168.230.1    4  65510         129394         5    0    0 01:03:57 Estab   819857 819857

As of August 2020, this feature is only supported on RIBD (so multi-agent mode doesn't work)