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Lab reference

Address plan

Segment Prefix Endpoints
Management 172.20.20.0/24 R1 .11, R2 .12, R3 .13, ISP .21, REN .22, H1 .31, H2 .32
R1 loopback 10.255.0.1/32 R1
R2 loopback 10.255.0.2/32 R2
R3 loopback 10.255.0.3/32 R3
R1–R2 10.255.1.0/30 R1 .1, R2 .2
R2–R3 10.255.1.4/30 R2 .5, R3 .6
R1–R3 10.255.1.8/30 R1 .9, R3 .10
R1–ISP 10.255.2.0/30 R1 .1, ISP .2
R1–REN 10.255.2.4/30 R1 .5, REN .6
H1 LAN 10.255.10.0/24 R2 gateway .1, DHCP .100–.199
H2 LAN 10.255.20.0/24 R3 gateway .1, DHCP .100–.199
Generated routes 10.64.0.0/10 Control plane only

Routing behavior

R1 retains the generated ISP and REN routes in BGP and does not redistribute them into OSPF. While the R1–ISP BGP session is established, R1 originates 0.0.0.0/0 into OSPF as external type 1 with metric 20. The default is withdrawn when ISP is unavailable even if REN remains established. Neither external speaker advertises a default or provides data-plane forwarding.

Route generation

The default generated advertisement set is deterministic:

Input Default Limit or relationship
SEED 20260812 Deterministic random seed
ISP_PREFIXES 500 Maximum 500
ISP_TRANSIT_LEARNED_PREFIXES 100 ISP transit-learned routes; cannot exceed ISP total
REN_PREFIXES 200 Maximum 200
REN_ADJACENT_ORIGIN_PREFIXES 100 Adjacent-origin + transit-learned must equal REN
REN_TRANSIT_LEARNED_PREFIXES 100 Adjacent-origin + transit-learned must equal REN
SHARED_PREFIXES 50 Cannot exceed either peer total

Preview a different advertisement set:

make generate \
  SEED=42 \
  ISP_PREFIXES=500 \
  ISP_TRANSIT_LEARNED_PREFIXES=100 \
  REN_PREFIXES=200 \
  REN_ADJACENT_ORIGIN_PREFIXES=100 \
  REN_TRANSIT_LEARNED_PREFIXES=100 \
  SHARED_PREFIXES=50

make deploy runs generation again. Pass the same values to make deploy if you want to deploy a customized set:

make deploy SEED=42

Generated files are:

  • generated/isp.conf
  • generated/ren.conf
  • generated/manifest.json
  • configs/routeros/r1.rsc
  • configs/routeros/r2.rsc
  • configs/routeros/r3.rsc

Generated prefixes come from 10.64.0.0/10 and have lengths from /16 through /24. Prefixes do not overlap, except for the shared set that both external peers advertise.

ISP and REN advertisements use four deterministic categories: ISP adjacent-origin, ISP transit-learned, REN adjacent-origin, and REN transit-learned. The speakers emit standard communities using 65000:10 for ISP, 65000:20 for REN, 65000:0 for adjacent-origin, and 65000:99 for transit-learned.

Every adjacent-origin path has depth 2: the external speaker followed by a generated origin AS. A transit-learned path has depth 3 through 6: the external speaker, one transit neighbor, zero through three intermediate ASes, and a generated origin AS. ISP uses transit neighbors AS 65010 through AS 65015; REN uses AS 65020 through AS 65022. Default generation uses every member of both pools, while transit-neighbor choice and path depth vary independently.

The seed controls prefix allocation, transit-neighbor selection, path depth, intermediate ASes, and origin ASes. Reusing all generation inputs reproduces the manifest and both ExaBGP configurations exactly. A different seed can change valid paths without changing route-category budgets or the deliberate shared-prefix overlap.

The private lab ASN plan reserves campus AS 65000, ISP AS 65001, REN AS 65002, and both transit-neighbor pools. Generated intermediate and origin ASNs come from the RFC 6996 16-bit private-use range, AS 64512 through AS 65534, excluding all reserved ASNs. Every path contains unique ASNs and excludes campus AS 65000. These values are synthetic and must not be copied into an operational network.

For each route, generated/manifest.json records its source category, path class, transit neighbor (when applicable), origin AS, path depth, complete expected AS path, communities, and the tail rendered into ExaBGP configuration.