3.5 KiB
3.5 KiB
Module 5: eBGP — Peering with the World
Course: ISP Backbone Lab Course Previous: Module 4: L3VPN Next: Module 6: Segment Routing
Network Diagram
eBGP peering at the IXP — route filtering, local-pref tiers, and the Big 9 best path selection
Peering Types at an ISP
| Type | What It Is | Relationship | Money |
|---|---|---|---|
| Transit | You pay a bigger ISP to reach the full internet | Customer → Provider | You pay them |
| Peering | Two ISPs agree to exchange traffic for free | Peer ↔ Peer | Free (settlement-free) |
| Customer | Someone pays YOU for connectivity | Provider → Customer | They pay you |
| IXP (Internet Exchange) | A shared switch where many ISPs peer at once | Many ↔ Many | Small port fee |
Our Lab Setup
PE-EDGE1 (AS 65000) and PE-EDGE3 (AS 65100) peer at the IXP. This simulates settlement-free peering between two ISPs.
Lab 5 Config: eBGP at the IXP
PE-EDGE1:
! IXP-facing interface
interface GigabitEthernet0/5
description TO IXP-SWITCH
ip address 172.16.0.1 255.255.255.0
no shutdown
!
router bgp 65000
neighbor 172.16.0.3 remote-as 65100
!
address-family ipv4 unicast
neighbor 172.16.0.3 activate
neighbor 172.16.0.3 route-map PEERING-IN in
neighbor 172.16.0.3 route-map PEERING-OUT out
neighbor 172.16.0.3 prefix-list PEER-IN-FILTER in
exit-address-family
!
! Only accept their customer prefixes, not the full internet
ip prefix-list PEER-IN-FILTER seq 10 permit 10.200.0.0/16 le 24
ip prefix-list PEER-IN-FILTER seq 999 deny 0.0.0.0/0 le 32
!
! Set local-pref lower for peering routes (prefer transit/customer)
route-map PEERING-IN permit 10
set local-preference 100
!
route-map PEERING-OUT permit 10
! Only advertise your customer routes, not routes learned from other peers
match community CUSTOMER-ROUTES
PE-EDGE3:
interface GigabitEthernet0/5
description TO IXP-SWITCH
ip address 172.16.0.3 255.255.255.0
no shutdown
!
router bgp 65100
neighbor 172.16.0.1 remote-as 65000
!
address-family ipv4 unicast
neighbor 172.16.0.1 activate
neighbor 172.16.0.1 route-map PEERING-IN in
neighbor 172.16.0.1 route-map PEERING-OUT out
exit-address-family
BGP Best Path Selection (The Big 9)
This is THE most important BGP concept. When a router has multiple paths to the same prefix, it picks the best one using this order:
| Priority | Attribute | Higher or Lower Wins? | Who Controls It? |
|---|---|---|---|
| 1 | Weight (Cisco-proprietary) | Higher | Local router only |
| 2 | Local Preference | Higher | Entire AS (via iBGP) |
| 3 | Locally originated | — | Prefer routes this router originated |
| 4 | AS Path length | Shorter | Neighbors (can be prepended) |
| 5 | Origin code | IGP > EGP > ? | Route origin |
| 6 | MED (Multi-Exit Discriminator) | Lower | Neighbor (suggestion only) |
| 7 | eBGP over iBGP | — | Prefer external routes |
| 8 | Lowest IGP metric to next-hop | Lower | Interior routing |
| 9 | Oldest route / Router ID | Varies | Tiebreakers |
Understanding Check
- Why do ISPs filter inbound routes with prefix lists? What could go wrong?
- What is AS-path prepending and when would you use it?
- Why set a lower local-preference on peering routes?
- What is a BGP community and how do ISPs use them for traffic engineering?
Next Module: Module 6: Segment Routing →