CCIE Enterprise Infrastructure · BGP

BGP — CCIE Enterprise Infrastructure

The complete BGP learning path for CCIE Enterprise Infrastructure: how sessions form, how prefixes are chosen and shaped, how the protocol scales with route reflection and confederations, and how it is designed, secured and repaired in production.

  • 24 complete lessons
  • 5 hr 20 min
  • Foundation

Address Families and MP-BGP · Lesson 01 of 01

Carrying more than IPv4 with MP-BGP

One BGP session can carry IPv4, IPv6, VPNv4 and more. Multiprotocol BGP is the capability that makes it possible.

  • 13 min lesson
  • 00:00 video
  • Foundation
Written lessonConfiguration

Written lesson

Address families on one session

Multiprotocol BGP negotiates, at OPEN time, which address families a session will carry — IPv4 unicast, IPv6 unicast, VPNv4, and others — using the MP_REACH_NLRI and MP_UNREACH_NLRI attributes to carry non-IPv4 reachability. This is why modern configuration is organised under address-family blocks and why a neighbour must be activated per family. One TCP session, several kinds of route, each with its own policy.

Configuration

IPv4 and IPv6 on the same peer

IOS / IOS XE
router bgp 65001
 neighbor 2001:db8::2 remote-as 65002
 address-family ipv4 unicast
  neighbor 2001:db8::2 activate
 address-family ipv6 unicast
  neighbor 2001:db8::2 activate

Written lesson

4-byte ASN interop

When a 32-bit ASN speaker peers with a 16-bit-only speaker, the old speaker cannot represent the large AS numbers. The 32-bit speaker substitutes AS_TRANS (23456) in the AS_PATH it sends, while carrying the real 4-byte path in the new AS4_PATH attribute, which the old speaker passes through transparently. The result is that the real path is reconstructed at the next 4-byte-capable router. It usually just works, but AS_TRANS showing up in a path is the fingerprint of a 16-bit speaker in the middle.

activate is the gotcha

Under the default no bgp default ipv4-unicast behaviour on some platforms, even IPv4 peers need explicit activation. When a session is Established but no routes move, check activation under the address family first.