Networking / Beginners

Network Distribution/Edge

This network layer provides the interface between the access and the core layers. It also provides the interface with external networks. At this layer, IGPs are used alongside BGP.

The same IGPs recommended for the core are suitable for the distribution/edge layer, too. In fact, the same RP often times covers both network layers. If the protocol used is OSPF, a hierarchical approach is suggested with the core in the backbone area and the distribution/edge split in multiple areas. IS-IS and EIGRP extend flatly over this layer.

BGP is also deployed at this network layer. iBGP is used between distribution/edge routers for the following reasons:

  • Tie up eBGP speakers together by propagating BGP-specific attributes (NEXTHOP, AS_PATH) across the core
  • Avoid overloading the core with routing information that is not relevant to it that could impact its convergence time
  • Exchange information with customers without having to expose the underlying IGP to the customer network complexities and instabilities
  • Enable services such as VPNs or 6PE

The iBGP is deployed in a full mesh realized with the help of route reflectors, confederations, or by fully meshing all the participating routers.

eBGP is used in the following deployment cases:

  • To exchange routing information between domains within large service provider or enterprise networks designed with multiple autonomous systems.
  • To exchange routing information with external networks. It enables the use of policies and aggregation.
  • To enable services such as "Carrier supporting Carrier" (CsC), which allow an ISP to be a transit for other ISPs.

Similar to IPv4 deployments, iBGP should be synchronized with the underlying IGP but, for scalability reasons, prefixes should not be distributed between them.

Network Access

The devices in the access layer are exposed to multiple users, but they tend to have less processing power than the ones in the other network layers. Moreover, because of IPv6 address assignment guidelines, it is likely that these devices will be exposed to large numbers of prefixes in the /64 to /48 range. For these reasons, complex and process-intensive RPs are not deployed in this network layer.

Although OSPF is used at the access layer, simpler routing protocols such as RIPng or EIGRP are sometimes favored. Static routing is also used actively. Static routes are manually configured or are dynamically installed during the address-assignment process (as is the case of DHCP-PD). The redistribution of static routes is common in this part of the network. Summarization is also an important aspect of the routing design. The upstream layers should be presented with a smaller number of prefixes to preserve resource in the aggregation nodes.

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