Routing protocols can be classified based on their support of variable-length subnet masking (VLSM) and classless interdomain routing (CIDR). Classful routing protocols do not advertise subnet masks in their routing updates; therefore, the configured subnet mask for the IP network must be the same throughout the entire internetwork. Furthermore, the subnets must, for all practical purposes, be contiguous within the larger internetwork. For example, if you use a classful routing protocol for network 200.170.0.0, you must use the chosen mask (such as 255.255.255.0) on all router interfaces using the 200.170.0.0 network. You must configure serial links with only two hosts and LANs with tens or hundreds of devices with the same mask, 255.255.255.0. The big disadvantage of classful routing protocols is that the network designer cannot take advantage of address summarization across networks (CIDR) or allocation of smaller or larger subnets within an IP network (VLSM). For example, with a classful routing protocol that uses a default mask of /25 for the entire network, you cannot assign a /30 subnet to a serial point-to-point circuit. The following are classful routing protocols:
- RIPv1
- IGRP (this protocol is not covered on the ENSLD 300-420 exam)
Classless routing protocols advertise the subnet mask with each route. You can configure subnetworks of a given IP network number with different subnet masks (using VLSM). You can configure large LANs with a smaller subnet mask and configure serial links with a larger subnet mask, thereby conserving IP address space. Classless routing protocols also allow flexible route summarization and supernetting (using CIDR). You can create supernets by aggregating classful IP networks. For example, 200.100.100.0/23 is a supernet of 200.100.100.0/24 and 200.100.101.0/24. The following are classless routing protocols:
- RIPv2
- OSPF
- EIGRP
- IS-IS
- RIPng
- OSPFv3
- EIGRP for IPv6
- BGP
IPv4 Versus IPv6 Routing Protocols
With the increasing use of the IPv6 protocol, a CCNP enterprise designer must be prepared to design networks using IPv6 routing protocols. As IPv6 was defined, routing protocols needed to be updated to support the new IP address structure. None of the IPv4 routing protocols support IPv6 networks, and none of the IPv6 routing protocols are backward compatible with IPv4 networks. But both protocols can coexist on the same network, each with its own routing protocol. Devices with dual stacks recognize which protocol is being used by the IP Version field in the IP header.
RIPng is the IPv6-compatible RIP routing protocol. EIGRP for IPv6 is the new version of EIGRP that supports IPv6 networks. OSPFv3 was developed for IPv6 networks, and OSPFv2 remains for IPv4 networks. Internet drafts were written to provide IPv6 routing using IS-IS. Multiprotocol extensions for BGP provide IPv6 support for BGP. Table 3-4 lists IPv4 and IPv6 routing protocols.
Table 3-4 IPv4 and IPv6 Routing Protocols
| IPv4 Routing Protocols | IPv6 Routing Protocols |
| RIPv2 | RIPng |
| EIGRP | EIGRP for IPv6 |
| OSPFv2 | OSPFv3 |
| IS-IS | IS-IS for IPv6 |
| BGP | Multiprotocol BGP |