Unicast Routing Protocols
Unicast routing protocols treat each IP network as an interface. The
interface corresponding to the primary subnet is the active interface, and the
interfaces corresponding to the secondary subnet are passive subnets.
For example, in the case of OSPF (Open Shortest Path First), the system
treats each network as an interface, and hello messages are not sent out or received
over the non-primary interface. In this way, the router link state advertisement (LSA)
includes information to advertise that the primary network is a transit network and the
secondary networks are stub networks, thereby preventing any traffic from being routed
from a source in the secondary network.
Interface-based routing protocols (for example, OSPF) can be
configured on per VLAN basis. A routing protocol cannot be configured
on an individual primary or secondary interface. Configuring a protocol
parameter on a VLAN automatically configures the parameter on all
its associated primary and secondary interfaces. The same logic
applies to configuring IP forwarding, for example, on a VLAN.
Routing protocols in the multinetted environment advertise the secondary
subnets to their peers in their protocol exchange process. For example, for OSPF the
secondary subnets are advertised as stub networks in router LSAs. RIP (Routing Information Protocol) also advertises secondary subnets to its peers residing on the
primary subnet.
This section describes the behavior of
OSPF in an IPv4 multinetting environment:
- Each network is treated as an interface, and hello messages are
not sent out or received over the non-primary interface. In this way, the router LSA
includes information to advertise that the primary network is a transit network and
the secondary networks are stub networks, thereby preventing any traffic from being
routed from a source in the secondary network.
- Any inbound OSPF control packets from secondary interfaces are
dropped.
- Direct routes corresponding to secondary interfaces can be
exported into the OSPF domain (by enabling export of direct routes), if OSPF is not
enabled on the container VLAN.
- When you create an OSPF area address range for aggregation, you
must consider the secondary subnet addresses for any conflicts. That is, any
secondary interface with the exact subnet address as the range cannot be in another
area.
- The automatic selection algorithm for the OSPF router ID
considers the secondary interface addresses also. The numerically highest interface
address is selected as the OSPF router-id.
This section describes the behavior of the Routing Information Protocol
(RIP) in an IP multinetting environment:
- RIP does not send any routing information update on the
secondary interfaces. However, RIP does advertise networks corresponding to
secondary interfaces in its routing information packet to the primary
interface.
- Any inbound RIP control packets from secondary interfaces are
dropped.
- Direct routes corresponding to secondary interfaces can be
exported into the RIP domain (by enabling export of direct routes), if RIP is not
enabled on the container VLAN.
There are no behavioral changes in the BGP (Border Gateway Protocol) in
an IP multinetting environment.
This section describes a set of recommendations for using BGP with IP
multinetting:
- Be careful of creating a BGP neighbor session with a BGP speaker
residing in secondary subnet. This situation can lead to routing loops.
- All secondary subnets are like stub networks, so you must
configure BGP in such a way that the BGP next hop becomes reachable using the
primary subnet of a VLAN.
- When setting the BGP next hop using an inbound or outbound
policy, ensure that the next hop is reachable from the primary interface.
- A BGP static network's reachability can also be resolved from
the secondary subnet.
- Secondary interface addresses can be used as the source
interface for a BGP neighbor.
- Direct routes corresponding to secondary interfaces can be
exported into the BGP domain (by enabling export of direct routes).
This section describes the behavior of IS-IS in an IPv4 multinetting
environment:
- IS-IS includes all the interface addresses in its reachability
information. Adjacency is established only based on the primary interface address.
If the adjacency-check option is disabled by the disable isis adjacency-check
command, then IS-IS adjacency is established irrespective of the subnet address
match.
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