L2 Routing
An L2 IS contains three routing tables: an L2 area-address routing table, an
internal-metric reachable-address-prefix table (internal), and an external-metric
reachable-address-prefix table (external).
The following summarizes L2 routing:
1. An L2 IS receives a packet and compares the destination address in the header
of the packet to the set of area addresses in the area address routing table. If a
match exists, the packet is forwarded to the next hop backbone router. If no
match exists, the router checks the internal routing table.
2. The internal routing table contains entries of reachable address prefixes that
lead to other domains. If the internal routing table contains a match, the packet
is forwarded along the backbone to the appropriate domain. If no match exists,
the router checks the external routing table.
3. The external routing table contains entries to reachable address prefixes that
also lead to other domains. If the external routing table contains a match, the
packet is forwarded along the path to the appropriate domain. If no match
exists, the packet is dropped.
Refer to “Internal and External Routing” for a detailed explanation of the internal
and external routing tables.
Routing Metric
A routing metric is a value associated with a function of the circuit to indicate the
cost of routing over that circuit. For example, the routing metric based on the
monetary expense of a circuit would use a low number to indicate a low monetary
expense and high number to indicate a high monetary expense of routing a packet
over that circuit.
The IS-IS routing protocol uses four routing metrics: default metric, delay metric,
expense metric, and an error metric.
The current implementation of the OSI protocol uses the IS-IS default metric only.
The default metric, by convention, is intended to measure the circuit’s capacity to
handle traffic.All ISs in the routing domain must be capable of calculating routes
based on the default metric. The other routing metrics are optional. Though they are
not used by this implementation of the OSI protocol, they are described below for
informational purposes only.
vThe delay metric measures the transit delay of the associated circuit.
vThe expense metric measures the monetary cost of utilizing the associated
circuit.
vThe error metric measures the residual error probability of the associated circuit.
Internal and External Routing
Internal or external routing involves an L2 IS routing a packet between two separate
domains. When a packet needs to be routed to another domain, the L2 IS tries to
match the address to a reachable address prefix in the internal or external routing
table. Internal and external routes are based on the cost (routing metric) to the
destination. An internal route’s cost considers the cost of routing within the domain
and the cost of routing to the destination. An external route’s cost is based only on
the cost of routing to the destination outside the routing domain. The IS chooses
the path with the lowest cost.
Using OSI/DECnet V
296 MRS V3.2 Protocol Config Ref Vol2