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Configuration of IP-Forwarding Media Cards 143
4.6.2 Simultaneous Routing and Bridging
Ascend’s transparent bridging does not preclude the use of IP packet routing
on the same physical interface.
Bridging as well as IP version 4 (IPv4) routing can both be enabled on the
same physical interface. In this circumstance, the GRF exchanges traffic
between bridging domains and routing domains that exist on the same
physical media.
A GRF interface may simultaneously bridge layer-2 frames and route layer-3
packets. That is, it can forward frames destined to a system attached to
another LAN at the MAC layer, but still receive IP packets destined for a
remote system attached to a non-broadcast GRF interface and route those
packets at the IP layer.
This unique capability eliminates the need for separate pieces of routing
equipment to transport packets between domains.
To perform the simultaneous functions, the GRF bridging interface examines
the destination MAC address of each arriving frame. If the address is
other
than
a GRF MAC address for any interface participating in the assigned
bridge group, the packet is submitted to the bridging engine for forwarding.
When the MAC address is a GRF MAC address, the packet is forwarded to
the GRF protocol forwarding engine for routing at the protocol layer.
4.6.3 Configuration Options
The GRF supports the configuration items specified in IEEE 802.1d. A GRF
functioning as a bridge will interoperate with other bridges, including
equipment of vendors in conformance with the IEEE 802.1d standard, to
allow forwarding of frames across multiple LAN hops.
Additionally, the GRF supports 16 active IEEE 802.1 bridge groups, and will
separate traffic between groups. For example, on a GRF with six attached
FDDI rings, rings A, B, and C could form one bridge group, rings D and E
could form a second bridge group, and ring F could stand alone, using only IP
routing for its packets.
A GRF functioning as a bridge will also interoperate with other bridges to
forward frames from one bridge to the other over ATM. This will allow two
independent bridged LANs at remote locations to function as one logical
network transparently connected by ATM. This encapsulated bridging follows
the Internet standard specification in RFC-1483.