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Cisco ASR 901 Series Aggregation Services Router Software Configuration Guide
OL-23826-09
Chapter 8 Configuring Ethernet Virtual Connections
Understanding EVC Features
DHCP Client on Switch Virtual Interface
The DHCP client retrieves the host information from the DHCP server and configures the SVI interface
of the Cisco ASR 901 router. If the DHCP server is unable to provide the requested configuration
parameters from its database to the DHCP client, it forwards the request to one or more secondary DHCP
servers defined by the network administrator. DHCP helps you to dynamically assign reusable IP
addresses to clients.
Hosts are connected to secondary VLANs, and the DHCP server assigns them IP addresses from the
block of addresses assigned to the primary VLAN. When new devices are added, the DHCP server
assigns them the next available address from a large pool of subnet addresses. In Cisco ASR 901 router,
the DHCP client is supported only on SVI interfaces and for IPv4 addresses.
Split-Horizon
The split-horizon feature allows service instances in a bridge domain to join groups. Service instances
in the same bridge domain and split-horizon group cannot forward data between each other, but can
forward data between other service instances that are in the same bridge domain, but not in the same
split-horizon group.
Service instances do not have to be in a split-horizon group. If a service instance does not belong to a
group, it can send and receive from all ports within the bridge domain. A service instance cannot join
more than one split-horizon group.
Use the bridge-domain bridge-id split-horizon group group_id service-instance command in the
configuration mode to configure a split-horizon group. The group_id is a number from 0 to31. All
members of the bridge-domain configured with the same group_id are part of the same split-horizon
group. EFPs that are not configured with an explicit group_id do not belong to any group.
You can configure no more than 12 service instances per bridge domain. When a bridge domain contains
a service instance that is part of a split-horizon group, this decreases the number of service instances
allowed to be configured in that split-horizon group. The router supports up to 32 split-horizon groups
plus the default (no group).
If a service instance joins split-horizon group, it can have no more than 12 members in split horizon
group in the same bridge domain. We recommend that you add split horizon groups in numerical order
to maximize the number of service instances that can belong to a group.
Rewrite Operations
Use the rewrite command to modify packet VLAN tags. You can also use this command to emulate
traditional 802.1Q tagging, where packets enter a router on the native VLAN and VLAN tagging
properties are added on egress. You can also use the rewrite command to facilitate VLAN translation
and QinQ.
Use the rewrite ingress tag pop 1symmetric service-instance configuration mode command to specify
the encapsulation adjustment to be performed on the frame ingress to the EFP. Entering pop 1 pops
(removes) the outermost tag.
Note The symmetric keyword is required to complete the rewrite configuration.